WO2018067507A1

WO2018067507A1 - Passive infrared detection of small temperature differences

Info

Publication number: WO2018067507A1
Application number: PCT/US2017/054859
Authority: WO
Inventors: Gerald J. DIEBOLD
Original assignee: Digimelt, Inc.
Priority date: 2016-10-04
Filing date: 2017-10-03
Publication date: 2018-04-12

Abstract

Infrared detectors can be operated to measure the temperature difference between two samples through use of an oscillating mirror, actuator or infrared transmitting optical fiber at a fixed frequency that moves the field of view of the detector between the two samples. In another embodiment, a superconducting or low temperature bolometer is used as the infrared detector without heterodyning. The apparatus can be employed for detecting differential cell or tissue growth, differential reaction of tissue to external stimuli, chemical reaction rate, and small temperature gradients along a surface.

Description

PASSIVE INFRARED DETECTION OF SMALL TEMPERATURE DIFFERENCES Background

[0001] As shown by J. Haber, M. N. Kashid, N. Borhani, J. Thorne, U. Krtschil, A. Renken, L. Kiwi-Minsker, in "Infrared imaging of temperature profiles in microreactors for fast and exothermic reactions", Chemical Engineering Journal 214, 97-105 (2013), infrared thermography can be used for the study of chemical reactions. As well, the use of infrared thermography has been reported by Salaimeh and coworkers for determining the rate of growth of bacteria. A. A. Salaimeh, J.J. Campion, B.Y. Gharaibeh, M.E. Evans, and K. Saito, "Real-time quantification of viable bacteria in liquid medium using infrared thermography", Infrared Physics and Technology 54, 517-524 (201 1). Both of these studies show the utility of using passive thermal detection of reaction and cell growth by monitoring temperature changes. However, these researchers employ ordinary thermal cameras. As a result, their measurements are not highly sensitive and thus have a restricted range of interest and applicability.

[0002] Designs of calorimeters employing pyroelectric films as the sensing element have been reported. Hagins et al. in US Patent 5,030,012 employ a pyroelectric film temperature sensor to measure light induced temperature changes. The change in the temperature is recorded when a sample is exposed to light, the amount of which is known. From the energy in the light pulse and the temperature change in the sample, as recorded by the pyroelectric film, the heat capacity of the irradiated sample can be determined. The output of the pyroelectric film, polyvinylidene difluoride (PVDF) in this case, is measured by recording its current output. Other thermal reactions induced by the light or that take place spontaneously can be recorded as well. The PVDF film detector is capable of measuring temperature changes.

[0003] Usamentiaga et al. review the use of passive infrared thermography for nondestructive testing. R. Usamentiaga, P. Venegas, J. Guerediaga, L. Vega, J. Molleda, and F. G. Bulnes; Infrared Thermography for Temperature Measurement and Non-Destructive Testing, Sensors 14 12305-12348 (2014).

[0004] They describe both active and passive infrared detection. Again, the technology does not deviate from the use of conventional infrared cameras. Bagavathiappan and coworkers give a review of real time temperature monitoring with a focus on the technology for nondestructive testing. S. Bagavathiappan, B. B. Lahiri, T. Saravanan and John Philio and T. Jayakumar, "Infrared thermography for condition monitoring-A review", Infrared Physics and Technology 60, 35-55 (2013).

[0005] Again, as in the papers reviewed by Usamentiaga there is only acceptance of the limited sensitivity of infrared cameras showing the capabilities of the existing infrared cameras for applications to testing metals under stress, heat loss by buildings, detection of subsurface water, heat production by machinery, and other industrial applications.

[0006] It would be useful to develop improvements in the detection of temperature changes using infrared technology.

Summary

[0007] One embodiment described herein is an apparatus comprising an infrared detector including a detection element, an oscillating device configured to rapidly alternatively direct infrared radiation emitted from a test specimen and infrared radiation emitted from a reference specimen to the detection element, and a driver for the oscillating device.

[0008] A further embodiment is an apparatus for heterodyning an infrared detector to improve a signal-to-noise ratio when measuring low intensity infrared radiation emitted from a test cell, the apparatus being configured to introduce a high frequency signal into the detector bias electronics while at the same time using at least one of a mechanical device and an electro-optical device that periodically switches the infrared radiation path between the test cell and a reference cell to produce an alternating signal voltage in the infrared detector. The resulting heterodyned and filtered voltage signal is detected with a lock-in amplifier at a sideband of the heterodyned signal.

[0009] Yet another embodiment described herein is an apparatus for heterodyning an infrared detector to improve a signal-to-noise ratio and hence sensitivity to low intensity infrared radiation based on introducing a high frequency signal into the detector bias electronics and modulation of the infrared radiation using any of an oscillating mirror, an oscillating linear actuator, an oscillating lens, a rotating disk containing the cells, an oscillating infrared transmitting optical fiber, or any other mechanical or electro-optical device that switches the radiation path from one cell to the other followed by lock-in detection of the heterodyned signal.

[0010] Another embodiment is an apparatus using modulation of passively emitted infrared radiation between two cells using any of an oscillating mirror, an oscillating linear actuator, an oscillating lens, a rotating disk containing the cells, an oscillating infrared transmitting optical fiber, or any other mechanical or electro-optical device that switches the radiation path from one cell to the other permitting lock-in or synchronous detection at the modulation frequency.

[0011] Yet another embodiment is an apparatus for detecting temperature differences, comprising a mirror assembly including an oscillating mirror with a driver configured to rapidly move the mirror between an infrared photon-emitting test sample and an infrared photon- emitting reference sample, and an optional photon transmitting mirror configured to receive reflected photons from the oscillating mirror. A photon detector is configured to detect photons emitted by at least one of the oscillating mirror and the optional photon transmitting mirror, and to output a first voltage. The apparatus also includes a first power source configured to provide the detector with a direct current (DC) bias, a second power source configured to provide the detector with an alternating current (AC) bias, a first filter configured to receive the first voltage from the detector and pass only a first upper sideband frequency to a pre-amplifier, a mixer configured to mix a signal from the driver for the oscillating mirror with a (high) frequency from a function generator, the function generator being configured to provide a carrier voltage at a high frequency, a second filter configured to filter out the frequency provided by the carrier voltage and a lower sideband frequency, and pass only a second upper sideband frequency as a reference frequency, and a lock-in amplifier. The lock-in amplifier is configured to receive the upper sideband reference frequency from the second filter, to receive the upper sideband frequency of the test sample from the pre-amplifier, and generate a direct current lock-in amplifier output. The lock-in amplifier output is proportional to the temperature difference between the test sample and the reference sample.

[0012] A further embodiment is an apparatus, comprising a mirror assembly, a photon detector and a lock-in amplifier. The mirror assembly comprising an oscillating mirror with a driver configured to rapidly move the mirror between an infrared photon-emitting test sample and an infrared photon-emitting reference sample, and an optional photon transmitting mirror configured to receive reflected photons from the oscillating mirror. The photon detector is configured to detect photons emitted by the test and reference samples reflected by at least one of the oscillating mirror and the optional photon transmitting mirror, and to output a first voltage. The lock-in amplifier is configured to receive a reference signal from the oscillating mirror driver, receive the signal from the detector, and generate a lock-in amplifier output, the lock-in amplifier output being proportional to the temperature difference between the test sample and the reference sample.

Brief Description of the Drawing

[0013] FIG 1 schematically shows a first embodiment of an infrared detection system described herein.

[0014] FIG. 2 schematically shows a second embodiment of an infrared detection system described herein.

[0015] FIG. 3 schematically shows a third embodiment of an infrared detection system described herein.

[0016] FIG. 4 schematically shows a fourth embodiment of an infrared detection system described herein.

[0017] FIG. 5 schematically shows a fifth embodiment of an infrared detection system described herein.

[0018] FIG. 6 schematically shows a sixth embodiment of an infrared detection system described herein.

[0019] FIG. 7 schematically shows a seventh embodiment of an infrared detection system described herein. [0020] FIG. 8 is a graph showing changes in voltage at various temperatures in accordance with an experimental example described herein.

Detailed Description Definitions:

[0021] As used herein, the "carrier" or "carrier voltage" is the signal from the local oscillator. This is the type of system used with radio, where the microphone signal from speech, which is in the kHz range, is too low a frequency to transmit. So the microphone signal is mixed with a MHz (or several hundred kHz) carrier which can be broadcast with a reasonably small antenna. The radio signal from the tower will thus be a strong carrier with two sidebands.

[0022] An apparatus is described herein for heterodyning conventional infrared detectors to improve sensitivity to low intensity infrared radiation based on introducing a high frequency signal into the detector bias electronics and modulation of the infrared radiation. The apparatus uses a mechanical or electro-optical device that switches the radiation path from one cell to the other, followed by lock-in detection of the heterodyned signal.

[0023] Another embodiment is an apparatus that uses modulation of passively emitted infrared radiation between two cells using a mechanical or electro-optical device that switches the radiation path from one cell to the other permitting lock-in or synchronous detection at the modulation frequency.

Problems with the Present Technology

[0024] The calorimeter described by Hagins et al. can be used on small samples. However the device has several deficiencies, the most important being its sensitivity and ability to measure differential temperatures. The use of a differential method permits high frequency operation, subtraction out of common mode temperature changes, lock-in detection, and provides a reference that is one element in the design of a detector with ultrahigh sensitivity. [0025] Merabet et al. shows how chemical reactions can be monitored using a PVDF calorimeter whose design is based on that of a standard titration calorimeter. They similarly use the current output of the PVDF detector to determine the temperature change. They show that titration of HCI with Tris buffer can be monitored in a reaction cell with a volume of 0.7 ml. They were able to measure the heat of reaction in their microcalorimeter that agreed with data reported earlier using conventional large volume techniques. Experiments were reported that showed their apparatus could measure energies of roughly one micro Joule. A serious limitation with this design from the perspective of the disclosed embodiments is that it involves contact; the disclosed embodiments provide a non-contact measurement of differential temperature change. As in the case of Hagins et al. there is no possibility of high frequency operation, subtraction out of common mode temperature changes, and provision of a reference that are elements in the design of an ultrahigh sensitivity detector.

[0026] The objective of combinatorial chemistry is to synthesize and test large numbers of chemical compounds for a desired activity. Combinatorial chemistry has been used most extensively in drug discovery, though applications in all fields of chemistry exist. In order to assay the large numbers of compounds synthesized in a typical library rapidly, high-throughput or ultra-high-throughput screens are carried out. A number of methods have been developed for high-throughput screening. Fluorescence, absorbance, and radioactivity based assays can all be carried out relatively easily by commercially available robotic systems. However, in many cases, and for a number of interesting targets it is difficult or impossible to adapt such optical or radioactive methods for screening. The apparatus described herein can act as a universal detector with the only restriction being that the reaction be exothermic, which is the most common case for reactions that proceed spontaneously.

[0027] Baffou et al. in their review of thermal detection of single cells cite over sixteen different publications where the temperatures of cells were monitored using various techniques. They solve the heat diffusion equation using known rates of nutrient uptake by cells in an aqueous environment and find that the temperature increment of a single cell where heat diffusion into the surroundings is taken into account gives a temperature increment of 10 microkelvins. They report as well that all sixteen of the measurements are meaningless. The problem with all of the technologies reviewed by Baffou et al. is that temperature sensitivity of the detection apparatus is inadequate to detect cell growth. The disclosed apparatus provides a means for achieving the required temperature sensitivity using heterodyne electronic processing or the use of a cryogenic bolometer, neither of which have ever before been explored for use in high differential temperature sensing based on passive emission of infrared radiation.

Infrared detectors and their characteristics

[0028] The manufacture of infrared detectors has been directed to obtaining materials with the highest value of the detectivity, denoted D*. Different materials are used for different regions of the infrared spectrum. The disclosed apparatus uses a heterodyning scheme (that can be used in any detector that requires a bias current) to ensure that amplification takes place at high frequency where 1/f noise is small, thus giving a higher signal-to-noise ratio in the detector which translates into the ability of the instrument to detect small differential temperatures. It is to be noted that given the relatively high frequency that resonant mirrors can operate (10 kHz), depending on the sensitivity desired, it is possible to use the resonant mirror for modulation alone without the use of heterodyning. Realization of this embodiment requires only that the infrared photon stream be modulated between the test and reference cells.

[0029] For the purpose of determining extremely small temperature differentials, a superconducting or low temperature bolometer has a superior performance relative to a conventional HgCdTe detector. That is, the D* of a typical cryogenic bolometer or its noise equivalent power, depending on the design of the device, are superior to those for a conventional HgCdTe detector in much of the near infrared wavelength region. The bolometer is thus a better choice for detection of small temperature differentials even though it does not easily admit to high frequency heterodyning (again depending on the frequency response of the detector element in the bolometer). Given its noise equivalent power at low frequencies, it is not necessary to use high modulation frequencies with the superconducting or low temperature bolometer.

[0030] The principles of passive infrared detection are well-known. Low and Hoffman some time ago gave an analysis of the noise in detection of infrared photons. Their analysis was specifically for detectors but the underlying physics is governed by photon statistics as given by Fellget and Lewis. There is an inherent noise in the detection of a stream of photons irrespective of wavelength. Basically, the cause of the noise is the statistical variation in the photon flux that varies depending on the magnitude of the flux. Irrespective of the responsivity of a detector (Volts/Watt), too small of a photon flux means that the statistical variations in the flux will be large so that small differences in the flux (from small temperature differentials) will be buried in the noise. The optical train between the object and the detector as well as the detector area can compensate for this problem to a certain extent: a large detector area and an optical system with a large numerical aperture result in a higher photon flux relative to a system where these parameters are small. Depending on the desired sensitivity, through use of appropriate optics there is an option for use of the certain embodiments as a passive infrared microscope.

[0031] HgCdTe detectors typically have D* in the range of 10⁹ to 10¹⁰ at wavelengths near 10 microns. The blackbody spectrum at a temperature of 300 K peaks at 9.6 microns. As well, the emissivity of liquid water in the range of 4 to 100 microns is approximately unity. For differential temperature recording, the change in radiation density with temperature dP/dT (where P is the emitted radiation power and T is the temperature) are required to be large for the highest sensitivity. Thus for differential detection of species that are primarily aqueous at room temperature, the signal-to-noise ratio in photon flux, dP/dT, and the sensitivity of the detector must be considered. A preliminary analysis indicates that the optimum wavelength for sensitive differential detection is at a somewhat shorter wavelength than the peak in the (300 K) blackbody emission maximum at 12 microns.

[0032] From the value of D*, the area of the detector, and the bandwidth of the measurement Δί, it is possible to determine the noise equivalent power (NEP) which is a measure of the smallest wattage that a detector can record. The expression NEP=(A Af)^{1 2}/D* gives the NEP in terms of Δί, and the area of the detector A. It can be seen from product literature, for instance, from Infrared Associates, Inc. that the highest values of D* and the highest vales of the responsivity (Volts/Watt) are found for the smallest area detectors. This argues for small detector areas. However, using the calculations in Low and Hoffman for the NEP and Wolfe for the power delivered to a 0.025 mm square HgCdTe detector (for instance an Infrared Associates MCT-13-.025) from a given optical train with a respectably high numerical aperture of 0.79, it is found that the S/N ratio in the detector would be only 0.0085 for a 10 micro kelvin differential temperature (in a 1 Hz bandwidth). Thus, if a temperature difference of this magnitude is to be recorded, larger area detectors are required. [0033] If, for instance, an Infrared Associates MCT-13-4 detector with the same optical train is used, the S/N ratio would be 1 .35 in the same detection bandwidth. Its NEP is calculated to be 20 pW. A superconducting hot electron bolometer (Model QFI/X) from QMC Terahertz Inc. has a NEP of only 500 fW so that it would be a factor of 40 more sensitive than the HgCdTe detector. Infrared Laboratories sells several low temperature bolometers, the most sensitive of which has a NEP of 3.6 fW. Note that it is this higher sensitivity that makes either the low temperature or superconducting bolometer the preferred detector for terahertz radiation (which spans part of the near infrared to the far infrared). As noted here, the lower NEP relative to an HgCdTe detector can be brought to bear on sensitive differential temperature recording.

Heterodyning

[0034] Heterodyning (see J. B. Hagen, Radio Frequency Electronics, Cambridge University Press (Cambridge, UK) 1996) is a technique used widely in radio electronics. The method consists of taking a weak incoming radio signal, for instance, and combining it with a large amplitude, high frequency alternating signal. Heterodyning has the property of increasing signal amplitude and shifting the frequency of the signal through a multiplicative effect. If, for instance, there is a weak signal to be detected of the form

where A_s is its amplitude, ω is its frequency, and t is the time, then the signal is fed to a mixer (a multiplier) along with a large voltage from a local oscillator (the carrier) of the form V_L0 = /4_LOcosnt . The effect of the mixer is to multiply the signals, which, according to elementary trigonometry, gives the multiplied voltage as

V(t)=A_L0A_s i[cos(Q-w)t + cos(Q+w)t] (1)

[0035] In so far as sensitive detection of temperature is concerned, it can be seen that heterodyning the HgCdTe detector signal before the first amplification stage has two salutary effects:

[0036] First, the signal is amplified by A_L0I2 so that both the signal and the noise in the detector can be made to dominate over the amplifier noise. As both the signal and noise in the detector will be amplified by a factor of A_L0/2, the S/N ratio will be governed by the intrinsic noise of the detector rather than that from noise originating in the first stage of amplification. Generally, if it is possible to achieve conditions where the detector noise is dominant over any other source of noise or pickup, the ultimate sensitivity of the detector is reached, and no more improvements in sensitivity are possible, short of using a lower noise detector.

[0037] Second, if Ω is made large, at, for instance, the extreme limit of the bandwidth of the detector, then 1/f noise whether generated by the preamplifier or the detector will be suppressed when the signal is fed to a lock-in amplifier set to detect a signal at Ω+ω. That is, the lock-in detection is carried out at a high frequency where the noise is the smallest.

[0038] Here, a method for achieving high sensitivity uses a double modulation scheme for operation with infrared detectors. Consider the HgCdTe detector as an example. HgCdTe detectors have a strong component of 1/f noise, so the noise is largest at low frequencies where conventional steady DC measurements are done. Measurements with a Fermionics Inc. detector shows that the amplitude of the voltage noise obeys almost perfectly a 1/f law. Heterodyning of the modulated infrared signal to a high frequency will provide an improved signal-to-noise ratio in the signal and hence high sensitivity differential temperature detection.

[0039] The wavelengths to which a HgCdTe detector responds range from roughly 6 to 13 microns (for 50% response). This wavelength range is useful for detection of radiation from aqueous (or primarily aqueous) samples at room temperature. The highest response to a temperature change varies with wavelength; it may be of value for increasing sensitivity if an appropriate wavelength filter is inserted in the optical train. If the differential temperatures of interest are at higher temperatures then detectors with higher sensitivity at shorter wavelength (e.g. indium arsenide (InAs)) would be appropriate. In some embodiments, others types of detectors, including but not limited to gallium arsenide (GaAs) and/or indium antimonide (InSb) may be useful.

First Embodiment - Using Conventional Infrared Detector

[0040] FIG. 1 shows an apparatus 20 with two cells, cell 1 in which a reference is placed, and cell 2 where the sample of interest is located. Both cells emit infrared photons in the form of rays 19a, 19b towards an oscillating mirror 3. The oscillating mirror 3 directs the photons to a parabolic mirror 4, which transmits the photons to the detector element 5. The oscillation of the mirror 3 or an equivalent device causes the detector element 5 to view alternately the reference and the sample at the modulation frequency of the mirror.

[0041] The detector 5 is given a DC bias at point 6 which causes a current to flow through a bias resistor 7 and into the detector 5 and finally to ground 37. An alternating signal at a high frequency is fed to the detector element 5 through a capacitor 8 through a coupling resistor 9. Thus, the voltage applied to the detector element 5 at point 10 is an alternating voltage. It is possible to adjust both the DC and AC voltages fed to the detector element at point 10 in the circuit. The high frequency carrier voltage at the frequency Ω is provided by an oscillator (or function generator) 11. The driver 12 for the oscillating mirror supplies power and controls the oscillating mirror 3 moving the field of view of the detector between the two cells at a frequency u>.

[0042] The parabolic mirror 4 can be replaced by a spherical mirror or an infrared transmissive lens that focuses the radiation onto the detector element 5. Depending on the geometry of the entrance port of the detector, the mirror 4 may not be necessary.

[0043] The voltage from the detector 5 is fed to a high pass filter 13 that filters out the lower sideband and the carrier frequencies, passing only the upper sideband to a preamplifier 14, whose output is fed to a lock-in amplifier 17. This voltage is the signal proportional to the temperature differential to be detected at the frequency Ω+ω. The lock-in amplifier requires a reference signal at this frequency, which is provided by mixing the signal from the mirror driver 12 with the high frequency (at frequency Ω) in a mixer 15. The mixer 15 generates sum and difference frequencies from 11 and 12. The carrier and the lower sideband are filtered out by the filter 16 leaving the upper sideband alone which is fed to the reference input of the lock-in amplifier 17. The lock-in amplifier output 18 gives the differential signal as a dc voltage that is proportional to the temperature differential between the cell and reference. Infrared photons from the sample and reference are shown as arrows with wavy tails. Arrow 19a shows radiation from the reference cell 1 , 19b shows radiation from the sample cell 2, 19c shows radiation from the oscillating mirror to the mirror 4, and 19d shows radiation from the mirror 4 to the infrared detector element 5. [0044] The oscillating mirror can be a resonant mirror such as an SC-30 scanner with an AGC 110 driver from Electro-Optical Products, Inc. Several different oscillation frequencies and mirrors are available for scanners from this company. The detector might be any liquid nitrogen cooled HgCdTe detector, such as those available from Infrared Associates, Inc., such as the MCT-13-4, with a detector element 4 mm on a side. Such detectors can be purchased with a range of detector element areas. There are numerous, highly stable, high frequency function generators such as the Agilent Inc. Model 33250 that are suitable for the device. The mixer can be based on the Analog Devices Inc. Model ADL 5391 high frequency mixer which has a frequency range far in excess of what is required here. Filters can be built using one or more surface acoustic wave crystals such as a Statek, Inc. CX1VSM crystal. Such crystals have quality factors on the order of 100,000. Commercially available crystal filters as sold by Filtronics, Inc. which have quality factors on the order of 20,000 are suitable for the instrument.

[0045] For the scheme shown in FIG. 1 , possible parameters for the various frequencies are:

Mirror driver 10 kHz

High frequency oscillator 990 kHz

Upper sideband frequency 1 ,000 kHz

[0046] This assumes that the detector can operate at the upper sideband frequency. If the detector cannot operate at 1 MHz, then lower frequencies must be chosen. For instance, if the detector would operate at 100 kHz, then suitable parameters are:

Mirror driver 10 kHz

High frequency oscillator 90 kHz

Upper sideband frequency 100 kHz

[0047] Note that for common infrared detectors such as the HgCdTe detector, the signal amplitude has a nearly linear dependence on bias current over a certain range. Rather than operate the detector with a fixed bias current as in done with DC detection, it is proposed here to modulate the bias current at a high frequency just below the maximum of the bandwidth limit of the detector. Since the HgCdTe detector acts as a photoconductor, which can in a primitive way be considered as a resistor whose resistance varies depending on light intensity, a heterodyning of the low frequency alternating signal resulting from switching the field of view of the detector between the two cells to a high frequency will take place. Essentially, the high frequency signal will act as V_L0(t), and the low frequency signal will act as V_s (t) to give the result in equation (1) above. The signal is thus multiplied by the high amplitude carrier voltage to make it large and that the amplification can be carried out at high frequency where 1/f noise is small. (When the tiny signal is mixed with a large one, the amplitude is the product of the two signals. Thus, there is amplification.) The result is an improved signal-to-noise ratio in the signal and thus a higher sensitivity of temperature detection relative to dc or low frequency modulation. Even in the embodiment where heterodyning is not used, modulation between the two cells at as high a frequency as possible results in an improved signal-to-noise ratio relative to low frequency or dc detection.

[0048] Note that in principle, a lock-in amplifier responds only to the signal at its reference frequency even in the presence of other voltages not at the reference frequency; however, if the carrier voltage is too large, the electronics in the lock-in amplifier cannot respond linearly. That is, the lock-in amplifier becomes overloaded-its dynamic range is exceeded. A crystal filter can be designed to eliminate or reduce the carrier and unwanted sideband so that only the wanted signal is passes. Alternatively, instead of using a crystal filter, a voltage at the carrier frequency can be electronically subtracted from the detector signal so that the signal fed to the lock-in amplifier does not exceed its dynamic range.

Second Embodiment - Employing a Superconducting or Low Temperature Bolometer

[0049] A second embodiment is based on a HgCdTe (or other conventional infrared detector) or a cryogenic bolometer. FIG. 2 shows the second embodiment, which is generally designated as 30. Infrared photons are shown at 29a, 29b, 29c and 29d. The infrared photons from the reference 21 and sample 22 cells strike the oscillating mirror 23, and are then directed by the mirror 24 onto the detector 25. This results in an alternating signal in the detector output when the temperatures of the reference and sample differ. The output from the oscillating mirror driver 26 is fed to the lock-in amplifier 27 to provide a lock-in reference. The signal from the infrared detector is fed to the input of the lock-in amplifier as well. The output 28 of the lock-in amplifier, assumed to be a dual phase instrument is proportional to the magnitude of the temperature differential between the reference and sample. This embodiment is particularly well suited for use with the superconducting or low temperature bolometer. Secondly this embodiment provides a lower cost version of the instrument depicted in FIG. 1 , usable with conventional infrared detectors such as HgCdTe.

[0050] Superconducting or low temperature bolometers typically operate only at low frequencies relative to detectors such as those based on HgCdTe. The operation of such a detector would be as shown in FIG. 2 where no heterodyning is used. FIG. 2 also is an embodiment using a conventional infrared detector such as HgCdTe where the ultrahigh sensitivity of heterodyning is not desired for reasons of cost or sensitivity. Here only modulation and detection at the mirror oscillation frequency is used. Secondly, if only moderate sensitivity is required for the measurement, then the single modulation technique may suffice. In this configuration, the noise from the detector and preamplifier would correspond to that at the frequency of the mirror oscillation frequency. This provides a superior signal-to-noise ratio in the final signal relative to ordinary (dc) detection.

[0051] The QMC, Ltd. Model QNbTES/X superconducting bolometer has a frequency response from 2 Hz to 1 kHz. The NEP of this device is 2 pW/vT i. A low temperature device sold by Infrared Laboratories, Inc. has a NEP of only 3.6 f W/ HZ and can operate up to 300 Hz. The remarkably low noise equivalent power of cryogenic bolometers indicates superior detection sensitivity relative to any infrared photon detector such as HgCdTe in the infrared range near 10 microns.

Third Embodiment

[0052] FIG. 3 shows an apparatus 40 with two cells: cell 31 in which a reference is placed, and cell 32 where the sample of interest is located. An infrared transmissive lens 34, such as a ZnSe lens, transmits the photons to the detector element 35. The detector element 35 is associated with a liquid nitrogen cooled HgCdTe detector 36.

[0053] As the field of view of a typical infrared detector is wide and since only the modulated signal passing through the optical train carries the desired temperature information, any radiation reaching the detector from outside the optical train constitutes unwanted noise. In some embodiments, the use of a cooled shroud 38 profoundly reduces the magnitude of the unwanted radiation which becomes much smaller in amplitude as it now emanates from a low temperature surface of the shroud 38. Infrared photons are shown at 39a and 39c.

[0054] The voltage from the detector is fed to a preamplifier 42 whose output is fed to a lock-in amplifier 43. The output 44 from the lock-in amplifier 43 is a steady DC voltage proportional to the temperature differential between the reference and sample cells. The power amplifier 46, which is connected to the oscillator 45, provides the appropriate voltage and current to the linear actuator 47. The position of the lens is made to move back and forth through use of the linear actuator 47. An optional modification of this embodiment is that the linear actuator be connected to move the reference and sample cells as a pair with the lens held stationary to give the same effect of modulating the photon flow to the detector.

Fourth Embodiment

[0055] FIG. 4 show an embodiment that combines the heterodyning components of the embodiment of FIG. 1 with the lens of the embodiment of FIG. 3. In this embodiment, an apparatus 50 includes a mixer 55 that generates sum (or difference) frequencies from a (1 MHz) first oscillator 59 and a (1 ,000,020 Hz) second oscillator 53. A low-pass filter 54 is used to eliminate (or reduce) the carrier frequency and the unwanted sideband. A linear actuator 52, which is connected to an actuator driver 57 and an amplifier 58, controls the movement of the lens 51. Infrared radiation modulated at 20 Hz is depicted at 56. Different frequencies also could be used.

Fifth Embodiment

[0056] FIG. 5 shows an embodiment that is a variation of the system shown in FIG. 1. In FIG. 5, an apparatus 60 has a mixer 65 that generates sum and difference frequencies from a (1 MHz) first oscillator 62 and a (1 ,010,000 Hz) second oscillator 64. A low-pass filter 66 is used. Infrared radiation modulated at 10 kHz is depicted at 68. Different frequencies also could be used. Output from low-pass filter 66 is fed to the input to the driver 67 for the resonant mirror 63. Depending on the manufacturer, an oscillating mirror is designed to be driven externally as is shown in FIG. 5, or the mirror power supply provides a signal synchronous at the oscillation frequency as in FIG.1. In either case, the principle of operation is identical: a sideband generated in the mixer corresponds to that of the oscillating mirror. Sixth Embodiment - An Optical System for use with Conventional, Superconducting, or Low Temperature Detectors

[0057] FIG. 6 shows an embodiment that is compatible with or without heterodyning, with a preferred optical train that requires only a single lens, a plane (or focusing) mirror, and an oscillating mirror. The advantage of this design is reduction of the number of optical elements, thus lowering the amount of radiation lost on reflection from surfaces or traversal of lenses. With this apparatus 70, the radiation from reference cell 161 and sample cell 162 first passes through an infrared objective lens 164, preferably with a large numerical aperture to gather as much infrared radiation as possible. The radiation then strikes a plane reflector 166 that directs it onto an oscillating mirror 163, finally being focused onto the detector element 165 of the infrared detector unit 167. The distances in the optical path are configured so as to focus radiation from the reference cell 161 and sample cell 162 onto the detector element.

[0058] FIG. 7 is a schematic of an optical system 80 for transmitting radiation from the reference and sample cells to the detector through use of an optical fiber 79. The method of alternating the position of the end 82 of the fiber 79 to view periodically the reference cell 71 and sample cell 72 requires no optical elements other than the fiber 79 itself. The fiber 79 can be made of an infrared transmitting material with an external cladding to trap radiation within the fiber, or can be a hollow fiber with coated reflecting surfaces. The path of the infrared radiation into the fiber 79 is shown at 74. The position of the fiber is made to move back and forth between the two cells by use of any of a linear actuator 73 such as a voice coil actuator, a rotating eccentric wheel, a periodically pulsed magnetic driver, or other equivalent mechanical or electrical device. The radiation is transmitted to the detector element 75 of the infrared detector unit 77 after traversing the length of the fiber.

[0059] The embodiments of FIGS. 6-7 are particularly useful with low temperature superconducting an cryogenic detectors, as they do not operate at high frequencies. Examples

[0060] Experiments were carried out using the configuration shown in Figure 6 but with the radiation focused on the oscillating mirror and with a second lens between the oscillating mirror and the detector. The experiment, which did not employ heterodyning was done to get an estimate of the sensitivity of the instrument to temperature changes by determining first the "responsivity" of the instrument, which is defined as the volts per degree difference between the cells. Next, the root mean square (RMS) noise in the instrument was recorded in the lock-in amplifier with no oscillation of the mirror. The noise recorded constitutes the limiting factor for recording small temperature differences. By multiplying the RMS noise by the inverse of the responsivity, the minimum temperature differential the instrument can record (with a signal-to- noise ratio of unity) is found.

[0061] The detector was a liquid nitrogen cooled HgCdTe detector, Infrared Associates, Inc., model MCT-13-1.00 that operated in the 12 micron region of the spectrum. A pair of black surfaces was substituted for the sample and reference cells, the latter equipped with a miniature electrical heater. The temperature difference between the two blocks was monitored with a conventional chromel-alumel thermocouple. The lens 32 was a conventional biconvex lens made from ZnSe; the flat mirror was built of copper with a gold coating. The oscillating mirror was an Electro-Optics, Inc. model SC-30 resonant scanning mirror with a gold surface that operated at 10 kHz. The signal from the resonant mirror driver at the mirror oscillation frequency was fed to the reference input of the lock-in amplifier. The output of the detector was fed to a Stanford Research Instruments, Inc. model SR 830 lock-in amplifier.

[0062] The temperature of the heated block was ramped up to approximately 4°C at which point the power to the heater was cut off. As the block slowly cooled to room temperature the signal from the lock-in amplifier was recorded versus temperature as indicated in FIG. 8. The responsivity was determined from the plot to be 45 mV/°C. Next, when the blocks were at room temperature the RMS noise from the lock-in amplifier was recorded. The RMS noise at 10 kHz was found to be 6 x 10^"6 V, thus giving the sensitivity of the instrument as 130 μΚ in a 1 Hz bandwidth.

[0063] FIG. 8 shows the output signal from the lock-in amplifier versus temperature differential as a heated black block (the sample cell) was heated and left to drift back to room temperature. The oscillating mirror operated at 10 kHz. The responsivity of the device (the slope of the curve) is 0.045 V/K. The noise with the sample cell and reference cells at room temperature was 6 microvolts, giving a sensitivity of 132 micro kelvins. In a second experiment an oscillating mirror with a larger surface area that operated at 430 Hz gave a responsivity of 0.7 V/K with a noise voltage of 6.3 microvolts. The sensitivity for this configuration was thus 9 micro kelvins.

[0064] An experiment was conducted using an oscillating optical fiber. The procedure was carried out with a Vigo, Inc. Model PVI-4TE-8 thermoelectrically cooled HgCdTe detector with a 0.5 x 0.5 mm detector element. The responsivity of the instrument was recorded to be 19.4 microvolts/Kelvin. The noise at the operating frequency of 4 Hz was recorded in a 1 Hz bandwidth to be 1.5 microvolts giving a temperature sensitivity of 77 mK. The lower sensitivity found in this experiment as compared with the previous experiment with the 10 kHz modulation can be attributed to the lower photon flux and different wavelength response of the detector.

[0065] In embodiments, the IR detector comprises a detector element, a coolant reservoir (such as a Dewar flask) for holding a refrigerant (e.g. liquid nitrogen or helium), or, as a substitute, an active, externally powered cooling device (such as a Peltier coller), an optional infrared optical element such as a lens adjacent to the detector element, and electrical leads that transmit voltages from the detector element through the flask wall to the electronics.

[0066] The embodiments described in the figures can be used to detect extremely small temperature changes and temperature differences. In embodiments temperature differences or changes in the range of 1 micro kelvin to 20 milli kelvins, or 10 micro kelvins to 10 milli kelvins, or 100 micro kelvins to 1 milli kelvin can be detected.

Objects of the Embodiments Described Herein

[0067] The disclosed embodiments relate to apparatus and method for improving the ability of conventional infrared detectors to record small temperature differences between two adjacent objects. An object of the disclosed embodiments with their high sensitivity is to monitor the rate of growth through temperature change of bacterial, plant, or mammalian cells so as to compare growth rates. An example would be monitoring cancer cell growth relative to healthy cell growth. Another example is to monitor bacterial growth in the presence of antibiotics wherein the object of the test is to determine the most effective antibiotic for patient treatment.

[0068] It is an object to implement a heterodyning method based on an electrical and optical interaction in the detector in order to enhance the sensitivity of infrared detection.

[0069] It is an object to detect temperature gradients, for example, in electronic circuits that have a current flow. Defects that result in non-uniform current flow giving rise to temperature gradients can be detected

[0070] An object is to provide a method and an apparatus for determining the progress and extent of small samples of solutions containing chemical reactants. The method relies on measurement of a temperature change in a cell that takes place as a result of heat evolution (or possibly consumption), that accompanies the reaction and which produces a temperature change in the solution. The apparatus provides a method for determining whether a reaction takes place at all.

[0071] The apparatus can be employed for detecting differential cell or tissue growth, differential reaction of tissue to external stimuli, chemical reaction rate, and small temperature gradients along a surface.

[0072] It is further an object to provide a method and apparatus for determination of the rate of chemical reaction of a large number of samples in an automated device that can be loaded and refilled rapidly with the infrared detector moved between samples, or through the employment of numerous infrared detectors.

[0073] It will be understand and appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

1. An apparatus comprising;

an infrared detector comprising a detection element,

an oscillating device configured to alternatively direct infrared radiation emitted from a test specimen and infrared radiation emitted from a reference specimen to the detection element, and

a driver for the oscillating device.

2. The apparatus of claim 1 , wherein the oscillating device comprises at least one of an oscillating mirror, an oscillating linear actuator, an oscillating lens, an oscillating infrared transmitting optical fiber and an electro-optic modulator to produce an alternating signal in the infrared detector.

3. The apparatus of claim 1 , further comprising a parabolic or spherical mirror disposed between the oscillating device and the infrared detection element.

4. The apparatus of claim 1 , wherein the oscillating device comprises a rotating disk containing the test specimen and the reference specimen.

5. The apparatus of claim 1 , wherein the oscillating device modulates the infrared radiation in order to permit lock-in detection in the infrared detector at a modulation frequency.

6. The apparatus of claim 1 , wherein the oscillating device is part of a heterodyning assembly.

7. The apparatus of claim 1 , wherein the infrared detector contains a heterodyne subsystem, wherein at least one of an electronic filter and a crystal filter is used to remove a carrier frequency and one sideband frequency, permitting a heterodyned signal to be detected with a lock-in amplifier.

8. The apparatus of claim 8, wherein subtraction of a voltage at the carrier frequency is employed to remove the carrier frequency, thereby reducing limitations based on the finite dynamic range of the lock-in amplifier.

9. The apparatus of claim 1 , wherein the infrared detector comprises a bolometer operating at or near the temperature of liquid helium.

10. The apparatus of claim 1 , wherein the infrared detector comprises at least one of a low temperature bolometer and a superconducting bolometer.

11. The apparatus of claim 1 , wherein the infrared detector comprises at least one of HgCdTe, PbSe and PbS.

12. The apparatus of claim 1 , wherein the infrared detector comprises at least one of gallium arsenide (GaAs), indium arsenide (InAs) and indium antimonide (InSb).

13. The apparatus of claim 1 , further comprising a first support configured to position the test specimen and a second support configured to position the reference specimen.

14. The apparatus of claim 7, wherein the heterodyning assembly includes two conventional electrical or crystal filters configured to remove a carrier frequency and one sideband frequency.

15. The apparatus of claim 1 , wherein the apparatus provides for detection of differences in infrared radiation intensity emitted by the reference sample and the test sample at a sensitivity of 100 micro kelvin or less.

16. The apparatus of claim 1 , wherein the apparatus provides for detection of differences in infrared radiation intensity emitted by the reference sample and the test sample at a sensitivity of 10 micro kelvin or less.

17. The apparatus of claim 1 , wherein the apparatus is used to determine the growth of at least one of bacterial cells, mammalian cells, plant cells and seeds.

18. The apparatus of claim 11 , wherein the apparatus is used to determine the growth of at least one of bacterial cells, mammalian cells, plant cells and seeds.

19. The apparatus of claim 4, wherein the apparatus is used to determine temperature gradients by scanning the focal point of the lens or mirror along a surface.

20. The apparatus of claim 1 , wherein the apparatus is used to determine differences in chemical reaction rate between two cells.

21. The apparatus of claim 1 , wherein the apparatus is used to determine the growth of bacterial cells in the presence of an antibiotic.

22. An apparatus for heterodyning an infrared detector to improve a signal-to-noise ratio when measuring low intensity infrared radiation emitted from a test cell, the apparatus being configured to:

introduce a high frequency signal into the detector bias electronics while at the same time using at least one of a mechanical device and an electro-optical device that periodically switches the infrared radiation path between the test cell and a reference cell to produce an alternating signal voltage in the infrared detector. The resulting heterodyned and filtered voltage signal being detected with a lock-in amplifier at a sideband of the heterodyned signal.

23. An apparatus using modulation of passively emitted infrared radiation between two cells using any of an oscillating mirror, an oscillating linear actuator, an oscillating lens, a rotating disk containing the cells, an oscillating infrared transmitting optical fiber, or any other mechanical or electro-optical device that switches the radiation path from one cell to the other permitting lock- in detection at the modulation frequency.

24. An apparatus for detecting temperature differences, comprising:

a mirror assembly comprising an oscillating mirror with a driver configured to move the mirror between an infrared photon-emitting test sample and an infrared photon-emitting reference sample, and an optional photon transmitting mirror configured to receive reflected photons from the oscillating mirror, a photon detector configured to detect photons emitted by at least one of the oscillating mirror and the optional photon transmitting mirror, and to output a first voltage,

a first power source configured to provide the detector with a direct current (DC) bias, a second power source configured to provide the detector with an alternating current (AC) bias,

a first filter configured to receive the first voltage from the detector and pass one of an upper sideband frequency or lower sideband frequency,

a mixer configured to mix a signal from the driver for the oscillating mirror with a (high) frequency from a function generator, the function generator being configured to provide a carrier voltage at a high frequency,

a second filter configured to filter out the frequency provided by the carrier voltage and a lower sideband frequency, and pass only a second upper sideband frequency as a reference frequency, and

a lock-in amplifier configured to receive the upper sideband reference frequency from the second filter, to receive the upper sideband frequency of the test sample from the preamplifier, and generate a direct current lock-in amplifier output, the lock-in amplifier output being proportional to the temperature difference between the test sample and the reference sample.

25. The apparatus of claim 24, wherein the driver for the oscillating mirror comprises at least one of an external driver and the difference frequency from the mixer.

26. The apparatus of claim 24, wherein at least one of an electronic inductor-capacitor- resistor filter or crystal filter is used to remove the carrier and one sideband permitting the heterodyned signal to be fed to a lock-in amplifier with a limited dynamic reserve.

27. The apparatus of claim 24, wherein subtraction of a voltage at the carrier frequency is employed instead of crystal filtering to remove the carrier thus permitting the heterodyned signal to be fed to a lock-in amplifier with a limited dynamic reserve.

28. The apparatus of claim 24, wherein the detector comprises at least one member selected from the group consisting of HgCdTe, PbSe, and PbS detectors.

An apparatus, comprising: a mirror assembly comprising an oscillating mirror with a driver configured to rapidly move the mirror between an infrared photon-emitting test sample and an infrared photon- emitting reference sample, and an optional photon transmitting mirror configured to receive reflected photons from the oscillating mirror,

a photon detector configured to detect photons emitted by at least one of the oscillating mirror and the optional photon transmitting mirror, and to output a first voltage, and

a lock-in amplifier configured to receive a reference signal from the oscillating mirror driver, receive the signal of the test sample and reference sample from the detector, and generate a lock-in amplifier output, the lock-in amplifier output being proportional to the temperature difference between the test sample and the reference sample.

30. The apparatus of claim 1 , 22, 23, 24 or 29, wherein the infrared detector comprises at least one member selected from the group consisting of HgCdTe, PbSe, and PbS detectors.

31. A method of determining the growth of bacteria, mammalian cell, human cells, or plant cells or seeds using the apparatus of claim 1 , 22, 23, 24 or 29.

32. A non-contact method of measuring small temperature differences using the apparatus of claim 1 , 22, 23, 24 or 29.

33. A method of determining differences in chemical reaction rate between two cells using the apparatus of claim 1 , 22, 23, 24 or 29.