Infrared detector

In at least one embodiment, an infrared (IR) detector for generating an image of an object is provided. The IR detector includes a plurality of thermal sensing elements that are arranged in an array of M columns and N rows. Each thermal sensing element is configured to receive at least one oscillating signal and detect at least a portion of a thermal output from the object. Each thermal sensing element is further configured to generate an electrical output signal that is indicative of at least a portion of detected thermal output and to modulate the electrical output signal with the at least one oscillating signal to generate a modulated output signal that is indicative of at least a portion of the image of the object.

BACKGROUND

1. Technical Field

The embodiments of the present invention generally relate to, among other things, an infrared (IR) detector.

2. Background Art

An IR detector is generally defined as a photodetector that responds to IR radiation. One type of an infrared detector is a thermal based detector. A thermal based detector may be implemented within a camera to generate an image of an object formed on the thermal properties generally associated with such an object. Thermal based detectors are known to include bolometers, microbolometers, pyroelectric, and thermopiles.

A microbolometer changes its electrical resistance based on an amount of radiant energy that is received from an object. Thermopiles include a number of thermocouples that convert thermal energy from the object into electrical energy. Such devices have been incorporated into cameras in one form or another for thermal imaging purposes.

SUMMARY

In at least one embodiment, an infrared (IR) detector for generating an image of an object is provided. The IR detector includes a plurality of thermal sensing elements that are arranged in an array of M columns and N rows. Each thermal sensing element is configured to receive at least one oscillating signal and detect at least a portion of a thermal output from the object. Each thermal sensing element is further configured to generate an electrical output signal that is indicative of at least a portion of detected thermal output and to modulate the electrical output signal with the at least one oscillating signal to generate a modulated output signal that is indicative of at least a portion of the image of the object.

DETAILED DESCRIPTION

Various embodiments of the present invention generally provide for, but not limited to, an IR detector that includes a thermal sensing device based array. The array includes a plurality of thermal sensing elements that each include a thermopile (or other suitable thermal sensing device) distributed into M columns and N rows (e.g., M×N thermopile array). A function generator (or other suitable device that is situated to generate an oscillating signal at a corresponding frequency) may drive each column (or row) of thermal sensing elements (to modulate an output of each thermopile) within the array with oscillating signals at a different frequency from one another such that an electrical output is provided for each column (or row). The modulated electrical output from each thermopile in the column (or row) may be provided on a single modulated electrical output and is amplified by an amplifier (or other suitable device) for the given column (or row). A demodulation circuit may receive each single modulated electrical output after amplification for each column (or row) and demodulate the amplified output (e.g., remove constant value from oscillating signal(s) for each column (or row)) to generate a constant electrical value. The constant electrical value may be indicative of a portion of the entire the detected image. The entire detected image can be reconstructed by assembling all of the constant electrical values that are read from each column (or row) within the array.

It is contemplated that the embodiments described herein may be utilized for purposes other than those described and that the challenges that may be noted herein are not intended to be an exhaustive list of challenges that may be overcome by the embodiments of the present invention. Such challenges that may be described herein are noted for illustrative purposes and that all of the challenges that may be overcome by the various embodiments of the present invention are not described for purposes of brevity. Moreover, it is contemplated that the embodiments described herein may provide for any number of advantages and that those noted are not intended to be an exhaustive list that may be achieved. Such advantages disclosed herein are noted for illustrative purposes and that all of the advantages achieved by the embodiments of the present invention are not described for purposes of brevity as well. Furthermore, the examples provided herein are disclosed for illustrative purposes and are not intended to be an exhaustive list of examples that are capable of being implemented and are not intended to limit the scope of the embodiments of the present invention in any manner.

The embodiments of the present invention as set forth inFIGS. 3-8generally illustrate and describe a plurality of circuits or other electrical devices. All references to the circuits and other electrical devices and the functionality provided by each, are not intended to be limited to encompassing only what is illustrated and described herein. While particular labels may be assigned to the various circuits or other electrical devices disclosed, such labels are not intended to limit the scope of operation for the circuits and the other electrical devices. Such circuits and other electrical devices may be combined with each other and/or separated in any manner based on the particular type of electrical implementation that is desired in the camera. It is recognized that any circuit or other electrical device disclosed herein may include any number of microprocessors, integrated circuits, memory devices (e.g., FLASH, RAM, ROM, EPROM, EEPROM, or other suitable variants thereof) and software which co-act with one another to generate the oscillating signals to perform analog-to-digital conversion of array outputs and to demodulate the array outputs.

FIG. 1depicts a conventional microbolometer based detector20. The detector20may be implemented within a camera. The detector20may comprise a plurality of pixels22that are arranged in 320×240 array (e.g., 320 columns and 240 rows). Each pixel22includes a microbolometer24, and a switch28. The switch28may be implemented as a field effect transistor (FET). It is known that the microbolometers24and the switches28, are formed on a semiconductor substrate. The detector20may be implemented with a pixel pitch of 45 um using a 3.3V 0.5 um Complementary Metal-Oxide Semiconductor (CMOS) technology.

A selectable DC based power supply (not shown) closes the switches28in sequence, row by row (e.g., all switches in a row are closed at the same time while all other switches in different rows are open) so that current from one microbolometer24in a column flows therefrom. The condition of measuring a single bolometer in a time slice that is 1/N (where N corresponds to the number of rows) before the cycle repeats is generally defined as time division multiplexing (TDM).

A capacitive trans-impedance amplifier (CTIA)30is coupled to the output of each pixel22for a given column. A capacitor32is coupled to each CTIA30. The size of the capacitor32controls the gain of the CTIA30output. Each CTIA30performs a current-voltage conversion by integrating a charge on the capacitor32. A switch34may serve to reset the current to voltage conversion performed by the CTIA30.

A switch36and capacitor42are coupled to an output of the CTIA30to perform a sample and hold (S&H) operation for a given column. When the proper amount of charge is integrated across the capacitor32, the switch36closes momentarily to transfer the charge to the capacitor42. The purpose of S&H operation is to hold the charge collected from the capacitor32to await digitization.

An additional amplifier38and switch40is provided so that the output from each column can be read. The switch40can be configured to close to enable the output for corresponding column to pass through a multiplexer. Once the output for a given column is ascertained, the switches28and40are opened and the switches28and40for a preceding row are closed so that a reading for such a row can be ascertained. This sequence occurs one at time for every row within the array. As noted above, the detector20employs a TDM approach such that the FET switch28for a given row is closed one at a time so that the corresponding output for the given row is ascertained. The outputs for each column are transmitted on a signal VIDEO_OUTPUT to an Analog to Digital (A/D) converter (not shown). The detector20as used in connection with the TDM approach may exhibit noise aliasing. Noise aliasing may change the shape of the noise spectrum (e.g., noise exhibits non-linear patterns) and become less predictable. As such, if one cannot predict the noise, then design options may be not able to be used that may otherwise eliminate the effects of such predictable noise to increase camera performance.

FIG. 2depicts a conventional thermopile based detector50. The detector50may be implemented within a camera. The detector50is generally packaged, mounted on a circuit board and enclosed by a cap in which a lens is arranged. The detector50includes a plurality of pixels52that may be arranged in 320×240 array (e.g., 320 columns and 240 rows). Each pixel52includes a thermopile sensor element54and a switch56. The switch56is implemented as a FET.

A column decoder58is provided and includes a DC power supply that selectively closes the switches56on a column wise bases, one column at a time (i.e., the detector50employs a TDM scheme). Each thermopile54in the corresponding column generates an output voltage in response to the switch56being closed. A low noise amplifier60is operably coupled to each thermopile54in a given row. The amplifier60is generally configured to provide a higher output gain than that of the amplifier used in connection with the detector20(e.g., the microbolometer based detector). A representative amplifier that may be used for increasing the gain from the thermopiles54is an LT6014 that is provided by Linear Technology of 1630 McCarthy Blvd., Milpitas, Calif. 95035-7417. A lead62is provided for distributing the output voltage from the thermopile54to a device that is not included within the detector50. The amplifier56increases the output voltage provided from the thermopile54.

In general, after each thermopile54within a given column is enabled by a corresponding FET switch56, each amplifier60that is coupled to the thermopile54requires a settling time. After such a settling time is achieved, the voltage output provided by the thermopile54is digitized so that the image can be rendered as an electronic image.

It is known that thermopiles generally have a good signal-to-noise ratio. It is also known that thermopiles generally exhibit a low response and low noise. In order to increase the response, the low noise amplifier60may be needed to increase the gain for a particular row of pixels52. However, the use of such low noise amplifiers may still add a significant amount of noise in the detector50readout. Particularly, for amplifiers that are incorporated on the same silicon substrate as the detector50. The detector50may also experience noise aliasing. As noted above, such a condition may reduce the ability to predict noise. If the noise cannot be predicted, then it may not be possible to implement design options that may mitigate the effects of predictable noise patterns.

FIG. 3depicts a thermopile IR detector70in accordance to one embodiment of the present invention. The detector70may be implemented within an imaging device69such as, but not limited to, a camera. The detector70is generally put into a package and mounted on a circuit board71. The detector70and the circuit board71are enclosed by a cap73in which a lens74is arranged. The detector70generally comprises a function generator72and a thermopile array76. The function generator72may drive each column (or row) of the thermopiles at the same time with an oscillating carrier (or oscillating signal). Each thermopile generates an electrical output in response to the thermal energy captured from the object. The corresponding electrical output that is generated by the thermopile is amplitude modulated with the oscillating carrier signal and transmitted therefrom. Each column (or row) of thermopiles is driven at a unique frequency from one another. In general, the function generator72is configured to activate all of the thermopiles in all of the columns (or rows) to amplitude modulate the output from each thermopile (e.g., through the use of one or more switches that may be coupled to each thermopile) with the oscillating carrier which is at a unique frequency for each column (or row). All of the thermopiles may be active at the same time. A gain circuit78that includes a plurality of amplifiers is operably coupled to the thermopile array76. Each amplifier is coupled to a particular column (or row) of thermopiles to increase the signal strength for each column (or row) of thermopiles. A demodulation circuit84is generally coupled to the gain circuit78and is configured to separate the orthogonal carriers for each column (or row) of thermopiles so that the corresponding voltage output from each column of thermopiles can be ascertained in order to generate an electronic image of the captured original image. It is contemplated that the embodiments of the present invention may utilize frequency modulation or phase modulation.

The concept of modulating all of the thermopiles for all columns (or rows) with an oscillating carrier at a unique frequency for each column in which all of the carriers are simultaneously presented to each column (or row) and modulated within an array is generally defined as a Frequency Division Multiplexing (FDM) approach. The FDM approach enables the use of a dedicated amplifier to be added to every row in the thermopile array76to increase the signal strength irrespective of the amount of noise generated by such amplifier. For example, a natural consequence of amplitude modulating each of the thermopiles for a given column (or row) with a unique carrier signal at a predetermined frequency and then simultaneously presenting such signals to the amplifiers with the gain circuit78is that the broadband noise of the channel becomes large (e.g., a standard deviation of the broadband noise grows by the square root of the number of thermopiles on the column (or row)). If the broadband channel noise is “large” compared to the broadband amplifier noise, the broadband noise created by the amplifier on the given column (or row) becomes insignificant due to the fact that the broadband noise for both the channel and the amplifier adds up as a quadrature sum (e.g., square root of the sum of squares of the noise standard deviations) so the amount of noise introduced by the electronics is considered to be inconsequential.

It is also contemplated that the materials used to construct the thermopiles in the array76may comprise compounds in the (Bi1-xSbx)2(Te1-ySey)3family (e.g., Bismuth—Tellurium family). The family of compounds will be denoted by Bi2Te3for brevity. The use of Bi2Te3to construct the thermopiles in the array76may cause the thermopile resistance to fall below 10 K Ohms, which can cause a decrease in the amount of thermopile (or detector) noise. While Bi2Te3based materials can be used to construct thermopiles for the TDM approach to reduce thermopile noise, such a reduction in noise may be minimized when compared to the amount of noise created by the amplifier (e.g., see amplifier60inFIG. 2). The large amount of noise created by the amplifier may be mitigated due in large part to the implementation of the FDM approach for the reasons noted above.

In general, the use of Bi2Te3may produce a very high performance thermopile based detector if the amplifier was ideal with no noise. Because the impedance (or resistance) of a Bi2Te3based thermopile is so low, its noise is also low. To read out a low impedance thermopile and not add any noise to the output signal may require a very low noise amplifier. This may be an issue with the TDM approach as it may be necessary to read out a high performance thermopile with a very high performance amplifier. High performance may mean high power because the noise from the amplifier is reduced the more power the input stage of the amplifier consumes. On the other hand, the FDM approach may incorporate low impedance (e.g., high performance) thermopiles that are in series (seeFIG. 5) to increase the overall noise presented to the amplifier. Since the total noise standard deviation is computed by the square root of the sum of the squares of the thermopile standard deviation (all in parallel or series (seeFIGS. 5 and 7)) and the amplifier standard deviation, the total noise may be primarily dominated by the noise from the thermopiles. While the overall signal before demodulation may be noisy, such a noisy signal may be averaged (e.g., by integrating) over a much longer time (e.g., the image frame rate time). Because the overall signal can be integrated over this longer period of time, the signal can be built back to the noise ratio of a single thermopile detector close to its original value after demodulation and the influence of the amplifier noise can be shown to nearly vanish. This condition may illustrate the notion of predicting the noise and using measures within the design to eliminate its effects.

The following illustrates the manner in which the FDM approach may reduce the electronic noise in comparison to the TDM approach. In particular, the signal to noise (SNR) ratio will be computed for the TDM approach and the FDM approach. The signal from a ithdetector (thermopile) under TDM can be written as:
ri(t)=vsi+nd(t)+ne(t)  (1)
where:ri(t)=Received signal from ithdetectorvsi=Signal voltage from ithdetector (V)nd(t)=zero-mean white Gaussian detector noise with spectral height

v*d2⁡(V2Hz)ne(t)=zero-mean white Gaussian electronics noise with spectral height

ve2*⁡(V2Hz)E[nd(t)ne(t)]=0E[•]=statistical expectationVar[•]=statistical variance
In the TDM approach, the detector is sampled for a fraction of the frame time, Tframe. The fraction of time is determined based on the number of detectors in a row that need to be multiplexed out, Ncolumn. The output of a standard integrator is:

VTDM=∫0TframeNcolumn⁢ri⁡(t)⁢ⅆt(2)
The SNR is given by the following equation:

SNRTDM2=∫0TframeNcolumn⁢E⁡[ri⁡(t)]2⁢ⅆtVar⁡[VTDM](3)
The SNR for TDM can now be evaluated:

For the FDM approach, each detector is modulated on a unique orthogonal carrier, si(t). It will be shown later that for the FDM approach, all of the detectors are present all the time on the row bus. The consequence of this is that the noise variance of each detector are added together. The signal on the row bus becomes:

r⁡(t)=∑i=1Ncolumns⁢[vsi⁢si⁡(t)]+nd⁡(t)+ne⁡(t)(5)
where:r(t)=Received signalsi(t)=Orthogonal carrier ivsi=Thermopile signal or orthogonal carrier i (V)n′d(t)=zero-mean white Gaussian detector noise with spectral height Ncolumns

Ncolumns·vd2⁡(V2Hz)n′e(t)=zero-mean white Gaussian electronics noise with spectral height

ve2⁡(V2Hz)E[n′d(t)n′e(t)]=0
and

{∫0Tframe⁢si⁡(t)⁢sj⁡(t)⁢ⅆt=Tframefor⁢⁢i=j∫0Tframe⁢si⁡(t)⁢sj⁡(t)⁢ⅆt=0for⁢⁢i≠j(6)
In FDM approach, the detector is sampled for the full frame time, Tframebecause all the detectors are on all the time. The output of a standard integrator is:

VFDM=∫0Tframe⁢r⁡(t)⁢si⁡(t)⁢ⅆt(7)
The SNR for FDM can now be evaluated for the ithcomponent:

Comparing Equation 8 to Equation 4, it can be seen that with the FDM approach, the electronic noise variance decreases based on the number of detectors that are multiplexed out (e.g., Ncolumn.).

In general, it is recognized that the oscillating carriers may include any orthogonal set of functions such as, but not limited to, Walsh Functions, sine and cosine functions.

The Walsh functions as used herein may be denoted by wal(0, θ), sal(i, θ) and cal(i, θ) (where θ is normalized time t/T). Walsh functions may generally form a complete system of orthonormal functions, which may be similar to the system of sine and cosine functions. There is a close connection between sal and sine functions, as well as between cal and cosine functions. In general, Walsh functions are known to form a complete orthonormal set and are therefore orthogonal.

FIG. 4depicts a function generator72implemented within the detector70ofFIG. 3in accordance to one embodiment of the present invention. The function generator72is configured to generate Walsh functions such as sal(x, t) and cal(y, t). For example, the function generator72generates the functions sal(1, t) through sal(8, t) and cal(1, t) through cal(8, t). In one example, the function generator72may be a 4-bit synchronous counter. It is recognized that the function generator72may be configured to accommodate for any number of bits and that the number of bits selected generally depends on the size (e.g., number of columns and/or rows) of the thermopile array. In addition, it is further recognized that the function generator72may be non-synchronous.

The function generator72includes a plurality of exclusive- or (XOR) gates86for receiving one or more bits (e.g., 4 bits) to generate the functions sal(1, t)-sal(8, t) and the functions cal(1, t)-cal (7, t) In general, the arrangement of the XOR gates86and the clock are configured such that each function of sal (x, t) and cal (y, t) is transmitted at a different period from one another so that a predetermined frequency is maintained between each function of sal(x, t) and cal(y, t). Each function of sal(x,t) and cal(y,t) is transmitted to a different column within the array76. For example, sal(1,t) and cal(1,t) may be transmitted to a first column of thermopiles within the array and so on, in which sal(8,t) and cal(8,t) are transmitted to an eight column within the array76. Because each function of sal (x, t) and cal (y, t) is transmitted at a different period from one another to maintain a predetermined frequency therebetween, such a condition may ensure that every column of thermopiles are modulated by the orthogonal set (e.g., of sal and/or cal functions) at a unique frequency.

FIG. 5depicts the thermopile array76implemented within the detector70ofFIG. 3in accordance to one embodiment of the present invention. The array76includes a plurality of pixels90(or thermal sensing elements) that are arranged in a 8×N array. For example, the array76includes 8 columns of pixels90and any number of rows of pixels90. Each pixel90includes a first pair of switches92, a second pair of switches94, a thermopile96, and a switch98. It is recognized that the quantity of switches and thermopiles within each pixel may vary based on the desired criteria of a particular implementation. The switches92and94may coact with the thermopile to modulate the output of thermopile onto the oscillating signals. The columns of pixels90are configured to receive the functions sal(1,t)-sal(8, t); and cal(1, t)-cal(8, t) from the function generator72. For example, pixel90in column1receives the function sal(1, t) and cal(1,t); pixel90in column2receives the functions sal(2,t) and cal(2, t) and so on such that the pixel in column8receives the functions sal(8,t) and cal(8,t).

It is recognized that the size of the array may vary and that the number of columns and rows may be selected based on the desired criteria of a particular implementation. It is also recognized that the number and configuration of switches92,94may vary based on the desired criteria of a particular implementation. The use of such functions may vary as well based on the desired criteria of a particular implementation. The circuit as depicted within the array76(or elsewhere in the detector70) is used for illustrative purposes and is not intended to demonstrate that the embodiments of the present invention are to be implemented in this manner alone.

As noted above, each function of sal (x, t) and cal (y, t) is transmitted at a unique frequency to each corresponding column of pixels90(e.g., column1receives sal(1, t) and cal(1,t) at a first frequency, column2receives sal(2,t) and cal(2,t) at a second frequency, column3receives sal(3,t) and cal(3,t) at a third frequency and so on). Each unique frequency may be separated by a predetermined amount to ensure that an output signal from each pixel90can be uniquely recovered during demodulation. In one example, the separation frequency may be 30 Hz.

In general, each column of pixels90is driven with the functions sal (x, t) and cal (y, t) and operate at a unique frequency from one another such that a voltage output from each thermopile96is read out on a row-wise basis. While each thermopile96may be a particular function of sal (x,t) or cal (y,t), half of the thermopiles96on a corresponding row may be in forward direction (+ side on row bus) and the other half of the thermopiles96may be in the reverse direction (− side on row bus) due to the cyclical nature of the orthogonal carriers (e.g, sal (x,t) and cal (y,t)). It is recognized that the voltage output from a given row is usually near ground because half of the thermopiles96may be in the forward direction while the remaining half of the thermopiles96are in the reverse direction. The overall dynamic range (e.g., the ratio of the highest measurable signal to the lowest measurable signal) is maintained. Each pixel90receives a sal(x,t) and cal(y,t) function because such functions provide non-overlapping clocks for suitable switching within each pixel90.

The array76transmits the voltage output for each row on the signal v1(t) through vn(t) (where N=the number of rows in the array). A gain circuit78includes a plurality of amplifiers102that receives the voltage outputs v1(t)-vn(t) and increases the amplitude for such to generate the voltage outputs v1′(t)-vn′(t). In one example, each amplifier102may be a CMOS amplifier similar to LMC6022 from National Semiconductor of 2900 Semiconductor Drive, Santa Clara, Calif. 95052. Each amplifier102may be integrated on the same silicon substrate as the array76. It is recognized that the type of amplifier used may vary based on the desired criteria of a particular implementation. As noted above in connection withFIG. 2, thermopiles generally exhibit a low response and require additional gain to increase the output. The thermopiles96are connected in series with one another in a given row and the corresponding voltage output is presented to the non-inverting input of the amplifier102. Due to such an arrangement, the switch98is added across each thermopile96to permanently close its corresponding pixel in the event the thermopile96is damaged. The coupling of the thermopiles96in series in a particular row and the presentation of the voltage output form that row to the non-inverting input of the amplifier102increases the gain voltage output and reduces the potential for 1/f noise because of the small current flow into the non-inverting input of the amplifier102.

A multiplexer80receives the output voltages v1′(t)-vn′(t) from the gain circuit80. An analog to digital (A/D) converter82receives an output voltage v1′(t)-vn′(t) over a single wire bus. The A/D converter82converts the output voltage v1′(t)-vn′(t) from an analog voltage signal into a digital voltage signal. The A/D converter82may include any combination of hardware and software that enables analog to digital conversion.

The demodulation circuit84is configured to receive a digital output from the A/D converter82for each row in the array76. The demodulation circuit84may be a matched filter, a Fast Walsh Transform or any other suitable circuit that includes any combination of hardware and software to determine the voltage output for a given row of thermopiles96in the array76. The output from the A/D converter82comprises a digital representation of the output voltage from a row of thermopiles96that is in the form of a constant that is multiplied to the corresponding orthogonal carriers (e.g., functions sal (x,t) and cal (y,t) that are transmitted at the unique frequency for each column).

Each of the unique orthogonal carriers includes the thermopile signal information. Multiplying the received signal by sal(i, t) (or cal(i, t)—if cal (i, t) is used, only a sign change will occur) performs the demodulation. The demodulated signal is then averaged to estimate the thermopile signal. The received signal from a row is given by Equation 9:

Depending on the scene and thermal time constant, m(t) can be considered to be either a constant or a random variable to be estimated. Assuming that the parameter to be estimated is a constant, the optimal estimator is given by:

m⋒i=1Tframe⁢∫0Tframe⁢r⁡(t)⁢sal⁡(i,t)⁢ⅆt(10)
where:
{circumflex over (m)}i=Estimated thermopile output signal from the ithdetector
Since sal(i, t) is either +1 or −1 implementation in a digital signal processor (DSP) or field-programmable gate array (FPGA) may be simple.

FIG. 6depicts a thermopile IR detector150in accordance to another embodiment of the present invention. The detector150includes a plurality of oscillators152(or function generator), an array154, a gain circuit156, a multiplexer circuit158, an A/D converter160, a memory circuit162, and a demodulation circuit164. The plurality of oscillators72is configured to generate oscillating carrier signals at a predetermined frequency for activating all thermopiles within a given column (or row) so that modulated signals are transmitted therefrom. For example, each oscillator72is configured to generate an oscillating signal at a unique frequency and to transmit the same to a corresponding column of pixels within the array154. Each of the columns of pixels is driven at the same time but at different frequency from one another. The detector150employs the FDM approach as noted in connection withFIG. 3.

The plurality of oscillators72is voltage controlled via a voltage source166. It is contemplated that different types of oscillators may be used instead of a voltage-controlled oscillator. For example, such oscillators may be coupled to a mechanical resonator (such as, but not limited to, a crystal). The type of device used to generate the oscillating signal at the unique frequency may vary based on the desired criteria of a particular implementation. A plurality of resistors155is positioned between the oscillators152and the voltage source166to adjust the voltage output of the voltage source. The resistance value for each resistor155may be selected to ensure such that a different voltage input is provided to each oscillator152. Such a condition may ensure that the oscillators152generate a unique frequency from one another in the event the oscillators152are voltage controlled. The oscillators72each generate an oscillating signal that is in the form of a sine function (e.g., sin (x, t)) or a cosine function (e.g., cos (y, t)).

FIG. 7depicts a more detailed diagram of the thermopile array154. The array154includes pixels202(or thermal sensing elements) that are arranged in an M×N array. Each pixel202includes a thermopile204and a FET based switch206. The number of thermopiles and switches implemented within a given pixel may vary based on the desired criteria of a particular implementation. All of the oscillators152are active all of the time such that all of the columns of pixels are amplitude modulated with a unique frequency. For example, the thermopiles204in column1are driven by a first oscillating signal at a first frequency and the thermopiles204in column M are driven by a second oscillating signal at a second frequency, where first frequency is different from the second frequency. In one example, the first frequency may be 30 Hz and the second frequency may be 60 Hz. The particular frequency used for each column is generally defined by:
f(i)=i*30 Hz,  (11)

where i corresponds to the column number.

It is recognized that metal film bolometers (or low resistance bolometers) may be implemented instead of the thermopiles with the FDM approach.

As noted in connection withFIGS. 3 and 5, each oscillator152is generally configured to activate all of the thermopiles for a corresponding column (or row) with an amplitude modulated orthogonal carrier at a unique frequency so that all of the thermopiles in such a column (or row) are on for the entire frame time. This may be performed for all columns within the array154. As such, it can be said that all of the thermopiles within the array154are active at the same time.

An amplifier208may increase the voltage output for each row. The multiplexer circuit158transmits each voltage output from a row on a single line to the A/D converter160. The A/D converter160converts the voltage output into a digital based output. The A/D converter160may include any combination of hardware and software to perform the conversion. A memory circuit162stores the digitalized output to enable transfer to the demodulation circuit164. In one example, the memory circuit162may be implemented as a Direct Memory Access (DMA) storage device or other suitable storage mechanism. The demodulation circuit164performs a Fast Fourier Transform (FFT) on the digitized output. The demodulation circuit164may include any combination of hardware and software to perform the FFT. An image result depicting the captured image is generated therefrom.

It is recognized that thermopile based arrays within the detectors70and150(or other suitable variants thereof) may exhibit increased levels of thermal stability and thus may be easy to maintain radiometric calibration over a wide range of ambient temperatures. It is also recognized that thermopile based arrays within the detectors70and150(or other suitable variants thereof) that utilize the FDM approach may be adaptable for a range of capabilities such as, but not limited to, fire fighting applications as such an array may not require special image processing techniques (e.g., combining higher noise low gain images with lower noise high gain images) to display images with both hot and cold objects in the capture image. It is also recognized that thermopile based arrays within the detectors70and150(or other suitable variants thereof) may respond linearly to incoming radiance from an object. Due to such a linear response, a low cost in-factory radiometric calibration may be achieved. It is also recognized that thermopile output based signals from the thermopiles within the detectors70and150(or other suitable variants thereof) are generally differential and unbiased and may not exhibit large drift offsets. As such, radiometric calibration may be easier to maintain over a wide range of ambient temperature. It is also recognized that that the detectors70and150(or other suitable variants thereof) may not exhibit 1/f noise due to the FDM approach, which nearly eliminates the 1/f noise from the amplifier by modulating the output of the thermopile at a high enough frequency where the 1/f noise of the amplifier is negligible. It is also recognized that the detectors70and150(or other suitable variants thereof) may be able to capture, but not limited to, short temporal events because all of the thermopiles within the array may be capturing energy all of the time.