JPS5918904B2 - Image sensor sensitivity adjustment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a device for adjusting the sensitivity of an image sensor;
More specifically, the present invention relates to an apparatus for adjusting the output level of a video signal obtained from an image sensor so as to minimize variations in the output level thereof.

Note that an image sensor here has the function of converting an image into an electric signal and the function of extracting the electric signal as an electric signal sequence corresponding to the image. Data are retrieved serially in an orderly manner using a semiconductor analog memory.

A phototransistor or photodiode is used as the element that converts the image of the subject into an electrical signal, and a CCD (charge couple device) using MOS technology is used as the analog memory. It can be seen as a MOS transistor with many gates between its drains, and is an element with two functions of storage and transfer, and is usually a shift register that transfers data using multiphase clock pulses. Also, BBD is MOS
This is a shift register with cascade-connected type transistors, and its operating function is almost the same as that of a CCD. In this way, unlike general photoelectric conversion elements such as CdS, image sensors can convert images of objects into video signals, and can therefore be applied to pattern recognition, mark readers, and the like.

For example, a pair of image sensors are placed horizontally a certain distance apart, images of a subject are formed on both image sensors, and video signals of the subject are extracted by each image sensor. By comparing the signals, the location of the subject can be determined. In other words, assuming that both image sensors are fixed, as the position of the subject changes,
The imaging position of the subject image on each image sensor changes. Therefore, when each image sensor simultaneously scans the subject image, the times at which the subject signals appear are shifted from each other, and by knowing the amount of shift, the position of the subject can be known. One method is to variably delay the scanning start timing of one image sensor relative to the other, depending on the amount of change in the subject's position, and then compare the output signal waves of both image sensors at each point in time. The position of the subject may be determined based on the amount of delay in the scan start timing when the same signal waves are synchronously obtained from both image sensors. By the way, the above-mentioned image sensor naturally has different output levels when the target object, that is, the object to be converted into a video signal by the image sensor, is in a bright state and when it is in a dark state. As a result, the levels of input and output signals of circuits that process the output change, and the operation and sensitivity of the circuits become unstable.

Note that, conventionally, Se, . In order to eliminate the above-mentioned problems in devices that use general photoelectric conversion elements such as CdS, a diaphragm plate is placed in front of the light-receiving surface of the element, and the diaphragm plate is used to adjust the amount of light incident on the diaphragm plate. Make it variable,
In addition, when such a diaphragm plate is not used, the output of the above element is amplified by passing it through a variable gain amplifier, and the gain of this amplifier is controlled to minimize fluctuations in the output level. . However, when the output level of the image sensor described above is adjusted by this variable gain amplifier, if the output signal of the image sensor reaches the saturation level, it will no longer operate, and if the output signal level is low, the SN The ratio becomes worse. On the other hand, the method of varying the amount of incident light using a diaphragm plate is relatively superior because it provides signals with good signal to noise ratio, but since it is mechanical, it is subject to restrictions in terms of placement, etc., and The problem is that the operating error becomes large. In view of the above-mentioned points, this invention makes use of the unique characteristics of an image sensor to adjust its sensitivity.The purpose of this invention is to adjust the surface illuminance of incident light by changing the amount of light incident on the image sensor. An object of the present invention is to provide a device that maintains the output level of an image sensor constant even if it increases or decreases.

The present invention will be explained below with reference to the drawings.

The saturation illuminance of the output signal of a phototransistor or photodiode, which is known as an element that converts an image of a subject into an electrical signal, can be changed by changing its light accumulation time.

In other words, even if the surface illuminance on the light-receiving surface of the light-receiving element is low, as the light accumulation time increases, the output signal will reach the saturation level, and as the surface illuminance increases, the output signal will reach the saturation level. The time it takes for light to accumulate until reaching the target is shortened. By the way, since an image sensor uses a clock pulse to transfer the charge accumulated in its light-receiving area, the amount of charge accumulated in the light-receiving area changes depending on the length of the scanning cycle. .

That is, when looking at one light-receiving area, when the charges accumulated therein are transferred by the first scan, the amount of charges in that area becomes zero. Therefore, in that area,
Immediately after one scan is completed, charge accumulation starts anew and continues until the next scan. Time will change. As is clear from this, if the number of scan bits of the lock pulse required to perform one scan of the image sensor is set, the frequency of the charge transfer clock pulse, that is, the scan frequency, can be changed by changing the light accumulation time. It can be changed by

Here, looking at the case where the surface illuminance of the light receiving area of the image sensor is higher than the arbitrary reference illuminance LxO and the case where it is lower than the above-mentioned standard illuminance, at the current standard illuminance LxO, the image sensor is As a result of scanning, the output voltage is V. , the frequency F at surface illuminance LXl higher than the reference illuminance LxO. When scanning is performed with the clock pulse of , the amount of accumulated charge in the light receiving area is greater than that at the reference illuminance, so the output voltage V1 is the output voltage V. be higher than Similarly, the frequency F at surface illuminance LX2 lower than the reference illuminance LxO. When scanning is performed with the clock pulse of , the output voltage V2 is the above output voltage. will be lower than Furthermore, by changing the frequency of the clock pulse at the reference illuminance LxO, the output voltage changes as shown in FIG. Therefore, each surface illuminance L mentioned above
Similarly, by changing the scanning frequency of Xl and LX2, the output voltage changes (see FIG. 1). Therefore, the above output voltage V. If is the reference voltage, then
As is clear from FIG. 1, when the surface illuminance of the image sensor is LXl, if it is scanned with a clock pulse of frequency f1, its output voltage becomes the reference voltage V. can be matched. Similarly, when the surface illuminance is LX2, by scanning it with a clock pulse of frequency F2, the output voltage becomes the reference voltage V. It can be made to equal ÷. This invention takes advantage of the characteristics of the image sensor as described above, and automatically changes its scanning frequency according to the amount of light incident on it, that is, the brightness of the subject, as described below. Minimize fluctuations in the output level of the subject's video signal obtained from the
The sensitivity is adjusted.

Note that the reference voltage V mentioned above.

can be set arbitrarily, so if the reference value is set to the optimal input value of a circuit that inputs the output signal of an image sensor, the above circuit can be operated under extremely stable conditions. I can do it. Furthermore, as mentioned above, the saturation level of the output signal of the light-receiving element changes by changing its light accumulation time. In the case where the output signal is saturated, it is possible to raise it to a level that does not saturate it. Therefore, according to the present invention, the range of surface illuminance in which a stable output signal can be obtained can be made wider than in the conventional sensitivity adjustment method. That is, the method using a variable gain amplifier is useless when its output signal has reached the saturation level, and the method using a diaphragm plate is useless when the surface illuminance is low. According to the sensitivity adjustment method, the saturation level of the output signal can be arbitrarily set by varying the frequency of the charge transfer clock pulse and changing the light accumulation time. Can be made wider.

Note that there are two methods for varying the frequency of the clock pulse for charge transfer that scans the image sensor: a method for varying the scanning frequency depending on the average value of the output signal of the image sensor itself; A possible method is to change the scanning frequency depending on the output signal of another light amount measuring device.

FIG. 2 shows an example of a means for adjusting the sensitivity of an image sensor using the above-described method. 1
Integrate the output signal of S with an integrating circuit C such as an operational amplifier,
The output signal of the integrating circuit C is used to change the frequency of the charge transfer clock pulse generated by the variable voltage oscillator 0.

That is, the output level of the integrating circuit C is the same as that of the image sensor 1.
It changes according to the change in the amount of light incident on S. For example, if the surface illuminance increases, the output level of the integrating circuit 1C increases,
Furthermore, if the surface illuminance decreases, the output level also decreases. This means utilizes this change to increase the frequency of the clock pulse generated by the voltage variable oscillator 0 when the output level of the integrating circuit 1C increases.
The variable voltage oscillator 0 is controlled by the output signal of the integrating circuit C in order to lower the frequency when the output level of the oscillator 0 decreases. In addition,
What is shown in FIG. 3 is an example of a means for adjusting the sensitivity of an image sensor according to the method described later, in which the scanning of the image sensor 1S is performed by the output signal of the voltage variable oscillator 0, as in the above-mentioned means. Image sensor 1
The part of the incident light to S is received by another independently provided light amount measuring device, for example, a light receiving element S such as CdS, and the output of this light receiving element S is amplified by an amplifier A. The variable voltage oscillator 0 is controlled by the output signal of the oscillator 0. Note that in the method shown in FIG. 3, the saturation level of the output signal of the light receiving element S determines the upper limit value of the control signal level that controls the voltage variable oscillator 0. Make sure that the output signal saturation illuminance is sufficiently higher than that of the image sensor S, or set the oscillation frequency of the voltage variable oscillator 0 to the highest value when the output signal of the light receiving element S approaches the saturation level. By setting, the range of surface illuminance of the image sensor 1S can be widened. In addition, in the means shown in FIG. 2, the output signal saturation illuminance of the image sensor 1S can be arbitrarily set by the oscillation frequency of the voltage variable oscillator 0, so the range of the surface illuminance is determined by the voltage. It is determined by the oscillation frequency range of variable oscillator 0.

For example, according to the capabilities of image sensors currently being developed, the scanning frequency can be increased from 5KHz to 3MHz.
Since scanning can be performed by varying up to z, the range of adjustable surface illuminance when SiO2 is used as the light receiving element is 2 lux (1x) to 200 lux (1x). As described above, according to the present invention, an increase or decrease in the surface illuminance of the image sensor with respect to the reference illuminance is detected, and the detected output is outputted to the image sensor of the charge transfer clock pulse generator so that the output of the image sensor becomes the reference value. Since the frequency of the charge transfer clock pulse is automatically controlled, the output level of the image sensor can be kept constant even if the amount of light incident on the image sensor changes.

[Brief explanation of the drawing]

FIG. 1 is an output characteristic diagram of an image sensor, FIG. 2 is a diagram showing an example of the sensitivity adjustment means of the image sensor according to the present invention, and FIG. 3 is a diagram showing another example of the sensitivity adjustment means. LxO? Lxl? Lx2llO surface illuminance 1DV1? V2・
...Output voltage, FO, fl, f2...Scanning frequency, IS...Image sensor, O...
...Variable voltage oscillator, IC...Integrator circuit,
S... Light receiving element.

Claims (1)

[Claims] 1. Detecting an increase or decrease in the surface illuminance of the image sensor with respect to a reference illuminance, and outputting the detection output to the image sensor of the charge transfer clock pulse generator so that the output of the image sensor becomes the reference value. 1. A sensitivity adjustment device for an image sensor, comprising means for automatically controlling the frequency of a clock pulse for charge transfer. 2. The means includes an integrating circuit that integrates the output signal of the image sensor, and automatically controls the frequency of the charge transfer clock pulse output to the image sensor of the charge transfer clock pulse generator based on the integrated output. An image sensor sensitivity adjustment device according to claim 1. 3. Claim 1, wherein the means comprises a light amount measuring means, and the frequency of the charge transfer clock pulse outputted to the image sensor of the charge transfer clock pulse generator is automatically controlled based on the detection output thereof. Sensitivity adjustment device for the image sensor described.