JP2000097767A - Thermal type infrared image pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct fixed pattern noises FPN generated due to sensitivity variations of each pixel in a thermal type infrared rays image pickup device which handles a microscopic signal and is easy to receive influences of a temperature drift. SOLUTION: This embodiment has an optical black OB not having sensitivity for incident infrared rays, an infrared detector 101 having normal valid pixels, and a memory 116 for accumulating drift coefficients of each pixel, and there is provided a drift correction circuit 117 for estimating a drift amount of the valid pixel from a variation amount of the optical black OB and the drift coefficients of each pixel to correct.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving device for an infrared imaging device, and more particularly to a driving device for a thermal infrared imaging device which captures incident infrared rays in the form of heat. It relates to correction of so-called drift that changes with time.

[0002]

2. Description of the Related Art The present invention is an improvement of the invention described in Japanese Patent Application No. 7-75264, filed by the same applicant as the present invention, which is the prior invention of the present inventor.

The thermal infrared imaging device of the prior application has a semiconductor substrate 700 and a scanning circuit 701 on the surface of the substrate, as shown in FIG. have. The circuit and the light receiving unit integrate a plurality of pixels, that is, thermoelectric conversion elements, so that a two-dimensional infrared image can be obtained. The light receiving section includes an infrared absorbing layer that absorbs infrared light, a diaphragm that prevents heat from escaping, and a thermoelectric conversion element that converts heat into an electric signal. The diaphragm forms a suspended film-like structure by removing the lower layer by etching. In this example, the thermoelectric conversion element uses a bolometer 707 whose electric resistance value changes depending on the temperature, and uses titanium as the bolometer. The infrared light incident on each pixel is absorbed by the infrared absorption layer of each pixel, and raises the temperature of the diaphragm of each pixel. This temperature rise is converted into an electric signal by the titanium bolometer 707, and is sequentially read out through a circuit on the substrate.

[0004] In general, regardless of the thermal type or the quantum type (photoelectric effect type using a semiconductor) in an infrared imaging device, there is a bias level variation due to a variation in a detector of each pixel. This is fixed pattern noise (FPN)
The correction is normally performed by a correction circuit. As an example of this, as described in the above-mentioned prior invention, a memory for holding a bias level variation amount is provided to correct fixed pattern noise.

Further, as shown in FIG. 9, in the quantum thermal infrared imaging apparatus, there is a so-called sensitivity variation in which the relationship between the infrared input energy and the detector output differs depending on the pixel. If the infrared input energy of the entire screen changes, such as when the background temperature of the subject changes in such a state,
Even if the fixed pattern noise is removed by x0 in FIG.
The fixed pattern noise corresponding to Δy1 appears again due to the sensitivity variation. As a measure against this, a configuration of a gain correction circuit as shown in FIG. 8 is provided, and the sensitivity variation Δy2 / y2 is stored in a sensitivity difference memory in advance.
Calculation of Δy1 = y1 · Δy2 / y2 is performed on the detector output y1, and the fixed pattern noise due to such sensitivity variation is corrected by adding the calculated value to y1 (for example, Japanese Patent Application Laid-Open No. Hei 2-107074). reference).

Further, as a thermal infrared detector, as shown in, for example, Japanese Patent Application Laid-Open No. 7-193752, an example has been proposed in which pixels which are insensitive to incident infrared rays, so-called optical black (OB), are provided to set a signal reference level. ing.

[0007]

However, the prior art described above has the following drawbacks.

The thermal infrared detector using a bolometer as shown in FIG. 7 has a problem unique to the bolometer type that, when the temperature of the device changes, the temperature change becomes a signal as it is. Moreover, since the amount of change varies depending on the sensitivity variation of each pixel, even if fixed pattern noise (hereinafter abbreviated as FPN) is removed at a certain point in time, the FPN will reappear due to a temperature change. The sensitivity variation is not small because it is caused by the variation of the temperature coefficient of the bolometer, the variation of the resistance value itself, and the like.

For example, since the temperature change of the diaphragm with respect to the temperature change of the object is very small, there is a signal change corresponding to a change of about 500 ° C. of the object by a change of 1 ° C. in the device. Assuming that the sensitivity variation of each pixel is 5%, the FPN becomes about 25 ° C. in terms of the object. When no temperature control is performed, the device temperature changes by several degrees immediately after the power is turned on and by about 1 degree even after the device is stabilized.

On the other hand, when applying the FPN correction in consideration of the sensitivity variation as conventionally performed in the quantum infrared detector, there are the following problems.

A thermal infrared detector generally needs to amplify a large signal because of its low sensitivity. However, since the drift of an amplifier that does this cannot be ignored with respect to the signal level, the absolute level of the signal is measured. Can not do. That is, only the relative level between pixels is known.

Further, a reference pixel having no sensitivity to incident infrared rays to be a reference level of a signal, that is, an optical black (hereinafter abbreviated as OB) is also affected by a change in device temperature similarly to an effective pixel. Would.

That is, the thermal infrared detector does not have a reference level for data processing, and the extension of the prior art cannot correct FPN generated due to sensitivity variations.

In the infrared detector of the infrared imaging device, variations occur in the OB pixel and the effective pixel due to a change in temperature, and the amount of the variation reflects a change in the temperature of the device.

The present invention has been made in view of the above-mentioned conventional circumstances, and therefore, an object of the present invention is to provide an infrared detector having an O detector.
To provide a novel thermal-type infrared imaging apparatus capable of eliminating the above-mentioned drawbacks inherent in the conventional technology by correcting the variation due to the temperature of the effective pixel using the variation due to the temperature of the B pixel. It is in.

[0016]

In order to achieve the above object, a thermal infrared imaging apparatus according to the present invention comprises a plurality of reference pixels having no sensitivity to incident infrared rays, so-called optical black, and a plurality of effective pixels. A memory for storing a coefficient of drift of each pixel, and a drift correction unit, wherein the drift amount of the effective pixel is corrected based on the variation amount of the plurality of reference pixels.

The present invention also has means for calculating an average of the output levels of the plurality of reference pixels, and means for calculating the displacement of each pixel from the average.

The present invention also has means for calculating the average of the absolute value of the displacement of each of the reference pixels.

Further, the present invention has a memory for storing an average of absolute values of displacements of the respective reference pixels obtained at the time of initialization.

Further, the present invention has a memory for storing the displacement of each effective pixel obtained at the time of initial setting.

Further, the present invention has means for dividing the average of the absolute values of the displacements of the reference pixels coming in real time by the average of the absolute values of the displacements of the reference pixels obtained at the time of initialization.

Further, the present invention has means for multiplying the result of the division by the displacement of each effective pixel obtained at the time of the initial setting, and means for subtracting each obtained multiplication result from each effective pixel.

In other words, the thermal infrared imaging apparatus according to the present invention blocks incident infrared rays during initial setting, and
After the correction, the average value of the output levels of the plurality of reference pixels having no sensitivity to the incident infrared ray is calculated by changing the device temperature, and the average value of the output levels of the plurality of reference pixels is calculated by the OB pixel average value calculation circuit. The output signal average value of the reference pixel is subtracted from the pixel signal by a first computing unit to obtain a drift coefficient for all the pixels, and among the reference pixels, an average value of the drift coefficient of the reference pixel is calculated. Calculating the average value of the output levels of the reference pixels, calculating the displacement of the reference pixel signal arriving in real time from the average value of the output levels of the reference pixels by the first computing unit, and calculating the average of the displacements of the reference pixels. OB value
The average value of the displacement of the reference pixel is calculated by the average value calculating circuit of the displacement, and the second arithmetic unit divides the average value of the displacement of the reference pixel by the average value of the drift coefficient of the reference pixel obtained at the time of initial setting. The effective pixel drift coefficient obtained for the effective pixel is multiplied by a third computing unit, and the multiplication result is subtracted by a fourth computing unit from the effective pixel signal arriving in real time.

[0024]

Drift is a phenomenon in which the DC level of each pixel fluctuates with time, as described above. If the amount of fluctuation is the same for all pixels, the drift is obtained by using the level of a reference pixel such as OB as a reference. Can be removed. However, in a thermal infrared sensor using a bolometer, as described above, the drift amount of each pixel varies depending on the sensitivity variation between pixels, and therefore, it is not possible to simply use the OB as a reference.

Drift is due to ambient temperature variations, which are almost linear with temperature. In other words, although the amount of change differs depending on the pixel, if there is a change Δvi in the pixel i at a certain temperature change ΔT, there will be a double change 2Δvi with a double temperature change 2ΔT. That is, if the amount of fluctuation of each pixel in a certain temperature change, the so-called drift coefficient, and the amount of temperature change are known, it is possible to return to the original screen with no variation by subtracting it.
The reference pixel, so-called OB, blocks incident infrared rays.
Variations between multiple OBs reflect temperature changes,
By measuring the variation of the OB, a temperature change can be estimated.

[0026]

Next, preferred embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing a first embodiment of a thermal infrared imaging apparatus according to the present invention.

Referring to FIG. 1, an infrared detector 101
Is a part of the conventional titanium bolometer type infrared detector,
Reference pixel having no sensitivity to incident infrared light, ie, OB
It is provided with pixels. Here, the OB pixel is realized by providing a shielding plate for blocking incident infrared rays on a package. As shown in FIG. 6, the OB pixel is desirably the first pixel for reading. It is sufficient that each row has one, but several rows can be formed as a spare in case of a defect. The method of providing the shielding plate on the package is advantageous in that no change is made to the device itself, and since there is no structural difference between the OB pixel and the effective pixel except for shielding incident infrared rays,
The effects of disturbances other than incident infrared radiation can be minimized.

Reference numeral 102 denotes an analog FPN correction circuit. The FPN correction circuit 102 removes FPN to some extent so that the level of each pixel falls within the dynamic range of the A / D converter 104. 105
Denotes a digital FPN correction circuit, which removes FPN that could not be removed by the analog FPN correction circuit 102 to make a screen with negligible variation.

In the present invention, the drift correction circuit 11
7. The drift correction circuit 117 is further composed of several blocks, as shown in FIGS.
The operation of FIG. 4 and the flowchart of FIG. 4 will be described together. FIGS. 2A and 2B show changes in the detector outputs of the OB pixel group and the effective pixel group as a function of the device temperature Td. Immediately after the FPN correction, the outputs of the detectors are the same for both the OB pixel group and the effective pixel group. When the temperature of the device changes, the detector output changes, and the output varies according to the sensitivity variation of each pixel. Since the sensitivity of each pixel is linear, the amount of signal variation also increases linearly.

Here, as an initial setting, the device temperature is changed while the incident infrared rays are blocked. This change may be changed by a Peltier element or the like, or a natural change in device temperature after power-on may be used. The temperature to be changed is not a problem, and it is sufficient that the effective number of bits for obtaining the drift coefficient varies. When some variation occurs, the drift coefficients of all the pixels including the OB pixel and the effective pixel are obtained. This is because the average value [vOB] of the output levels of all the OB pixels is obtained by the OB average value calculation circuit 110 and the result Δvi0 obtained by subtracting [vOB] from each pixel including the OB pixel and the effective pixel by the subtractor 111 is used as a drift coefficient memory. It is stored in the OB section of 116. This Δvi0 becomes the drift coefficient of the pixel i. The drift coefficient of the OB pixel is particularly called Δvi0, OB. The drift coefficient of the effective pixel is Δvi0. Effective pixel drift coefficient Δv
i0 is also obtained in the same manner as the OB pixel, and is stored in the effective pixel section of the drift coefficient memory 116. It is preferable to obtain the drift coefficient by averaging the results obtained over several frames in order to reduce the influence of random noise. OB
The average value register [Δvi0, OB] of the absolute value of the drift coefficient Δvi0, OB of each OB pixel is stored in the average value register 113 of the partial drift coefficient.

That is, assuming that the number of OB pixels is NOB,

(Equation 1) OB part drift coefficient average value register 113
To store. In this calculation, as shown by the broken line in FIG.
The B displacement average value calculation circuit 112 may be used, or an external arithmetic device or the like may be used.

The above is the initial setting in a state where the incident light is cut, and the normal operation is performed as follows. Similarly to the above, the average value [vOB] of the output level of the OB pixel at that time is obtained by the OB average value calculation circuit 110, and the average output level [vv] of the OB pixel is obtained by the average value calculation circuit 112 of the OB displacement.
An average value [Δvi, OB] of the absolute value of the displacement from [OB] is obtained. This value indicates the variation amount of the OB pixel, and reflects the device temperature. That is, the absolute value of the displacement of the OB pixel changes every moment according to the drift. OB
The average value [Δvi0, OB] of the absolute value of the drift coefficient Δvi0, OB of the OB pixel acquired in the initial setting is stored in the average value register 113 of the partial drift coefficient.
Xd = [Δvi, OB] / [Δvi0, OB]. The value xd indicates the current level of the device temperature with respect to the initial setting, and the value xd is multiplied by the drift coefficient of the effective pixel acquired by the initial setting to the multiplier 11d.
By multiplying by 5, the drift amount of the effective pixel can be estimated. Since the effective pixel normally receives incident infrared rays, it is necessary to make such an estimation. The drift correction is completed by subtracting the estimated drift amount of each effective pixel from each effective pixel by the subtractor 107.

The frequency of this correction depends on the drift level and the time constant, but may be performed once every several frames. The scan converter 108 is used to match the operation timing of the infrared detector 101 with the timing of the NTSC output. An NTSC composite signal is output by the D / A converter 109 and other circuits.

The drive circuit 118 applies a pulse such as a clock for operating the infrared detector 101. Reference numeral 119 denotes a Peltier element for keeping the device temperature of the infrared detector 101 constant, and reference numeral 120 denotes its controller. Temperature control usually fluctuates about 100mK,
In addition, since there is fluctuation of an amplifier and the like other than the device, the drift does not disappear, but the drift is considerably reduced. This allows a margin for designing the dynamic range of the drift correction circuit 117. For the same purpose, it is possible to put the whole or a part of the circuit of FIG. 1 in a temperature-controlled device such as a thermostat.

FIG. 3 is a block diagram showing the drift correction circuit 117 shown in FIG. 1 more specifically.

Referring to FIG. 3, the infrared detector 101 is assumed to be a two-dimensional sensor of about 128 × 128, and is assumed to have 128 OB pixels, one for each row. The signal resolution is assumed to be 14 bits. O
The B average value calculation circuit 110 includes an ADDER 301, a latch 302, a RAM 303, and a timing generator 3 for an OB portion.
04. OB timing generator 30
4 to control each circuit, 128 OBs
Is calculated, and the upper 14 bits are used to obtain 128
And the average value of OB is obtained. The value is stored in the RAM 303.

The output of the subtraction circuit 111 includes the OB of each OB.
A deviation from the mean appears. This displacement, that is, the variation of the OB is assumed to be about 1/16 with respect to the dynamic range of 14 bits, and the number of bits here is 10 bits.
It may be it.

The average OB displacement calculation circuit 112 further averages the absolute values of the variation amounts of the 128 OBs. The absolute value may simply take the sign bit. Here, the addition result is not divided by the number of OBs in order to increase the number of significant digits in the subsequent division circuit 114. That is, the average value [Δvi, OB] of the OB variation amount is multiplied by 128, and further shifted by 3 bits to be multiplied by 1024 in total.

Average value register 1 of drift coefficient of OB section
13 stores the average value [Δvi0, OB] of the absolute value of the OB variation amount acquired at the time of the initial setting as described above. The output [Δvi, OB] of the average value calculation circuit 112 of the OB displacement coming in real time is divided by the average value [Δvi0, OB] obtained at the time of the initial setting (114), and how much the output drifts from the initial setting. Ask for
At this time, since the real-time data [Δvi, OB] is multiplied by 1024, if the drift amount is the same as that at the time of the initial setting, the value here is 1024. Also, as [Δvi, OB] can be up to twice as large as [Δvi0, OB],
The output of the division circuit 114 is 11 bits.

The output of the dividing circuit 114 is multiplied by the drift coefficient Δvi0 of each effective pixel obtained at the time of initial setting (115), and the drift amount Δvi of each effective pixel can be estimated. The reason why the upper 11 bits of the output of the parallel multiplier 115 are taken is that the lower 10 bits are discarded and the result of the previous 1024 multiplication is restored. The address generator 309 is
The addresses of the effective pixels are sequentially generated.

When configuring a thermal infrared imaging apparatus, it is necessary to consider how many bits should be allocated to the minimum temperature resolution. Excessive allocation will increase the size of the device. In order to make full use of the performance of the device, the FPN needs to be sufficiently small with respect to random noise. FPN is 1 effective value for random noise
If it is about / 4,

(Equation 2) Only a small increase in total noise is required. The control of FPN is set to 1 / random of random noise (corresponding to the minimum temperature resolution).
In the case of performing with an accuracy of 4, at least 2 bits must be allocated to the minimum temperature resolution. For example, when the temperature resolution of the device is 0.1 ° C., 0.02 per bit
The control accuracy of the FPN may be set to 0.025 ° C. at 5 ° C.

The block configuration shown in FIGS. 1 and 3 is a block diagram showing an example of the present invention, and it is of course possible to configure other circuits in accordance with the gist of the present invention.

FIG. 5 is a block diagram showing a second embodiment according to the present invention. In the second embodiment, all the above calculations are performed by the CPU 501, and the results are stored in the image memory 502 as needed. What is necessary is just to comprise a program according to the flowchart of FIG. 503, 5
Reference numeral 04 denotes a ROM and a RAM for operating the CPU.

In the above description, the level of the drift (xd) is estimated from the displacement ｖvi0, OB of each OB at the time of initial setting and each OB displacement Δvi, OB in real time as follows: By xd

(Equation 3) Seeking. In addition to this estimation of xd, [Equation 4]
[Equation 5]

(Equation 4)

(Equation 5) And so on. [Equation 4] has a problem that miscalculation becomes large. [Equation 5] has a problem that the calculation becomes complicated.

[0046]

As described above, according to the thermal infrared imaging apparatus of the present invention, the correction of the fixed pattern noise FPN generated due to the sensitivity variation of the thermal infrared detector, which was difficult with the extension of the prior art, was performed. And the temperature resolution can be further improved.

The temperature drift of each pixel includes the temperature drift of the amplifier circuit in addition to the temperature drift of the device itself. According to the present invention, this effect can be eliminated.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of a thermal infrared imaging apparatus according to the present invention.

FIG. 2 is a diagram illustrating an operation of the thermal infrared imaging device according to the present invention.

FIG. 3 is a block diagram showing the first embodiment of the thermal infrared imaging apparatus according to the present invention shown in FIG. 1 more specifically;

FIG. 4 is a flowchart illustrating an embodiment of a thermal infrared imaging apparatus according to the present invention.

FIG. 5 is a block diagram showing a second embodiment of the thermal infrared imaging apparatus according to the present invention.

FIG. 6 is a circuit configuration diagram showing a specific example of an infrared detector used in one embodiment of the thermal infrared imaging device according to the present invention.

FIG. 7 is a cross-sectional view of a conventional thermal infrared detecting element.

FIG. 8 is a block diagram of a conventional infrared imaging device.

FIG. 9 is a diagram illustrating the operation of a conventional infrared imaging device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 101 ... Infrared detector 102 ... Analog FPN correction circuit 103,106,107,111 ... Subtractor 104 ... A / D converter 105 ... Digital FPN correction circuit 108 ... Scan converter 109 ... D / A converter 110 ... OB average value Calculation circuit 112 ... Average value of OB displacement calculation circuit 113 ... Average value of OB section drift coefficient 114 ... Divider 115 ... Multiplier 116 ... Drift coefficient memory 117 ... Drift correction circuit 118 ... Driving circuit 119 ... Peltier element 120 ... Peltier controller 301, 305 Adder 302, 306 Latch 303, 307 RAM 304, 308 Timing generator for OB part 309 Address generator 501 CPU 502 Image memory 503 ROM 504 RAM 601 Titanium bolometer 602 Pixel Switch (N-type MOSFET) 603 vertical AND 604 transfer gate 605 horizontal AND 606 common source line 607 vertical signal line 608 horizontal signal line 609 vertical shift register 610 horizontal shift register 611 output 612 integration Circuit 613 Integration transistor 614 Integration capacitance C1 615 Reset transistor 700 Semiconductor substrate 701 Scanning circuit 702 Silicon oxide film 703 Cavity 704, 705 Aluminum 706 Polysilicon 707 Titanium bolometer 708 Silicon oxide film 709 Titanium nitride

Claims (3)

[Claims] 1. An infrared detector comprising a plurality of reference pixels having no sensitivity to incident infrared light and a plurality of effective pixels, a means for obtaining an average of output levels of the plurality of reference pixels, and the average value Means for calculating the displacement of each pixel from the memory, a memory for storing the displacement of each pixel, means for performing a division by inputting the displacement of the reference pixel during normal operation and the displacement of the reference pixel obtained at the time of initial setting, and the division A thermal infrared imaging apparatus comprising: means for multiplying a result by inputting a displacement of an effective pixel obtained at the time of initial setting; and means for inputting and subtracting the multiplication result and an effective pixel signal during normal operation. . 2. A thermal infrared imaging apparatus according to claim 1, further comprising means for obtaining an average of said division results. 3. An OB pixel average value calculation circuit calculates an average value of output levels of a plurality of reference pixels having no sensitivity to the incident infrared light by blocking incident infrared rays and changing a device temperature after FPN correction. And a first calculator for subtracting the average output level value of the reference pixel from all pixel signals including the reference pixel and a plurality of effective pixels to obtain a drift coefficient for all the pixels, The thermal infrared imaging apparatus according to claim 1, wherein the drift coefficient of the effective pixel is stored in an OB pixel drift coefficient memory, and the drift coefficient of the effective pixel is stored in an effective pixel drift coefficient memory.