JPH03270569A - Picture signal correction method - Google Patents

Abstract

PURPOSE:To efficiently correct the dispersion between picture elements of an image sensor at a required exposure range (read range) by correcting the dispersion in the characteristic of the image sensor at a low exposure region within an exposure range required for reading a picture. CONSTITUTION:An ND filter 18 is put in an optical path, a read signal LS executing the shading correction processing as to an output signal from each picture element of a line sensor 16 at this time is inputted to a correction value arithmetic circuit 24 via a differential amplifier 20 and an A/D converter 22, a difference DELTASi between an output of each picture element and a reference value in an ideal output characteristic R is obtained and stored in a correction value storage memory 26 as a correction signal. Then the ND filter 18 is moved at the outside of the optical path to light an original 10 by a light source 12, a correction signal stored in the correction value storage memory 26 is converted into an analog signal by a D/A converter 28, a signal resulting from subtracting the difference DELTASi from the picture read signal LS is converted into a digital value at the A/D converter 22 and it is outputted as a picture signal PS.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image signal correction method when reading an image using an image sensor such as a line sensor.

[Prior Art] The photoelectric output characteristics of each pixel, which is a photoelectric conversion unit, constituting an image sensor such as a line sensor is determined according to the fourth specific issue A.
, B, and C, the characteristics are not necessarily linear over the entire area with respect to the exposure amount, and the characteristics tend to vary greatly especially at the exposure level near the dark time. Further, each pixel has a unique dark current even when no light is incident thereon, and an output voltage corresponding to this dark current appears as a dark output.

Therefore, in order to correct this variation in photoelectric output characteristics, so-called shading correction is performed in areas with a large amount of exposure, as disclosed in, for example, Japanese Patent Application Laid-Open No. 62-2 [2673]. The photoelectric output characteristics are matched, and then the dark output, that is, when the exposure amount is zero, is corrected so that the photoelectric output characteristics match the zero value. The results of this correction are shown in the fifth fixed numbers A', B', and C'.

[Problems to be Solved by the Invention] However, even with this correction, because the photoelectric output characteristics differ in the low exposure area close to the dark level, streak-like unevenness occurs in the sub-scanning direction on the output image. There are drawbacks that appear as follows.

As a technique for solving this drawback, the present applicant has proposed a technique for image reading in No. 6 Patent Application No. A' and B'C', as disclosed in Japanese Patent Application No. 63-2468'64. There is one in which the output value of the image sensor is made the same for all pixels at the minimum value of a necessary exposure range (hereinafter referred to as reading range as necessary) or an exposure amount X smaller than the minimum value.

However, when the reading range is determined in this way, a new difficulty arises in that the dynamic range of the reading range becomes narrow. In this case, even if the reading range is expanded to a low exposure region, there remains the drawback that sliver-like unevenness is observed because visibility to the human eye is extremely good in the low exposure region.

This invention was made in view of the above problems, and
Corrects variations in image sensor characteristics in low exposure areas within the exposure range required for image reading,
It is an object of the present invention to provide an image signal correction method that can obtain a wide reading range without generating visible sushi-like unevenness on an output image.

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides an image signal correction method that is applied when reading an image with an image sensor consisting of a plurality of pixels, which comprises: At an exposure amount that is larger than the minimum value of the exposure range and smaller than the maximum value of the exposure range,
The first step is to obtain a correction signal such that the output value is the same for all pixels constituting the image sensor; and and a second step of calculating the correction signal in a predetermined manner with respect to the output signal, thereby correcting variations between pixels.

[Function] In the image signal correction method of the present invention configured as described above, when correcting variations in the pixel output characteristics of the image sensor, the exposure amount, which is the correction standard for making the output value of each pixel constant, is adjusted. The corresponding output value is not the minimum value of the exposure range required for dark conditions or image reading, or the output value at an exposure amount slightly smaller than that, but the value that is lower than the minimum value of the exposure range required for image reading. By setting the correction standard to the output value at an exposure amount that is large and smaller than the maximum value, the reading range is expanded and images are reproduced that make streak-like unevenness invisible in the low exposure area within this reading range. You can.

[Example] An example will be given regarding the image signal correction method according to the present invention,
A detailed description will be given below with reference to the accompanying drawings.

FIG. 1 shows an example of an apparatus using analog signal correction processing for carrying out the method of the present invention, in which a document 10 such as a color reversal film is illuminated with an illumination light source 12 which is a fluorescent lamp, and passed through a lens 14 to a line sensor 16. It is designed to be scanned and read. Further, an ND filter 18 is arranged between the lens 14 and the line sensor 16 so as to be movable forward and backward. The output signal of the line sensor 16 is converted into a read signal LS via a shading correction circuit 17.
is input to the differential amplifier 20, and its differential output is A/
The D converter 22 converts it into a digital image signal PS and outputs it.

Note that when the required density range of the ND filter 18 is 0 to 3.0, the required exposure range is 1 to 1 if the white level exposure amount is 1000.
000, so as shown in FIG. 2, the density of the ND filter 18 is selected at an exposure level such that the density range is 2 to 3, that is, the exposure range is 100 to 1000. This is because the pixel output characteristics of the image sensor become consistent at the exposure level. In other words, the output value of the ND filter 18 is the same for all pixels constituting the image sensor at an exposure amount that is larger than the minimum value of the exposure range necessary for image reading and smaller than the maximum value of the exposure range. The density is selected to obtain such a correction signal (see FIG. 2). Further, the shading correction circuit 1
7, a transparent @document is placed in place of the document 10, and a well-known shading correction signal for the output signal of the line sensor in a state in which the ND filter 18 is removed from the optical path is stored in advance.

The output of the A/D converter 22 is input to the correction value calculation circuit 24, and the correction value holding memory 26 stores the reference value related to the ideal output characteristic R (see FIG. 2) calculated by the correction value calculation circuit 24. ΔSi is held as a correction signal. and,
The difference ΔS which is a correction signal held in the memory 26 at the time of reading the original 1° is converted into an analog quantity by the D/A converter 28 and inputted to the differential amplifier 2o.

The apparatus to which the image signal correction method according to the present embodiment is applied is basically configured as described above, and its operation will be explained next.

First, the ND filter 18 is introduced into the optical path, and the read signal LS, which has undergone shading correction processing on the output signal from each pixel of the line sensor 16 at this time, is transmitted through the differential amplifier 2o and the A/D converter 22. The correction value calculation circuit 24 inputs the difference ΔS1 between the output value of each pixel and the reference value in the ideal output characteristic R, and stores it in the correction value holding memory 26' as a correction signal (first process).

After that, move the ND filter 18 out of the optical path and
0 is illuminated by the light source 12, and the image reading signal LS outputted via the line sensor 6 and the shading correction circuit 17 is inputted to the differential amplifier 20, and the difference ΔSi held in the correction value holding memory 26 is inputted. The correction signal is converted into an analog signal by the D/A converter 28 and inputted to the differential amplifier 20, and the signal obtained by subtracting the difference ΔSi from the image reading signal LS is converted into a digital value by the A/D converter 22. This is output as an image signal PS (second process).

The optical output characteristic of the image signal corrected in this way is E
FIG. 2 shows the relationship between R and the ideal output characteristic R. As can be understood from Figure 2, if the required density range is 0 to 3, the required exposure range is 1 to 10.
00, and the photoelectric conversion characteristics of each pixel are the same at the exposure amount Y. Therefore, in other respects, there are some errors compared to the conventional technology, but in the easily visible low exposure area, the deviations are smaller compared to the characteristics shown in Figure 6, so the output image When looking at the image with the naked eye, it is impossible to visually recognize the sushi-like exposure unevenness.

The embodiment shown in FIG. 1 described above is based on analog signal correction processing, but when performing digital signal correction processing, as shown in FIG. A/D signal LS
The converter 40 converts it into a digital value and the correction value calculation circuit 42
The difference ΔS1 is obtained by hand, and this difference ΔSi is held in the correction value holding memory 44. When reading the original 10, the output of the A/D converter 40 is input to the subtracter 46, and the difference ΔSi read from the correction value holding memory 44 is subtracted, and the subtraction result is output as an image signal PS. Just do it like this.

In the above embodiment, the original 10 is a transparent original, and its transmitted light is read, but it may be a reflective original. Furthermore, although a line sensor is used as the image sensor in the above embodiment, the same method can be applied to an area sensor as well.

[Effects of the Invention] As described above, according to the image signal correction method of the present invention,
Variations in image sensor characteristics in low-exposure areas are corrected within the exposure range required for image reading, so variations between image sensor pixels in the required exposure range (reading range) can be corrected efficiently. Can be well corrected. Therefore, the reading range is widened, and it is possible to prevent the occurrence of sliver-like unevenness (in the sub-scanning direction) in the case of reading by the line sensor.

[Brief explanation of drawings]

FIG. 1 is a block configuration diagram showing an example of an apparatus for implementing the method of the present invention, FIG. 2 is a characteristic diagram explaining the operation of the apparatus shown in FIG. 1, and FIG. 3 is a diagram showing another embodiment of the present invention. A block diagram of the apparatus and FIGS. 4 to 6 are characteristic diagrams for explaining a conventional image sensor correction method. 10... Original 12... Light source 14... Lens 16... Line sensor 17... Shading correction circuit 18... ND filter 20... Differential amplifier 22.40... A/D Converter 24.42...Correction value calculation circuit 26.44...Correction value holding memory 28...D/A converter 46...Subtractor FIG, 6 Minimum value of L reading range

Claims (1)

[Claims] (1) An image signal correction method applied when reading an image with an image sensor consisting of multiple pixels, which involves exposure that is larger than the minimum value of the exposure range necessary for image reading and smaller than the maximum value of the exposure range. In quantity,
The first step is to obtain a correction signal such that the output value is the same for all pixels constituting the image sensor; and An image signal correction method comprising: a second step of calculating the correction signal in a predetermined manner with respect to the output signal.