Color filter array - AbsoluteAstronomy.com

Photography

Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...

, a color filter array (CFA), or color filter mosaic (CFM), is a mosaic of tiny color filters placed over the pixel

Pixel

In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

sensors of an image sensor

Image sensor

An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

to capture color

Color

Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...

information.

Color filters are needed because the typical photosensors detect light intensity with little or no wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

specificity, and therefore cannot separate color information.
Since sensors are made of semiconductors they obey solid-state physics

Solid-state physics

Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state physics studies how the large-scale properties of solid materials result from...

.

The color filters

Filter (optics)

Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....

filter the light by wavelength range, such that the separate filtered intensities include information about the color of light.
For example, the Bayer filter

Bayer filter

A Bayer filter mosaic is a color filter array for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image...

(shown to the right) gives information about the intensity of light in red, green, and blue (RGB) wavelength regions. The raw image data captured by the image sensor is then converted to a full-color image (with intensities of all three primary color

Primary color

Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....

s represented at each pixel) by a demosaicing

Demosaicing

A demosaicing algorithm is a digital image process used to reconstruct a full color image from the incomplete color samples output from an image sensor overlaid with a color filter array...

algorithm which is tailored for each type of color filter.
The spectral transmittance

Transmittance

In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...

of the CFA elements along with the demosaicing algorithm jointly determine the color rendition. The sensor's passband

Passband

A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....

quantum efficiency

Quantum efficiency

Quantum efficiency is a quantity defined for a photosensitive device such as photographic film or a charge-coupled device as the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's electrical sensitivity to...

and span of the CFA's spectral responses are typically wider than the visible spectrum

Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

, thus all visible colors can be distinguished. The responses of the filters do not generally correspond to the CIE

CIE 1931 color space

In the study of color perception, one of the first mathematically defined color spaces is the CIE 1931 XYZ color space, created by the International Commission on Illumination in 1931....

color matching functions, so a color translation is required to convert the tristimulus values into a common, absolute color space

Absolute color space

In color science, there are two meanings of the term absolute color space:* A color space in which the perceptual difference between colors is directly related to distances between colors as represented by points in the color space....

.

The Foveon X3 sensor

Foveon X3 sensor

The Foveon X3 sensor is a CMOSimage sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductorand Dongbu Electronics....

uses a different structure such that a pixel utilizes properties of multi-junctions to stack blue, green, and red sensors on top of each other. This arrangement does not require a demosaicing algorithm because each pixel has information about each color. Dick Merrill

Richard B. Merrill

Richard Billings Merrill was an American inventor, engineer, and photographer.Merrill was born in New York City, the son of Dr. Frederick Hamilton Merrill and Joan Williams Merrill. He was the adoptive grandson of Richard Billings Richard Billings Merrill (aka Dick Merrill) (1949–2008) was an...

of Foveon distinguishes the approaches as "vertical color filter" for the Foveon X3 versus "lateral color filter" for the CFA.

List of color filter arrays

Name	Description	Pattern size (pixels)
Bayer filter Bayer filter A Bayer filter mosaic is a color filter array for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image...	Very common RGB RGB color model The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors... filter. With one blue, one red, and two green.	2×2
RGBE filter RGBE filter In digital photography, the RGBE filter is an alternative color filter array to the Bayer filter . It similarly uses a mosaic of pixel filters, of red, green, blue and "emerald" , and so also requires demosaicing to produce a full-color image...	Bayer-like with one of the green filters modified to "emerald"; used in a few Sony cameras.	2×2
CYYM filter	One cyan, two yellow, and one magenta; used in a few cameras of Kodak.	2×2
CYGM filter CYGM filter In digital photography, the CYGM filter is an alternative colour filter array to the Bayer filter . It similarly uses a mosaic of pixel filters, of cyan, yellow, green and magenta, and so also requires demosaicing to produce a full-colour image....	One cyan, one yellow, one green, and one magenta; used in a few cameras.	2×2
RGBW Bayer	Traditional RGBW similar to Bayer and RGBE patterns.	2×2
RGBW #1	Three example RGBW filters from Kodak, with 50% white. (See Bayer filter#Alternatives)	4×4
RGBW #2		4×4
RGBW #3		2×4

Bayer CFA

The Bayer CFA is named for its inventor, Dr. Bryce E. Bayer from Eastman Kodak. It is realized by application of color filters over the photodiodes used in sensors, cameras, videocameras, and scanners, for creation of the color image.

RGBW sensor

An RGBW matrix (from Red, Green, Blue, White) is a CFA that includes "white" or transparent filter elements that allow the photodiode to respond to all colors of light; that is, some cells are "panchromatic", and more of the light is detected, rather than absorbed, compared to the Bayer matrix. Kodak announced several RGBW CFA patterns in 2007, all of which have the property that when the panchromatic cells are ignored, the remaining color filtered cells are arranged such that their data can be processed with a standard Bayer demosaicing

Demosaicing

A demosaicing algorithm is a digital image process used to reconstruct a full color image from the incomplete color samples output from an image sensor overlaid with a color filter array...

algorithm.

CYGM sensor

A CYGM matrix (Cyan, Yellow, Green, Magenta) is a CFA that uses mostly secondary color

Secondary color

A secondary color is a color made by mixing two primary colors in a given color space. Examples include the following:-Light : red + green = yellowgreen + blue = cyan blue +...

s, again to allow more of the incident light to be detected rather than absorbed. Other variants include CMY and CMYW matrices.

Manufacture of the CFA

Diazonaphthoquinone (DNQ)-novolac photoresist

Photoresist

A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....

is one material used as the carrier for making color filters from color dyes. There is some interference between the dyes and the ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

light needed to properly expose the polymer, though solutions have been found for this problem. Color photoresists sometimes used include those with chemical monikers CMCR101R, CMCR101G, CMCR101B, CMCR106R, CMCR106G, and CMCR106B.

A few sources discuss other specific chemical substances, attending optical properties, and optimal manufacturing processes of color filter arrays.

For instance, Nakamura said that materials for on-chip color filter arrays fall into two categories: pigment

Pigment

A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

and dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

. Pigment based CFAs have become the dominant option because they offer higher heat resistance and light resistance compared to dye based CFAs. In either case, thicknesses ranging up to 1 micrometre are readily available.

Theuwissen says "Previously, the color filter was fabricated on a separate glass plate and glued to the CCD (Ishikawa 1981), but nowadays, all single-chip color cameras are provided with an imager which has a color filter on-chip processed (Dillon, 1978) and not as a hybrid." He provides a bibliography focusing on the number, types, aliasing effects, moire patterns, and spatial frequencies of the absorptive filters.

Some sources indicate that the CFA can be manufactured separately and affixed after the sensor has been manufactured, while other sensors have the CFA manufactured directly on the surface of the imager. Theuwissen makes no mention of the materials utilized in CFA manufacture.

At least one early example of an on-chip design utilized gelatin filters (Aoki et al., 1982). The gelatin is sectionalized, via photoliothography, and subsequently dyed. Aoki reveals that a CYWG arrangement was used, with the G filter being an overlap of the Y and C filters.

Filter materials are manufacturer specific. Adams et al. state "Several factors influence the CFA's design. First, the individual CFA filters are usually layers of transmissive (absorptive) organic or pigment dyes. Ensuring that the dyes have the right mechanical properties—such as ease of application, durability, and resistance to humidity and other atmospheric stresses—is a challenging task. This makes it difficult, at best, to fine-tune the spectral responsivities.".

Given that the CFAs are deposited on the image sensor surface at the BEOL

BEOL

Back-end-of-line denotes the second portion of IC fabrication where the individual devices get interconnected with wiring on the wafer. BEOL generally begins when the first layer of metal is deposited on the wafer...

(back end of line, the later stages of the integrated circuit manufacturing line), where a low-temperature regime must be rigidly observed (due to the low melting temperature of the aluminum metalized "wires" and the substrate mobility of the dopants implanted within the bulk silicon), organics would be preferred over glass. On the other hand, some CVD silicon oxide processes are low temperature processes.

Ocean Optics has indicated that their patented dichroic filter

Dichroic filter

A dichroic filter, thin-film filter, or interference filter is a very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors tend to be characterized by the color of light that they...

CFA process (alternating thin film

Thin film

A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....

s of ZnS

ZNS

ZNS or ZnS may refer to:In entertainment:* ZNS , a popular entertainer in China and Chinese speaking parts of the world...

and Cryolite

Cryolite

Cryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987....

) can be applied to spectroscopic CCDs. Gersteltec sells photoresist

Photoresist

s that possesses color filter properties.

Some pigment and dye molecules used in CFAs

In U.S.P.# 4,808,501, Carl Chiulli cites the use of 5 chemicals, three of which are C.I. #12715, AKA Solvent Red 8; Solvent Yellow 88; and C.I. # 61551, Solvent Blue 36. In U.S.P. # 5,096,801 Koya et al., of Fuji Photo Film company, list some 150-200 chemical structures, mainly azo dyes and pyrazolone-diazenyl, but fail to provide chemical names, CAS Registry numbers, or Colour Index numbers.

Optically efficient CFA implementation

Nakamura provides a schematic and bibliographic items illustrating the importance of microlens

Microlens

A microlens is a small lens, generally with a diameter less than a millimetre and often as small as 10 micrometres . The small sizes of the lenses means that a simple design can give good optical quality but sometimes unwanted effects arise due to optical diffraction at the small features...

es, their f-number

F-number

In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...

, and the interplay with the CFA and CCD

Charge-coupled device

A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

array.
Further, a short discussion of anti-reflection films is offered, though Janesick's
work appears is more concerned with photon–silicon interaction. Early work on microlenses
and on the three-CCD/prism cameras
stress the importance of a fully integrated design solution for CFAs. The camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

system, as a whole, benefits from careful consideration of CFA technologies and their interplay with other sensor properties.