US20040028286A1 - Image reading apparatus and computer readable storage medium for correcting image signal - Google Patents
Image reading apparatus and computer readable storage medium for correcting image signal Download PDFInfo
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- US20040028286A1 US20040028286A1 US10/638,443 US63844303A US2004028286A1 US 20040028286 A1 US20040028286 A1 US 20040028286A1 US 63844303 A US63844303 A US 63844303A US 2004028286 A1 US2004028286 A1 US 2004028286A1
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- H04—ELECTRIC COMMUNICATION TECHNIQUE
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- H04N1/407—Control or modification of tonal gradation or of extreme levels, e.g. background level
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- the present invention relates to an image reading apparatus for optically reading an image and a computer readable storage medium which stores program codes for realizing the function of the image reading apparatus, in particular, to the apparatus and storage medium suitable for reading of an image of an X-ray film of medical use.
- a CCD line sensor which is used as the solid state image pickup device has a variation in pixel data, called a dark output distribution (fixed pattern noises) caused by dark current fluctuation or the like.
- a dark output distribution fixed pattern noises
- the dark output distribution fixed to respective pixels as different from random noises appears on the image as artifacts which linear in the scan direction. Since a medical X-ray film image reading apparatus outputs density values, such artifacts become conspicuous in a high density area. Therefore, if a CCD line sensor is used as the solid state image device, it is necessary to check fixed pattern noises at each pixel and correctly compensate for such noises.
- an output of each pixel of a CCD line sensor when a light source is not turned on is stored in a memory or the like, and an offset value corresponding to the fixed pattern noises is subtracted from pixel data by a subtractor.
- an output of each pixel picked up without light may be collected a plurality of times to use an average of the plurality outputs as fixed pattern noise correction data.
- the invention has been made to solve the above problems and it is an object of the present invention to reduce the influence of random noises upon fixed pattern noise correction data to thereby prevent artifacts from being superposed upon an image and execute a black correction process with simple circuits.
- an image reading apparatus which comprises: image pickup means for optically reading an image and converting the read image into an electrical image signal; calculating means for calculating correction data in accordance with dark output distribution data obtained by the image pickup means; and subtracting means for subtracting the correction data from the image signal obtained by the image pickup means by reading the image.
- a computer readable storage medium storing a program, the program executing: an image pickup step of optically reading an image with image pickup means and converting the read image into an electrical image signal; a calculating step of calculating correction data in accordance with dark output distribution data obtained by the image pickup means; and a subtracting step of subtracting the correction data from the image signal obtained by the image pickup means by reading the image.
- FIG. 1 is a block diagram showing the structure of an image reading apparatus according to the invention.
- FIG. 2 is a block diagram showing the structure of a dark distribution correction data calculation circuit according to a first embodiment of the invention.
- FIG. 3 is a block diagram showing the structure of a dark distribution correction data calculation circuit according to a second embodiment of the invention.
- FIG. 4 is a flow chart illustrating the processes to be executed by the first and second embodiments.
- An X-ray film image reading apparatus is used by way of example in the following embodiments.
- FIG. 1 is a block diagram showing the structure of a shading correction circuit of an X-ray film image reading apparatus according to the first embodiment of the invention.
- the X-ray film image reading apparatus has: a light source 1 such as a fluorescent lamp and a halogen lamp; an X-ray film 2 from which an image is read; an optical system lens 3 , a CCD line sensor 4 ; an amplifier 5 for amplifying an output of the CCD line sensor; an A/D converter 5 , a dark distribution correction circuit 7 ; a logarithm conversion look-up table 8 ; a bright distribution correction circuit 9 ; a dark distribution correction data calculation circuit 10 ; a memory 11 for storing shading corrected data; a control circuit 12 having a CPU and the like; a storage medium 13 which also constitutes the invention and may be a RAM, a ROM, an HD or the like storing programs and the like which execute the processes shown in FIG. 4 for running the X-ray film image reading apparatus; and a bus line 14 for transfer address, data, control signal and the like in this apparatus.
- a light source 1 such as a fluorescent lamp and a halogen lamp
- the dark distribution correction circuit 7 is constituted of a subtractor 7 a and a dark distribution memory 7 b having a capacity corresponding to the number of lines of the CCD line sensor 4 .
- the bright distribution correction circuit 9 is constituted of a subtractor 9 a and a bright distribution memory 9 b having a capacity corresponding to the number of lines of the CCD line sensor 4 .
- An output of the CCD line sensor 4 is supplied via the amplifier 5 to the A/D converter 6 , and an output of the A/D converter 6 is supplied to the dark distribution correction circuit 7 and dark distribution correction data calculation circuit 10 .
- An output of the dark distribution correction data calculation circuit 10 is supplied to the dark distribution memory 7 b of the dark distribution correction circuit 7 , an output of the dark distribution correction circuit 7 is supplied via the logarithm conversion look-up table 8 to the bright distribution correction circuit 9 , and an output of the bright distribution correction circuit 9 is stored in the memory 11 .
- the dark distribution correction calculation circuit 10 Connected via the bus line 14 of this apparatus to the control circuit 12 are the dark distribution correction calculation circuit 10 , dark distribution memory 7 b of the dark distribution correction circuit, memory 11 and storage medium 13 .
- a shading correction process to be described hereinunder includes a pre-processing and a main processing.
- pre-processing correction data is collected, calculated, and stored in the memory.
- main processing a subject X-ray film is read and corrected in accordance with the data stored during the pre-processing.
- FIG. 4 is a flow chart illustrating the shading correction process.
- the pre-processing includes Steps S 31 to S 39 shown in FIG. 4, and the main processing includes Steps S 21 to S 28 .
- Step S 31 an unrepresented turn-on control circuit turns off the light source 1 .
- Step S 32 A/D converted output data of the CCD line sensor 4 is acquired.
- Step S 33 in accordance with the data acquired at Step S 32 , dark distribution correction data such as an average value, a mode value and a value obtained through spatial low-pass filtering is calculated.
- Step S 34 the calculation result at Step S 33 is stored as dark distribution correction data.
- Step S 35 the unrepresented turn-on control circuit turns on the light source 1 .
- Step S 36 A/D converted output data of the CCD line sensor 4 is acquired.
- Step S 37 the data stored at Step S 34 is subtracted from the data acquired at Step S 36 .
- Step S 38 the data obtained at Step S 37 is logarithmically converted.
- Step S 39 the data logarithmically converted at Step S 38 is stored as bright distribution correction data.
- Step S 21 the light source 1 is turned-on by a turn-on control circuit (not shown).
- Step S 22 transport of an X-ray film is stared by transport means (not shown).
- Step S 23 A/D converted output data of the CCD line sensor 4 is acquired.
- Step S 24 the data stored at Step S 34 is subtracted from the data acquired at Step S 23 .
- Step S 25 the data acquired at Step S 24 is logarithmically converted.
- Step S 26 the data logarithmically converted at Step S 25 is subtracted from the data stored at Step S 39 .
- Step S 27 the data acquired at Step S 26 is stored.
- Step S 28 it is judged whether the film transport is completed, and if not, Steps S 23 to S 27 are repeated until the film transport is completed, to thereby obtain one image.
- the turn-on control circuit (not shown) turns off the light source 1 . Since the light source 1 is turned off, the CCD line sensor 4 outputs a dark voltage. The dark voltage output from the CCD line sensor 4 is amplified by the amplifier 5 . Noises in the dark voltage of the CCD line sensor 4 are reduced by a noise reduction circuit (CDS, not shown) and thereafter, an output of the noise reduction circuit is supplied to the A/D converter 6 . The dark voltage is converted by the A/D converter 6 into n-bit digital data B i (1 ⁇ i ⁇ p, p is the number of pixels of one line) and supplied to the dark distribution correction circuit 7 and dark distribution correction data calculation circuit 10 .
- CDS noise reduction circuit
- FIG. 2 is a block diagram showing the structure of the dark distribution correction data calculation circuit 10 .
- the dark distribution correction data calculation circuit 10 has an adder 10 a , a register 10 b for storing an addition result, and a bit shifter 10 c for dividing an output of the register 10 b.
- the register 10 b for storing an addition result is reset by the control circuit 12 immediately before dark distribution data is collected.
- the adder 10 a adds an output of the register 10 b to the dark distribution voltage of the first pixel of the CCD line sensor 4 converted into a digital value by the A/D converter 6 , the addition result being stored in the register 10 b .
- the addition result stored in the register 10 b is fed back and added to the next output of the A/D converter 6 , the addition result being again stored in the register 10 b.
- the dark distribution correction data B may use line data which is acquired under the conditions that the dark distribution correction data is 0, that the logarithm conversion look-up table 8 is linear, and that the bright distribution correction calculation is not performed, and is averaged by the control circuit 12 over a plurality of pixels in one line, e.g., 2 to 500 pixels. In this case, data is not necessary to be collected from a plurality of lines.
- N the number of calculation points for calculating the average value
- the average value is used for the dark distribution correction data
- the correction data is not limited only to the average value.
- the average value may not represent the real dark distribution level. In such a case, not the average value but a mode value of distribution is used.
- the mode value may take 0 level in some case. In this case, the mode value different from 0 level may be used.
- the dark distribution memory 7 b is structured not as a one-line memory but as a one-pixel register, and the dark distribution correction circuit 7 may be structured to always subtract each pixel value by a value in the one-pixel register.
- the turn-on control circuit (not shown) turns on the light source 1 without setting the X-ray film 2 . Since the light source 1 is turned on, the CCD line sensor 4 outputs a bright distribution voltage. The bright distribution voltage output from the CCD line sensor 4 is amplified by the amplifier 5 . Noises in the bright distribution voltage of the CCD line sensor 4 are reduced by the noise reduction circuit (CDS, not shown) and thereafter, an output of the noise reduction circuit is supplied to the A/D converter 6 . This voltage is converted by the A/D converter 6 into n-bit digital data L i (1 ⁇ i ⁇ p, p is the number of pixels of one line).
- the dark distribution correction circuit 7 performs dark distribution correction by using the stored dark distribution correction data B i ′ and outputs L i ⁇ B i ′ (1 ⁇ i ⁇ p). This output is supplied to the logarithm look-up table 8 of an n-bit input ⁇ to-n-bit output type.
- bright distribution correction data L i ′ subjected to the dark distribution correction and logarithmic conversion can be obtained as:
- the turn-on control circuit (not shown) turns on the light source 1 .
- Light from the light source 1 is transmitted through the X-ray film 2 , converged by the optical lens 3 , and focussed upon the CCD line sensor 4 .
- the X-ray film 2 is sequentially transported in a direction indicated by an arrow by transport means (not shown).
- the CCD line sensor 4 receives a one-dimensional image perpendicular to the transport direction so that the X-ray film 2 is scanned by the CCD line sensor 4 and all pixel data are read.
- the subtractor 7 a of the dark distribution correction circuit 7 subtracts the dark distribution correction data B i ′ calculated by the pre-processing from the output data of each pixel of the CCD line sensor.
- An output of the dark distribution correction circuit 7 is given by:
- the subtractor 9 a of the bright distribution correction circuit 9 subtracts the output Yi of the logarithm conversion look-up table 8 from the output data of each pixel or bright distribution correction data Li collected at the pre-processing. This subtraction by the subtractor 9 a corresponds to division which readily calculates a transmission factor of the X-ray film.
- This logarithmic output of the subtractor 9 a is a density value Z i given by:
- the density value Z i is stored in the memory 11 .
- the shading correction process is performed in the above manner while an X-ray film is read.
- a line sensor is used, an area sensor may also be used by performing similar processes described above, if a variation in the dark output distribution is small.
- an average value of dark output distribution data is calculated to obtain the black correction data. It is possible to prevent artifacts from being generated by random noises superposed upon the black correction data, and to simplify circuits.
- This embodiment assumes that variation in fixed pattern noises to be caused by dark current fluctuation or the like is negligibly small. If a pixel has a random noise level such that absolute value of difference between that random noise level and a reference random noise level does not enter a range predetermined by the random noise characteristics anticipated from the physical characteristics of a solid state image pickup device and its peripheral circuit or by the random noise characteristics actually measured, then the black correction of such the pixel may be performed by using the dark output value of that pixel in place of the reference random noise level.
- the reference random noise level is B and the random noise characteristics anticipated from the physical characteristics of a solid state image pickup device and its peripheral circuit or random noise characteristics actually measured are represented by a standard deviation ⁇ N .
- the black correction for this pixel is performed by using the dark output value B i of this pixel in place of the reference random noise value B.
- a CCD line sensor is generally fixedly mounted on an optical system. Since a heat dissipation efficiency at the contact portion between the fixedly mounted portion of the CCD line sensor and the optical system is different from that at the other portions, a temperature distribution of the CCD line sensor becomes uneven. Since a variation in the dark current depends on temperature, the fixed value for dark distribution correction described above may not correct the dark current over the whole area of the CCD line sensor.
- a change in the dark output distribution, caused by the uneven temperature distribution, is generally gentle relative a CCD line sensor. Therefore, in order to correct a change in the dark output distribution, collected fixed pattern noises are subjected to a spatial low-pass filtering process to remove and correct random noises.
- the structure of the second embodiment is similar to that shown in FIG. 1, except that a dark distribution correction data calculation circuit 10 takes the structure shown in FIG. 3.
- the turn-on control circuit (not shown) turns off the light source 1 . Since the light source 1 is turned off, the CCD line sensor 4 outputs a dark distribution output voltage. The dark voltage output voltage from the CCD line sensor 4 is amplified by the amplifier 5 . Noises in the dark voltage of the CCD line sensor 4 are reduced by the noise reduction circuit (CDS, not shown) and thereafter, an output of the noise reduction circuit is supplied to the A/D converter 6 . The dark voltage is converted by the A/D converter 6 into n-bit digital data B i (1 ⁇ i ⁇ p, p is the number of pixels of one line).
- FIG. 3 is a block diagram showing the structure of the dark distribution correction data calculation circuit 10 .
- the dark distribution correction data calculation circuit 10 has an adder 10 a , a register 10 b for storing an addition result, a bit shifter 10 c for dividing an output of the register 10 b , a subtractor 10 d , and sixteen shift registers SR 1 , SR 2 , . . . , SR 16 .
- the register 10 b for storing an addition result and the shift registers SR 1 , SR 2 , . . . , SR 16 are reset by the control circuit 12 immediately before dark distribution data is collected. Outputs from the CCD line sensor 4 converted into digital values by the A/D converters are sequentially input to the register, starting from the first pixel.
- the adder 10 a adds an output of the register 10 b to the dark distribution voltage of the first pixel of the CCD line sensor 4 converted into the digital value by the A/D converter 6 .
- the subtractor 10 d subtracts the output of the shift register SR 16 from an output of the adder 10 a , the subtraction result being stored in the register 10 b .
- the subtraction result stored in the register 10 b is fed back and added to the next output of the A/D converter 6 , and subtracted by an output of the shift register SR 16 , the subtraction result being again stored in the register 10 b.
- the dark distribution moving average output B i ′ is stored in the memory 7 b .
- the number of shift registers may be increased to 32 , 64 , . . . , with the number of bits to be shifted by the shift register 10 c being changed correspondingly, to thereby suppress the influence of random noises.
- the number of shift registers may be decreased to simplify the circuit structure.
- the dark distribution correction data B i ′ may use line data which is acquired under the conditions that the dark distribution correction data is 0, that the logarithm conversion look-up table 8 is linear, and that the bright distribution correction calculation is not performed, and is subjected to a moving averaging process given by the following equation (5) by the control circuit. In this case, data is not necessary to be collected from a plurality of lines.
- m is the number of moving averaging points.
- the moving averaging process is used as the spatial low-pass filtering, other processes may also be used which allow the spatial low-pass filtering.
- the collected dark output distribution data subjected to the spatial low-pass filtering is used as the dark distribution correction data. Therefore, without collecting dark distribution data of a plurality of lines, artifacts to be caused by random noises superposed upon the dark distribution correction data can be prevented from being generated, and a change in the dark output distribution data, caused by an uneven temperature distribution, can be corrected.
- the present invention can be achieved also by supplying another system or apparatus with the storage medium 13 storing program codes achieving the function of the invention, and by making a computer of the system or apparatus read and execute the program codes stored in the storage medium 13 .
- a dark output distribution value of a pixel having a particular random noise level may be used for data subtraction. Accordingly, it is possible to effectively correct a variation in fixed pattern noises such as dark current fluctuation of a CCD line sensor.
- Collected dark output distribution data subjected to spatial low-pass filtering may be used as the correction data. Accordingly, without collecting dark distribution data of a plurality of lines, artifacts to be caused by random noises superposed upon the dark distribution correction data can be prevented from being generated, and a change in the dark output distribution, caused by an uneven temperature distribution, can be corrected.
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Abstract
In an X-ray film image reading apparatus, dark output distribution data of a CCD line sensor is corrected by using correction data. In order to suppress the influence of random noises on the correction data, a dark distribution correction data calculation circuit 10 calculates an average value or mode value of dark distribution outputs of all or some pixels of a CCD line sensor or a dark distribution output of the CCD line sensor subjected to a spatial low-pass filtering process, and stores it in a dark distribution memory as correction data. Next, a light source is turned on, and a subtractor subtracts the stored correction data from an output of the line sensor read from an X-ray film while the light source is turned on.
Description
- 1. Field of the Invention
- The present invention relates to an image reading apparatus for optically reading an image and a computer readable storage medium which stores program codes for realizing the function of the image reading apparatus, in particular, to the apparatus and storage medium suitable for reading of an image of an X-ray film of medical use.
- 2. Related Background Art
- Various types of image reading apparatuses have been developed recently. Also in the medical field, apparatuses have been developed which detect an image, particularly an image of an X-ray film of medical use and digitalize it for use such as electronic filing, remote diagnosis and automatic diagnosis.
- With such an image reading apparatus, light from a light source such as a halogen lamp and a fluorescent lamp is applied to an X-ray film, and light transmitted through the X-ray film is received by a solid state image pickup device and converted into an electric signal which is then converted into a digital signal to be output to an image processing circuit for shading correction and the like. The image pickup device may be a CCD line sensor and the like.
- A CCD line sensor which is used as the solid state image pickup device has a variation in pixel data, called a dark output distribution (fixed pattern noises) caused by dark current fluctuation or the like. As a two-dimensional image is scanned and read with a CCD line sensor, the dark output distribution fixed to respective pixels as different from random noises appears on the image as artifacts which linear in the scan direction. Since a medical X-ray film image reading apparatus outputs density values, such artifacts become conspicuous in a high density area. Therefore, if a CCD line sensor is used as the solid state image device, it is necessary to check fixed pattern noises at each pixel and correctly compensate for such noises.
- In a know method of compensating for fixed pattern noises, an output of each pixel of a CCD line sensor when a light source is not turned on is stored in a memory or the like, and an offset value corresponding to the fixed pattern noises is subtracted from pixel data by a subtractor. In order to suppress the influence of random noises, an output of each pixel picked up without light may be collected a plurality of times to use an average of the plurality outputs as fixed pattern noise correction data.
- With this method, however, if random noises are mixed with the fixed pattern noise correction data, an image is corrected by using the fixed pattern noise correction data containing random noise components, and these noises appear on the image as linear artifacts.
- If fixed pattern noises are collected a plurality of times and these noises are averaged in order to suppress the influence of random noises, it takes a long time to collect data, memory means capable of storing data of a plurality of lines becomes necessary, and the circuit structure becomes complicated.
- The invention has been made to solve the above problems and it is an object of the present invention to reduce the influence of random noises upon fixed pattern noise correction data to thereby prevent artifacts from being superposed upon an image and execute a black correction process with simple circuits.
- According to one aspect of the present invention, there is provided an image reading apparatus which comprises: image pickup means for optically reading an image and converting the read image into an electrical image signal; calculating means for calculating correction data in accordance with dark output distribution data obtained by the image pickup means; and subtracting means for subtracting the correction data from the image signal obtained by the image pickup means by reading the image.
- According to another aspect of the present invention, there is provided a computer readable storage medium storing a program, the program executing: an image pickup step of optically reading an image with image pickup means and converting the read image into an electrical image signal; a calculating step of calculating correction data in accordance with dark output distribution data obtained by the image pickup means; and a subtracting step of subtracting the correction data from the image signal obtained by the image pickup means by reading the image.
- FIG. 1 is a block diagram showing the structure of an image reading apparatus according to the invention.
- FIG. 2 is a block diagram showing the structure of a dark distribution correction data calculation circuit according to a first embodiment of the invention.
- FIG. 3 is a block diagram showing the structure of a dark distribution correction data calculation circuit according to a second embodiment of the invention.
- FIG. 4 is a flow chart illustrating the processes to be executed by the first and second embodiments.
- First, the principle upon which the present invention is based will be described.
- With recent improvements on manufacture techniques of CCD line sensors, a variation in fixed pattern noises such as dark current fluctuation is becoming negligibly small. From this reason, in this invention, an average value or mode value (most frequently occurring value) of dark outputs of all or some pixels of a CCD line sensor or a dark output of the CCD line sensor subjected to a spatial low-pass filtering process is used for each pixel as correction data for fixed pattern noises such as dark current fluctuation.
- Since an average value or mode value (most frequently occurring value) of dark outputs of all or some pixels of a CCD sensor or a dark output of the CCD sensor subjected to a spatial low-pass filtering process is used as correction data for fixed pattern noises such as dark current fluctuation, it is possible to prevent artifacts from appearing on an image by using simple circuits, which artifacts may be caused by random noises superposed upon the correction data.
- Embodiments of the invention will be described with reference to the accompanying drawings.
- An X-ray film image reading apparatus is used by way of example in the following embodiments.
- FIG. 1 is a block diagram showing the structure of a shading correction circuit of an X-ray film image reading apparatus according to the first embodiment of the invention.
- Referring to the block diagram of FIG. 1, the X-ray film image reading apparatus has: a
light source 1 such as a fluorescent lamp and a halogen lamp; anX-ray film 2 from which an image is read; anoptical system lens 3, aCCD line sensor 4; anamplifier 5 for amplifying an output of the CCD line sensor; an A/D converter 5, a dark distribution correction circuit 7; a logarithm conversion look-up table 8; a bright distribution correction circuit 9; a dark distribution correctiondata calculation circuit 10; amemory 11 for storing shading corrected data; acontrol circuit 12 having a CPU and the like; astorage medium 13 which also constitutes the invention and may be a RAM, a ROM, an HD or the like storing programs and the like which execute the processes shown in FIG. 4 for running the X-ray film image reading apparatus; and abus line 14 for transfer address, data, control signal and the like in this apparatus. - The dark distribution correction circuit7 is constituted of a
subtractor 7 a and adark distribution memory 7 b having a capacity corresponding to the number of lines of theCCD line sensor 4. The bright distribution correction circuit 9 is constituted of asubtractor 9 a and abright distribution memory 9 b having a capacity corresponding to the number of lines of theCCD line sensor 4. - An output of the
CCD line sensor 4 is supplied via theamplifier 5 to the A/D converter 6, and an output of the A/D converter 6 is supplied to the dark distribution correction circuit 7 and dark distribution correctiondata calculation circuit 10. An output of the dark distribution correctiondata calculation circuit 10 is supplied to thedark distribution memory 7 b of the dark distribution correction circuit 7, an output of the dark distribution correction circuit 7 is supplied via the logarithm conversion look-up table 8 to the bright distribution correction circuit 9, and an output of the bright distribution correction circuit 9 is stored in thememory 11. - Connected via the
bus line 14 of this apparatus to thecontrol circuit 12 are the dark distributioncorrection calculation circuit 10,dark distribution memory 7 b of the dark distribution correction circuit,memory 11 andstorage medium 13. - Next, the operation of the apparatus will be described.
- A shading correction process to be described hereinunder includes a pre-processing and a main processing. In the pre-processing, correction data is collected, calculated, and stored in the memory. In the main processing, a subject X-ray film is read and corrected in accordance with the data stored during the pre-processing.
- FIG. 4 is a flow chart illustrating the shading correction process.
- The pre-processing includes Steps S31 to S39 shown in FIG. 4, and the main processing includes Steps S21 to S28.
- At Step S31, an unrepresented turn-on control circuit turns off the
light source 1. - At Step S32, A/D converted output data of the
CCD line sensor 4 is acquired. - At Step S33, in accordance with the data acquired at Step S32, dark distribution correction data such as an average value, a mode value and a value obtained through spatial low-pass filtering is calculated.
- At Step S34, the calculation result at Step S33 is stored as dark distribution correction data.
- At Step S35, the unrepresented turn-on control circuit turns on the
light source 1. - At Step S36, A/D converted output data of the
CCD line sensor 4 is acquired. - At Step S37, the data stored at Step S34 is subtracted from the data acquired at Step S36.
- At Step S38, the data obtained at Step S37 is logarithmically converted.
- At Step S39, the data logarithmically converted at Step S38 is stored as bright distribution correction data.
- The pre-processing is thus completed.
- At Step S21, the
light source 1 is turned-on by a turn-on control circuit (not shown). - At Step S22, transport of an X-ray film is stared by transport means (not shown).
- At Step S23, A/D converted output data of the
CCD line sensor 4 is acquired. - At Step S24, the data stored at Step S34 is subtracted from the data acquired at Step S23.
- At Step S25, the data acquired at Step S24 is logarithmically converted.
- At Step S26, the data logarithmically converted at Step S25 is subtracted from the data stored at Step S39.
- At Step S27, the data acquired at Step S26 is stored.
- At Step S28 it is judged whether the film transport is completed, and if not, Steps S23 to S27 are repeated until the film transport is completed, to thereby obtain one image.
- With the above operations, an X-ray film image can be obtained.
- First, the pre-processing performed before an X-ray film is read will be described in some detail.
- First, the turn-on control circuit (not shown) turns off the
light source 1. Since thelight source 1 is turned off, theCCD line sensor 4 outputs a dark voltage. The dark voltage output from theCCD line sensor 4 is amplified by theamplifier 5. Noises in the dark voltage of theCCD line sensor 4 are reduced by a noise reduction circuit (CDS, not shown) and thereafter, an output of the noise reduction circuit is supplied to the A/D converter 6. The dark voltage is converted by the A/D converter 6 into n-bit digital data Bi (1≦i≦p, p is the number of pixels of one line) and supplied to the dark distribution correction circuit 7 and dark distribution correctiondata calculation circuit 10. - FIG. 2 is a block diagram showing the structure of the dark distribution correction
data calculation circuit 10. - The dark distribution correction
data calculation circuit 10 has anadder 10 a, aregister 10 b for storing an addition result, and abit shifter 10 c for dividing an output of theregister 10 b. - Next, the operation of the dark distribution correction
data calculation circuit 10 will be described. - The
register 10 b for storing an addition result is reset by thecontrol circuit 12 immediately before dark distribution data is collected. Theadder 10 a adds an output of theregister 10 b to the dark distribution voltage of the first pixel of theCCD line sensor 4 converted into a digital value by the A/D converter 6, the addition result being stored in theregister 10 b. The addition result stored in theregister 10 b is fed back and added to the next output of the A/D converter 6, the addition result being again stored in theregister 10 b. -
- where 2k≦p (p: the number of pixels of one line). The dark distribution correction data B is calculated in the above manner.
- The dark distribution correction data B may use line data which is acquired under the conditions that the dark distribution correction data is 0, that the logarithm conversion look-up table8 is linear, and that the bright distribution correction calculation is not performed, and is averaged by the
control circuit 12 over a plurality of pixels in one line, e.g., 2 to 500 pixels. In this case, data is not necessary to be collected from a plurality of lines. -
- where 2≦N>p (N: the number of calculation points for calculating the average value). Although the calculation point starts from the first pixel, the above calculation may be executed by using N pixels around the center pixel of the CCD line sensor.
- Although the average value is used for the dark distribution correction data, the correction data is not limited only to the average value. For example, if the distribution of noises is not symmetrical, the average value may not represent the real dark distribution level. In such a case, not the average value but a mode value of distribution is used.
- If the dark output is limited to 0 level, the mode value may take 0 level in some case. In this case, the mode value different from 0 level may be used.
- The dark distribution correction data B calculated as above is stored in the
dark distribution memory 7 b as the correction data Bi′=B (1≦i≦p) for the i-th pixel. - The same value is subtracted from each pixel. Therefore, the
dark distribution memory 7 b is structured not as a one-line memory but as a one-pixel register, and the dark distribution correction circuit 7 may be structured to always subtract each pixel value by a value in the one-pixel register. - Next, the turn-on control circuit (not shown) turns on the
light source 1 without setting theX-ray film 2. Since thelight source 1 is turned on, theCCD line sensor 4 outputs a bright distribution voltage. The bright distribution voltage output from theCCD line sensor 4 is amplified by theamplifier 5. Noises in the bright distribution voltage of theCCD line sensor 4 are reduced by the noise reduction circuit (CDS, not shown) and thereafter, an output of the noise reduction circuit is supplied to the A/D converter 6. This voltage is converted by the A/D converter 6 into n-bit digital data Li (1≦i≦p, p is the number of pixels of one line). - The dark distribution correction circuit7 performs dark distribution correction by using the stored dark distribution correction data Bi′ and outputs Li−Bi′ (1≦i≦p). This output is supplied to the logarithm look-up table 8 of an n-bit input−to-n-bit output type. By acquiring the output of the logarithm look-up table 8 without bright distribution correction calculation, bright distribution correction data Li′ subjected to the dark distribution correction and logarithmic conversion can be obtained as:
- L i ′=[A·log 10(L i −B i′+1)] (3)
- where 1≦i≦p and A=(2n−1)/log10(2n). This bright distribution correction data Li′ is stored in the
bright distribution memory 9 b. - The pre-processing is completed by the above processes.
- Next, the main processing of reading an X-ray film will be described.
- The turn-on control circuit (not shown) turns on the
light source 1. Light from thelight source 1 is transmitted through theX-ray film 2, converged by theoptical lens 3, and focussed upon theCCD line sensor 4. - The
X-ray film 2 is sequentially transported in a direction indicated by an arrow by transport means (not shown). TheCCD line sensor 4 receives a one-dimensional image perpendicular to the transport direction so that theX-ray film 2 is scanned by theCCD line sensor 4 and all pixel data are read. - Light received by the
CCD line sensor 4 is photoelectrically converted and output as a voltage value of each pixel. This output is amplified by theamplifier 5. Noises of the output signal are reduced by the noise reduction circuit (not shown) and thereafter, the output of the noise reduction circuit is supplied to the A/D converter 6 which converts it into n-bit digital data Di (1≦i≦p, p is the number of pixels of one line). - The
subtractor 7 a of the dark distribution correction circuit 7 subtracts the dark distribution correction data Bi′ calculated by the pre-processing from the output data of each pixel of the CCD line sensor. An output of the dark distribution correction circuit 7 is given by: - D i ′=D i −B i′(1≦i≦p).
- This output is supplied to the logarithm conversion look-up table8 for division calculation. An output of the logarithm conversion look-up table 8 of the n-bit input−to-n-bit output type is given by:
- Y i =[A·log 10(D′+1)]
- where 1<i<p and A=(2n−1)/log10(2n).
- The
subtractor 9 a of the bright distribution correction circuit 9 subtracts the output Yi of the logarithm conversion look-up table 8 from the output data of each pixel or bright distribution correction data Li collected at the pre-processing. This subtraction by thesubtractor 9 a corresponds to division which readily calculates a transmission factor of the X-ray film. This logarithmic output of thesubtractor 9 a is a density value Zi given by: - Z i =L i ′−Y i(1≦i≦p)
- The density value Zi is stored in the
memory 11. - The shading correction process is performed in the above manner while an X-ray film is read. In this embodiment, although a line sensor is used, an area sensor may also be used by performing similar processes described above, if a variation in the dark output distribution is small.
- With this embodiment, an average value of dark output distribution data is calculated to obtain the black correction data. It is possible to prevent artifacts from being generated by random noises superposed upon the black correction data, and to simplify circuits.
- This embodiment assumes that variation in fixed pattern noises to be caused by dark current fluctuation or the like is negligibly small. If a pixel has a random noise level such that absolute value of difference between that random noise level and a reference random noise level does not enter a range predetermined by the random noise characteristics anticipated from the physical characteristics of a solid state image pickup device and its peripheral circuit or by the random noise characteristics actually measured, then the black correction of such the pixel may be performed by using the dark output value of that pixel in place of the reference random noise level.
- More specifically, it is assumed that the reference random noise level is B and the random noise characteristics anticipated from the physical characteristics of a solid state image pickup device and its peripheral circuit or random noise characteristics actually measured are represented by a standard deviation σN. In this case, if a pixel has a random noise level not entering the range of B+/−kσN, then the black correction for this pixel is performed by using the dark output value Bi of this pixel in place of the reference random noise value B.
- Next, the second embodiment of the invention will be described.
- A CCD line sensor is generally fixedly mounted on an optical system. Since a heat dissipation efficiency at the contact portion between the fixedly mounted portion of the CCD line sensor and the optical system is different from that at the other portions, a temperature distribution of the CCD line sensor becomes uneven. Since a variation in the dark current depends on temperature, the fixed value for dark distribution correction described above may not correct the dark current over the whole area of the CCD line sensor.
- A change in the dark output distribution, caused by the uneven temperature distribution, is generally gentle relative a CCD line sensor. Therefore, in order to correct a change in the dark output distribution, collected fixed pattern noises are subjected to a spatial low-pass filtering process to remove and correct random noises.
- In the second embodiment, a shading correction circuit capable of correcting a variation in the temperature distribution will be described.
- The structure of the second embodiment is similar to that shown in FIG. 1, except that a dark distribution correction
data calculation circuit 10 takes the structure shown in FIG. 3. - First, the pre-processing to be performed before an X-ray film is read will be described.
- First, the turn-on control circuit (not shown) turns off the
light source 1. Since thelight source 1 is turned off, theCCD line sensor 4 outputs a dark distribution output voltage. The dark voltage output voltage from theCCD line sensor 4 is amplified by theamplifier 5. Noises in the dark voltage of theCCD line sensor 4 are reduced by the noise reduction circuit (CDS, not shown) and thereafter, an output of the noise reduction circuit is supplied to the A/D converter 6. The dark voltage is converted by the A/D converter 6 into n-bit digital data Bi (1≦i≦p, p is the number of pixels of one line). - FIG. 3 is a block diagram showing the structure of the dark distribution correction
data calculation circuit 10. - The dark distribution correction
data calculation circuit 10 has anadder 10 a, aregister 10 b for storing an addition result, abit shifter 10 c for dividing an output of theregister 10 b, asubtractor 10 d, and sixteen shift registers SR1, SR2, . . . , SR16. - Next, the operation of the dark distribution correction
data calculation circuit 10 will be described. - The
register 10 b for storing an addition result and the shift registers SR1, SR2, . . . , SR16 are reset by thecontrol circuit 12 immediately before dark distribution data is collected. Outputs from theCCD line sensor 4 converted into digital values by the A/D converters are sequentially input to the register, starting from the first pixel. Theadder 10 a adds an output of theregister 10 b to the dark distribution voltage of the first pixel of theCCD line sensor 4 converted into the digital value by the A/D converter 6. Thesubtractor 10 d subtracts the output of the shift register SR16 from an output of theadder 10 a, the subtraction result being stored in theregister 10 b. The subtraction result stored in theregister 10 b is fed back and added to the next output of the A/D converter 6, and subtracted by an output of the shift register SR16, the subtraction result being again stored in theregister 10 b. - This operation is repeated by 16 times and thereafter, the output of the
register 10 b is shifted by four bits by thebit shifter 10 c. Correction data Bi′ for the first pixel is therefore stored in thedark correction memory 7 b. -
- where 1≦i≦p−16. The dark distribution moving average output Bi′ is stored in the
memory 7 b. Although sixteen shift registers are used, the number of shift registers may be increased to 32, 64, . . . , with the number of bits to be shifted by theshift register 10 c being changed correspondingly, to thereby suppress the influence of random noises. Alternatively, the number of shift registers may be decreased to simplify the circuit structure. -
- where m is the number of moving averaging points. Although the moving averaging process is used as the spatial low-pass filtering, other processes may also be used which allow the spatial low-pass filtering.
- The bright distribution data collection process and the main processing of the second embodiment are similar to those of the first embodiment.
- In the second embodiment, the collected dark output distribution data subjected to the spatial low-pass filtering is used as the dark distribution correction data. Therefore, without collecting dark distribution data of a plurality of lines, artifacts to be caused by random noises superposed upon the dark distribution correction data can be prevented from being generated, and a change in the dark output distribution data, caused by an uneven temperature distribution, can be corrected.
- The present invention can be achieved also by supplying another system or apparatus with the
storage medium 13 storing program codes achieving the function of the invention, and by making a computer of the system or apparatus read and execute the program codes stored in thestorage medium 13. - As described above, according to the embodiments of the invention, it is possible to prevent artifacts from being generated by random noises superposed upon the fixed pattern noise correction data, and to simplify circuits.
- Instead of the correction data, a dark output distribution value of a pixel having a particular random noise level may be used for data subtraction. Accordingly, it is possible to effectively correct a variation in fixed pattern noises such as dark current fluctuation of a CCD line sensor.
- Collected dark output distribution data subjected to spatial low-pass filtering may be used as the correction data. Accordingly, without collecting dark distribution data of a plurality of lines, artifacts to be caused by random noises superposed upon the dark distribution correction data can be prevented from being generated, and a change in the dark output distribution, caused by an uneven temperature distribution, can be corrected.
- Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.
Claims (20)
1. An image reading apparatus comprising:
image pickup means for optically reading an image and converting the read image into an electrical image signal;
calculating means for calculating correction data in accordance with dark output distribution data obtained by said image pickup means; and
subtracting means for subtracting the correction data from the image signal obtained by said image pickup means by reading the image.
2. An image reading apparatus according to claim 1 , wherein said calculating means calculates an average value of the dark output distribution data of all or some pixels and uses the average value as the correction data.
3. An image reading apparatus according to claim 1 , wherein said calculating means calculates a statistical mode value of the dark output distribution data of all or some pixels and uses the mode value as the correction data.
4. An image reading apparatus according to claim 1 , wherein said calculating means calculates, if the dark output distribution data is limited to a predetermined level, a mode value having a level different from the predetermined level.
5. An image reading apparatus according to claim 1 , wherein said subtracting means does not subtract the correction data from the image signal for a specific pixel, but subtracts a pixel value of the specific pixel from the image signal.
6. An image reading apparatus according to claim 5 , wherein the specific pixel is a pixel having a random noise level such that an absolute value of an output of said subtracting means does not enter a range relative predetermined by the random noises anticipated from the physical characteristics of said image pickup means and peripheral circuits or by adding a standard deviation multiplied by k (k being an integer) of the random noises actually measured.
7. An image reading apparatus according to claim 1 , wherein said calculating means uses the dark output distribution data subjected to a spatial low-pass filtering process as the correction data.
8. An image reading apparatus according to claim 7 , wherein the spatial low-pass filtering process is a moving averaging process.
9. An image reading apparatus according to claim 1 , wherein the dark distribution output data is obtained by a single data acquisition operation.
10. An image reading apparatus according to claim 1 , wherein said image pickup means includes a line sensor using solid state image pickup elements.
11. A computer readable storage medium storing a program, the program executing:
an image pickup step of optically reading an image with image pickup means and converting the read image into an electrical image signal;
a calculating step of calculating correction data in accordance with dark output distribution data obtained by the image pickup means; and
a subtracting step of subtracting the correction data from the image signal obtained by the image pickup means by reading the image.
12. A computer readable storage medium storing a program according to claim 11 , wherein said calculating step calculates an average value of the dark output distribution data of all or some pixels and uses the average value as the correction data.
13. A computer readable storage medium storing a program according to claim 11 , wherein said calculating step calculates a statistical mode value of the dark output distribution data of all or some pixels and uses the mode value as the correction data.
14. A computer readable storage medium storing a program according to claim 11 , wherein said calculating step calculates, if the dark output distribution data is limited to a predetermined level, a mode value having a level different from the predetermined level.
15. A computer readable storage medium storing a program according to claim 11 , wherein said subtracting step does not subtract the correction data from the image signal for a specific pixel, but subtracts a pixel value of the specific pixel from the image signal.
16. A computer readable storage medium storing a program according to claim 15 , wherein the specific pixel is a pixel having a random noise level such that an absolute value of an output of said subtracting step does not enter a range predetermined by the random noises anticipated from the physical characteristics of the image pickup means and peripheral circuits or by adding a standard deviation multiplied by k (k being an integer) of the random noises actually measured.
17. A computer readable storage medium storing a program according to claim 11 , wherein said calculating step calculates, uses output distribution data subjected to a spatial low-pass filtering process as the correction data.
18. A computer readable storage medium storing a program according to claim 17 , wherein the spatial low-pass filtering process is a moving averaging process.
19. A computer readable storage medium storing a program according to claim 11 , wherein the dark distribution output data is obtained by a single data acquisition operation.
20. A computer readable storage medium storing a program according to claim 11 , wherein the image pickup means includes a line sensor using solid state image pickup elements.
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US10/638,443 US20040028286A1 (en) | 1998-05-12 | 2003-08-12 | Image reading apparatus and computer readable storage medium for correcting image signal |
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JP10-128862 | 1998-05-12 | ||
JP10128862A JPH11331592A (en) | 1998-05-12 | 1998-05-12 | Image reader and computer readable storing medium |
US09/309,872 US7196725B1 (en) | 1998-05-12 | 1999-05-11 | Image reading apparatus and computer readable storage medium |
US10/638,443 US20040028286A1 (en) | 1998-05-12 | 2003-08-12 | Image reading apparatus and computer readable storage medium for correcting image signal |
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US09/309,872 Division US7196725B1 (en) | 1998-05-12 | 1999-05-11 | Image reading apparatus and computer readable storage medium |
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US20040028286A1 true US20040028286A1 (en) | 2004-02-12 |
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US10/638,443 Abandoned US20040028286A1 (en) | 1998-05-12 | 2003-08-12 | Image reading apparatus and computer readable storage medium for correcting image signal |
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Also Published As
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US7196725B1 (en) | 2007-03-27 |
JPH11331592A (en) | 1999-11-30 |
EP0957628A2 (en) | 1999-11-17 |
EP0957628A3 (en) | 2002-01-16 |
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