Single sensor color video camera with blurring filter

• US 4,663,661 A
• Filed: 05/23/1985
• Issued: 05/05/1987
• Est. Priority Date: 05/23/1985
• Status: Expired due to Term

• Current Assignee: Eastman Kodak Company
• Original Assignee: Eastman Kodak Company
• Inventors: Weldy, John A., Kristy, Stephen H.
• Primary Examiner(s): Groody, James J.
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US06/737,359
• Time in Patent Office: 712 Days
• Field of Search: 358/37, 358/41, 358/43, 358/44
• US Class Current: 348/242
• CPC Class Codes: G06T 3/4015 Image demosaicing, e.g. colour filter arrays [CFA] or Bayer patterns
• CPC Class Codes: H04N 23/84 for processing colour signals
• CPC Class Codes: H04N 25/134 based on three different wavelength filter elements

Abstract

A single sensor video camera includes an optical blurring filter having a known spatial (or frequency) function and a color filter interposed between the blurring filter and an image sensor. The color filter has red and blue filter elements intermingled among a majority of green elements. An accurate estimate of the blurred green light blocked by the non-green elements is recovered by applying the image signal from the sensor to a digital filter and substituting the resultant signal for the missing green portions. The coefficients of the filter are selected in an optimization process that seeks to match the known function of the blurring filter to a like function of the digital filter (as represented by the coefficients). To do this the color filter must be configured to provide at least two green samples on either side of a non-green element.

Claims

1. Video apparatus for converting light from an image into an electrical signal and for processing the electrical signal, comprising:

sensing means for generating the electrical signal from a blurred likeness of the image light;

color filter means for partially blocking light of a particular color from activating said sensing means; and

electrical filter means for processing the electrical signal according to a set of weighting coefficients that are selected so as to generate a filtered signal representative of the brightness distribution of said light of said particular color that is blocked by said color filter means.

2. Apparatus as claimed in claim 1 in which said sensing means comprises:

a sensor for generating a light-induced signal corresponding to the brightness of the image light striking said sensor; and

an optical filter for blurring the light directed to said sensor.

3. Apparatus as claimed in claim 2 in which said color filter means comprises a color filter interposed between said optical filter and said sensor.

4. Apparatus as claimed in claim 3 in which said color filter has a pattern of filter elements, some elements blocking light of said particular color from striking said sensor and other elements allowing light of said particular color to strike said sensor.

5. Apparatus as claimed in claim 4 in which the elements blocking light of said particular color comprise a majority of said filter elements.

6. Apparatus as claimed in claim 4 in which the elements blocking light of said particular color comprise approximately seventy-five percent of said filter elements.

7. Apparatus as claimed in claim 1 in which said sensing means comprises:

a pair of sensors comprised of photosites for generating a pair of light-induced signals corresponding to the brightness of the image light striking said photosites, one sensor offset from the other sensor by one-half the width of a photosite; and

means for interleaving the pair of light-induced signals to form the electrical signal of which the signal segments derived from said sensor photosites are in sequential time relation and represent a blurred likeness of the image light.

8. Apparatus as claimed in claim 7 in which said color filter means comprises:

respective color filters, positioned in the path of the light striking said sensors, for selectively passing light of said particular color, one color filter passing only light of said particular color to one said sensor and the other color filter having a pattern that periodically blocks light of said particular color from reaching said other sensor.

9. Video apparatus for converting light into an electrical signal and for processing the electrical signal, comprising:

a sensor for generating the electrical signal corresponding to the brightness of light striking said sensor;

an optical filter for blurring the light directed to said sensor according to a known response function;

a color filter, interposed between said optical filter and said sensor, said color filter having a plurality of filter elements that pass certain colors and block others, the elements being so arranged along one dimension that an element blocking a selected color is surrounded by at least four elements, two on either side thereof, that pass said selected color;

means for sampling the electrical signal;

filter means for generating a filtered signal from a linear combination of samples from a string thereof corresponding to said at least four elements surrounding said filter element blocking said selected color, the samples being weighted according to a set of coefficients that are selected in a process that substantially equates them to the response function of said optical filter; and

means for substituting the filtered signal for that portion of the electrical signal corresponding to the filter element blocking said selected color.

10. Apparatus as claimed in claim 9 in which the known response function of said optical filter is a spatial function and said filter means is constituted by a set of coefficients so selected that the convolution of the coefficients with the spatial function of said optical filter yields a close approximation of the spatial function of said optical filter.

11. Apparatus as claimed in claim 9 in which the known response of said optical filter is a frequency response and said filter means has a frequency response so selected that the filter frequency response multiplied by the optical filter frequency response yields a close approximation of the optical filter frequency response.

12. A video camera of the type that generates a signal corresponding to selected light of a particular color received from areas of a scene, said camera comprising:

an image sensor having a plurality of photosites corresponding spatially to areas of the scene;

an optical filter that spreads light from each scene area over a region occupied by several photosites according to known spatial and frequency response functions;

a spectrally-selective filter interposed between said image sensor and said optical filter, said spectrally-selective filter having one type of filter element that blocks the selected light and another type of filter element that passes the selected light;

means for generating an image signal corresponding to the selected light striking said image sensor, said signal comprising periods during which the selected light is not received on said sensor and other periods during which the selected light is received through the light-passing filter elements;

a digital filter characterized by a set of weighting coefficients capable of approximately reconstructing the spatial response function of said optical filter when convolved therewith or the frequency response function when multiplied therewith;

means for applying the image signal to said digital filter and for obtaining a filtered signal therefrom; and

means for substituting said filtered signal for the signal periods during which the selected light is not received.

13. A color video camera of the type that generates a signal corresponding to colored light received from areas of the scene, said camera comprising:

an image sensor having a plurality of photosites corresponding spatially to areas of the scene;

an optical filter that spreads light received from each scene area over a sensor region occupied by several photosites according to a known response function;

a color selective filter interposed between said image sensor and said optical filter, said color filter having one type of filter element that blocks light of a selected color and another type of filter element that passes light of said selected color;

means for generating an image signal corresponding to light striking said image sensor, said signal comprising a sequence of samples corresponding to the light striking discrete photosites, including photosites for which said color filter blocks light of the selected color;

means for filtering said image signal by generating a filtered signal from a linear combination of the image samples and weighting each sample according to a function that approximates the effect of the optical filter response function upon the scene; and

means for substituting the filtered signal for the image signal during sample periods for which said color filter blocks light of the selected color.

14. A single sensor color video camera of the type that generates a luminance signal from the green information in a scene and a color signal from a combination of the green information with other color information in the scene, said camera comprising:

an optical filter that generates a blurred image of the scene according to known spatial and frequency characteristics;

a color filter having at least twice as many green filter elements as non-green filter elements, said color filter elements forming a line pattern in which substantially all non-green filter elements are separated by at least two green filter elements;

an image sensor having photosensitive image sites that correspond spatially to said filter elements;

means for so arranging said color filter between said optical filter and said image sensor that green light spatially corresponding to a non-green filter element is blurred and spread over several filter elements, said several filter elements including a non-green filter element and a green element on either side thereof;

means for recovering an image signal from said image sensor; and

an electrical filter for processing said image signal, said filter comprising means for assembling a linear combination of weighted samples of said image signal, said linear combination being weighted according to a set of coefficients so selected that the convolution of the spatial characteristic of said optical filter with the spatial characteristic of said electrical filter, or the multiplication of the frequency characteristic of said optical filter with the frequency characteristic of said electrical filter, approximately reproduces the corresponding spatial or frequency characteristic of said optical filter.

15. A single sensor color video camera as claimed in claim 14 in which said color filter elements form a line pattern in which substantially all non-green filter elements are separated by three green filter elements and in which said green light spatially corresponding to a non-green filter element is blurred and spread over a non-green element and at least one green element on either side thereof.

16. A single sensor color video camera as claimed in claim 14 in which said optical filter is a birefringent filter that separates at least the green light spatially corresponding to a non-green filter element into three separate green rays that strike said non-green filter element and a green element on either side thereof.

17. A digital filter for use with a color video system in which a color selective signal representative of areas of a scene is generated by the combination of:

an optical filter for blurring light received from the scene according to a known spread function;

an image sensor having many photosites for generating a discrete set of signals from the blurred light; and

a color filter interposed between the optical filter and the image sensor, said color filter having an array of filter elements permitting light of a particular color to strike a majority of photosites of the image sensor and blocking said colored light from a minority of photosites;

said digital filter characterized by means for generating a linear combination of said discrete signals according to a set of weighting coefficients applied to said signals, the weighting coefficients selected so that said known spread function is substantially replicated when said selected coefficients are convolved with said known spread function.