Patent Application
20130327942
Conventional systems for detecting methane use simple “point” collectors that spectrally detect the presence of the methane. These devices are limited in their ability to survey very large areas while simultaneously providing adequate spectral and radiometric sensitivity for high probability of detection with a low rate of false alarms. Other approaches have used multiple imaging spectrometers, each configured to cover a separate spectral band. However, these systems typically have large size, weight and power requirements due to multiple spectrometer optics and multiple imaging detectors.
Aspects and embodiments are directed to a system and method for remotely detecting a hydrocarbon gas, such as methane, from any remote sensing platform, such as a ground-based, space-based or airborne platform.
According to one embodiment, a multi-band imaging spectrometer comprises an objective optical system, an optical spectrometer sub-system including a diffraction grating, the optical spectrometer sub-system configured to receive and collimate an input beam from the objective optical system to provide a collimated beam at the diffraction grating, the diffraction grating configured to disperse the collimated beam into at least two spectral bands, a single entrance slit positioned between the objective optical system and the optical spectrometer sub-system and configured to direct the input beam from the objective optical system to the optical spectrometer sub-system, and a single focal plane array optically coupled to the diffraction grating and configured to receive the at least two spectral bands and to produce an image from the at least two spectral bands.
In one example the objective optical system includes a primary objective minor of positive optical power configured to reflect the input beam, a secondary objective mirror of negative optical power configured to receive the input beam from the primary objective minor and to reflect the input beam, and a third objective minor of positive optical power configured to receive the input beam from the secondary objective mirror and to reflect the input beam to the single entrance slit. In another example, the optical spectrometer sub-system includes a double-pass reflective triplet. The single focal plane array may be positioned at an image plane between the single entrance slit and the double-pass reflective triplet. In one example, the diffraction grating has a single blaze angle. In one example, the at least two spectral bands include the short-wavelength infrared band and the mid-wavelength infrared spectral band. In another example, the at least two spectral bands include the short-wavelength infrared band and the long-wavelength infrared spectral band. In another example, the at least two spectral bands include the mid-wavelength infrared band and the long-wavelength infrared spectral band.
The single focal plane array may include at least one photo-detector coupled to at least one read-out integrated circuit. In one example, the single focal plane array includes a monolithic photo-detector coupled to a monolithic read-out integrated circuit. In another example, the single focal plane array includes at least two discrete photo-detectors coupled to a monolithic read-out integrated circuit. In another example, the single focal plane array includes a monolithic photo-detector coupled to at least two discrete read-out integrated circuits. In another example, the single focal plane array includes at least two discrete photo-detectors coupled to a corresponding at least two read-out integrated circuits.
According to another embodiment, a method of remote hydrocarbon gas detection using an imaging spectrometer comprises directing an input light beam through a single entrance slit, collimating the input light beam to provide a collimated beam, dispersing the collimated beam into at least two spectral bands, the spectral bands being separated in the spectral dimension, directing the at least two spectral bands to an imaging detector, and imaging and providing a spectral analysis of the at least two spectral bands at the imaging detector.
In one example the method provides remote detection of methane, the input light beam includes infrared light, and dispersing the collimated beam into the at least two spectral bands includes dispersing the infrared light into at least two of the short-wavelength infrared band, the mid-wavelength infrared band, and the long-wavelength infrared band. In one example, dispersing the collimated beam into the at least two spectral bands includes diffracting the collimated beam with a diffraction grating to provide at least two diffraction orders. In another example, directing the input light beam through the single entrance slit includes reflecting the input light beam with a reflective objective optical system to direct the input light beam to the single entrance slit.
Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments are discussed in detail below. Embodiments disclosed herein may be combined with other embodiments in any manner consistent with at least one of the principles disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment.
Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures:
Aspects and embodiments are directed to a compact multi-band imaging spectrometer, and to the use of a single such spectrometer to cover multiple discrete spectral regions for hydrocarbon gas detection. Imaging spectrometers are used to provide an image and also a spectral analysis of the image in a selected wavelength band of interest. These images and spectral analyses may be used to remotely detect the presence of various chemical compounds, including hydrocarbon gases, such as methane, ethane and/or propane, for example. Methane gas has multiple spectral absorbance features in the infrared wavelength band, including features in the short-wavelength infrared (SWIR) spectral region from 2.1 to 2.6 micrometers (μm), mid-wavelength infrared (MWIR) region from 3.1 to 3.5 μm, and long-wavelength infrared (LWIR) region from 7.2 to 8.2 μm. Remote sensors attempting to detect methane, or other gases, from any appreciable distance (for example, airborne or space-based sensors) will suffer from signal attenuation due to atmospheric absorption. As a result, accounting for absorption in the atmosphere, useful spectral features for methane detection include SWIR features from 2.1 to 2.5 μm, MWIR features from 3.3 to 3.5 μm, and LWIR features from 7.7 to 8.2 μm. Accordingly, a spectrometer capable of collecting and analyzing all three of these discrete spectral regions, with little to no band-to-band mis-registration, may be desirable for methane detection. Similarly, a compact spectrometer capable of collecting and analyzing discrete spectral regions containing useful features associated with other hydrocarbon gases may also be desirable.
Some approaches for multi-band detection have included using a multi-band spectrometer with multiple entrance slits to spatially separate the different spectral bands. For example, a three-band spectrometer may use three entrance slits positioned between the foreoptics and the focal plane array.
Since in this example, the three discrete spectral bands are offset from one another in the spatial dimension, a slit re-formatter is used to co-align the fields of view of the regions 110 of the focal plane array 100.
In contrast to the above-discussed approach, aspects and embodiments are directed to a compact multi-band imaging spectrometer in which different diffraction orders from a diffraction grating are used to offset discrete spectral regions in the spectral dimension, rather than the spatial dimension. A single entrance slit may be used to provide spatial co-registration of the spectra from all discrete spectral bands, and avoid band-to-band mis-registration. As discussed in more detail below, a single blaze angle diffraction grating may be used to provide high diffraction efficiency for all selected discrete spectral bands. In addition, multiple detector configurations may be implemented to optimize system-level performance, such as the use of monolithic or discrete detectors and/or read-out integrated circuits (ROICs) for the multiple spectral regions. Embodiments of the imaging spectrometer are scalable with multiple configurations to provide any two or all three discrete infrared spectral bands suitable for methane detection. Similarly, embodiments of the imaging spectrometer may be configured to provide any or all of the discrete spectral bands suitable for other hydrocarbon gas detection, such as detection of ethane or propane, for example. Furthermore, certain embodiments may use a variable number of spectral channels per discrete spectral band to optimize spectral ranges and sampling intervals, as discussed in more detail below.
It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, elements and features discussed in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiment.
Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Any references to embodiments or elements or acts of the systems and methods herein referred to in the singular may also embrace embodiments including a plurality of these elements, and any references in plural to any embodiment or element or act herein may also embrace embodiments including only a single element. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms.
Referring to
According to one embodiment the foreoptics 320 includes an all-reflective objective, which may be made solely of minors and with no lenses. The spectrometer 330 may include a reflective triplet spectrometer, one example of which is disclosed in U.S. Pat. No. 7,382,498.
In one embodiment, the reflective triplet 430 includes a primary mirror 435 having a positive optical power, a secondary mirror 440 having a negative optical power, and a tertiary minor 445 having a positive optical power. The three mirrors of the spectrometer 330 collimate the incoming beam 450 received via the entrance slit 420 and provide a collimated output beam 455 at a dispersive element 460. As discussed above, in one example, the dispersive element 460 is a diffraction grating. The dispersive element 460 is configured and oriented to receive and disperse the collimated output beam 455 and to direct the dispersed light 465 back through the double-pass reflective triplet to be incident on the detector 340 at the image plane 425. The angular direction of the dispersed light is determined by the spatial orientation of the diffraction grating 460. The blaze angle and diffraction order(s) of the diffraction grating determines the spectral dispersion of the collimated output beam 455.
Thus, the reflective triplet of the spectrometer 330 is referred to as a “double-pass” optical component because the light beams travel through the reflective triplet 430, and are collimated on the way to the dispersive element 460. Then, on the return path from the dispersive element 460, the light travels through the reflective triplet 430 and is imaged on the image plane 425. Although not shown in
As discussed above, the focal plane array forming detector 340 may have numerous different configurations selected to optimize system performance over the spectral bands of interest.
According to one embodiment, the focal plane array 500 may be further configurable in that it may include monolithic or discrete detector materials and/or read-out integrated circuits (ROICs). For example, referring to
In another example, the focal plane array 500 includes multiple discrete photo-detectors 630, 635 coupled to a monolithic ROIC 620, as shown in
Referring to
Thus, embodiments of the multi-band imaging spectrometer may be configured and optimized in various ways to provide good multi-band spectral performance for remote detection of a gas of interest. A single entrance slit may be used to direct the incoming electromagnetic radiation to the spectrometer components 330, thereby avoiding band-to-band mis-registration. The radiation is dispersed into its spectral components using a diffraction grating and multiple diffraction orders to achieve spectral separation of two or more spectral bands of interest. The detector 340 may be configured in various ways, as discussed above, using different materials and any of several photo-detector and ROIC configurations to achieve good performance for all spectral bands.
The function and advantages of these and other embodiments will be more fully understood from the following examples. The examples are intended to be illustrative in nature and are not to be considered as limiting the scope of the systems and methods discussed herein. Table 1 below provides a summary of the following four examples which are discussed in more detail below. For simplicity, each of the four examples used a fixed number of spectral channels, namely 256, per discrete spectral band (e.g. the short wave infrared, SWIR). However, as discussed above, in other embodiments, the number of channels per band may be varied to optimize performance.
In each example, the design should obtain separation (in the spectral) direction of each band, such that the bands do not land on the same part of the focal plane array (see
This example demonstrates performance of an embodiment of a dual-band spectrometer configured to detect and analyze at least a portion of each of the SWIR and MWIR spectral bands. In particular, referring to
In this example, the third diffraction order was used for the MWIR spectral band, and the fourth diffraction order was used for the SWIR spectral band. The spectral sampling interval (SSI) was 1.4 nm for the third diffraction order and 1.05 nm for the fourth diffraction order. Table 2 provides the center wavelengths (CWL) detected by each of several example channels in the focal plane array 500.
In this example, a diffraction grating (for dispersive element 460) having a blaze wavelength of 9.7 μm was used, resulting in a peak diffraction efficiency (DE) of approximately 95% at the grating blaze wavelength. Table 1 above provides the average diffraction efficiency for each spectral band. Table 3 below provides the diffraction efficiencies at certain wavelengths within each of the SWIR and MWIR spectral bands for this example configuration.
This example demonstrates performance of an embodiment of a dual-band spectrometer configured to detect and analyze at least a portion of each of the SWIR and MWIR spectral bands. In particular, referring to
In this example, the fifth diffraction order was used for the MWIR spectral band, and the second diffraction order was used for the LWIR spectral band. The spectral sampling interval (SSI) was 2.5 nm for the second diffraction order and 1 nm for the fifth diffraction order. Table 4 provides the center wavelengths (CWL) detected by each of several example channels in the focal plane array 500.
In this example, a diffraction grating (for dispersive element 460) having a blaze wavelength of 16.8 μm was used, resulting in a peak diffraction efficiency (DE) of approximately 95% at the grating blaze wavelength. Table 5 below provides the diffraction efficiencies at certain wavelengths within each of the MWIR and LWIR spectral bands for this example configuration.
This example demonstrates performance of an embodiment of a dual-band spectrometer configured to detect and analyze at least a portion of each of the SWIR and LWIR spectral bands. In particular, referring to
In this example, the third diffraction order was used for the SWIR spectral band, and the first diffraction order was used for the LWIR spectral band. The spectral sampling interval (SSI) was 5 nm for the first diffraction order and 1.7 nm for the third diffraction order. Table 6 provides the center wavelengths (CWL) detected by each of several example channels in the focal plane array 500.
In this example, a diffraction grating (for dispersive element 460) having a blaze wavelength of 9.6 μm was used, resulting in a peak diffraction efficiency (DE) of approximately 95% at the grating blaze wavelength. Table 7 below provides the diffraction efficiencies at certain wavelengths within each of the MWIR and LWIR spectral bands for this example configuration.
This example demonstrates performance of an embodiment of a three-band spectrometer configured to detect and analyze at least a portion of each of the SWIR, MWIR and LWIR spectral bands. In particular, referring to
In this example, the third diffraction order was used for the SWIR band, the second diffraction order was used for the MWIR spectral band, and the first diffraction order was used for the LWIR spectral band. The spectral sampling interval (SSI) was 3 nm for the first diffraction order, 1.5 nm for the second diffraction order, and 1 nm for the third diffraction order. Table 8 provides the center wavelengths (CWL) detected by each of several example channels in the focal plane array 500.
In this example, a diffraction grating (for dispersive element 460) having a blaze wavelength of 6.9 μm was used, resulting in a peak diffraction efficiency (DE) of approximately 95% at the grating blaze wavelength. Table 8 below provides the diffraction efficiencies at certain wavelengths within each of the SWIR, MWIR and LWIR spectral bands for this example configuration.
Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.
