JPS62279478A - Logarithmic conversion circuit - Google Patents

Abstract

PURPOSE:To execute an arithmetic conversion of a high luminance of an image sensor output, and to obtain a good image signal extending over a wide dynamic range, by setting an average value of every picture element of a dark current at the time of light-shielding of the image sensor, as a reference voltage value of a logarithmic conversion. CONSTITUTION:First of all, an image sensor IS is set to a light-shielding state, a dark current signal of an effective photosensitive picture element is read out plural times by a sensor driving controlling circuit DV and AD/converted by an ADC converter, and based on an obtained digital dark current signal, an average value of a dark current is calculated digitally at every picture element by an arithmetic circuit CPU, dark current information of every picture element of this result of operation is DA/converted by a DA converter DAC, and this converted data inputted as reference voltage information to a non- inversion input side of a logarithmic conversion use operational amplifier OP2. In such a way, even if a dark current of the sensor IS is uneven, the corresponding dark current can be set, and a logarithmic conversion output having a high accuracy can be obtained. Also, an influence of random noise of the sensor IS can be lowered remarkably.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a logarithmic conversion circuit that logarithmically converts the signal output of a storage type image sensor using a CCD (charge coupled device) or the like with high accuracy.

[Prior Art] When an image sensor such as a CCD is used to photoelectrically read an image, there is a drawback that the output signal of the image sensor becomes significantly small on the low luminance side, resulting in poor detection accuracy. Therefore, in order to obtain image signals with substantially the same precision over a wide dynamic range from high brightness to low brightness, it is conceivable to logarithmically transform the output signal of the image sensor.

Conventional logarithmic conversion circuits of this type are configured so that a certain reference signal level is determined in advance and a signal higher than this reference signal level is input.

FIG. 3 shows an example of the configuration of such a conventional logarithmic conversion circuit. Here, IS is an accumulation type image sensor such as CCD, D
V is a sensor drive control circuit (driver) that drives the image sensor Is. Further, 1 is a differential amplifier circuit composed of resistors R1 to R4 and an operational amplifier, op 1, which outputs a signal from the image sensor IS and a reference signal for removing noise in phase with the output. As a result, a signal output from which common mode noise has been removed is produced at the output of the operational amplifier apl.

RF is a reference voltage generation circuit that generates a reference voltage VRF equivalent to the dark current signal level of the image sensor Is via a variable resistor VR1, and the difference between this reference voltage VRF and the signal voltage of the operational amplifier apl is equal to the logarithm of the resistor R5. Conversion element LD
The signal current is converted into a signal current by a logarithmic conversion circuit 2 consisting of I and an operational amplifier circuit op2, and is subjected to logarithmic conversion.

The logarithmically converted signal output from the operational amplifier op2 is connected to the resistor R.
6 and a temperature compensation logarithmic conversion element LD2, the resistor R7 and the operational amplifier o
The output impedance is converted in the buffer amplifier circuit 4 consisting of p3.

FIG. 4 shows an example of the waveform of the output signal of the main part of the conventional circuit shown in FIG. 3 described above. In FIG. 4, φS□ is a shift pulse for reading image information of the image sensor Is supplied from the sensor drive control circuit DV, and reading of information from the image sensor IS is started with the high level pulse signal of this shift pulse φSH. be done. Further, the outputs E1 to E3 of the operational amplifier opl are a blank feed signal section without a sensor section of the image sensor 25, followed by <DI, D2 is a light-shielded pixel signal section,
Following this <St~SN is an effective photosensitive signal section, following this <D
3. D4 is a dummy pixel signal section, and reading of the signal from the image sensor IS is completed at time t3. Further, the last outputs E4 to E9 of the operational amplifier apl are the idle feed signal portion during signal accumulation until the next signal readout begins.

[Problems to be Solved by the Invention] However, in such a conventional logarithmic conversion circuit, when logarithmically converting the signal of the image sensor Is, a signal below the reference signal level may be input due to variations in the characteristics of the image sensor Is. Therefore, it was often impossible to perform a good logarithmic transformation.

For example, if the reference voltage level generated by RF is set to a constant value of ■□, a significant error occurs in the logarithmically converted output signal when the input signal is small due to uneven dark current for each pixel.

SUMMARY OF THE INVENTION An object of the present invention is to provide a logarithmic conversion circuit capable of solving the above-mentioned problems and obtaining a highly accurate and good logarithmic conversion output of a signal output of a storage type image sensor.

Means for Solving Problem E 1 In order to achieve the above object, the present invention provides a logarithmic conversion circuit that logarithmically converts the signal output output from the storage type image sensor, in which the image sensor is placed in a light-shielded state and the image sensor is activated multiple times. A first circuit reads out the dark current of the photosensitive pixel, and the average value of the dark current read out from the first circuit is determined for each pixel, and the average value is used as a reference voltage value for logarithmic conversion and is generated for each pixel. The present invention is characterized by comprising a second circuit.

[Function] In the present invention, since the average value for each pixel of R' when the image sensor is shaded is used as the reference voltage value for logarithmic conversion, highly accurate logarithmic conversion of the image sensor output is possible. .

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows the configuration of an embodiment of the logarithmic conversion circuit of the present invention.

Note that parts having the same functions as those of the conventional example shown in FIG. 3 will be described with the same reference numerals.

In FIG. 1, ADC is an analog-to-digital (AD) converter that converts the analog output signal of the operational amplifier apl of the differential amplifier circuit 1 into a digital signal, and CPU is the average value for each pixel of the digital output value of the AD converter ADC. The arithmetic circuit that calculates DAC is a digital/analog (DAC) that converts the output average value signal of the arithmetic circuit CPU into an analog signal.
DA) converter, and the output of this DA converter DAC is used as a reference voltage to the operational amplifier o of the logarithmic conversion circuit 2A.
Apply power to the inverting input terminal of p2.

First, the image sensor IS is placed in a light-shielded state, and the sensor drive control circuit DV reads out the dark current signal of the effective photosensitive pixel multiple times, and the ADC converter performs AD conversion.Based on the converted digital dark current signal, the arithmetic circuit CPU digitally calculates the average value of the dark current for each pixel, converts the dark current information for each pixel as a result of this calculation from DA to DA converter DAC, and converts this DA converted data to an operational amplifier for logarithmic conversion.
Manually input the reference voltage information to the non-inverting input side of p2.

In this case, it is possible to set the reference level for logarithmic conversion based on the average value of the @ current for each pixel, so there is no unevenness (variation) in the dark current of the image sensor Is as shown in Figure 4.
Even if there is a problem, the corresponding dark current can be set appropriately, and a highly accurate logarithmic conversion output can thereby be obtained.

Furthermore, since the average value of the dark current signal for each pixel is detected, the influence of random noise of the image sensor 2 can be significantly reduced.

FIG. 2 shows the configuration of another embodiment of the present invention. In FIG. 2, DA is a differential amplifier circuit, which takes the difference between the output of the operational amplifier apl and the average value signal of the dark current for each pixel of the DA converter DAC, and converts the output signal of the image sensor Is to T.
Convert to a level-based signal. The output of the differential amplifier circuit DA is input via a resistor R5 to a non-inverting input terminal of a logarithmic conversion operational amplifier op2 whose level is set as a reference voltage. The other configurations are the same as in the first embodiment.

In the above configuration, the differential amplifier circuit DA calculates the difference between the DA conversion value of the dark current information for each pixel detected in the same manner as in the embodiment shown in FIG. 1 and the output value of the image sensor IS. A signal is converted to a signal with a constant voltage level, for example, a level (ground level) as a reference value in the embodiment, and an operational amplifier for logarithmic conversion with the level as a reference voltage is used.
Manpower p2.

Similar to the embodiment shown in FIG. 1, this embodiment also has the remarkable effect of being able to obtain a highly accurate logarithmic conversion output that eliminates the influence of current unevenness of the image sensor IS.

[Effects of the Invention] As explained above, according to the present invention, the average value of the dark current for each pixel during light shielding of the image sensor is used as the reference voltage value for logarithmic transformation, so that the output of the image sensor is High-precision logarithmic transformation is possible. This makes it possible to obtain a good image signal over a wide dynamic range from high brightness to low brightness.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a circuit diagram showing the configuration of a logarithmic conversion circuit according to an embodiment of the present invention, FIG. 2 is a circuit diagram showing the configuration of a logarithmic conversion circuit according to another embodiment of the present invention, FIG. 3 is a circuit diagram showing an example of the configuration of a conventional logarithmic conversion circuit, and FIG. 4 is a waveform diagram showing an example of output waveforms of the main parts of the circuit shown in FIG. Is...Storage type image sensor, DV...Senna drive control circuit, ADC...Analog/digital converter, DAC/
...Digital-to-analog converter, CPU...arithmetic circuit, DA...differential amplifier, apl, op2. op3... operational amplifier, LDI, L
D2... Logarithmic conversion element, 1... Differential amplifier circuit, 2.2A, 2B... Logarithmic conversion circuit, 3... Temperature compensation circuit, 4... Buffer amplifier circuit.

Claims (1)

[Scope of Claims] 1) A logarithmic conversion circuit that logarithmically converts a signal output output from an accumulation type image sensor, comprising: a first circuit that reads out the dark current of an effective photosensitive pixel a plurality of times with the image sensor in a light-shielded state; , a second circuit that calculates the average value of the current read out from the first circuit for each pixel, and generates the average value for each pixel using the average value as a reference voltage value for logarithmic conversion. A logarithmic conversion circuit featuring: 2) The circuit according to claim 1, wherein the reference voltage value obtained by the second circuit is supplied to a non-inverting input side of a logarithmic conversion amplifier. 3) The circuit according to claim 1, wherein the difference between the reference voltage value obtained by the second circuit and the output value of the image sensor is supplied to a logarithmic conversion amplifier. conversion circuit.