JPH0444457B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ripple correction method for correcting ripples in an image signal due to light intensity ripples of a light source for exposing an original in an original reading device using a photoelectric conversion element in a copying machine, a facsimile, or the like.

The document to be recorded is exposed to light using a lamp, and the reflected light is passed through an optical system including mirrors and lenses.
After converting it into an electrical signal using a photoelectric conversion element such as a CCD or photodiode array, the electrical signal is used to form an electrostatic latent image on a photoconductor or dielectric material using an optical fiber tube or needle-like electrode. Recording devices that create recorded images by developing electrostatic latent images are already known and have been put into practical use. Furthermore, as a method of varying the output of a photoelectric conversion element such as a CCD, a method is known in which the charge accumulation time on the light receiving surface of the photoelectric conversion element is changed according to the illuminance of the light receiving surface. The output of a photoelectric conversion element such as a CCD used in the document reading device of such a recording device can read out an image signal in a short charge accumulation time when a fluorescent lamp or the like that is turned on by an AC power source is used as the light source for exposure. If this is repeated, the level of the image output signal will fluctuate. In other words, in Fig. 1, waveform a indicated by a broken line corresponds to the commercial frequency (50Hz or
60Hz) AC power supply, waveform b shown by a solid line is the light intensity of a fluorescent lamp, and C is a transfer control signal of a CCD for reading an original, and the interval between transfer control signals indicates the charge accumulation time. In the figure, since the area of the shaded portion represents the exposure amount, it is easy to understand that the level of the image output signal fluctuates every day. Therefore, as an AC power source to light up the fluorescent lamp used as the light source for exposure,
One possible method is to use a high frequency power source of about 20 KHz to keep the exposure amount approximately constant during each charge accumulation time.

For example, if a high frequency power source of 20 KHz is used, the cycle of light intensity change of the fluorescent lamp is 1/20 x 10 ³ x 2 = 25 x 10 ^-6 (s) = 25 (μs). Also, if the number of pixels of the CCD is 2048 and the video frequency is 1MHz, the minimum charge accumulation time is 1/1 × 10 ⁶ × 2048 = 2048 × 10 ^-6 (s) = 2.048 (ms), which is 2.048 (ms). Approximately 82 waves enter each time during this period, so there is almost no level fluctuation in the image output signal. However, in reality, there is a problem in that the level of the image output signal fluctuates because the commercial frequency (50Hz or 60Hz) is applied to the 20KHz high-frequency power supply.

The present invention has been made in view of the above-mentioned points.In a document reading device that repeatedly reads output signals from a photoelectric conversion element, the light intensity of a light source for document exposure is detected, and the document image is detected at a frequency that does not affect fluctuations in the image signal. Integrate for each main scan, extract only the ripple amount from the integrated value, hold this ripple amount for one main scan, and set the reference voltage of the A/D converter based on this ripple amount. This paper proposes a ripple correction method that performs quantization without the influence of ripples.

The present invention will be explained below based on the drawings.

FIG. 2 schematically shows an example of the configuration of a document reading section of an electrophotographic copying machine having a document reading device with a movable document table. The reflected light from the document 2 is incident on a photoelectric conversion element 6 via a mirror 4 and a lens 5, and is converted into an electrical image signal. In this example, a sensor 7 such as a photodiode for detecting the amount of light is provided in a non-image area of the lamp 3 (an area outside the effective image area at one longitudinal end of the lamp).

Here, the principle of ripple correction will be explained with reference to FIG. 2. The sensor 7 detects the amount of light from the lamp 3, and integrates a frequency component that does not affect output fluctuations of the photoelectric conversion element 6. For example, if the number of pixels of a CCD is 2048 and the video frequency is 1MHz, the minimum charge accumulation time is 2.048ms as explained above, and as long as the period of light intensity ripple is 2.048ms or less, it has no effect on output fluctuations. In this case the frequency is approximately 500Hz
Therefore, if we integrate over 500Hz, we get the third
The result will be as shown in Figure A. If only the ripple component is extracted from this signal, it will be as shown by the solid line in FIG. 3B. When a CCD is used as the photoelectron conversion element 6,
The charge accumulated in a certain charge accumulation time is shown in Figure 3 (c).
It is transferred to an analog shift register by a CCD transfer control signal and output as an image signal within the next charge accumulation time. Therefore, it is necessary to maintain the ripple waveform information shown in Figure 3B at the level shown by the chain line in the same figure.In the present invention, the ripple waveform is held, and after level conversion, it is set to a preset constant voltage V _C. Addition results in A/ as shown in Figure 3 D.
The reference voltage _VB of the D converter is determined.

FIG. 4 shows an embodiment of an apparatus for carrying out the ripple correction method according to the present invention, and in the figure,
8 is an integration circuit that integrates the signal from the sensor 7 by a specific frequency; 9 is a ripple extraction circuit that extracts only ripples from the signal from the integration circuit 8; 10 is a circuit that performs one scan based on the CCD transfer control signal. A sample-and-hold timing circuit outputs a time hold signal; 11 is a sample-and-hold circuit that holds the ripple amount using the hold signal output from the sample-and-hold timing circuit 10; and 12 is a sample-and-hold circuit that holds the ripple amount to an appropriate level. 13 is a constant voltage generator that generates a constant voltage V _C that is a reference for quantization of the A/D converter, which will be described later; 14 is the ripple amount of the level conversion circuit 12 and the reference voltage 13;
15 is an A/ _D converter for quantizing the image signal from the photoelectric conversion element 6; It is a D converter.

As shown in FIG. 3D, the reference voltage of the A/D converter 15 is set to a large value where the level of the ripple waveform is high. In addition, A/D converter 1
It is set in advance so that quantization errors do not occur due to the resolution of 5 and the level fluctuation of the image output signal from the photoelectric conversion element 6.

In the above embodiment, the ripple waveform is extracted from the signal obtained by integrating the signal from the sensor 7 over a frequency that does not affect fluctuations in the image output signal. It is also possible to extract a ripple waveform from a signal.

In the above embodiment, the sensor 7 for detecting the brightness of the lamp 3 is attached directly to the lamp 3, but
The sensor may detect the amount of light from the lamp via an optical fiber in order to prevent the influence of heat from the lamp.

As explained above, in the present invention, a change in the light intensity of a light source for exposing an original is detected, a ripple amount is extracted from this detection signal, this ripple amount is held for one main scan, and the original is scanned based on this ripple amount. Since the reference voltage for quantizing the read image output signal is set, the image signal can be quantized regardless of ripples. According to the present invention, since the image signal is not directly corrected, there is no signal deterioration due to correction.

In addition to the type in which reflected light from an original is directly incident on the photoelectric conversion element, the present invention may be of a type in which an optical image of the original is incident through a fiber. Furthermore, the ripples that are the object of the present invention include not only power supply ripples caused by fluctuations in the commercial frequency, but also relatively short-term changes in light intensity caused by fluctuations in the power supply voltage.

[Brief explanation of drawings]

Fig. 1 is a diagram illustrating fluctuations in the image output signal level of a photoelectric conversion element due to ripples, Fig. 2 is a schematic configuration diagram of a document reading device of an electrophotographic copying machine, and Fig. 3
The figures are various signal waveform diagrams for explaining the ripple correction method according to the present invention, and FIG. 4 is a block diagram of an embodiment of an apparatus for carrying out the ripple correction method according to the present invention. DESCRIPTION OF SYMBOLS 1...Original table, 2...Original, 3...Lamp, 4...Mirror, 5...Lens, 6...Photoelectric conversion element, 7...Sensor, 8...Integrator circuit, 9...Ripple extraction circuit, 10
...Sample hold timing circuit, 11...Sample hold circuit, 12...Level conversion circuit, 13
...Constant voltage generator, 14...Reference voltage setting circuit, 1
5...A/D converter.

Claims (1)

[Claims] 1. Irradiate the original with a light source for exposing the original, enter the reflected light into the CCD, read the charges accumulated in the CCD at predetermined time intervals, sequentially A/D convert the read charges, and output them as image signals. In the document reading device, the document exposure light source is driven using a commercial AC power supply as a power source with a high frequency having a cycle shorter than the readout time interval of the CCD, and the amount of irradiated light is electrically detected by the light amount sensor. The detected amount of irradiation light is integrated for each main scan of the original image, the ripple amount caused by the commercial AC power source is calculated based on the integrated output, the ripple amount is held for one main scan, and the ripple amount is A/ Quantizes the original read image signal based on the amount
A ripple correction method characterized by performing ripple correction by changing a reference voltage of a D converter.