Solid-State Imaging Device With Pixels Having Weighted Sensitivity Distribution

1. Field of the Invention

The present invention relates to a solid-state imaging device used for photoelectric conversion.

2. Related Background Art

Conventionally such solid-state imaging device is constructed by arranging pixels of lateral lengths a, b of several microns to several tens of microns as shown in FIG. 1(A) by a number of several hundred to several thousands as shown in FIG. 1(B). Each pixel has uniform sensitivity distribution over the entire area as shown in FIG. 2. Said sensitivity distribution is one-dimensionally shown in FIG. 3, and is represented by: ##EQU1##

When an aperture having such distribution is illuminated with light, there is obtained a resolution as shown by a solid line in FIG. 4, by diffraction.

The equation (1) can be Fourier transformed as follows: ##EQU2## and the trajectory of the equation (3) corresponds to the solid line in FIG. 4. In comparison with the complete resolution shown by a broken line in FIG. 4, the actually obtained resolution becomes deteriorated as indicated by the solid line, so that the read image becomes blurred.

The object of the present invention is to provide a solid-state imaging device not associated with the above-mentioned drawbacks and provided with an improved resolution.

The above-mentioned object can be achieved, according to the present invention, by a solid-state imaging device in which each pixel has a weighted distribution of sensitivity.

A simple structure having small pixels on both sides of a pixel improves the effective sensitivity distribution and enables effective edge enhancement.

FIG. 1(A) is a schematic view of a pixel of a conventional solid-state imaging device;

FIG. 1(B) is a schematic view showing arrangement of pixels in a conventional solid-state imaging device;

FIG. 2 is a view showing sensitivity distribution of a pixel in a conventional solid-state imaging device;

FIG. 3 is a chart of one-dimensional representation of sensitivity distribution of a conventional pixel;

FIG. 4 is a chart of a resolution curve of the pixel shown in FIG. 3;

FIG. 5 is a schematic view of a pixel of a solid-state imaging device embodying the present invention;

FIG. 6 is a chart showing a sensitivity distribution of the embodiment shown in FIG. 5;

FIGS. 7 and 8 are schematic views showing the structures of a pixel in other embodiments;

FIG. 9 is a schematic view of a copying machine employing a CCD sensor embodying the present invention;

FIG. 10 is a block diagram of the electric circuit of a printer employing a CCD sensor embodying the present invention;

FIG. 11 is a chart comparing the modulation transfer functions of an embodiment of the present invention and a conventional solid-state imaging device; and

FIG. 12 is a chart showing the change in the modulation transfer function when the ratio of widths of pixels 2A and 2B varies.

Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings.

FIG. 5 schematically shows an embodiment of the solid-state imaging device of the present invention. A pixel 2 of said solid-state imaging device is composed of a pixel 2A and two pixels 2B, 2B placed on both sides of pixel. The output Y of said pixels 2B, 2B is converted by a converter circuit 3 into a negative signal and is synthesized with the output signal X of the pixel 2A. As the result, the pixel 2 has a sensitivity distribution as shown in FIG. 6. In FIGS. 5 and 6, .alpha. and .beta. respectively indicate the distances from the center of the pixel 2 to the ends of the pixels 2A and 2B. Also the value h in FIG. 6 can be arbitrarily varied by suppressing the converted output signal of the pixels 2B.

FIGS. 7 and 8 illustrate other embodiments of the solid-state imaging device of the present invention, having circular pixels. In FIG. 7 a pixel 2A is surrounded by a pixel 2B, while in FIG. 8 a pixel 2A is placed between two pixels 2B, 2B.

FIGS. 9, 10 illustrate examples of application of CCD sensors embodying the present invention.

FIG. 9 shows a copying machine employing a CCD sensor of the present invention, wherein shown are an original document 11; an original supporting glass 12; an illuminating system 13; a mirror 14; an imaging optical system 15; and a CCD sensor 16 of the present invention.

In response to the actuation of an unrepresented start button, the original support 12 starts to move in a direction indicated by the arrow, whereby the original document 11 is read. The original document 11 is illuminated by the illuminating system 13, and a reflected light beam from the original document 11 is transmitted through the mirror 14 to the imaging optical system 15 and focused on the CCD sensor 14, for conversion of the received optical information into electrical signals. FIG. 10 is a block diagram of an electric circuit for processing the output of the CCD sensor to a printer. The output signal X from the pixel 2A of the CCD sensor 16 and the output signals Y from the pixels 2B are supplied to an operation circuit 17.

In said operation circuit 17, the signal Y from the pixels 2B is converted in sign, suppressed in intensity and synthesized with the signal X from the pixel 2A. Consequently a pixel in the CCD sensor shows a sensitivity distribution as shown in FIG. 6. The output of the operation circuit 17 is digitized by an A/D converter 18 and supplied to a printer 19 for obtaining a printout according to the output signal of the CCD sensor.

FIG. 11 compares the modulation transfer functions (MTF) of the solid-state imaging device of the present invention and of a conventional solid-state imaging device. A curve A is the MTF of a solid-state imaging device having circular pixels as shown in FIG. 3, wherein the pixel 2A has a diameter of 50 .mu.m while the pixel 2B has an outer diameter of 100 .mu.m (corresponding to .alpha.=25 .mu.m and .beta.=50 .mu.m in FIG. 5) while a curve B is the MTF of a conventional solid-state imaging device. The negative areas of the MTF's are illustrated with inverted sign. In comparison with the conventional device, the solid-state imaging device of the present invention shows a higher value by 1.5 times or more at a spatial frequency of 10 lines/mm.

FIG. 12 shows the change in MTF when the ratio of the width a of the pixel 2A to the width b.sub.0 of the pixels 2B varies. Curves C, D, E, F, G and H respectively correspond to ratios a.sub.0 :b.sub.0 =2:1, 3:1, 4:1, 5:1, 6:1 and 1:1. A ratio a.sub.0 :b.sub.0 =2:1 provides a largest improvement in the MTF, thus enabling most effective edge enhancement regardless of the magnitude of h.

For an original image containing a large amount of high frequency components such as a screen tone image, the signal of the pixels 2B should not be inverted to negative but is used in the positive state. This change corresponds to an increase in the aperture of a pixel and permits the reduction of the Moire pattern caused by the high frequency components of the screen pattern, arrangement of the apertures and the size of the apertures.

As explained in the foregoing, these embodiments allow, by a simple weighting structure having two small pixels on both sides of a pixel, to improve the effective sensitivity distribution, thereby achieving an effective edge enhancement.