Patent Application
20240361246
The invention relates to systems and methods for measuring chlorophyll fluorescence. In particular the invention relates to an installation suitable for use in commercial greenhouses, vertical farms, or outdoor farms for measuring the chlorophyll fluorescence of plants located across a target area using optical analysis.
In plant growth, chlorophyll fluorescence is the phenomenon by which absorbed light is re-emitted by chlorophyll molecules in the plant during de-excitation. Light absorbed by a plant can generally contribute to electron transport, heat, or fluorescence, and the ratio of these can vary based on environmental factors and factors associated with the plant physiology. Measuring chlorophyll fluorescence can be used as an indicator for several aspects of plant growth, including rates of CO2 assimilation by the plant via photosynthesis (electron transport through the photosystem), perception of biotic or abiotic stress, and the impact environmental changes (e.g. temperature, humidity, irrigation, carbon dioxide concentration) are having on the growth efficiency of the plant. As such, chlorophyll fluorescence is a useful technique for evaluating the yield, nutrition, and quality of agricultural crops.
Chlorophyll fluorescence is usually assessed by firstly exposing a dark-adapted plant to a short high intensity flash of light in order to saturate the photochemical processes in the plant, which allows the fluorescence levels to reach their maximum. The fluorescence then returns to its minimum levels in the dark adapted state. Subsequently, an actinic light is applied, and saturation pulses applied again. Fluorescence rises to a new light-adapted maximum in response to the pulses, before falling to a light-adapted minimum. The ratio of these fluorescence levels provides various indications of the state of the plant, including its efficiency, i.e. the proportion of incident light that is contributing to useful electron transport.
A description of the theory behind chlorophyll fluorescence analysis can be found in “Chlorophyll fluorescence analysis: a guide to good practice and understanding some new applications.”, by E. Murchie and T. Lawson, Journal of experimental botany, 64 13 (2013): 3983-98, DOI: 10.1093/jxb/ert208.
In commercial plant growth, it is known to bathe a plant bed in light and optically monitor the plants for fluorescence. Close-proximity measurement techniques are also known, by which the pulses of light and the subsequent fluorescence are carried through an optical fiber clamped or held close to a region of interest. Both of these techniques suffer from problems which it would be desirable to overcome. In particular, the former techniques are very energy demanding, owing to the need for regular bright pulses of light, and also generally have relatively low sensitivity to fluorescence. On the other hand, the close-proximity techniques typically require manual operation and can damage the plant being measured. The present invention aims to provide a system for performing chlorophyll fluorescence analysis in a commercial setting that does not suffer from the above-noted drawbacks associated with known chlorophyll measuring systems.
In accordance with a first aspect of the present invention, there is provided a system for measuring chlorophyll fluorescence, the system comprising: at least one light source configured to emit substantially collimated or convergently focused light so as to irradiate an area with light; an actuator configured to controllably direct the light from the light source so as to enable the light source to irradiate a plurality of different areas with light; and an optical sensor configured to detect chlorophyll fluorescence from each area irradiated with light.
This system uses an actuated light source that emits substantially collimated or convergently focused light in order to irradiate small areas of a canopy of a plant crop. The system will typically be mounted over a crop and will irradiate down onto the canopy in small areas corresponding to measurement target position. Irradiating small areas of a canopy at a time allows the light intensity threshold for saturation and measurement pulses to be reached at much lower overall energy cost than systems which bathe the canopy in light pulses. Actuating this directed light source enables regions of interest to be measured across a wider area of the crop canopy, ensuring a large-scale picture of crop growth can still be produced. Additionally, actuating the light source to illuminate target areas off the vertical axis, i.e. with some horizontal component, can allow you to make equally accurate measurements of targets under the canopy. This targeted measurement system can be implemented without physical contact with the plant, and without the need for a manual operator and so offers significant advantages over known systems for measuring plant fluorescence.
It will be appreciated that the system does not require precise collimation of the light source, and indeed commercially available lasers have some degree of beam divergence. The term “substantially collimated” will be understood to encompass a light source with a beam divergence of 10° or less. More tightly collimated light will, of course, be preferred, and indeed, it will be preferred to have a beam divergence of 5° or less, more preferably 2° or less, even more preferably 1° or less, most preferably 0.1° or less. Commercially available lasers will typically have a beam divergence of much less than 0.1°, but as will be explained further below, the present system can be implemented with light sources that have much more loosely collimated light.
As noted above, the system includes an actuator configured to controllably direct the light from the light source so as to enable the light source to irradiate a plurality of different areas with light. This may typically involve an actuator that is configured to move the light source so as to direct the light from the light source. That is, by moving the whole light source, it is possible to change the area irradiated with light from that light source. Alternatively, or additionally, the actuator may be configured to move one or more optical elements, such as mirrors, relative to the light source so as to direct the light from the light source. The use of, for example, a scanning mirror or a light-steering microelectromechanical system may allow the light from the light source to be directed between different areas without movement of the light source.
It is particularly preferred that the system includes an actuator configured to move the optical sensor and/or to move one or more optical elements, such as mirrors, relative to the optical sensor so as to enable the optical sensor to detect chlorophyll fluorescence from each area irradiated with light. This is preferred as it enables the optical sensor to be aimed directly at the measurement point, and also allows for smaller field of view sensors to be used, as will be discussed in more detail below. While this is preferred, alternatively, a fixed optical sensor may be used, e.g. whose field of view covers the entire space accessible to the movable light source.
Typically, the actuator configured to move the optical sensor and/or to move one or more optical elements relative to the optical sensor is the same actuator configured to direct light from the light source, i.e. one actuator directs both the light source and the optical sensor. For example, the light source and optical sensor may both be mounted to the same body or frame, which is then moved by the actuator to simultaneously point both the light source and the optical sensor at different targets. Alternatively, the actuator may control one set of optical elements that are used to direct light from the light source and to the optical sensor. While preferred, different actuators could also be used for the light source and optical sensor. It should be noted that this does not preclude the use of additional actuators to move the light source and optical sensor relative to one another, e.g. to accommodate varying measurement distances, but it is typically simpler and therefore preferred for the at least one light source and optical sensor to be fixedly mounted relative to one another.
As noted above, having a movable optical sensor enables you to use a sensor with a smaller field of view. In particular, the optical sensor may receive light from a field of view that is smaller than the total space accessible for irradiation by the at least one light source, preferably less than 10%, more preferably less than 5%, further preferably less than 1%, most preferably less than 0.5% of the total space accessible for irradiation by the light source. The total space accessible to the light source refers to the way the actuator is able to direct the light from the light source in order to irradiate different areas of a crop canopy, which will define a working area, which may be a solid angle, within which the light can be directed to make a measurement. In absolute terms, the optical sensor may receive light from a field of view of no more than 400 deg2, preferably no more than 200 deg2, more preferably no more than 50 deg2, most preferably no more than 25 deg2. A typical camera that may be used as the optical sensor may have a field of view of 18°×12° or less, or more preferably 6°×4°. The use of an optical sensor with a relatively small field of view is particularly preferred for a number of reasons. Significantly, this reduces the signal to noise ration of the chlorophyll fluorescence measurement, i.e. since the irradiated area covers a greater proportion of the field of view of the sensor. Additionally, a smaller field of view means less data to transfer and process when analysing the chlorophyll fluorescence measurements, which speeds up operation of the system and limits power usage.
In many embodiments, it is preferred that the optical sensor comprises an imaging sensor, such as a camera. This is preferred for a number of reasons. Firstly, this allows the position of the irradiated area within the field of view of the sensor to be determined, which, as explained below, allows the distance to the measurement target to be determined. Additionally, this can also allow the system to construct an image of the canopy, which can be useful for assessing the state of the crop and identifying points of interest. While an imaging sensor is preferred, the system could also use a non-imaging sensor and still obtain acceptable chlorophyll fluorescence measurements. Indeed, as will be explained below, a non-imaging sensor may be preferred in some contexts.
It may also be preferred in some cases to provide an optical sensor capable of high measurement frequency, which would permit more precise measuring of the response curve of the target to the irradiating light. A high measurement frequency in this context may mean a frequency of at least 1 kHz, preferably at least 10 kHz, more preferably at least 100 kHz, most preferably at least 1 MHz, which corresponds to a time interval between measurements of at most 1 ms, preferably at most 100 μs, more preferably at most 10 μs, most preferably at most 1 μs. Generally a higher measurement frequency will be preferred, as it will provide better resolution on the fluorescence response curve. Cameras are typically less preferred in this context as they will often have lower frames per second. Instead an optical sensor may be used which is non-imaging and/or outputs an analogue signal, as these are typically able to achieve higher measurement frequencies. Suitable optical sensors may include silicon photodetectors and/or a photodiode.
One problem faced by chlorophyll fluorescence measurement systems is that these are often installed in settings that use pulse width modulated grow lights. Such grow lights commonly include light in the same far-red wavelengths associated with chlorophyll fluorescence, and the pulse width modulation of this light source introduces a flicker into any images taken of the plants. This background flicker makes it difficult to determine in any one image the proportion of light coming from chlorophyll fluorescence and the proportion that is merely reflected from the grow lights. Conventionally, this has been addressed by changing the grow lights to remove the use of pulse width modulation, so that any background is constant for all images. However, systems that use an imaging sensor may further comprise a controller configured to distinguish between a target area containing the area irradiated with light from the at least one light source and at least one background area in a measurement image, the controller further being configured to adjust the detected intensity in the target area based on the detected intensity in the background area. The target area may typically be identified based on its brightness and its position and shape, i.e. it will typically be a bright point in the image whose position will be confined to certain areas dependent on the distance to the target. Computer vision techniques may be used to outline the target region more accurately, this could include image subtraction, contouring, averaging or denoising. Any point outside of this area in the image may be expected to contain predominantly reflected light from a grow light and so may be used to establish a background intensity level in the target area. It will be noted that the advantages of this technique are permitted by the way the system irradiates only a small area of the target and known techniques that bathe the target in light cannot get an instantaneous comparison with an unilluminated background area.
In these embodiments, the background area may be substantially the entire of the measurement image outside of the target area. This may make the background compensation computationally simple. Alternatively, the controller may be configured to select a part of the measurement image outside of the target area as the background area, preferably based on the detected intensities across the measurement image. For example, this may involve computer vision techniques that look to identify an area of the same plant or same leaf as at the target area, or may be based on intensity and proximity in the image to the target area, or may target a particular vertical or horizontal portion of the image to avoid background differences caused by, for example, a rolling shutter.
Adjusting the detected intensity in the target area will typically involve the controller calculating an average background intensity across the background area and subtracting the average background intensity from the detected intensity in the target area. For example, an average background intensity may be calculated by summing the intensity of each pixel in the background area and dividing by the number of pixels. Alternatively, the fiftieth percentile could be taken as the median background pixel intensity. The background value may then be subtracted from each pixel in the target area to compensate for artificial background lighting.
Another advantage of using an imaging sensor, particularly in the context of systems in which the light source comprises a laser, is that this can improve the quality of the chlorophyll fluorescence measurement by compensating for the laser irradiation profile. Preferably, the system further comprises a controller configured to identify a target area in a measurement image of the imaging sensor, the target area being a portion of the area irradiated by the one or more light sources, wherein preferably a laser is configured to irradiate a spot-shaped area with light and the target area is a central portion of the irradiated spot-shaped area. Because a laser will typically not have a flat top irradiance profile, some portions of the area irradiated with laser light may not reach the saturation threshold. Therefore, an imaging sensor may allow the controller to isolate a portion of the area irradiated with light in which this threshold is met, and the chlorophyll fluorescence measurement may be detected from this area. The target area may be determined in advance based on the characteristics of the laser or may be determined directly from the measurement image based on detection of reflected laser light or the detected chlorophyll fluorescence profile.
As set out above, there are various advantages to using both imaging and non-imaging sensors. Therefore, in some embodiments, it may be preferred to use both an imaging sensor and a non-imaging sensor. Preferably, the above optical sensor, i.e. which detects the chlorophyll fluorescence and is preferably actuated by the actuator, is a non-imaging sensor, such as a photodiode, and the system further comprises an imaging sensor, such as a camera. The non-imaging sensor will preferably output an analogue signal and/or will have a measurement frequency of at least 1 kHz, preferably at least 10 kHz, more preferably at least 100 kHz, most preferably at least 1 MHz. In this embodiment, the non-imaging sensor preferably has a small field of view, as defined above. In the context of a non-imaging sensor, the field of view refers to the area from which light impinges upon the sensor, and this may be controlled by one or more optical elements, such as lenses, associated with the non-imaging sensor. Preferably, the non-imaging sensor or one or more optical elements, such as mirrors, associated with the non-imaging sensor are movable by an actuator (preferably the same actuator as directs the light from the light source) to enable the optical sensor to detect chlorophyll fluorescence from each area irradiated with light. The imaging sensor preferably has a wider field of view than the non-imaging sensor, and may be used to locate the irradiated area within the space accessible for irradiation by the light source, e.g. the crop canopy, as well as performing visual assessments of the crop. For example, the non-imaging optical sensor may have a field of view of no more than 400 deg2, preferably no more than 200 deg2, more preferably no more than 50 deg2, most preferably no more than 25 deg2, whereas the imaging optical sensor may have a field of view of at least 1000 deg2, preferably at least 2000 deg2, more preferably at least 5000 deg2, most preferably at least 10000 deg2. The imaging sensor could be substantially fixed within the system, for example, so as to provide a fixed image of the crop canopy. However, alternatively both the imaging sensor and non-imaging sensor, or optical elements associated therewith, could be actuated by the same or different actuators to be directed at each area irradiated with light. The imaging sensor may be used to determine the distance to the measurement target based on the position of the area irradiated with light in the field of view, as will be described below.
Preferably, the system further comprises an optical filter arranged to block reflected light from the at least one light source and allow the optical sensor to detect light resulting from chlorophyll fluorescence. This may be performed by locating the filter in front of the optical sensor (i.e. between the optical sensor and the area irradiated by the light source), or otherwise in the optical path between the target area and the optical sensor. Typically the light source will emit light having a different wavelength or range of wavelengths than is re-emitted as chlorophyll fluorescence. For example, a light source may be 450 nm and fluorescence may be in the range 650 nm to 750 nm. Providing an optical filter for filtering out the wavelength or wavelength range emitted by the light source enables more accurate measurements of the chlorophyll fluorescence. The wavelengths re-emitted in fluorescence are longer than those emitted by the light source, and so preferably the filter is a long-pass filter, preferably configured to block light having a wavelength of approximately 650 nm or less, or may be a band-pass filter, preferably configured to block light outside of the range 650 nm to 750 nm, preferably outside of the range 650 nm to 700 nm or 700 nm to 750 nm. The use of separate band-pass filters of 650 nm to 700 nm and 700 nm to 750 nm, independently movable into the optical path of the sensor may allow for PSI and PSII fluorescence measurements to be made separately. A particularly preferred type of filter is an interference filter. Interference filters are inexpensive, but the filtered wavelengths depend on the angle of incidence. Because only a small area is irradiated with light, the light from across this area will be incident on the filter will substantially the same angle of incidence, meaning an interference filter can be particularly effective. This is especially the case when the optical sensor is movable, since pointing the sensor at the irradiated area will mean light is always incident on the sensor from substantially the same direction, regardless of where the measurement is being made on the canopy.
In some cases it may be preferably to be able to selectively use the filter. For example, the optical filter may be movable relative to optical path of light impinging on the optical sensor in order to selectively block reflected light from the at least one light source, and preferably the optical filter may be located on a filter wheel. This can allow the systems to make additional measurement to assess the state of the crop. For example, it may be desirable to detect the amount of incident light that is reflected from the target position, or to build an accurate colour picture of the canopy, in order to infer more about the state of the crop. Alternatively, as mentioned above, it may be preferred to switch between different filters, such as a band-pass filters of 650 nm to 700 nm and 700 nm to 750 nm.
As alluded to above, another advantage of the present system is that it can build a picture of the height of the crop canopy. This may be achieved by providing a controller configured to determine a target distance based on the position the size, and/or the shape of the irradiated area within the field of view of the optical sensor. For example, if the light source is a laser with a 3 mm spot size, then this can be used to infer the distance to the laser spot based on the size of the spot in the field of view of the optical sensor. Alternatively or additionally, the position of the spot in the field of view of the optical sensor may change depending on distance to the target based as a result of parallax, which can also be used to determine the distance to the target. If the relative direction that the light source and the optical sensor are pointing is changed, for example by actuation of the light source, then the position of the actuator may also be needed to determine the distance to the measurement target. Finally, the shape of the spot can be used to determine distance variance across the target area, e.g. the angle of the target with respect to the direction of the beam. For example, a circular spot may imply a target that is normal to the beam, whereas an elliptical spot may imply an inclination, with one side of the target being further from the light source than the other, and the magnitude of this inclination may be determined by the degree of elongation of the spot. This distance and inclination can be factored in to the chlorophyll fluorescence measurement, to compensate for lower detected intensity at greater distances or higher inclinations, and also as another measurement for plant growth, or to build up a 3D model of the canopy. Additionally, preferably, the power of the light source is adjusted based on the position the size, and/or the shape of the irradiated area within the field of view of the optical sensor. This may allow the system to compensate for a lower energy density of the light at greater distances or steeper inclination angles.
Preferably, the actuator has two degrees of freedom so as to enable the at least one light source (and the optical sensor, if also directed by the actuator) to irradiate different areas across two dimensions with light. The system will preferably operate by taking chlorophyll fluorescent measurements across a two-dimensional array of measurement positions. Other embodiments could have only one dimension of movement, either by measuring areas along only one direction, or by using a line focused laser swept across the canopy.
While the system could operate with an actuator that translates the light source (and optionally the optical sensor), for example, along orthogonal directions, it is much preferred for the actuator to be configured to rotate the light source about one or more directions. For example, preferably the actuator has variable yaw and pitch so as to enable the at least one light source to irradiate different areas across two dimensions with light. That is, the actuator may rotate the light source about two different preferably orthogonal rotation axes, which are preferably substantially perpendicular to the direction along which light is emitted. It will be appreciated that the actuator may not directly rotate the light source, and could rotate a body to which the light source (and optical sensor) is mounted. This form of actuator enables a system to be mounted in a fixed position while still taking chlorophyll measurements from a relatively wide area. Alternatively, the system may have one direction of translational movement and one direction of rotational movement of the light source in order to take measurements from a two-dimensional area.
In some particularly preferred embodiments, the at least one light source and optical sensor are mounted within an enclosed housing comprising a transparent window through which the at least one light source and optical sensor may operate. In some embodiments, the actuator may also be mounted inside the housing, or alternatively the actuator may move the housing with light source and optical sensor inside. This arrangement helps to protect the system from the environment, which may be humid, for example, as well as from contact by other equipment or workers close to the system. This is particularly preferred in cases in which the actuator rotates the light source, as this allows the light source to take measurements from a wide area with only a small volume within which the light source is moved. In such embodiments, it may be preferred for the transparent window to be provided by an at least partially transparent dome-shaped element of the housing. The term “partially transparent” may be understood to mean that at least part of the dome-shaped housing portion is transparent, so that the light source and optical sensor may operate therethrough. The dome-shaped housing element may be arranged so that the light from the light source passes through part of the dome-shaped housing element in a direction substantially perpendicular to that part of the dome-shaped housing element, for substantially any direction in which the light source (and optical sensor) is oriented. This may minimise distortion of the light by the window, and also minimise distortion of light received by the optical sensor, particularly where this uses a relatively small field of view.
As has been mentioned above, preferably, the at least one light source comprises a laser. Preferably, the laser is configured to irradiate a spot-shaped area; however, alternatively a line focused laser configured to irradiate a line-shaped area could be used. A laser is particularly useful as it produces tightly collimated light and typically irradiates a very small area, and can be selected to use light of a desired wavelength. This is therefore able to meet the required energy density threshold for irradiated areas while using a comparatively low amount of power, and is also capable of working across a wide range of distances. Preferably the laser is an adjustable power laser. This would enable the same laser to perform both the saturation and measurement pulses. Alternatively, or additionally, this may allow the laser to adjust power to compensate for the distance to the measurement target and/or variation in irradiation area due to the target being inclined relative to the direction of incidence. It will be appreciated that the light source may comprise more than one laser, for example, if it is desired to take separate measurements using light of different wavelengths. Where a laser is used, preferably the system further comprises at least one beam shaping optical element, such as a holographic diffuser. Such beam shapers may be configured to achieve a desired spot size, shape and uniformity of the laser and so improve the chlorophyll fluorescence measurement.
Another preferred type of light source for use as an alternative or in addition to a laser would be one or more LEDs (light emitting diodes). LEDs have the advantage over lasers that they are typically less expensive and require fewer safety considerations. As will be explained in more detail below, some preferred embodiments may use both one or more LEDs and one or more lasers. The use of supplementary LEDs may reduce the required power of the laser, i.e. since less power is required from the laser to reach the required energy density for saturation and measurement pulses, and so this can help make the system safer for use around workers. Nonetheless, the system may still benefit from lower power usage.
Indeed, the at least one light source may comprise a diverging light source, of which an LED may be an example, and focusing optics for substantially collimating or convergently focusing the light emitted from the diverging light source. A diverging light source, such as an LED bulb, may have at least part of its light, which is initially emitted over a wide angular range, focused by focusing optics, such as one or more lenses, to increase the energy density at the measurement target. It will be preferred for the light to be substantially collimated by the focusing optics. It will be appreciated that light collimated by focusing optics will typically be more loosely collimated than a laser, for example, and so may suffer from a greater variation in the size of the irradiated area depending on the distance to the measurement target. In some cases, it may be preferred to convergently focus the light. This may be useful for irradiating a small area, comparative with a laser spot, at a predetermined distance. In other cases, the focusing optics may be adjustable, and may vary the focus of the light depending on the distance to the measurement target.
Regardless of the type of light source used, preferably the light source is an adjustable power light source, to allow the light source to perform both saturation and measurement pulses, and to adjust the power based on the distance to the target.
As mentioned above, the system may comprise a plurality of light sources, each configured to emit substantially collimated or convergently focused light and arranged so as to irradiate a common area with light. Irradiating of a “common area” will be understood to mean that the two light sources at least partially overlap where they impinge upon the target. It should be noted that the light sources do not necessarily have to irradiate precisely the same area. For example, it may be common for the spot from a focused LED to be larger than the spot from a laser, which could nonetheless be directed at the same target and would define a common area irradiated by both light sources. It should be noted that it is not necessarily required that all light sources carried by the system irradiate one and the same common area. For example, the system may comprise a first laser and one or more LEDs emitting light at a first wavelength for a first measurement type, and a second laser and/or one or more LEDs for emitting light at a different second wavelength for a second measurement type, and the light from these sources would not necessarily need to overlap, and indeed it may be preferred that they do not overlap.
As alluded to above, different types of light sources have their own advantages, and so it may be preferred to integrate different types in the same system in order to benefit from the strengths of each. Therefore, preferably a first of the plurality of light sources comprises a laser, preferably an adjustable power laser, and a second of the plurality of light sources comprises a diverging light source (e.g. one or more LEDs) and focusing optics for substantially collimating or convergently focusing the light emitted from the diverging light source. As noted above, supplementing a laser with one or more focused light sources of this type can reduce the required laser power in order to reach the energy density needed for saturation and measurement pulses, and so make the system safer for use around workers. Indeed, in most cases, more than only two light sources will be used. Preferably a third of the plurality of light sources comprises a diverging light source and focusing optics for substantially collimating or convergently focusing the light emitted from the diverging light source. In these preferred cases, you may have an array of diverging light sources and focusing optics, and at least one laser, all configured to irradiate a common area with light in order to measure chlorophyll fluorescence. The plurality of light sources, particularly the plurality of divergent light sources (e.g. LEDs) and associated focusing optics, may be arranged in a ring array around the optical sensor. A ring array of light sources is advantageous, since it improves the uniformity of the light on the measurement target at different irradiation directions.
Where a laser is used in combination with a diverging light source (e.g. one or more LEDs) and focusing optics for substantially collimating or convergently focusing the light emitted from the diverging light source, preferably the laser is an adjustable power laser and the system further comprises a controller configured to adjust the power of the laser based on a target distance and/or based on the power of the second light source and/or the beam divergence of the second light source. As mentioned above, a diverging light source with focusing optics will typically suffer from a greater variation in the illuminated area as a function of the distance to the measurement target. By using an adjustable laser, the laser power may be varied in order to accommodate this. For example, where the target is close and the light from the diverging light source with focusing optics is concentrated in a small area, a lower power laser spot may be needed to reach the energy density required for a saturation pulse or measurement pulse. On the other hand, where the target is further away, because collimation may not be perfect or because the target is beyond the focal point of a convergent focus, the light may be spread out over a larger area and so require more power from the laser in order to reach the required energies. Adjustment of the laser may also be based on the position, the size and/or the shape of the irradiated area (either by the laser or by the diverging light source and focusing optics) within the field of view of the optical sensor, which may be indicators of distance to the target and its inclination, as explained above.
As has been mentioned above, it may be desired to use light of different wavelengths or wavelength ranges in measurements made using the system, a system with a plurality of light sources may therefore comprise at least two of the plurality of light sources which emit light having a wavelength or range of wavelengths different from each other. While this is possible, it would also be possible to have a system that used light only of one wavelength, or range of wavelengths.
Many advantages of the present system come from constraining the system to act on small target areas during any one measurement. Preferably, the or each light source is configured to irradiate an area of less than 50 cm2, preferably less than 25 cm2, more preferably less than 5 cm2, even more preferably less than 1 cm2, most preferably less than 5 mm2, at a predetermined working distance. It will be appreciated that a focused LED, for example, may irradiate a larger area than a laser, with a laser typically having a spot size of less than 1 cm2 and often less than 5 mm2. Generally, a smaller irradiation area is preferred, as this allows a smaller field of view sensor to be used, which reduces noise and data processing requirements, and also reduces energy used by the light source in reaching the target energy density. The predetermined working distance may be in the range 0.1 m to 20 m, preferably in the range 0.1 m to 10 m, more preferably in the range 0.1 m to 5 m, most preferably in the range 0.5 m to 1 m.
The size of the irradiated area is determined by the collimation or convergence of the emitted light and the distance from the light source that the light impinges upon the target. Typically, the system will be configured to work across a range of distances, and so it may be preferred that the target irradiation area size is achieved across the whole working range, which may be 0.1 m to 20 m, preferably 0.1 m to 10 m, more preferably 0.1 m to 5 m, most preferably 0.5 m to 1 m. This may be simply achieved by ensuring that a substantially collimated light source meets the target irradiation area size at both extremes of the working range. In one notable example, the or each light source is configured to irradiate an area of less than 50 cm2, at least at one working distance in the range 0.1 m to 20 m, preferably across the whole working range. Where a laser is used, the area irradiated may be less than 1 cm2 across the whole range 0.1 m to 20 m. It is, however, more likely as a result of limitations on detection of the fluorescence at large distances that the system will have a smaller working range, and as a result smaller irradiated areas will be achievable. In one such example, the or each light source is configured to irradiate an area of less than 25 cm2 at least at one working distance in the range 0.1 m to 10 m, preferably across the whole of said range. In another example, the or each light source is configured to irradiate an area of less than 10 cm2 at least at one working distance in the range 0.1 m to 5 m, preferably across the whole of said range. Again, a laser in particular may irradiate an area of less than 1 cm2 across working ranges of 0.1 m to 10 m, or 0.1 m to 5 m. Of course, it will be apparent that the working range and irradiated area can be configured as required for a particular installation by appropriate selection of the light source and the optical sensor.
In many embodiments, there is also provided a controller configured to: a) irradiate a first area with light using the at least one light source; b) detect chlorophyll fluorescence from the first area using the optical sensor; c) redirect the at least one light source using the actuator; d) irradiate a second area with light using the at least one light source; and e) detect chlorophyll fluorescence from the second area using the optical sensor. The controller will typically repeat steps c), d) and e) for further measurement targets, i.e. by successively irradiating different areas with light using the light source and detecting chlorophyll fluorescence from said areas across a two-dimensional array of measurement positions. This allows the system to build a picture of plant growth across the canopy of the crop with a limited number of measurements.
The system may be connected to the controller and/or a data processing and/or storage device, via a network, preferably the internet. For example, the data collected by the system may be communicated to a cloud processing and/or storage network via the internet. The system may comprise a WiFi or ethernet connection module, for example, for connecting to such a network.
A plurality of systems may be provided, each as described above, for measuring chlorophyll fluorescence of plants in different areas. Each of these systems may be connected to the same network for control and/or for collecting the measurement data.
Measurements may be reported to an operator, e.g. grower, through an interface, such as a display, connected to one or more systems, or may be reported on an operator's device via an API. Where multiple systems are provided, measurements may be reported and attributed to the respective system from the which the detections were made.
An advantage of the present system is that the same light source can be used for both saturation pulses and measurement pulses. Preferably, the controller is configured to irradiate each of the first and second areas with a plurality of pulses of light of different intensities and/or durations. In particular, this may involve a first measurement phase comprising irradiating each area with at least one, preferably a plurality of measurement pulses having lower intensity, lower frequency and/or shorter duration (relative to a second measurement phase) to provide a so-called F0 measurement. In this first measurement phase, the light will typically be modulated in intensity with a low average intensity (again relative to the second phase). This will typically involve modulation between low intensity on states and an off state, although it could be varied between two different intensities, both of which may be low relative to pulses in the second measurement phase. Then, the second measurement phase may typically involve irradiating said area with at least one saturation pulse having higher intensity, higher frequency and/or longer duration than the pulses in the first measurement phase to provide a so-called Fm measurement. In this second measurement phase, the light will typically be modulated in intensity with a high average intensity relative to the first phase. This may comprise varying the light between two different intensities, both of which will typically be higher than any of the intensities used in the first measurement phase. After performing the second measurement phase, a third phase will typically follow, in which the lower intensity, lower frequency and/or shorter duration measurement pulses resume, e.g. in which the light will typically be modulated in intensity with a low average intensity (relative to the second measurement phase), such as between low intensity on states and an off state.
In one particularly preferred embodiment the controller is configured to control the at least one light source to irradiate each area with light such that, in a first measurement phase, the light is modulated in intensity to define one or more pulses of light with a low average intensity (relative to a second measurement phase), and in the second measurement phase, the light is modulated in intensity to define a plurality of discrete different pulse types, together having a high average intensity. This is different to measurements made with a saturation pulse in a conventional measurement process, as the saturation pulse (i.e. the second measurement phase in the present terminology) will typically comprise a single light pulse that is formed by the superposition of a number of different light pulses.
Preferably the intensity of irradiated light at the target area returns to substantially zero between each pulse type. It is preferable for the light intensity to return to zero between each pulse type, as this helps with the linearity requirements of the light source. However, alternatively, the intensity could return to a low baseline intensity between pulses.
In the present example, preferably the second measurement phase comprises a plurality of light pulses of a first pulse type and a plurality of light pulses of a second pulse type, wherein the light pulses of the second pulse type occur between the light pulses of the first pulse type, wherein preferably the first and second pulse types alternate. The first pulse type preferably involves the light being modulated from a low value, preferably an off state, to a first high value, such that said first pulse type has a first intensity and duration before returning to the low value, and the second pulse type preferably involves the light being modulated from the same low value, preferably the off state, to a second high value (which may be the same or different from the first high value) such that at least one, preferably both, of the intensity and duration of the second pulse type differs from that of the first pulse type in the second measurement phase. Indeed, typically, the pulses will differ in duration by several orders of magnitude. The second measurement phase here is analogous to a conventional saturation pulse; however, in the present example, the target is irradiated with a saturation pulse, which is then switched off for a short period and during this period in which the saturation pulse is off, the target is irradiated with a measurement pulse.
The above measurement profile has a number of advantages. The pulse profile is not sensitive to a rise or fall time, and is not highly sensitive to amplitude, i.e. intensity. Linearity requirements of the light source are reduced and the switch on current sensitivity is reduced. Laser safety is also improved, since two laser pulses are not superimposed.
In another preferred embodiment, the controller is configured to control the at least one light source to irradiate each area with light such that, in a first measurement phase, the light is modulated in intensity to define one or more discrete pulses of light making up a first proportion of the first measurement phase and having a low average intensity (relative to a second measurement phase), and such that in a second measurement phase, the light is modulated in intensity to define a plurality of discrete pulses making up a second proportion of the second measurement phase greater than the first proportion and having a high average intensity. The proportion of the measurement phase refers to the amount of time at which the light intensity is at a high value, e.g. the light being on, compared with the amount of time at which the light is at a low value, e.g. the light being off. The second measurement phase should have a higher proportion at the high intensity value that the first measurement phase to provide a higher average intensity. For example, these different proportions may be provided by providing the pulses in the first measurement phase with a lower frequency that the pulses in the second measurement phase. Preferably the pulses in the second measurement phase have a frequency of between 5 and 1000 kHz, preferably between 10 and 1000 kHz, more preferably between 10 and 500 kHz, so that the biological response of a crop is not affected by the light frequency and only responds to the average intensity.
Preferably, the light pulses in the first measurement phase have the same intensity as the light pulses in the second measurement phase. Preferably the light pulses in the first measurement phase have the same duration as the light pulses in the second measurement phase. Preferably the light pulses in the first measurement phase are the same as the light pulses in the second measurement phase and differ in frequency in order to provide the different average intensities.
In this embodiment, the conventional saturation pulse (equivalent to the present second measurement phase) is replaced with a train of high frequency pulses. These pulses can be the same as the measurement pulses used in the F0 measurement (the first measurement phase in the present terminology). The pulse amplitude, duration and duty cycle during the first measurement phase can be calculated to achieve a low average illumination during the first measurement phase so that photosynthesis is not triggered, and to achieve saturation of reaction centres during the second measurement phase.
An advantage of using only one pulse type, as in this embodiment, is that the driver for generating the pulses can be simple. The pulse profile is also not sensitive to rise or fall time and is not highly sensitive to amplitude of the pulse.
Linearity requirements of the light source are reduced and the switch on current sensitivity is reduced. This technique means the detector can use a lock-in amplifier. Laser safety may be improved since a single amplitude may be used, which may be lower than the peak amplitude required in a typical saturation pulse. Finally, an important advantage of this measurement technique is that the high frequency pulses allow measurements to be made at high frequency, particularly with non-imaging sensors. This allows measurements of chlorophyll fluorescence kinetics, such as OJIP.
The conversion of light intensities captured by a detector system into a chlorophyll fluorescence measurement (Fo, Fm) may require additional processing steps to obtain a representative fluorescence intensity from the plant leaf. This is due to a typical laser spot profile having a gaussian distribution of power intensity and only the central region will have sufficient light intensity for measurement. As such, the isolated region for calculating fluorescence intensity should be constrained to the central region. The region should be calculated to ensure non-overlapping, symmetrical normal distributions for the fluorescence intensities returned by the detector at each excitation intensity. In the case of an image sensor, computer vision techniques can be used to first detect the region of interest as described above, and then refine the specific pixels of interest to ensure accurate measurement.
In many embodiments, the optical path between the or each light source and the target and the optical path between the target and the optical sensor impinge upon the target area at different angles. This may be a result of spacing the light source and the optical sensor apart and aiming them at the same target. In some contexts, such an arrangement is advantageous and indeed we have already explained how this parallax may be used to help determine the distance to the target. However, in other contexts, it may be preferred to have a co-axial arrangement whereby the optical paths are substantially the same at the target. Therefore, some embodiments further comprise a beam splitter or optical coupler, wherein the light source is arranged so as to irradiate said area, i.e. the target area, along a first optical path through the beam splitter or optical coupler and wherein the optical sensor is arranged to receive chlorophyll fluorescence emitted from said area along a second optical path different from the first optical path through the beam splitter or optical coupler, wherein the light source and the optical sensor are arranged such that the optical paths are substantially co-axial, i.e. the same, between the beam splitter or optical coupler and said irradiated area. In other words, the irradiation direction at the target is substantially the same as the sensing direction of the sensor. Both a beam splitter and an optical coupler convert each input direction to two output directions and the present arrangement uses one of the two outputs from the light source to then receive the fluorescent light as an input, with the optical sensor being arranged on a vacant output of this fluorescent light. This allows the optical sensor to be provided effectively on axis with the light source via the beam splitter or optical coupler. This may provide that the fluorescence from the target area is always incident on the optical sensor in the same place, e.g. regardless of the nature of the target, which can make measurements faster by removing the need to identify the target.
In these embodiments, where an optical filter is used, the filter should be arranged between the beam splitter or optical coupler and the optical sensor, i.e. so that the first optical path does not pass through the filter. In this way, the irradiating light is not affected by the filter, and only the fluorescent light bound for the optical sensor is filtered.
One advantage of the use of a beam splitter is that this may further comprise focusing optics between the beam splitter and said area to be irradiated, the focusing optics being configured to substantially collimate or convergently focus light from the light source and to collimate or convergently focus chlorophyll fluorescence from said area towards the optical sensor. Hence, one set of optics can be used for shaping the light from the light source as is used to shape the light being directed onto the sensor. This may make the system less expensive to produce.
One advantage of the use of an optical coupler is that the light source may be configured to emit light into a first end of a first optical fiber and the optical sensor configured to receive light from a first end of a second optical fiber, said first and second optical fibers being coupled together by the optical coupler so that a second end of either the first or second optical fiber emits light from the light source to irradiate said area and said second end receives chlorophyll fluorescence from said area, wherein preferably the actuator is configured to controllably direct the light from the light source by moving said second end of either the first or second optical fiber In particular, this arrangement means that the light source and the optical sensor can remain stationary, and the directing of the light and aiming of the optical sensor performed by moving the optical fibers. In such an embodiment, it may be preferred to include focusing optics for collimating or convergently focusing light emitted from the second end of either the first or second optical fiber towards the target and also for focusing chlorophyll fluorescence for coupling it into the optical fiber.
The above embodiments are particularly advantageous when used with non-imaging optical sensors. This is because the chlorophyll fluorescence will always be incident in the same place on the optical sensor and no imaging is required to identify the target area within a field of view. As mentioned above, non-imaging sensors are typically capable of a higher measurement frequency, which can be useful for measuring the pulse response curve.
In accordance with a second aspect of the invention, there is provided a method of measuring chlorophyll fluorescence, the method comprising: a) irradiating a first area with light using at least one light source configured to emit substantially collimated or convergently focused light; b) detecting chlorophyll fluorescence from the first area using an optical sensor; c) moving the at least one light source using an actuator; d) irradiating a second area with light using the at least one light source; and e) detecting chlorophyll fluorescence from the second area using the optical sensor.
This method corresponds to a method of measuring chlorophyll fluorescence using the system according to the first aspect of the invention, in which the light source is movable by an actuator in order to selectively irradiate different areas for testing. Each of the advantageous features described above with respect to the first aspect of the invention may likewise be implemented in the method according to this aspect.
It will be appreciated that steps c), d) and e) may be repeated for one or more further areas. These further areas may include areas different from the first and second areas, but could also include repeating measurements at substantially the same first and/or second areas, e.g. at a later time, in order detect changes in the chlorophyll fluorescence over time. The present method may be used to provide real time monitoring of plant photosynthetic efficiency, the conversion of light photons into excited electronic states in a photochemical process, by measuring chlorophyll fluorescence over time and reporting changes to an operator, e.g. grower.
This method may be used to determine common chlorophyll fluorescence metrics, such as Fv/Fm, from the detected chlorophyll fluorescence. The method may also be used to monitor and report plant growth and productivity. For example, chlorophyll fluorescence metrics may be measured over time and changes identified and reported to an operator, e.g. the grower. From these monitored metrics, it may be possible to recommend changes to the environment of the plant measured. For example, the method may involve recommending changes to one or more of lighting, e.g. artificial lighting, temperature, water, humidity, CO2 or O2 concentration, soil nutrient concentration and soil pH based on the detected chlorophyll fluorescence or the change in the detected chlorophyll fluorescence over time.
In accordance with a further aspect of the invention, there is provided a method of measuring chlorophyll fluorescence, the method comprising: irradiating a first area with light using at least one light source, wherein irradiating the first area comprises irradiating the first area with light such that, in a first measurement phase, the light is modulated in intensity to define one or more pulses of light with a low average intensity, and in a second measurement phase, the light is modulated in intensity to define a plurality of discrete different pulse types, together having a high average intensity, wherein preferably the intensity of irradiated light at the target area returns to substantially zero between each pulse type in the second measurement phase; and detecting chlorophyll fluorescence from the first area using an optical sensor. Preferably the second measurement phase comprises a plurality of light pulses of a first pulse type and a plurality of light pulses of a second pulse type, wherein the light pulses of the second pulse type occur between the light pulses of the first pulse type, wherein preferably the first and second pulse types alternate. The first pulse type preferably involves the light being modulated from a low value, preferably a substantially off state, to a first high value, such that said first pulse type has a first intensity and duration before returning to the low value, and the second pulse type preferably involves the light being modulated from the same low value, preferably the substantially off state, to a second high value (which may be the same or different from the first high value) such that at least one, preferably both, of the intensity and duration of the second pulse type differs from that of the first pulse type in the second measurement phase.
In accordance with another aspect of the invention, there is provided a method of measuring chlorophyll fluorescence, the method comprising: irradiating a first area with light using at least one light source, wherein irradiating the first area comprises irradiating the first area with light such that in a first measurement phase, the light is modulated in intensity to define one or more pulses of light making up a first proportion of the first measurement phase and having a low average intensity, and such that in a second measurement phase, the light is modulated in intensity to define a plurality of discrete pulses making up a second proportion of the second measurement phase different from the first proportion of the first measurement phase and having a high average intensity. Again, the proportion of the measurement phase refers to the amount of time at which the light intensity is at a high value, e.g. the light being on, compared with the amount of time at which the light is at a low value, e.g. the light being off. The second measurement phase should have a higher proportion at the high intensity value that the first measurement phase to provide a higher average intensity. For example, these different proportions may be provided by providing the pulses in the first measurement phase with a lower frequency that the pulses in the second measurement phase. Preferably the pulses in the second measurement phase have a frequency of between 5 and 1000 kHz, preferably between 10 and 1000 kHz, more preferably between 10 and 500 kHz, so that the biological response of a plant is not affected by the light frequency and only responds to the average intensity. Preferably, the light pulses in the first measurement phase have the same intensity as the light pulses in the second measurement phase. Preferably the light pulses in the first measurement phase have the same duration as the light pulses in the second measurement phase. Preferably the light pulses in the first measurement phase are the same as the light pulses in the second measurement phase and differ in frequency in order to provide the different duty cycle ratio and hence the different average intensities. In this embodiment, the conventional saturation pulse (equivalent to the present second measurement phase) is replaced with a train of high frequency pulses. These pulses can be the same as the measurement pulses used in the F0 measurement (the first measurement phase in the present terminology).
The invention will now be described with reference to the accompanying drawings, of which:
A first embodiment of the invention will now be described with reference to
As shown in
Within the housing 10 are control electronics 60. This includes a controller for the actuator 50, a controller for the laser 20, and a controller for the camera 30, which are coordinated to take a series of measurements of different targets in the canopy C below the system. These control electronics are mounted on the base 12 of the housing 10 so that they are stationary during use.
Also within the housing 10, as mentioned, are the laser 20, camera 30, support 40 and actuator 50, which are shown in isolation in
The support 40 is a piece of moulded plastic that attaches to the housing of the tilt motor 55 of the actuator 50 and supports both of the laser 20 and the camera 30, along with the filter 35. The support 40 comprises, in particular, a camera-holding portion 41 and a laser holding portion 42. These are arranged so that both the laser 20 and the camera 30 are pointed along the same direction, i.e. parallel with one another, with the laser 20 being spaced a few centimetres above the camera 30.
The camera-holding portion 41 includes a rear plate 41a, to which the camera 30 is attached, pointing away from the rear plate and towards the target. It also comprises two arms 41b, which extend forwards from the rear plate 41a, beneath the camera barrel. The two arms connect at their forward end to the filter assembly 33. The filter assembly comprises a filter wheel 34 connected to a filter motor 36 mounted on end of the arms 41b. The filter motor 36 operates to rotate the filter wheel 34 so that either filter 35 or filter 37 within the filter wheel may be placed in front of the camera 30. The filter 35, which is held in front of the lens of the camera, may be a long-pass interference filter, which filters out perpendicularly incident light having a wavelength of less than 650 nm, to allow the camera 30 to measure the fluorescence in the range 650 to 750 nm, or may be a band-pass filter, blocking all light outside of the range he range 650 to 750 nm. Filter 37 may be a different filter, such as a band-pass filter of 650 to 700 nm or 700 nm to 750 nm, for acquiring PSI or PSII fluorescence in isolation. Alternatively, filter 35 may be a band-pass filter of 650 to 700 nm and filter 37 a band-pass filter of 700 nm to 750 nm to allow the system to measure both PSI and PSII fluorescence independently of one another.
The laser-holding portion 42 of the support 40 is a substantially cylindrical sleeve that is open at both ends, with a smaller opening at the front end, through which the laser beam is emitted. The laser-holding portion 42 receives a substantially cylindrical laser in the open rear end, which points forwards through the open front end.
The laser 20 itself is configured to emit collimated light having a wavelength of approximately 450 nm, with a beam diameter of 3 mm and should be capable of delivering saturation pulses lasting approximately one second to the target having an average photosynthetic photon flux density (PPFD) of 8000 μmol m−2 s−1. The laser should also be capable of delivering measurement pulses lasting between 1 μs and 10 ms.
The camera 30 may be an OV9281 manufactured by OmniVision® of 4275 Burton Drive, Santa Clara, California 95054 USA. This camera may be configured with a relatively small field of view of 6°×4°, which decreases the noise in the signal and increases the sensitivity to the area irradiated by the laser. As mentioned previously, the field of view of the camera must be large enough for the laser spot to be visible across the working range of the system. In embodiments in which the laser 20 and the camera 30 are pointed parallel to one another, the laser spot will be in the centre of the field of view at infinity, and will be closer to the top of the field of view the closer the target is to the system. Indeed, as we have mentioned before, the position of the laser in the field of view of the camera, as well as the spot size, can be used to determine the distance to the target. In other embodiments, the laser and the camera may not be parallel and may instead define a small but fixed angle in the direction they are pointed, so that the laser spot is in the middle of the field of view of the camera roughly in the centre of the working range of the system. This may cause the spot to move across the full field of view of the camera depending on the distance to the target, and so offer greater sensitivity in measuring the distance to the target. An angle of approximately 8° has been found to be suitable for a typical separation distance of camera and laser and for a working distance in the range 0.5 to 1 m, although it will be appreciated that these can be configured as needed depending on the particular system installation, including the working range and the spacing of the camera and laser. In other embodiments, the angle between the laser and the camera could be adjustable. The control system may adjust the angle between the laser 20 and the camera 30 until the spot is in the centre of the field of view, and then the distance to the target determined by the angle between the camera and the laser needed to achieve this centring of the laser spot.
Now that the laser and camera are pointed at the target, in step S200, the target is irradiated with light using the laser. Typically, a first measurement (so-called F0 measurement) will comprise a series of pulses, which in one example have an amplitude of up to 6000 μmol m−2 s−1, pulse width of 0.1 μs to 100 μs and period of up to 10 s so as to deliver an average PPFD at the target of as low as approximately 0.1 μmol m−2 s−1 over a measurement interval. To measure saturation (so-called Fm measurement), instead, a saturation pulse is delivered. The intensity of the saturation pulse may be varied, but in one example this is configured to provide an average PPFD at the target of approximately 8000 μmol m−2 s−1, over an interval of approximately 0.8 s. The saturation pulse may itself comprise a series of pulses from a lower amplitude to a higher amplitude, both in the range 8000 to 14000 μmol m−2 s−1, having a pulse width of 0.1 μs to 10 ms and a period of between 10 and 100 ms.
As mentioned above, before executing the required irradiation profile, the controller may first determine the distance to the target by irradiating the target and detecting the position of the light source in the field of view of the sensor, before adjusting the power of the light source so that the correct PPFD is reached accounting for any known variation in the spot size with distance as a result of beam divergence, and accounting for the inclination angle of the target relative to the beam.
In step S300, fluorescence is detected from the target using the camera. While this step is shown after step S200, it will be appreciated that these steps will largely be running in parallel, with the camera continuously detecting the fluorescence levels over time during the course of the irradiation step. The camera may take a series of images of the target during this step, and this may correspond to different measurement types. For example, the light source may initially irradiate the target with the lower energy pulses for an F0 measurement, during which time the camera may take a series of images, and then the light source may irradiate the target with higher energy pulses for an Fm, measurement, during which time, again, the camera may take a series of measurement images.
The method may also involve identifying from an image, such as shown in
In step S400, the controller checks whether there are any additional targets which require measurement. If there are, the process returns to step S100, and the controller points the light source and the sensor at a new target before repeating steps S200 and S300. If there are no targets remaining, then the process ends.
In this embodiment, the light source comprises a laser 20, which is substantially as described before in relation to the first embodiment, along with three further light sources 22a, 22b, and 22c. While only three further light sources are shown in this embodiment, it will be appreciated that more may be used.
Each of these further light sources is an LED along with associated focusing optics. Each LED should emit the same wavelength of light as the laser 20, in this case approximately 450 nm, if they are intended to be used together in the same measurement process. The focusing optics comprises, in this case, a collimating lens so that each light source 22a, 22b, 22c emits substantially collimated light, but could also comprise a condensing lens for emitting converging light targeting a focal point in the centre of the working range, or an adjustable set of lenses for adjustably focusing the light at a target distance.
The further light sources are arranged in a ring array surrounding the camera 30 so that they are each equidistant from the camera 30 located in the centre. In some embodiments, each alternating LED may have one of two wavelengths or wavelength ranges, so that the further light sources can perform measurements using two different wavelengths of light. In such an embodiment, this may be supplemented by a second laser having the second wavelength, and an adjustable filter wheel 35 may also be provided to accommodate the different types of measurement utilising different wavelengths of interest.
Each light source 22a, 22b, 22c irradiates a respective area 23a, 23b, 23c in the canopy below. For most measurements, the irradiated areas will only partially overlap and this is demonstrated in an exaggerated manner in
As shown in
The light sources 22a, 22b, 22c will typically have a greater beam divergence than the laser 20. As a result, the spot size will increase more as the distance to the target increases, and so the energy density at the target will decrease with distance more so than with a more tightly collimated laser. To compensate for this, the control electronics may adjust the power of the laser so that energy density at the target, i.e. in the area of the laser spot, remains at a desired level for the measurement being made. The energy output of each of the light sources will be known, and by knowing the beam divergence, it will be possible to determine for each light source the size of the area that is irradiated at any particular distance, which will allow the energy density at the target to be determined. As mentioned above, the distance to the target can be determined by the position and/or size of the irradiated area(s) in the field of view of the camera, with the laser spot 21 being particularly preferred for determining distance. Accordingly, the control system is able to increase the power of the laser to compensate for the lower energy density that normally accompanies a greater distance to the target and hence a larger spot size for the light sources 22a, 22b, 22c. The control system can thus achieve the target energy density for the measurement being made at any distance within the working range.
A further embodiment will now be described with reference to
This embodiment comprises a light source 20, again in the form of a laser. This laser is configured to emit collimated light having a wavelength of approximately 450 nm, with a beam diameter of 3 mm and should be capable of delivering saturation pulses lasting approximately one second to the target having an average photosynthetic photon flux density (PPFD) of 16000 μmol m−2 s−1. The laser should also be capable of delivering measurement pulses lasting between 1 μs and 1 ms. It will be noted that the laser in this embodiment is more powerful than in the preceding embodiment owing to the use of the beam splitter.
The higher powered laser is required because the laser irradiates a beam splitter 70, also carried on the support 40. In the Figure, the laser emits light along a vertical axis, with the beam being incident on the mirror at 45° angle. The beam splitter may be a half-silvered mirror, for example. The beam splitter should be configured so that, as close as possible, 50% of incident light is transmitted through the beam splitter and 50% of incident light is reflected. In the present arrangement, the light from the laser 20 that is transmitted through the beam splitter is used to irradiate a target area 21, and hence twice the irradiation power is required in order for the split beam to reach the required energy densities at the target. In this example, the light from the laser 20 that is reflected into the horizontal axis by the beam splitter is not used.
Also carried on the support 40 is an optical sensor arrangement comprising a non-imaging optical sensor 30, such as a photodiode, an optical filter 35, which may be a filter wheel substantially as described with respect to the first embodiment, and a condensing lens 38. These components are arranged on another side of the beam splitter, i.e. on the horizontal axis in the Figure and on the side of the beam splitter 70 to which none of the laser light is directed. The condensing lens 38 is preferably motorised so as to be movable in the direction towards and away from the optical sensor 30 in order to compensate for varying distance to the sample. This increases the working range of the detector and improves the resulting signal. A motorised condensing lens may be used in any of the embodiments described herein, e.g. either as part of a camera or, as in the present embodiment, spaced from a non-imaging detector. By this arrangement, chlorophyll fluorescence within a detectable area 31 is incident on the beam splitter 70 in the vertical axis, and the light that is reflected by the beam splitter into the horizontal axis is directed towards the optical sensor 30. The light reflected into this horizontal axis is incident first upon the condensing lens 38, which is arranged to focus the light reflected from the beam splitter 70 towards the optical sensor 30. As the light is focused from the condensing lens 38 towards the optical sensor 30, it passes through the optical filter 35, which may filter out all light outside of the range 650 nm to 750 nm for example. Thus, the intensity of the light received at the optical sensor 30 will be indicative of the level of chlorophyll fluorescence at the target area 21. It should be noted that the condensing lens 38 in this embodiment is not essential and could be omitted, provided that the optical sensor 30 and the filter 35 are large enough to receive sufficient light from the area 31. Additionally, while a non-imaging sensor is used, it would also be possible to use an imaging sensor, such as a camera; however depth perception of the spot due to parallax would be lost.
As can be seen in
This embodiment comprises a laser light source 20, which again should be configured to emit collimated light having a wavelength of approximately 450 nm, with a beam diameter of 3 mm and should be capable of delivering saturation pulses lasting approximately one second to the target having an average photosynthetic photon flux density (PPFD) of at least 16000 μmol m−2 s−1. A greater power may be required depending on the losses from the optical fibers. This laser injects light into a first end 81a of an optical fiber 81. This optical fiber conveys the light to an optical fiber coupler 71, which splits this input light so that, as near as possible, 50% continues towards the second end 81b of the first fiber 81 and 50% is directed into a second optical fiber 82, whereupon it travels towards a second end 82b of the second optical fiber 82. The light directed towards the second end 82b of the second optical fiber 82 is not used further in this embodiment.
The second end of the first optical fiber is mounted on a movable platform 40. The second end of the fiber is coupled to a fiber collimator 81c, which ensures that the light emitted from the fiber is converted into a substantially collimated light beam that may be directed at a target 21. This fiber collimator 81c may be a separate component attached to the end of the fiber, or may be integrated into the second end 81b of the first optical fiber 81. Preferably, the optical fibers 81, 82 have diameters on the order of 100 nm, so as to have a low numerical aperture. A relatively narrow optical fiber and low numerical aperture ensures tight collimation of the output light can be more readily achieved. The movable platform may be moved by an actuator and thus move the end of the optical fiber and the collimator in order to controllably direct the light at different target areas.
Chlorophyll fluorescence from the target 21 will be incident upon the fiber collimator 81c, which will act to couple this light into the second end 81b of the optical fiber, whereupon it will be transmitted to the optical fiber coupler 71. The optical fiber coupler thus splits the chlorophyll fluorescence so that, as near as possible, 50% continues towards the first end 81a of the first fiber 81 and 50% is directed into the second optical fiber 82, whereupon it travels towards a first end 82a of the second optical fiber 82.
The chlorophyll fluorescence travelling towards the first end 82a of the second optical fiber is passed through a fiber Bragg grating or thin film filter 35, which should be configured to transmit only light in the range 650 nm to 750 nm, to filter out light from the laser. Finally, the filtered light is received at a coupled optical sensor 30, which detects the intensity of the chlorophyll fluorescence.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21194432.7 | Sep 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2022/052237 | 9/1/2022 | WO |
