JPS5887971A - Shading compensation system - Google Patents

Abstract

PURPOSE:To compensate the shading according to the 4th power of cosine of a lens in an excellent way, by multiplying a sinusoidal wave signal synchronized with the storage time of a CCD with a photoelectrically converted output from the CCD. CONSTITUTION:An image illuminance E is proportional to the 4th power of cosine of an angle theta of an incident ray to an optical axis (E=kcos<4>theta). The reflected light from an original 4 is shrinked at a lens 5, projected to a CCDb and photoelectrically converted. A rectangular wave signal generated from a rectangular wave signal generator 8 synchronized with the storage time of the CCD is inputted to a band pass filter 9 using the reciprocal of the storage time as a center frequency to obtain a sinusoidal signal synchronized with the storage time of the CCD. Based on this sinusoidal wave signal, the output of a picture signal processing circuit 7 is multiplied with the picture signal at a multiplication processing circuit 10, allowing to compensate shading.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for correcting shading caused by the cos fourth law of a lens using a sine wave signal synchronized with the storage time of a CCD.

A facsimile i using a CCD performs photoelectric conversion by optically scanning an original, reducing the reflected light from the original using a lens, and projecting the light onto the CCD.

In this photoelectric conversion, due to the cos 4 of the lens, the conversion gain decreases along a cosine curve from the center to the periphery, and shading occurs.

Conventionally, methods for correcting this shading include inserting a shielding plate in the center of the fluorescent lamp, and installing reflectors on the periphery to relatively increase the amount of light at the periphery.
There is a method that stores the image information when an all-white document is inserted in memory and performs image signal processing based on that information to correct it, but the former method uses mechanical correction, so it has the disadvantage that it is difficult to obtain high accuracy. Although the latter provides high accuracy, it has the disadvantage of complicating the circuit configuration.

The purpose of the present invention is to use a rectangular wave signal (a signal indicating the width of an effective image signal) synchronized with the COD accumulation time to
An object of the present invention is to provide a shading correction method that can easily correct shading caused by the s-fourth law.

The shading correction method of the present invention generates a rectangular wave color code that is generated in synchronization with the accumulation time of a CCD that performs photoelectric conversion on an original image signal inputted through an optical system, and uses the reciprocal of the period of the accumulation time as a center frequency. shading caused by the optical system is electrically corrected by multiplying the image signal photoelectrically converted by the CCU by the output obtained by passing it through a band-pass filter. shall be.

Hereinafter, the present invention will be specifically explained with reference to the drawings. FIG. 1(5) is an explanatory diagram of the cos fourth power law of a lens. This law states that the angle θ that the image plane illuminance E makes with the optical axis of the incident ray is
It is proportional to the fourth power of the cosine (cos) of , and is expressed by the following equation.

E=k cos'θ (Constant) FIG. 1(C) shows the original image signal projected on the COD when the original is completely white.

FIG. 2 (5) shows the waveform of the image signal when the CCD storage time is T (seconds). FIG. 2(c) shows a rectangular wave signal synchronized with the storage time of the CCD. Figure 2 (C'1 is the band pass P of the center frequency 1/T (Hz) of the rectangular wave signal.
The waveform of a sine wave signal passed through a wave generator is shown.

FIG. 3 shows a principle diagram of the present invention. The reflected light from the original 4 is reduced by a lens 5, projected onto a CCD 6, and photoelectrically converted. A rectangular wave signal generated from a rectangular wave signal generator 8 synchronized with the accumulation time of the CCD is input to a bandpass P-wave generator 9 whose center frequency is the reciprocal of the accumulation time, and a sine wave signal synchronized with the accumulation time of the CCD is generated. obtain. Based on this sine wave signal, the output of the image signal processing circuit 7 is multiplied by the multiplication processing circuit 10.
The shading is corrected by performing multiplication processing on both signals.

FIG. 4 shows a first embodiment of the present invention. C.C.
The image signal photoelectrically converted by the DII passes through the sample and hold circuit 12 and is output to A B C (Autcma-tic).
Background Control) The maximum value of the amplitude is set to a constant value in the circuit 13, and the A7D conversion circuit 14
is input. A sine wave signal synchronized with the accumulation time of 90 CDs obtained through the rectangular wave generation circuit 15 and bandpass P-wave circuit 16 is converted to an appropriate level by the level conversion circuit 17, and then sent to the comparator of the A/D conversion circuit 14. Entered as the maximum rate level threshold. Each comparator level follows a sinusoidal curve of the image signal that has undergone sinusoidal shading, and distortion is corrected.

FIG. 5 shows a second embodiment of the invention. 18
is a CCD, 22 is a rectangular wave generation circuit synchronized with the CCD accumulation time, 23 is a bandpass p-wave circuit whose center frequency is the reciprocal of the CCD accumulation time, 19 is a sample-and-hold circuit, 21 is an ABC circuit, and 24 is a Level conversion circuit, 20 is A
It is a GC circuit.

The sine wave signal synchronized with the accumulation time of the CCD is appropriately level-converted by the level conversion circuit 24, and then sent to the AGC circuit 20.
is applied as a control signal. Distortion is corrected by changing the gain sinusoidally for an image signal that has undergone sinusoidal shedding.

As described above, according to the present invention, the cos of the lens
Shading due to the fourth power law can be corrected with good accuracy using a simple circuit configuration.

[Brief explanation of the drawing]

5- Fig. 1 (5) and (c) are plan views and characteristic diagrams for questioning and clarifying the cos fourth law, Fig. 2 (A) is a waveform diagram of the image signal of the CCD accumulation time, and Fig. 2 Figure (B) is a waveform diagram of a rectangular wave signal synchronized with the COD accumulation time, Figure 2 (C) l-1
Waveform diagrams of sine wave signals synchronized with the accumulation time of ccD, Figure 3 is a block diagram showing the principle of the present invention, Figures 4 and 5.
The figure is a block diagram showing an embodiment of the present invention. 1.4...Original, 2,5...Lens,
3,6°11.18...CCD, 7...
・Picture signal processing circuit, 8.15.22...COD
Square wave generation circuit synchronized with the accumulation time of 9, 1.6.2
3...CCD17) Bandpass Okiha circuit whose center frequency is the reciprocal of the accumulation time, 10...Multiplication processing circuit, 12.19...Sample/hold circuit, 13. 21...ABC circuit, 14...
- A/D conversion circuit, 17.24...Level conversion circuit, 20...AGC circuit. Agent Patent Attorney Susumu Uchihara 6- Figure 1 (A) Figure 1 (B) Illuminance 1°1. Figure 2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] A rectangular wave signal generated in synchronization with the accumulation time of a CCD that performs photoelectric conversion on an original image signal inputted through an optical system is passed through a bandpass filter whose center frequency is the reciprocal of the period of the accumulation time. A shading correction method, characterized in that shading caused by the optical system is electrically corrected by multiplying an output obtained by the optical system and an image signal photoelectrically converted by the CCD.