EP0199738A1

EP0199738A1 - Single-chip solid-state color image sensor and camera incorporating such a sensor

Info

Publication number: EP0199738A1
Application number: EP85904379A
Authority: EP
Inventors: David Roy Cok
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1984-09-10
Filing date: 1985-08-28
Publication date: 1986-11-05
Also published as: WO1986001678A2; WO1986001678A3

Abstract

Capteur d'images en couleurs à semi-conducteurs monopuce possédant un réseau (12) de filtres de couleurs utilisés avec une puce capteur d'images servant à définir un réseau d'éléments capteurs de luminance (38) intercalés avec des éléments capteurs de chrominance (R et B). Le capteur se caractérise par le fait que les éléments capteurs de luminance comprennent 75% des éléments capteurs de la puce, et que chaque élément d'échantillonnage de chrominance (R ou B) est totalement entouré par les éléments d'échantillonnage de luminance (38). Le capteur est associé à un organe de traitement de signaux fournissant des valeurs d'interpolation entre des valeurs d'échantillon de chrominance en interpolant des valeurs de tonalité. Le capteur d'images résultant possède un pouvoir séparateur amélioré sans aucune perte décelable de la qualité des couleurs. Cette invention est utile dans un appareil électronique de mise en images, tel que des caméras vidéo à semi-conducteurs.A single-chip solid-state color image sensor having an array (12) of color filters used with an image sensor chip for defining an array of luminance sensor elements (38) interposed with chrominance sensor elements (R and B). The sensor is characterized by the fact that the luminance sensor elements comprise 75% of the sensor elements of the chip, and that each chrominance sampling element (R or B) is completely surrounded by the luminance sampling elements (38 ). The sensor is associated with a signal processor providing interpolation values between sample chrominance values by interpolating tone values. The resulting image sensor has improved resolving power without any detectable loss in color quality. This invention is useful in electronic imaging apparatus, such as solid-state video cameras.

Description

SINGLE-CHIP SOLID-STATE COLOR IMAGE SENSOR AND CAMERA INCORPORATING SUCH A SENSOR Technical Field

The present invention relates to single- chip solid-state color image sensors of the type comprising a solid-state image sensing chip having an array of image sensing elements and an array of color filter elements registered on the array of image sensing elements, and more particularly to the pattern of color filter elements employed with the image sensor. The invention also relates to a color camera having built-in such a sensor. Background Art

It is known to make a so-called single-chip color image sensor by providing a solid-state image sensing chip with a three color array of color filter elements arranged over the image sensing elements of the image sensor to make the image sensor color sensitive. To take advantage of the fact that the human visual system is most sensitive to luminance detail in an image, it is also known to arrange the color filter elements so that a majority of the image sensing elements of the solid-state image sensor are sensitive to luminance information. Sometimes the color "green" is chosen for the luminance sensitive elements, and sometimes white. As used herein, the term "luminance" refers to the predominant color of the image sensor, whether that color be green, white or some other color. The other two colors will be referred to as "chrominance components" to distinguish them from luminance.

U.S. Patent No. 4,065,785 issued December 27, 1977 to Adcock et al shows such a single-chip color image sensor having a vertical stripe pattern of color filters. The stripe filters are arranged in repeated groups of green, green-blue and green-red filter elements. In use, a decoder circuit extracts blue and red information produced by the image sensor by subtracting the value of adjacent green samples from the green-blue, and green-red samples respectively. In one briefly mentioned embodiment, the stripe pattern groups noted above are alternated with groups of four green stripes, resulting in the image sensor having 75% luminance sensing elements. Although this high sampling rate for luminance information improves the luminance resolution of the image sensor in the horizontal direction, in the vertical direction there are repeated lines that contain no luminance information at all. Furthermore, the chrominance sampling pattern is nonsymmetrical in both the horizontal and vertical directions. Because of this lack of symmetry in luminance and chrominance sampling, colored artifacts, are produced in abundance around the colored details in the image. Disclosure of the Invention

It is the object of the invention to provide a single-chip solid-state color image sensor of the type having an array of color filter elements with 75% luminance sensing elements, that is free of the problems noted above. The object is achieved according to the invention by arranging the color filter elements in a two-dimensional array so that each sensing element which is next to a chrominance sensing element is a luminance sensing element. In such a manner, each chrominance component sensing element is totally surrounded by luminance sensing elements. Brief Description of the Drawings

Modes of practicing the invention are described with respect to the drawings, wherein: Fig. 1 is a schematic diagram showing one arrangement of luminance and chrominance sensitive image sensing elements on a si gle-chip solid state image sensor according to the present invention;

Fig. 2 is a schematic diagram illustrating a portion of an image sensor of the interline transfer type, useful with the present invention; Fig. 3 is a schematic diagram showing electronic imaging apparatus having a single chip solid state color image sensor, signal processing electronics and image display means;

Fig. 4 is an example of a portion of a color filter array having the pattern shown in Fig. 1;

Fig. 5 is is a schematic diagram showing an alternative arrangement of luminance and chrominance image sensing elements on a single-chip solid state image sensor; and Fig. 6 is an example of a portion of a color filter array having the pattern shown in Fig. 5. Modes of Carrying Out the Invention

Fig. 3 is a schematic diagram illustrating an electronic imaging apparatus representative of the prior art, having a single-chip solid-state color image sensor. The imaging apparatus includes a lens 10 for projecting an image of an object 0 through a color filter array 12 onto a single-chip solid-state image sensor 14. The color filter array 12 is formed directly on the light receiving surface of the image sensor 14 or alternatively is formed on a transparent support and is optically projected onto the image sensor by an optical relay (not shown) . The image sensor 14 is a solid-state image sensor comprising a two dimensional array of image sensing elements such as photocapacitors or photodiodes. The image sensor may, for example comprise a CCD frame transfer image sensor, a CCD interline transfer image sensor, a CID (charge injection device) image sensor, or an X-Y addressed photodiode array.

A sampled analog color image signal produced by the image sensor on line 16 is supplied to signal processing electronics 18. The signal processing electronics includes, for example, an analog-to-digital converter 20 for converting the sampled analog signal to a digital signal, a frame store 22 for storing the unprocessed and processed image signal, a central processing unit 24 for performing the signal processing on the digital image signal, and random access and read only memories (RAM and ROM 26 and 28) for storing the control programs for the central processing unit. The elements of the signal processing electronics 18 are electrically connected via a bus 30. Processed video-image signals are supplied to .a digital-to-analog converter 32 to convert the signals from digital to analog form. The analog color image signals are displayed on display means such as a CRT color monitor 34, or a graphic arts scanner 36. The description of the electronic imaging apparatus is intended to be generic. The present invention is not limited to use with any particular type of optical system, signal processing apparatus, or display means. A color sampling pattern according to the present invention for a single-chip color image sensor for use with the imaging apparatus of Fig. 3 will now be described with reference to Figs. 1, 2 and 4. Fig. 2 shows a portion of an interline transfer type image sensor 14, having columns of image sensing elements 38 interspersed with interline readout shift registers 40. The arrows illustrate schematically the transfer path of the image information supplied by the sensing elements . Fig. 1 shows a pattern of image sensing elements for use with the image sensor in Fig. 2. The squares labeled with a 0 represent the locations of luminance sensing elements, and the squares labelled with an X represent the locations of chrominance sensing elements. As can be seen from Fig. 1, each chrominance sensing element is surrounded on all sides by luminance sensing elements, and the color sampling pattern is symmetrical in both the horizontal and vertical directions. Fig. 4 shows a portion of a color filter array 12 for use with the image sensor 14 to produce a sampling pattern of the type shown in Fig.^' 1. The color filter comprises a field of green, indicated by G, in which red R and blue B filter elements are arranged. .The locations of image sensing elements 38 with respect to the filter elements are shown in phantom. Some of the image sensing element locations are numbered from 1-16 to facilitate later explanation of signal processing.

When a single-chip color image sensor according to the present invention is employed in imaging apparatus such as shown in Fig. 3, the signal processing electronics interpolates the luminance values at the chrominance component sample locations and the values of the chrominance components at the luminance sample location.

Since each chrominance component sample location is completely surrounded by luminance sample locations, the accuracy of interpolation of luminance information is greatly facilitated. Because the chrominance component samples are uniformly distributed in the horizontal and vertical directions, the appearance of color fringes in the color signal is greatly reduced. Best results have been achieved when the interpolation of the chrominance component values is performed as a function of the value of the luminance signal. As described in copending PCT International Patent Application based on U.S.

Patent Application Serial No. 648,999 filed

September 10, 1984, a preferred method for interpolating chrominance component values employs interpolation of hue components produced at neighboring chrominance component sample locations.

The interpolated hue component values are then used with the luminance values to determine interpolated chrominance component values at each sample locatio . For example, for the color sampling pattern shown in Fig. 4, the signal processing to interpolate the red chrominance component for element position 11, proceeds as follows.

First, interpolated luminance values are determined at positions 6 and 16," for example by taking the average of the eight surrounding luminance sample values surrounding each chrominance component sample location.

Next, a red hue component HR is determined for positions 6 and 16 as follows:

HRe Λ6

G¹ where

Re is the chrominance component value at position 6,

G'₆ is the interpolated luminance value, and similarly for HRiβ.

The hue component values for the sample locations between the chrominance component sample locations are determined for example by linear interpolation as follows: -T-

Finally, the interpolated chrominance component val¬ ues are found by multiplying the interpolated hue component values by the luminance values as follows:

R' 11 HR' 11 . JI 1

The other chrominance component (blue) is reconstructed in a similar manner.

10 By employing this method of interpolation, the red and blue chrominance component signals are reconstructed without noticeable degradation.

Another color sampling pattern according to the invention, wherein each chrominance component

^•*^■ sample location is surrounded by luminance sample locations is shown in Fig. 5. It is to be noted that both Fig. 1 and Fig. 5 patterns can be considered as a mosaic repeating the same elementary square group of 4 elements, three of which being

20 luminance elements. The 0's indicate luminance sample locations, and the X's indicate chrominance sample location. An example of a color filter array 12 for yielding such a sampling pattern with the image sensor 14 is shown in Fig. 6. In this

25 example, the chrominance sample locations are shifted by one pixel from column to column. Technical Effect and Industrial Applicability

By providing a single-chip solid-state color image sensor with a color filter array configured to provide 75% luminance sensing elements, the resolution of the single-chip solid-state color image sensor is improved. By making the color sampling pattern symmetrical in the horizontal and vertical directions, the appearance

35 of color fringes in the output image is greatly reduced. When employed with signal processing apparatus that interpolates the chrominance signals as a function of the luminance signal, the chrominance signals are reconstructed with no noticeable degradation. Since each chrominance sample is surrounded by luminance samples, the missing luminance samples can be more accurately interpolated due to the greater number of nearest neighboring luminance samples to each luminance interpolation location. The invention is useful in electronic imaging apparatus such as solid-state video cameras, and in graphic arts apparatus employing solid state image sensors.

Claims

CLAIMS :

1. A single-chip solid-state color image sensor of the type having a squared array of color filter elements matched to a solid-state image sensor having a corresponding array of respective chrominance and luminance sensing elements, characterized in that each sensing element of the array which is next to a chrominance sensing element is a luminance sensing element.

2. The sensor claimed in claim 1, wherein luminance sensing elements sense green light, one-half of the chrominance sensing elements sense red light, and the other half of the chrominance sensing elements sense blue light.

3. The sensor claimed in claim 1 or 2, wherein the array of sensing elements is made of a mosaic repeating a square elementary group of 4 sensing elements, three of which being luminance sensing elements.

4. A solid-state color camera of the type having a solid-state color image sensor having a squared array of luminance and chrominance sensing elements, and signal processing means for processing the signal produced by the image sensor to provide interpolated values between luminance signal values and between chrominance signal values, characterized in that each sensing element of the array which is next to a chrominance sensing element is a luminance sensing element, and in that the signal processing means for providing interpolated values between chrominance sample values comprising means for interpolating hue values between chrominance sampling locations.

5. The camera claimed in claim 4, wherein said signal processing means for providing interpolated luminance values employs all of the luminance values from the luminance sampling locations surrounding a chrominance sampling location.

6. The camera claimed in claim 5, wherein the signal processing means computes the average value of the surrounding luminance samples.

7. The camera claimed in claim 6, wherein the signal processing means computes the hue value at a chrominance sampling location by dividing the chrominance component value by the average value of the surrounding luminance samples.

8. The camera claimed in any one of claims 4 to 7, wherein the luminance sensing elements sense green light and the chrominance sensing elements sense red or blue light.