WO2008044883A1

WO2008044883A1 - Black level compensating apparatus for a camera and method thereof

Info

Publication number: WO2008044883A1
Application number: PCT/KR2007/004973
Authority: WO
Inventors: Sae Hoon Lee
Original assignee: Nucron Co., Ltd.
Priority date: 2006-10-11
Filing date: 2007-10-11
Publication date: 2008-04-17
Also published as: KR100822849B1

Abstract

This invention relates to a camera black level compensating apparatus and method which compensates the black level of non-linearly distorted video signal from the image sensor influence by radiation through the vision of reference image sensor. When acquiring the image of subject through the image sensor, the image can be distorted by the influent radiation. This black level change can be compensated through the signal acquired by the reference image sensor in the same condition except the interception of lights. In this way, this equipment effectively compensates the non-linear black level distortion to improve the image quality and reliability of radiation-proof camera. Moreover, it filters the outputs of reference image sensor with low band filtering and integrates them to show the radiation quantity. With this information, we can enhance the video quality in the radiative environment and comprehend the radiation quantity of the corresponding area at the same time. So, by providing the radiative quantity as well as the contemporary video information, it enables to check the radiation quantity selectively and to check whether the dedicated radiation detector is working properly, presenting the replacing time of the detector. Moreover, it also has an effect to use above radiation quantity for the post treatment of vision.

Description

BLACK LEVEL COMPENSATING APPARATUS FOR A CAMERA AND METHOD THEREOF

Technical Field

[1] This invention relates to Black level compensating apparatus for a camera and mehtod thereof, and more particularly, to a camera black level compensating apparatus and method which is capable of compensating a black level of an image sensor image, which is nonlinearly deformed due to radiation, by using an image of a reference image sensor. Background Art

[2] Although many efforts have been widely made to develop or discover the next generation enegry to substitute existing fossil energy in preparation for depletion of fossil energy, such efforts do not show visible effects. Rather, in reality, energy is increasingly depending on nuclear power.

[3] Currently, although the proportion of nuclear energy produced in many countries including Korea is relatively low, it is commonly expected that it is gradually increasing until epoch-making energy source is be developed or discovered.

[4] With expectation for nuclear power, establishments of new nuclear power plants and enlargements and reconstructions of existing nuclear power plants are occurring frequently throughout the world, but the industries related to nuclear power are still very low in their relative importance in entire industries and their technical developments are also very slow because the subjects of their utilizations are limited.

[5] For instance, in the case of the cameras to monitor the areas in nuclear power plants where radiation occurs, the dedicated cameras with a high radiation-proof property are being used because common cameras can't provide normal images and would be easily damaged by even low levels of irradiation, but technical improvements for such dedicated cameras are progressing slowly compared to the extent of the demand for the improvements.

[6] FIG. 1 shows the structure of an existing camera tube sensor. As illustrated, a camera tube sensor 10 obtains video signals by transforming the electric phases generated when an image obtained from a subject is formed on the thin film in a photoelectric area 11 into electronic signals using the electron beams scanned by an electron gun 12. Deflection yokes 13 are used to control the direction of the electron beams. Since the camera tube sensor has a high radiation-proof property (2x10 rad), it is still used in some areas where the radiation-proof property is required although it is the oldest commercial image sensor. But, since its size is big and its operation is complicated, currently it is used only in very limited places for the purpose of monitoring in radiation areas including those in nuclear power plants.

[7] FIG. 2 shows a simple structure of a radiation-proof camera 20 using a solid-state imaging device image sensor 23 such as CCD or CMOS. Since a CCD or CMOS image sensor 23 can't act normally if it is directly exposed to gamma rays, this camera works in a manner where the image sensor 23 and an image sensor signal processing unit 24 are protected by a radiation shielding housing 20 made of a material that can shield against radiation and images of subjects are provided to the image sensor 22 indirectly using a mirror 22. In some cases, this camera uses a structure that prevents reversal of the top and bottom of images using a mirrored structure like a periscope with multiple mirrors.

[8] FIG. 3 is a block diagram showing a basic configuration of a camera using a solid- state imaging device image sensor. As illustrated, the camera includes an image sensor unit 30 where images are formed, a synchronized signal inserting unit 32 that inserts synchronized waveforms into the video signals provided by the image sensor unit 30 in accordance with image dimensions, and a timing generating unit 31 that provides the image sensor unit 30 with a timing to obtain images and provides the synchronized signal inserting unit 32 with a timing. The video signals provided through the synchronized signal inserting unit 32 become analog TV video signals (NTSC, PAL etc).

[9] However, as illustrated, gamma rays among radioactive rays may flow into the image sensor unit 30 and this will cause nonlinear increases of the black level.

[10] FIG. 4 shows a pattern of increase of a black level occurring when gamma rays flow into the image sensor unit. As illustrated, the pattern can be seen where a screen becomes nonlinearly darker in its parts closer to the left bottom side.

[11] FIG. 5 graphically shows the phenomenon of nonlinear increase of the black level caused by gamma rays as illustrated in FIG. 4. As illustrated, it can be seen that the reference black level not affected by gamma rays maintains a constant voltage regardless of line positions whereas the actual black levels measured from a case where an image sensor unit was exposed to gamma rays increase nonlinearly on later lines.

[12] If the black levels of video signals change as such, images become darker or turn into grey and thus good images can't be obtained. This phenomenon becomes a factor to greatly impair the reliability of radiation-proof cameras and those radiation-proof cameras that can't process digital signals under radiation environments will not be able to easily remove this distortion immediately in real time within the cameras. Disclosure of Invention Technical Problem [13] In order to prevent the phenomenon of image distortion caused by the influx of radioactive rays as mentioned above, it is an object of the invention to provide a camera black level compensating apparatus and method that can improve the image quality and reliability of radiation-proof cameras by compensating the variation of black levels caused by the radioactive rays flowing into image sensors when obtaining images of subjects through the image sensors using the signals obtained by a reference image sensor to which light is blocked under the same condition.

[14] It is another object of the invention to provide a camera black level compensating apparatus and method that can selectively compensate only image distortions without increases of local noises caused by influxes of radioactive rays by compensating the image distortions caused by influxes of radio active rays in the image sensors outputting analog signals by obtaining the outputs produced by the influxes of radioactive rays into an image sensor of the same specification to which light is blocked and then using the values of the outputs less high frequency ingredients.

[15] It is still another object of the invention to provide a camera black level compensating apparatus and method that can realize the auto iris function that is actively adjusted depending on radiation environments and the compensation of nonlinear brightness distortions by compensating the analogous video signals in radiation environments based on the outputs of an image sensor to which light is blocked in the same environment.

[16] It is still another object of the invention to provide a camera black level compensating apparatus and method that can compensate the variation in black levels caused by the radioactive rays flowing in when obtaining images of subjects through image sensors using the signals obtained by a reference image sensor to which light is blocked under the same condition and also can simultaneously provide information on the dose of radioactivity and on images as well as selectively checking the dose of radioactivity in the place to which the camera is directed, substituting dedicated radioactivity dose measuring devices, or subsidiarily rechecking the accuracy of the dose of radio activity measured by dedicated radioactivity dose measuring devices so that the dose of radioactivity can be used also in post-treatment of images by grasping the dose of radioactivity with the values obtained by integrating the outputs of above mentioned reference image sensor after low pass filtering and thereby simultaneously improving screen qualities in radiation environments and grasping the dose of radio activity. Technical Solution

[17] In order to accomplish the above objects, according to an aspect of the invention, there is provided a camera black level compensating apparatus including an image sensor unit that can obtain images of subjects in the state it is exposed to radiation and outputs the images in analogous video signals; a reference image sensor unit that can output only the effect of radioactive rays in analogous video signals in the state light is blocked; a filtering unit that will remove high frequency components from the analogous video signals of above mentioned reference image sensor unit; and a compensating unit that will output the analogous video signals for which image distortions have been compensated by compensating above mentioned analogous video signals of image sensor unit based on the output of above mentioned filtering unit.

[18] Preferably, the image sensor unit and the reference image sensor unit have the same specifications and external exposure environment and that above mentioned filtering unit includes a low pass filter that will pass only the frequencies below the line frequency to prevent the impact of variations in pixel brightness voluntarily occurring due to radioactive rays.

[19] Preferably, the camera black level compensating apparatus further includes an integrating unit that integrates the outputs of above mentioned filtering unit for a certain time to quantify the analogous video signals output by above mentioned reference image sensor unit for corresponding time and that outputs the signals in the voltage corresponding to the dose of radioactivity.

[20] Acccording to another aspect of the invention, there is provided a camera black level compensating apparatus including an image sensor unit that will obtain images of subjects in the state it is exposed to radiation and outputs the images in analogous video signals; a reference image sensor unit that will output only the effect of radioactive rays in analogous video signals in the state light is blocked; a first filtering unit that will remove high frequency ingredients from the analogous video signals of above mentioned reference image sensor unit; a compensating unit that will reduce the analogous video signals of above mentioned image sensor unit by the output of above mentioned first filtering unit through differential amplifications and then will increase above mentioned differentially amplified signals by the reference black level voltage to output compensated analogous video signals; a second filtering unit that will receive the outputs of above mentioned first filtering unit; and a integration unit that will integrate the outputs of above mentioned second filtering unit based on the frames of above mentioned images to output them in the voltage corresponding to the dose of radioactivity.

[21] Acccording to still another aspect of the invention, there is provided a camera black level compensating method including a first step to obtain the video signals of subjects distorted by influxes of radioactive rays from the first image sensor unit with a radiation- proof property; a second step to obtain the video signals affected by radioactive rays from the second image sensor that has been arranged in the same en- vironment as that of above mentioned first image sensor in the state it has been shielded with a material that blocks light but passes radioactive rays; and a third step to compensate the analogous video signals obtained from above mentioned first image sensor based on the analogous video signals obtained through above mentioned second image sensor unit in order to output the compensated signals.

[22] Preferably, the third step uses the video signals from which high frequency ingredients have been removed among the video signals from the second image sensor obtained through the second stepe.

[23] Preferably, the camera black level compensating method further includes a fourth step to remove high frequency ingredients from the video signals obtained through above mentioned second image sensor and then integrate them to output the voltage information to measure the dose of radioactivity.

[24] Acccording to still another aspect of the invention, there is provided a camera black level compensating method including a first step to obtain the video signals of subjects distorted by influxes of radioactive rays from each of one or more image sensor units with a radiation-proof property; a second step to obtain the video signals affected by radioactive rays from a reference image sensor that has been shielded with a material that blocks light but passes radioactive rays and has been arranged in the same environment as that of the one or more image sensors; and a third step to compensate each of the analogous video signals obtained from the one or more image sensors based on the analogous video signals obtained through the reference image sensor to output the compensated analogous video signals.

Advantageous Effects

[25] As described in detail above, the camera black level compensating apparatus and method produces the effect to improve the images and reliability of radiation-proof cameras by effectively compensating the nonlinear black level distortions of images through compensating the variation of black levels caused by the radioactive rays flowing in when obtaining images of subjects through image sensors using the signals obtained through the reference image sensors to which light is blocked under the same condition.

[26] In addition, the camera black level compensating apparatus and method produces the effect to greatly improve video qualities in radiation environments with out increased noises caused by influxes of radioactive rays by compensating the video distortions caused by the influxes of radioactive rays into the image sensors that will output analogous signals using the outputs produced by the radioactive rays flowing into an image sensor with the same specifications to which light has been blocked after removing high frequency ingredients from above mentioned outputs. [27] In addition, the camera black level compensating apparatus and method produces the effect to compensate the outputs of the image sensors that will output analogous video signals in radiation environments based on the outputs of the image sensors to which light is blocked in the same environments and thereby the auto iris function that will be actively adjusted depending on radiation environments will be realized and black levels will be effectively compensated and thus the video quality of above mentioned camera that has to use analogous signals in order to improve its radiation- proof property will be improved and especially, the effect to provide clear identification of dark subjects will be produced.

[28] Further, the camera black level compensating apparatus and method produces the effect to compensate the variation of black levels using the signals obtained through the reference image sensor to which light is blocked under the same condition and also will integrate the outputs of above mentioned reference image sensor after low pass filtering so that the dose of radioactivity can be grasped with the values and thereby screen qualities in radiation environments will be improved and at the same time, the dose of radioactivity in corresponding area will be grasped and excellent effects will be produced as well including simultaneous provisions of information on the dose of radioactivity and on images , selective identifications of the does of radioactivity in the place to which above mentioned camera is directed, substitutions of dedicated radioactivity does measuring devices, or subsidiary rechecking of accuracy of dedicated radioactivity dose measuring devices and enabling the use of above mentioned dose of radioactivity in after treatment of images too. Brief Description of the Drawings

[29] FIG. 1 shows a structure of a radiation-proof camera using a camera tube.

[30] FIG. 2 shows a structure of a radiation-proof camera using a solid-state imaging device.

[31] FIG. 3 shows a block diagram of a camera using a solid-state imaging devices.

[32] FIG. 4 shows an example of change in images of a camera exposed to a radiation environment.

[33] FIG. 5 is a graph showing changes in black levels of a camera exposed to radioactive rays.

[34] FIG. 6 is a block diagram showing the configuration of an embodiment of the invention.

[35] FIG. 7 is a graph showing video signals produced by the operation of an embodiment of the invention.

[36] FIG. 8 is a graph showing video signals of outputs obtained through an embodiment of the invention. [37] FIG. 9 is a block diagram showing the configuration of another embodiment of the invention.

[38] FIG. 10 is a view showing a system configuration to which embodiments of the invention are applied. Best Mode for Carrying Out the Invention

[39] Hereinafter, the invention will be described in detail through exemplary embodiments with reference to the attached drawings.

[40] FIG. 6 is a block diagram showing the configuration of one embodiment of the invention. As illustrated, the embodiment includes a first image sensor unit 110 that obtains an image of a subject and outputs the obtained image as an anolog signal, and a second image sensor unit 120 with the same size as that of the first sensor unit 110 added to the first sensor unit 110 as an additional part, wherein the sensing part of the second sensor unit 120 is shielded with a material 121 such as aluminum foil which intercepts all lights except radiation and is laid to be exposed to the same environment as the first sensor unit 110. The configuration is made to calculate the output from the first sensor unit 110 and the output from the second sensor unit that is filtered through a low-pass filter 130 using an calculation unit 140 so as to compensate the image distortion.

[41] The first image sensor unit 110 is constructed with an image sensor that converts optical signal to electrical signal, and a module that provides driving timing to the image sensor, and inserts sync so as to produce analogous image signal. As the image sensor, diverse kinds of solid image sensor (CCD or CMOS type sensor) can be used. As the image sensor, it is desirable to use a type of sensor that can endure a radiative environment, and more desirable to use CMOS APS (Active Pixel Sensor).

[42] When the first image sensor unit 110 is exposed to a radiative environment, gamma rays with the shortest wavelength cause the black level to be distorted nonlinearly as depicted in FIGs. 4 and 5.

[43] Accordingly, in order to compensate the distortion, the second image unit 120 with the same size as the first image sensor unit 110 is added. In this, the shielding object 121 made of metallic materials that intercept all lights except gamma rays is applied to the sensing part of second image sensor unit 120 to cut away lights. As such, the second image sensor unit works as a reference sensor for measuring the black level's distortion caused by pure gamma rays.

[44] So, when the analog output from the second image sensor unit 12 is subtracted from the analog output from the first image sensor unit 110 with a distortion by means of the calculation unit 140, nonlinear black level distortion can be compensated.

[45] However, it is worth to note that video distortion by gamma rays is not causing only black level's nonlinear distortion. That is, gamma rays influence any image sensor cells in random, and this changes for every video field and frame. In other words, the influence from the gamma rays are mingled with video signals in a form of high frequency noises, and one of the results is the black level's nonlinear distortion. Accordingly, if the output from the second image sensor unit 120 is subtracted from the output from the first image sensor unit 110, the high frequency noises appear in an overlap, and image distortion can be doubled in this case because noises become doubled in this case in the output image, even though black level's nonlinear distortion is compensated to some degree.

[46] Accordingly, as shown in the figure, a low pass filter 130 should be added to the output of the second image sensor unit 120 so as to cut away the high-frequency component. The second image sensor unit 120 measures the nonlinear distortion caused by gamma rays and what is important is that of low-frequency as shown in FIG. 5. Therefore, there is no problem even if the gamma rays' influences working on the cells in random are removed for compensating the black level distortion. In order to select the low-frequency components and remove the high-frequency components, it is desirable to cut away frequency components higher than that of the line used for obtaining each image line of each of the frames, and this can be determined automatically or manually depending on the desired image and compensation level. Such filtering and compensation of reference signal can be facilitated because the above- mentioned signals are analog video signals with a same sync signal.

[47] As depicted in the figure, the analog video signal including the gamma ray noises from the first image sensor unit 110 is applied to the non-inverting terminal of the calculation unit 140, and the analogous signal from the second image sensor unit 120 removed of high-frequency components is applied to the inverting terminal of the calculation unit 140, and then calculation is conducted so as to only the black level's nonlinear signals are compensated in the output from the first image sensor unit 110, whereby compensated analogous video signals can be outputted. However, there is one more consideration at this stage. The electrical power should be shifted, or corresponding power should be added by the amount of the reference black level so that the above-mentioned analogous video signal can become a practically usable video signal.

[48] FIG. 7 shows the degree of black level distortion obtained from the first image sensor and an image of the signal obtained from the second image sensor. As shown, mutually symmetric signals are obtained because image sensors of the same size are exposed to a same environment. Accordingly, when the two are added, the obtained signal becomes nearly zero. That is, even when the real video information is included, the black level signal becomes zero, so by itself it cannot be a normal video signal. [49] In order to obtain desirable video signal as in FIG. 8, the reference black level power should be added to the result from the signal of FIG. 7 that has been operated, or the signal should be shifted by the corresponding power.

[50] Accordingly, the calculation unit 140 may be composed of an operational amplifier capable of subtraction and addition, or a differential amplifier that can perform corresponding function. The calculation unit 140 can be either exposed to the radiation environment along with the image sensor units 110 and 120, or located outside radiation environment through a cable, but it is more desirable for it to be exposed to radiation environment along with the image sensor units 110 and 12Of or easier manipulation and modulation. For this, the calculation unit 140 should be constructed with radiation- proof components. It is difficult to obtain highly radiation-proof characteristics when ordinary calculation chips are used. So, it is desirable to use discrete components with radiation-proof characteristics such as manual components and bipolar transistors for the same functions.

[51] The above-mentioned function can be practiced according to the influences from radiations, and especially from gamma rays, to compensate the black level's distortion, and such a function is corresponding to the function of auto iris. That is, brightness and black level are compensation by itself according to the environment, without needing a separate control or adjustment, and this provides means to very outstanding image quality in the field of radiation-proof cameras that have to use analog signals in order to improve radiation-proof characteristics.

[52] FIG. 9 shows a block diagram of the configuration of another embodiment of the invention, in which an additional application is possible with the second image sensor unit 120, beside the above-mentioned application for compensating the distortion of the output from the first image sensor unit 110 caused by radiations.

[53] It is for using the second image sensor unit 120 in addition to the above-mentioned application in which the output filtered by the low-frequency pass filter 130 so as to remove the high-frequency components, and it is used to compensate distortions, and for the purpose, the signal void of the high-frequency components through the low- frequency filter 130 is integrated through an integrator 160 over a certain duration, so as to output the radiation amount as a conformed voltage value.

[54] The voltage value may depend on the amount of integration, and the output direct voltage (radiation amount measured in DC) maybe of nonlinear, but this can be remedied by making it of quantitative nature by making the integration period coinciding with the period of analogous video signal outputted from the second image sensor unit 120 (for example, one field (odd or even field) or one frame (sum of odd field and even field), or its multiple combination), and the integrated value obtained in such a way may be used along with a corresponding table according to measured radiation amount, or by matching the value through a specific formula or algorithm, whereby making it possible to use the second image sensor unit 120 to measure the radiation in its location. Here, as shown in the figure, the measured value can be stabilized further if, in processing the analog image signal from the second image sensor part 120, the output from the low-frequency pass filter 130 is not just integrated by the integrator 160 but integrated after the high-frequency components are removed with a second low-frequency filter 150 added in-between, thus filtering out the high- frequency components further (frequencies lower than those passed through the first low-frequency pass filter 130).

[55] As shown in the figure, by using only this camera at a remote distance, radiation amount can be confirmed, while adjusting the brightness through the automatic iris of radiation-proof camera and compensating nonlinear black level distortion.

[56] Especially, as the second image sensor unit 120 compensates the black level distortion in each image sensor in the current radiative environment, it is not necessary to increase its number but just one is enough even though multiple image sensors are used. Yet, the calculation unit 140 in the drawing must be configured according to the above image sensor unit.

[57] FIG. 10 shows the whole structure of the radiation-proof camera system. As shown in this figure, all the components in the radioactive area have radiation-poof characteristics. Here, a camera unit 200 is installed to output the radiation quantity and the video signal compensated of black level distortion by the radiation through the reference sensor. A management unit 300 is also arranged in the non-radioactive area to receive the video signal from the camera unit 200 and the voltage corresponding to the radiation quantity, and to provide the control signal and power to the camera unit 200. A display unit 310 is connected with the management unit 300 to show the video signal to users.

[58] The management unit 300 can have a relay function providing the video signal to the display unit 310 internally, and can convert the analog signal to a digital video signal if necessary to provide the signal to the display unit of digital display format, or can have capabilities of storing/image-filtering of the digital video signal. Also, a function may be provided in which the potential representing the radiation amount may be converted using a matching table, whereby the converted value is provided to the display unit 310.

[59] In addition, if necessary, the radiation amount may be provided through on-screen display OSD method on top of the analogous video signal or digital video signal, and if the image signal from the camera unit 200 is to be processed later, the measured radiation amount may be utilized in adjusting the removal rate of the noises as an image process. [60] Especially, the radiation value represented by the potential may be individually compensated utilizing a calibration procedure through actually measured values, after a reference table is generated, whereby the functions and performances of radiation- hardened camera can be enhanced in a small-quantity market where highest reliability and quality are deemed most important.

Claims

[1] A camera black level compensating apparatus comprising: an image sensor unit that acquires an image of projected object in an exposed status by radiation and outputs the image into an analog video signal; a reference image sensor unit that outputs only a part influenced by the radiation, with all other lights excluded, into the analogous video signal; a filtering unit that removes high frequency components in the analogous video signal from the above reference image sensor unit; and a compensating unit that outputs an analog video signal compensated in its distorted part on the basis of outputs from the filtering unit.

[2] The camera black level compensating apparatus of Claim 1, wherein the compensating unit includes am operational amplifier which receives the signal inputted by the filtering unit with inverting terminal and the analogous video signal inputted by the image sensor unit with non-inverting terminal, and then subtracts the signal of the filtering unit from that of sensor units.

[3] The camera black level compensating apparatus of Claim 1, wherein the compensating unit locates in radiation range and comprises discrete devices including passive elements and a transistor instead of the operational amplifier.

[4] The camera black level compensating apparatus of Claim 1, wherein the compensating unit subtracts the output of the filtering unit from the analog video signal of the image sensor unit and adds the reference black level power to generate a compensated video signal.

[5] The camera black level compensating apparatus of Claim 1, wherein the image sensor unit and the reference image sensor unit have the same specification and the same exposure environment.

[6] The camera black level compensating apparatus of Claim 1, wherein the image sensor unit and compensation unit are multiple in number, and the reference image sensor unit is single.

[7] The camera black level compensating apparatus of Claim 1, wherein the filtering unit includes the low-band passing filter that only allows the passage of video signal with frequencies lower than the line frequency.

[8] The camera black level compensating apparatus of Claim 1, further comprising an additional integration unit which integrates the outputs of filtering unit for a specified time and quantifies the analogous video signal outputs of reference image sensor unit with it to output the power corresponding to the radiation quantity.

[9] The camera black level compensating apparatus of Claim 8, wherein the in- tegration time uses the frame period of vision outputted by the reference image sensor as its unit.

[10] A camera black level compensating apparatus comprising: an image sensor unit which acquires the image of subject in the exposed status to the radiation and outputs it as analog video signal; a reference image sensor unit which only outputs the part influenced by the radiation with all other lights excluded into the analog video signal; a first filtering unit which removes the high frequency components from the analog video signal of reference image sensor unit; a compensating unit which reduces the analog video signal of image sensor unit by the output of first filtering unit through the differentiate amplification, and raises the differentiate amplification signal by adding the reference black level power to it to output the compensated analogous video signal; a second filtering unit which only passes the signal with lower frequencies on the basis of output from the first filtering unit; and an integrating unit which integrates the outputs of second filtering unit according to the video frame and outputs them as the power according to the radiation quantity.

[11] The camera black level compensating apparatus of Claim 10, wherein the compensation unit and integration unit are installed in the area without radiation.

[12] A camera black level compensating method comprising: a first step in which a video signal of a subject distorted by influx of radiation is acquired from a first image sensor with radiation-proof capacity; a second step in which the video signal affected by the radiation is acquired from a second image sensor installed in the same condition of the first image sensor and enclosed by the wall that can intercept all lights but radiation; and a third step in which the analogous video signal from the first image sensor is compensated on the basis of signal from the second image sensor and outputted.

[13] The camera black level compensating method of Claim 12, wherein the third step uses for the compensation the video signal that the high frequency components are removed from the video signal from the second image sensor acquired at the second step.

[14] The camera black level compensating method of Claim 12, further comprising a fourth step in which the video signal removed of high frequency components in the signal from the second image sensor is integrated to output the power information for the measurement of radiation.

[15] The camera black level compensating method of Claim 12, wherein the third step includes an additional step in where the analogous signal from the fist image sensor is added by the inverted phase of analogous video signal from the second image sensor as well as by the reference black level power to output the compensated analogous video signal. [16] A camera black level compensating method comprising: a first step in which a video signal of a subject distorted by influx of radiation is acquired from one or more image sensors with radiation-proof capacity; a second step in which the video signal affected by the radiation is acquired from a reference image sensor installed in the same condition of the one or more image sensors and enclosed by the wall that can intercept all lights but radiation; and a third step in which the analogous video signal from the one or more image sensors is compensated on the basis of signal from the reference image sensor and outputted.