JP2755150B2 - Infrared imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an infrared imaging device.

[0002]

2. Description of the Related Art FIG. 4 is a schematic structural view of a conventional infrared imaging device shown in P83 of "Solid-state imaging device" edited by the Institute of Television Engineers of Japan. In this infrared imaging device, a conversion / accumulation unit 6 composed of a detector 1 for converting the intensity of infrared rays into a magnitude of a current and a storage capacitor 3 for accumulating the current constitutes one pixel. Are arranged in a matrix in a two-dimensional manner in a horizontal direction and a vertical direction in the drawing. One end of each storage capacitor 3 is connected to a vertical switch 4 composed of a MOS transistor for each pixel. The other ends of the vertical switches 4 are commonly connected in the column direction, and are connected to a horizontal switch 5 including MOS transistors provided for each column. The other end of the horizontal switch 5 is commonly connected to the output terminal 100. The horizontal shift register is a horizontal switch 5
Are sequentially turned on, and a signal in each column direction is led to the output terminal 100. The vertical shift register 8 sequentially turns on the vertical switches 4 and guides the signal of each pixel to a common horizontal switch 5 in the column direction for each row. The horizontal and vertical shift registers 7 and 8 and the switches 5 and 4 constitute horizontal and vertical read circuits.

A conversion / accumulation unit 6 when a photodiode is used as a photoelectric conversion element for converting an infrared ray into an electric signal.
FIG. 5 shows a circuit diagram of one example. The photodiode 9 is made of HdCgTe or InP which is sensitive to infrared wavelengths.
Use a semiconductor such as b. The photodiode 9 generates a photogenerated current according to the intensity of the incident infrared light. The current is integrated and stored in the storage capacitor 3 over a predetermined time. Subsequently, the data is read out to the outside by a reading circuit using the vertical switch 4 and the horizontal switch 5. After the data of each pixel is read by the read circuit from the conversion / accumulation unit 6 forming each pixel, the storage capacitor 3 is reset. The resetting of the storage capacitor 3 is described in detail in the above-mentioned document “Fixed imaging device”.

Next, a case where a bolometer is used as a photoelectric conversion element will be described. The bolometer is a resistor using a semiconductor or a metal, and is made of a material whose electric resistance changes relatively greatly with temperature. This element detects a temperature rise due to infrared absorption as a change in electrical resistance. FIG. 6 shows a conversion / accumulation unit 6 when a bolometer is used as the detector 1.
FIG. 3 is a circuit diagram of an example of FIG. The bolometer 10 has a constant current source 1
2, a bias current is supplied to convert a change in resistance in the bolometer 10 into a voltage. Then, the voltage is converted into a current by the MOS transistor 16 and injected into the storage capacitor 3 for integration. Reading is performed by sequentially switching the vertical switch 4 and the horizontal switch 5 as in the case of the photodiode shown in FIG.

In the above description, a dedicated storage capacitor 3 is prepared in the storage section, and a circuit type infrared imaging device using MOS type transistor switches 4 and 5 has been described as an example for reading. It should be added that a CCD can be used and a vertical CCD can be used for charge storage.

[0006]

When an infrared imaging device is used to image a background or room near room temperature with an infrared imaging device, most of the infrared energy incident on the infrared imaging device is due to background radiation, and is radiated from an object of interest. It is said that the infrared ray, that is, the signal component is only about 1% of the total incident infrared ray in energy ratio. Therefore, also for the photodiode 9 shown in FIG. 5, most of the output is the background radiation component.

On the other hand, the storage capacitor 3 in each pixel has an upper limit due to dimensional restrictions, and the upper limit is several PF. Accordingly, the photo diode # 9 does not saturate the storage capacitor 3 over a full frame period such as 1/30 second.
It was not possible to accumulate and integrate the output currents.
Due to this limitation of the signal accumulation period, the upper limit of the sensitivity and dynamic range of the conventional infrared imaging device has been determined.

In the circuit shown in FIG. 6, as the bias current flowing from the constant current source 12 to the bolometer 10 increases, the sensitivity and the S / N ratio can be improved. Was. Furthermore, in order to suppress the effect of noise generated in the MOS transistor 16 for voltage and current conversion, the drain current of the transistor 16 must be increased. And sensitivity and S / N
Deterioration is inevitable. As described above, the conventional infrared imaging device has problems to be solved with respect to sensitivity, S / N, and dynamic range.

[0009]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a detector for photoelectrically converting infrared rays to generate an electric signal corresponding to the intensity of the infrared rays, and a charge corresponding to the electric signal. And a readout circuit for reading out the charge stored in the storage capacitor. The detector and the storage capacitor form a pair of conversion / storage units, and the conversion / storage units are respectively In the infrared imaging element in which a plurality of pixels are arranged two-dimensionally, the electric signal of the output of the detector of a certain pixel and the electric signal of the output of the detector of a pixel adjacent to the pixel Is provided for each pixel, and a subtraction circuit for obtaining the difference between the pixels and storing the difference in the storage capacitor is provided.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an embodiment of the infrared imaging device of the present invention. In this embodiment, a conversion / storage unit 6 that converts infrared rays into a current, stores the current in a capacitor, and outputs a voltage proportional to the intensity of the infrared rays is arranged in a two-dimensional matrix. Further, the infrared imaging device includes a readout circuit including a vertical switch 4, a horizontal switch 5, a vertical shift register 8, and a horizontal shift register 7. The readout circuit is used to store the data in the storage capacitor 3 of the conversion / storage unit 6. and outputs a voltage V _n corresponding to the amount of electric charge from the output terminal 100 to the outside. The conversion / accumulation unit 6 is electrically connected to the conversion / accumulation unit 6 of the adjacent pixel for transmitting / receiving a signal to / from the adjacent pixel, and has a built-in subtraction circuit 2 for calculating a signal difference between pixels. . By the subtraction processing in the subtraction circuit 2, most of the background radiation and the component of the bias of the detector 1 can be canceled. Reference numeral 11 indicates a reference signal source. FIG. 2 is a circuit diagram showing a specific example of the conversion / accumulation unit 6 for one pixel when a photodiode is used as the detector 1 which is a photoelectric conversion element. Field effect transistors Q ₁ , Q ₂
Form a first current mirror 13a, field effect transistors Q ₁ and Q _{4 form a} second current mirror 13b,
The field effect transistors Q ₅ and Q _{6 form a} third current mirror 14. Photodiode 9 generates a Mitsunari raw current I _n receives the infrared (n = 1,2, ...,
N). I _n the a for taking the difference between the previous pixel, a current mirror 13a for sending to the next pixel, is made two systems at 13b. I _n one line are sent to the next pixel by reversing the direction of current in the current mirror 14. I _n the other one system is taken the difference between the Mitsunari raw current I _n-1 of the previous pixel, the differential current I _n -I _n-1
Is injected into the storage capacitor 3 and integrated.

FIG. 3 shows a conversion / accumulation unit 6 for one pixel when a bolometer is used as the detector 1 which is a photoelectric conversion element.
FIG. 3 is a circuit diagram showing one specific example. The field effect transistors Q ₇ , Q ₈ , Q ₉ , Q ₁₀ and the constant current source 22 form a transconductance amplifier 15. Constant current driven bolometer 10 receives infrared changing the electrical resistance, voltage V _n across the bolometer 10 in proportion to the change in its electrical resistance changes. V _n is connected to one gate of the transconductance amplifier 15 of the differential, the adjacent pixels at the same time also connected to the other gate of (be adjacent to the left side of FIG. 3, pixels which are not shown) I have.
Transconductance amplifier 15 is injected, the gate voltage V _n, V _n-1 current I _n corresponding to, and converted to I _n-1, both of the difference between the current (I _n -I _n-1) to the storage capacitor 3 And integrate.

In the above, the difference / accumulation operation between the pixels has been described by citing a circuit in which one pixel is constituted by the conversion / accumulation unit 6 using two types of photoelectric conversion elements as the detector 1. In any of these circuits, a pixel at the right end (in FIGS. 2 and 3) in the row direction (n = 1, which is referred to as an extreme pixel) is provided with some reference signal (I _{0 in} the circuit of FIG. 2). , V ₀ ) must be given in the circuit of FIG. In the present invention,
As the reference signal source 11, a fixed current source or a fixed voltage source or an optical black element which is a light-shielded photoelectric conversion element is provided. Therefore, the rightmost pixel of each row in FIG. 1, that is, the pixel (first pixel) where n = 1, has a difference from the reference signal of the output of the reference signal source 11. Output terminal 1
00 can not be displayed as an image be provided to directly display the pixel signal V _n read from must be supplied to the display device after restoring the pixel signal before taking the difference. FIG. 7 is a block diagram showing a restoration circuit for restoring the pixel signal. The restoration circuit includes a delay circuit 17 for delaying a signal by one pixel, and an adder 18.
, A reference signal generator 19, and a switch 20. The circuit of FIG. 7 is applied to an infrared imaging device in which the conversion / accumulation unit 6 is configured by the circuit of FIG. 2 or FIG. Nth
Signal V _n of the pixel is restored by being added at the _n-1 pixel signal V _n-1 and the addition circuit 18 through the delay circuit 17. The reference signal generator 19 outputs a signal V corresponding to the output signal of the reference signal source 11 built in the infrared imaging device.
₀ is generated. For the first pixel signal V ₁ will be adding the V ₀ and V ₁ switches the switch 20 to V ₀ side, it is possible to DC level restored correctly.

[0013]

As described above, the infrared imaging device of the present invention stores only the difference signal between adjacent pixels in the storage capacitor of each pixel, so that a sufficient bias is applied to the photoelectric conversion device and full Even if accumulation is performed during the frame period, the accumulation capacity does not saturate. Therefore, according to the present invention, it is possible to realize an infrared imaging device having significantly improved sensitivity, S / N, and dynamic range.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific example of a conversion / accumulation unit in the embodiment of FIG.

FIG. 3 is a circuit diagram showing another specific example of the conversion / accumulation unit in the embodiment of FIG.

FIG. 4 is a schematic configuration diagram of a conventional infrared imaging device.

FIG. 5 is a circuit diagram showing an example of a conversion / accumulation unit in the infrared imaging device of FIG.

FIG. 6 is a circuit diagram showing another example of the conversion / accumulation unit in the infrared imaging device of FIG. 4;

7 is a diagram showing a circuit for restoring a pixel signal from an inter-pixel difference signal output from an output terminal 100 in the embodiment of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Detector 2 Subtraction circuit 3 Storage capacity 4 Vertical switch 5 Horizontal switch 6 Conversion / accumulation part 7 Horizontal shift register 8 Vertical shift register 9 Photodiode 10 Bolometer 11 Reference power supply 12 Bias current source 100 Output terminal 13a, 13b, 14 Current mirror 15 Transconductance amplifier 16 MOS transistor

Claims (2)

(57) [Claims] 1. A detector for photoelectrically converting an infrared ray to generate an electric signal corresponding to the intensity of the infrared ray, a storage capacitor for storing a charge corresponding to the electric signal, and a charge stored in the storage capacitor. A read-out circuit for reading out, wherein the detector and the storage capacitor form a pair of conversion / storage units, and the conversion / storage units form pixels, and a plurality of infrared imaging units are arranged two-dimensionally. A subtraction circuit for calculating a difference between the electric signal of the output of the detector of a certain pixel and the electric signal of the output of the detector of a pixel adjacent to the pixel, and accumulating the difference in the storage capacitor; An infrared imaging device comprising: 2. The two-dimensionally arranged pixels are linearly arranged in a row direction and a column direction to form a matrix, and are located at one end in the row direction among the two-dimensionally arranged pixels. 2. The infrared imaging device according to claim 1, further comprising a reference power supply or an optical black pixel for obtaining a difference between an output of a detector of the first pixel and the electric signal by the subtraction circuit. 3.