Patent Application
20110255075
The invention relates to an assembly especially for optical spectroscopy with means for spectral splitting of incidental light and an optical detector for spectrally resolved detection of a spectral region of the incident light as well as a method for determining a temperature value of an optical detector for spectrally resolved detection of a spectral region of incident light.
The sensitivity of an optoelectrical light detector, for instance a CCD or CMOS sensor depends especially on the temperature of the detector. This temperature-dependency of the detector sensitivity inhibits the measuring accuracy of spectrometers, especially near its upper limiting wavelength (at the long-wave end of the region of application).
In order to compensate for temperature swings, it is known to acquire the detector temperature by means of a thermal sensor. In DE 10 2005 003 441 A1 it is further described that a second temperature sensor can be utilized for more exact determination of the influence of the ambient temperature on the detector. Based on the determined detector temperature, it is possible to control a temperature unit of the detector, for instance heating or cooling, such that the detector temperature remains constant. In this way the accuracy of the measurement of the incidental luminous power on the detector can be improved. A temperature sensor, however, has the disadvantage that the temperature of the detector can only be determined with limited accuracy as the sensor cannot be arranged in or inside the detector but must always be arranged at a distance from the detector. Correspondingly also the accuracy of the compensation of temperature swings is limited.
Detectors with various materials for different wavelength regions are used for spectrometers. The upper limiting wavelength, especially, is a material property that must be taken into account in the development of spectrometers.
Materials with higher limiting wavelengths generally have the advantages of a larger usable spectral region and a lower temperature dependency of the sensitivity at a given wavelength. However, they are more expensive and have the disadvantage of a greater noise that can only be reduced by cooling.
The object of the invention is to improve the assembly mentioned at the start so that the detector temperature can be measured with a high degree of accuracy with little effort, especially the for the purpose of the compensation of temperature swings.
The task is solved by means of an assembly with the features mentioned in claim 1 and by a method that possesses the features mentioned in claim 12.
Advantageous embodiments of he invention are given in the subsidiary claims.
According to the invention, in addition to means for spectral division of incident light and an optical detector for spectrally resolved detection of a spectral range of the split light, a second optical detector is provided for detection of a partial range of this spectral range as a reference detector, whereby a sensitivity of the reference detector is substantially temperature-independent or at least significantly less temperature dependent than a corresponding sensitivity of the first detector. By means of the first optical detector there is determined a detector signal for a part region of the spectral region to be detected and by means of a second optical detector an (almost) temperature-independent reference signal for the same part region of the spectral region.
The luminous power falling on the reference detector is in almost constant relationship to the luminous power that the first detector acquires in the part region used for reference. Because of the substantial temperature independence of the sensitivity of the reference detector, the relationship of the signals of both detectors is a highly accurate dimension for the relative temperature of the first detector.
In order to determine the relationship, an integrated luminous power above the part spectral region is sufficient so that the reference detector does not need to be spectrally or spatially resolving. Thus, in comparison with the first detector, it can be implemented with substantially less size and with substantially simpler structure. Because of the, at least, approximate temperature-independence of the sensitivity of the reference detector, a cooling of the reference detector for the application according to the invention is not necessary. A thermal temperature sensor is also not necessary either for the first detector or for the reference detector.
Appropriately, the reference detector possesses a higher upper limiting wavelength than the first detector. In this way a lesser temperature-dependency of the reference detector is achieved.
Especially advantageous are embodiments in which the first detector (main detector) is silicon (Si detector) and the reference detector is an indium-gallium-arsenide-semiconductor detector (InGaAS detector).
Advantageously, the whole of the spectral region to be detected by means of the first detector and the part region of the spectral region to be detected by the reference detector are detectable simultaneously. The highest possible accuracy of the temperature value can be achieved with this simultaneity. The simultaneous detection of the whole of the part spectral region is arrived at in different ways.
In a first advantageous embodiment, the reference detector is assembled such that the light that is reflected from the first detector and spectrally resolved by it is detectable. In this manner a beam splitter and a band pass filter can be dispensed with. In addition, the effective sensitivity of the assembly is high; because the reference detector only receives light not detectable to the first detector. The luminous power detectable by the first detector remains unchanged despite the reference measurement.
In a second advantageous embodiment the means for spectral splitting include a depletive grid in which the reference detector is assembled such that because of it light is detectable in a different diffraction of the grid than through the first detector. In this manner a beam splitter can be dispensed with. A band pass filter can be dispensed with when the reference detector is not arranged in the zero order. However, with the placing of the reference detector in the direction of the zero diffraction order, the spectrometer can be made compact, which is especially advantageous for the assembly in a measuring head housing. In this case a band pass filter for the part region to be detected is necessary.
In a third advantageous embodiment a beam splitter is provided, by means of which a first fraction of the incident light is led to the first detector and a second fraction is led to the reference detector whereby the reference detector is provided with a band pass filter for the part region to be detected. Typically, the beam splitter is arranged in front of the entry slit of a spectrometer. This assembly can be set up with little effort, as the placing of the reference sensor in the incident light can be relatively coarse.
Usefully, a control unit is provided which based on a detection signal of the reference detector and on at least one detection signal that corresponds to the part region of the first detector, determines a relative temperature value for the first detector. This control unit, for instance, can be assembled together with the detectors in measuring head housing. The control unit can output the determined temperature value especially for later processing over an interface. For example, both the detection signals of the first detector as well as those of the reference detector can be passed on immediately to a primary control computer, that later corrects the temperature swings of the detection signals on the basis of the reference signals.
Advantageously, on the basis of the temperature values and on a sensitivity function of the first detector, the control unit corrects the direct detection signals of the first detector. In this way there is achieved the compensation of a temperature independence of a sensitivity of a spectrally resolved optical detector with a high degree of accuracy and with little effort.
Usefully, the temperature value is determined as a quotient from the reference signal and the detection signal. The correction signal from the output detection signal of the first detector of temperature-referenced swings can, for example, be carried out purely mathematically with little effort. In this way an expensive tempering unit is not required. The control unit will preferably output the corrected detection signal in place of the uncorrected detection signal, for example to a primary control computer or on to a memory medium. In this way the correction is transparent for the control computer or another form of further processing. The assembly according to the invention can therefore also be used with existing control computers without these having to be adapted.
Also possible are embodiments in which the control unit, in dependence of the temperature value, controls a tempering unit for the first detector. This embodiment also permits the compensation of a temperature dependence of a sensitivity of a spectrally resolved optical detector with a high degree of accuracy.
Advantageously, a sensitivity function of the first detector is measured during a warming up phase or is measured after a warming-up phase in order to avoid influences of swings due to production tolerances. Alternatively a detector manufacturer can use prescribed sensitivity functions.
The invention also includes a spectrometer with a light source and an assembly as described above. In addition, the spectrometer can be encapsulated as measuring head in an enclosed housing that can be connected for output of corrected or uncorrected detection signals via an interface to an electronic bus system. Besides this, the invention also includes a computer program and a control unit that is arranged for implementing a method according to the invention.
In the following, the invention is described in more detail on the basis of embodiment examples.
The drawings show:
In all drawings the same parts have the same reference numbers.
In the sequence of the path of light, the spectrometer 10 consists of an entrance slot 1, an imaging grating 2 and a first detector 3 that is used for the detection of different spectral sub-spectral region, lines or matrix-shaped arranged detector elements 3.1, 3.2 . . . (Pixels). In addition the first detector 3 can be equipped with a tempering unit 4, namely a heating or cooling or both. For example, in its simplest form, it is a heating resistance in the neighborhood of the first detector 3. A control and evaluation unit 5 is connected to the first detector 3 and tempering unit 4. The control and evaluation unit 5 acquires a respective detection signal Di for each detector element 3.1 and controls the tempering unit 4. For the sake of simplicity the drawing shows only a first connection in place of the whole of the detection signals. For example, the spectral region to be acquired by the spectrometer 10 is 600 nm to 1800 nm. An SI-detector is correspondingly used as a first detector 3 whose upper limiting wavelength of, for instance λmax=1100 nm is just sufficient for acquiring the required spectral region.
For this purpose, the imaging grating 2 splits the incident light L arriving at the entrance slot 1 spectrally and depicts the entrance slot 1 on the first detector 3, which is arranged such that it only acquires spectrally split light S of the +1st diffraction of the grating 2 in a spectral region λmin . . . λmax. Each of its detector elements acquire one optical luminous power in a respective sub-spectral region and outputs it in the form of a respective electric detection signal Di to the control and evaluation unit 5. In this respect the structure of the spectrometer 10 is known from prior art.
Beyond the prior art, the spectrometer 10 possesses a single InGaAs detector as a reference detector 6 with a higher upper limiting wavelength than the first detector 3 of silicon. For instance, the upper limiting wavelength is 1700 nm. The reference detector 6 receives spectrally split light, S reflected from the surface of the first detector 3. Thus, a part of the incident light L is used that cannot be detected in principle by the first detector 3. Due to its surface dimension and positioning, the reference detector 6 acquires light, for instance 1040 nm to 1600 nm, which is a part region in the neighborhood of the long-wave end λmax of the spectral region of the first detector 3. Because the reference detector 6 only possesses a single detector element, it integrates the luminous power above this part spectral region. The reference detector 6 passes the acquired luminous power in the form of a detection signal R as a temperature Independent reference to the control and evaluation unit 5.
The sensitivity of the second detector 6, at the wavelength of 1050 nm, because of the higher limiting wavelength has a significantly lower temperature dependence than the Si detector 3 so that the temperature dependence of the reference detector 6 can be ignored. The luminous power in the reference detector 6 is then in a constant relationship to the luminous power that the first detector 3 acquires in the same part region of the overall detected spectral region λmin . . . λmax. The relationship of this luminous power of the two detectors 3 and 6 is therefore a relative dimension for the temperature of the first detector 3.
This temperature value can be used by the control and evaluation unit 5 for controlling the tempering unit 4, especially for regulating the heating or cooling of the first detector 3 or for the mathematical correction of the detection signal Di of the first detector 3. For this purpose, the control and evaluation unit 5 first adds up those detection signals Di of the individual detector elements 3.1 of the first detector 3 that that have a wavelength of 1040 nm to 1060 nm, i.e. the part region that corresponds to that acquired by the individual InGaAs detector 6. Then it forms the relationships of this sum ΣDi to the detection signal R of the InGaAs reference detector 6 in order to define a temperature value Trel=R/ΣDi for the first detector 3. In the case that the sub-spectral region acquired by the relevant detector elements 3.1 do not agree exactly with the limits of the part region acquired by the reference detector 6 of the overall spectral region λmin . . . λmax to be detected, then corresponding weighting factors gi are used (ΣgiDi) in the addition of the detection signal Di so that the relative temperature value is calculated as Trel=R/ΣDi. If the temperature of the Si first detector 3 is to be kept constant, then the heating must be reduced of the cooling increased when this relationship exceeds or falls below a prescribed nominal value for the relative temperature value Trel. This can occur in a control circuit known in the industry.
The alternative mathematical correction of the detection signal Di occurs with a wavelength and temperature-independent sensitivity function provided by the detector manufacturer or determined in the climate chamber. Alternatively, it can be advantageous due to unavoidable production tolerances to measure the sensitivity curve for a concrete spectrometer 10 immediately after the switching-on of the spectrometer 10 and to measure the associated inherent heating, In addition, if available, a heating can be switched on in order to measure the wavelength and temperature-dependent sensitivity function. Under certain conditions it may be advantageous to leave an existing heating switched off during the normal measuring operation of the spectrometer 10 and only to use the mathematical sensitivity compensation because a higher detection temperature also leads to higher detector noise.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 054 056.0 | Oct 2008 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/07664 | 10/27/2009 | WO | 00 | 3/24/2011 |
