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61-479: The initials CIE come from the French name "Commission Internationale de l'éclairage" , which has maintained and developed many of the standards in use today relating to colorimetry . The CIE color spaces were created using data from a series of experiments, where human test subjects adjusted red, green, and blue primary colors to find a visual match to a second, pure color. The original experiments were conducted in

122-886: A 1 nm -interval dataset of CIE 1931 and CIE 1964 provided by Wyszecki 1982. A CIE publication in 1986 appears also to have a 1 nm dataset, probably using the same data. Like the regular 5 nm dataset, this dataset is also derived from interpolation. The derivation of the CIE standard observer from color matching experiments is given below , after the description of the CIE RGB space. The CIE's color matching functions x ¯ ( λ ) {\displaystyle {\overline {x}}(\lambda )} , y ¯ ( λ ) {\displaystyle {\overline {y}}(\lambda )} and z ¯ ( λ ) {\displaystyle {\overline {z}}(\lambda )} are

183-457: A color with a spectral radiance L e,Ω,λ are given in terms of the standard observer by: where λ {\displaystyle \lambda } is the wavelength of the equivalent monochromatic light (measured in nanometers ), and customary limits of the integral are λ ∈ [ 380 , 780 ] {\displaystyle \lambda \in [380,780]} . The values of X , Y , and Z are bounded if

244-424: A combination of the three primaries at relative intensities r ¯ ( λ ) {\displaystyle {\bar {r}}(\lambda )} , g ¯ ( λ ) {\displaystyle {\bar {g}}(\lambda )} , and b ¯ ( λ ) {\displaystyle {\bar {b}}(\lambda )} respectively, then

305-487: A one-lobe function. The CIE XYZ color matching functions are nonnegative, and lead to nonnegative XYZ coordinates for all real colors (that is, for nonnegative light spectra). Other observers, such as for the CIE RGB space or other RGB color spaces , are defined by other sets of three color-matching functions, not generally nonnegative, and lead to tristimulus values in those other spaces, which may include negative coordinates for some real colors. The tristimulus values for

366-406: A projection of an infinite-dimensional spectrum to a three-dimensional color . International Commission on Illumination The International Commission on Illumination (usually abbreviated CIE for its French name Commission internationale de l'éclairage ) is the international authority on light , illumination , colour , and colour spaces . It was established in 1913 as a successor to

427-496: A tabulation of these values at various λ will estimate three functions of wavelength. These are the RGB color-matching functions. Any spectral distribution can be thought of as a combination of a number of monochromatic sources at varying intensities, so that (by Grassmann's laws ) integrating the color matching functions with that spectral distribution will yield the intensities of the three primaries necessary to match it. The problem

488-427: A test color was projected while on the other an observer-adjustable color was projected. The adjustable color was a mixture of the three monochromatic primary colors, each with adjustable brightness. The observer would alter the brightness of each of the three primary beams until a match to the test color was observed. If the test color were simply a monochromatic color at wavelength λ, and if it could be matched by

549-435: A tristimulus specification of the objective color of the light spectrum. The three parameters, denoted "S", "M", and "L", are indicated using a 3-dimensional space denominated the " LMS color space ", which is one of many color spaces devised to quantify human color vision . A color space maps a range of physically produced colors from mixed light, pigments , etc. to an objective description of color sensations registered in

610-404: Is known as the "1931 CIE standard observer". Rather than specify the brightness of each primary, the curves are normalized to have constant area beneath them. This area is fixed to a particular value by specifying that The resulting normalized color matching functions are then scaled in the r:g:b ratio of 1:4.5907:0.0601 for source luminance and 72.0962:1.3791:1 for source radiance to reproduce

671-498: Is limited, covering only 35.9% of the CIE 1931 gamut. While this allows the use of a limited bit depth without causing color banding , and therefore reduces transmission bandwidth, it also prevents the encoding of deeply saturated colors that might be available in an alternate color spaces. Some RGB color spaces such as Adobe RGB and ProPhoto intended for the creation, rather than transmission, of images are designed with expanded gamuts to address this issue, however this does not mean

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732-461: Is often arbitrarily chosen so that Y = 1 or Y = 100 is the brightest white that a color display supports. In this case, the Y value is known as the relative luminance . The corresponding whitepoint values for X and Z can then be inferred using the standard illuminants . Since the XYZ values are defined much earlier than the characterization of cone cells in the 1950s (by Ragnar Granit ),

793-447: Is recommended when dealing with more than about a 4° field of view. Both standard observer functions are discretized at 5 nm wavelength intervals from 380 nm to 780 nm and distributed by the CIE . All corresponding values have been calculated from experimentally obtained data using interpolation . The standard observer is characterized by three color matching functions . There is also

854-431: Is that the three primaries can only produce colors which lie withinin their gamut - the triangle in color space formed by the primaries, which never touches the monochromatic locus nor the purple line except at the three primaries. In other words, there is no monochromatic source that can be matched by a combination of the three primaries, except at the wavelengths of the three primaries themselves. However, by adding one of

915-430: Is the wavelength of the equivalent monochromatic light (measured in nanometers ), and the standard limits of the integral are λ ∈ [ 380 , 780 ] {\displaystyle \lambda \in [380,780]} . Since the human eye has three types of color sensors that respond to different ranges of wavelengths , a full plot of all visible colors is a three-dimensional figure. However,

976-487: The CIE 2° standard observer . RGB color spaces are well-suited to describing the electronic display of color, such as computer monitors and color television . These devices often reproduce colours using an array of red, green, and blue phosphors agitated by a cathode-ray tube (CRT), or an array of red, green, and blue LCDs lit by a backlight, and are therefore naturally described by an additive color model with RGB primaries. Early examples of RGB color spaces came with

1037-562: The ISO or IEC . RGB color spaces RGB color spaces is a category of additive colorimetric color spaces specifying part of its absolute color space definition using the RGB color model . RGB color spaces are commonly found describing the mapping of the RGB color model to human perceivable color, but some RGB color spaces use imaginary (non-real-world) primaries and thus can not be displayed directly. Like any color space, while

1098-454: The LMS color space , but not restricted to non-negative sensitivities, associates physically produced light spectra with specific tristimulus values. Consider two light sources composed of different mixtures of various wavelengths. Such light sources may appear to be the same color; this effect is called " metamerism ." Such light sources have the same apparent color to an observer when they produce

1159-408: The RGB color spaces , imply negative values for at least one of the three primaries because the chromaticity would be outside the color triangle defined by the primary colors. To avoid these negative RGB values, and to have one component that describes the perceived brightness , "imaginary" primary colors and corresponding color-matching functions were formulated. The CIE 1931 color space defines

1220-417: The Y tristimulus value: The figure on the right shows the related chromaticity diagram. The outer curved boundary is the spectral locus , with wavelengths shown in nanometers. The chromaticity diagram is a tool to specify how the human eye will experience light with a given spectrum. It cannot specify colors of objects (or printing inks), since the chromaticity observed while looking at an object depends on

1281-437: The luminance of a color. The chromaticity is then specified by the two derived parameters x and y , two of the three normalized values being functions of all three tristimulus values X , Y , and Z : That is, because each tristimulus parameter, X , Y , Z , is divided by the sum of all three, the resulting values, x , y , z , each represent a proportion of the whole and so their sum must be equal to one. Therefore,

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1342-413: The 1920s, two independent experiments on human color perception were conducted by W. David Wright with ten observers, and John Guild with seven observers. Their results laid the foundation for the trichromatic CIE XYZ color space specification. The experiments were conducted by using a circular split screen (a bipartite field) 2 degrees in diameter, which is the angular size of the human fovea. On one side

1403-523: The CIE 1931 model, Y is the luminance , Z is quasi-equal to blue (of CIE RGB), and X is a mix of the three CIE RGB curves chosen to be nonnegative (see § Definition of the CIE XYZ color space ). Setting Y as luminance has the useful result that for any given Y value, the XZ plane will contain all possible chromaticities at that luminance. The unit of the tristimulus values X , Y , and Z

1464-431: The CIE 1931 space. This standard is not currently realisable with current LCD technology, and alternative architectures such as quantum dot or OLED based devices are currently in development. gamma The CIE 1931 color space standard defines both the CIE RGB space, which is a color space with monochromatic primaries , and the CIE XYZ color space, which is functionally similar to a linear RGB color space, however

1525-500: The CIE XYZ color matching functions can be approximated by a sum of Gaussian functions , as follows: Let g ( x ) denote a piecewise-Gaussian function, defined by That is, g ( x ) resembles a bell curve with its peak at x = μ , a spread/standard deviation of 1 / τ 1 {\displaystyle 1/\tau _{1}} to the left of the mean, and spread of 1 / τ 2 {\displaystyle 1/\tau _{2}} to

1586-610: The CIE published the CIELUV and CIELAB color spaces, which are derived from XYZ, and are intended to provide more uniform predictions relative to human perception. The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in short ("S", 420 nm – 440 nm ), medium ("M", 530 nm – 540 nm ), and long ("L", 560 nm – 580 nm ) wavelengths. These cone cells underlie human color perception in conditions of medium and high brightness; in very dim light color vision diminishes, and

1647-642: The Commission Internationale de Photométrie, which was founded in 1900, and is today based in Vienna, Austria . The CIE has six active divisions, each of which establishes technical committees to carry out its program: Two divisions are no longer active. The President of the CIE from 2023 is Jennifer Veitch from Canada. CIE publishes Technical Reports (TRs), International Standards (ISs) and Technical Notes (TNs). International Standards (ISs) are often further developed as dual standards with

1708-451: The LMS cone responses of the human eye. Due to the distribution of cones in the eye, the tristimulus values depend on the observer's field of view . To eliminate this variable, the CIE defined a color-mapping function called the standard (colorimetric) observer , to represent an average human's chromatic response within a 2° arc inside the fovea . This angle was chosen owing to the belief that

1769-454: The adoption of the NTSC color television standard in 1953 across North America, followed by PAL and SECAM covering the rest of the world. These early RGB spaces were defined in part by the phosphor used by CRTs in use at the time, and the gamma of the electron beam. While these color spaces reproduced the intended colors using additive red, green, and blue primaries, the broadcast signal itself

1830-925: The amounts of primaries needed to match the monochromatic test primary. These functions are shown in the plot on the right (CIE 1931). r ¯ ( λ ) {\displaystyle {\overline {r}}(\lambda )} and g ¯ ( λ ) {\displaystyle {\overline {g}}(\lambda )} are zero at 435.8 nm , r ¯ ( λ ) {\displaystyle {\overline {r}}(\lambda )} and b ¯ ( λ ) {\displaystyle {\overline {b}}(\lambda )} are zero at 546.1 nm and g ¯ ( λ ) {\displaystyle {\overline {g}}(\lambda )} and b ¯ ( λ ) {\displaystyle {\overline {b}}(\lambda )} are zero at 700 nm , since in these cases

1891-452: The color-sensitive cones resided within a 2° arc of the fovea. Thus the CIE 1931 Standard Observer function is also known as the CIE 1931 2° Standard Observer . A more modern but less-used alternative is the CIE 1964 10° Standard Observer , which is derived from the work of Stiles and Burch, and Speranskaya. For the 10° experiments, the observers were instructed to ignore the central 2° spot. The 1964 Supplementary Standard Observer function

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1952-408: The concept of color can be divided into two parts: brightness and chromaticity . For example, the color white is a bright color, while the color grey is considered to be a less bright version of that same white. In other words, the chromaticity of white and grey are the same while their brightness differs. The CIE XYZ color space was deliberately designed so that the Y parameter is also a measure of

2013-857: The green and blue matching functions have rather small negative values. Although Wright and Guild's experiments were carried out using various primaries at various intensities, and although they used a number of different observers, all of their results were summarized by the standardized CIE RGB color matching functions r ¯ ( λ ) {\displaystyle {\overline {r}}(\lambda )} , g ¯ ( λ ) {\displaystyle {\overline {g}}(\lambda )} , and b ¯ ( λ ) {\displaystyle {\overline {b}}(\lambda )} , obtained using three monochromatic primaries at standardized wavelengths of 700 nm (red), 546.1 nm (green) and 435.8 nm (blue). The (un-normalized) color matching functions are

2074-463: The green and red primaries, some blue must be added and b ¯ ( λ ) {\displaystyle {\bar {b}}(\lambda )} will be negative. For wavelengths below the wavelength of the blue primary, or above the wavelength of the red primary, some green must be added and g ¯ ( λ ) {\displaystyle {\bar {g}}(\lambda )} will be negative. In each case,

2135-419: The human eye, typically in terms of tristimulus values, but not usually in the LMS color space defined by the spectral sensitivities of the cone cells . The tristimulus values associated with a color space can be conceptualized as amounts of three primary colors in a tri-chromatic, additive color model . In some color spaces, including the LMS and XYZ spaces, the primary colors used are not real colors in

2196-412: The larger space has 'more colors". The numerical quantity of colors is related to bit depth and not the size or shape of the gamut. A large space with a low bit depth can be detrimental to the gamut density and result in high Δ E {\displaystyle \Delta E} errors . More recent color spaces such as Rec. 2020 for UHD-TVs define an extremely large gamut covering 63.3% of

2257-404: The light source as well. Mathematically the colors of the chromaticity diagram occupy a region of the real projective plane . The chromaticity diagram illustrates a number of interesting properties of the CIE XYZ color space: When two or more colors are additively mixed, the x and y chromaticity coordinates of the resulting color (x mix ,y mix ) may be calculated from the chromaticities of

2318-432: The low-brightness, monochromatic "night vision" receptors, denominated " rod cells ", become effective. Thus, three parameters corresponding to levels of stimulus of the three kinds of cone cells, in principle describe any human color sensation. Weighting a total light power spectrum by the individual spectral sensitivities of the three kinds of cone cells renders three effective values of stimulus ; these three values compose

2379-421: The luminance values (L 1 , L 2 , etc.) one can alternatively use any other photometric quantity that is directly proportional to the tristimulus value Y (naturally meaning that Y itself can also be used as well). As already mentioned, when two colors are mixed, the resulting color x mix , y mix will lie on the straight line segment that connects these colors on the CIE xy chromaticity diagram. To calculate

2440-410: The mid 1920s by William David Wright  [ ja ] using ten observers and John Guild using seven observers. The experimental results were combined, creating the CIE RGB color space. The CIE XYZ color space was derived from CIE RGB in an effort to simplify the math. The CIE 1931 XYZ color space is still widely used, even though it is not perceptually uniform in relation to human vision. In 1976

2501-403: The mixing ratio of the component colors x 1 ,y 1 and x 2 ,y 2 that results in a certain x mix ,y mix on this line segment, one can use the formula where L 1 is the luminance of color x 1 ,y 1 and L 2 the luminance of color x 2 ,y 2 . Because y mix is unambiguously determined by x mix and vice versa, knowing just one or the other of them is enough for calculating

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2562-464: The mixing ratio. In accordance with the remarks concerning the formulas for x mix and y mix , the mixing ratio L 1 /L 2 may well be expressed in terms of other photometric quantities than luminance. The first step in developing the CIE XYZ color space is the measurement of the CIE RGB color space. The CIE RGB color space is one of many RGB color spaces , distinguished by a particular set of monochromatic (single-wavelength) primary colors . In

2623-417: The mixture components (x 1 ,y 1 ; x 2 ,y 2 ; …; x n ,y n ) and their corresponding luminances (L 1 , L 2 , …, L n ) with the following formulas: These formulas can be derived from the previously presented definitions of x and y chromaticity coordinates by taking advantage of the fact that the tristimulus values X, Y, and Z of the individual mixture components are directly additive. In place of

2684-417: The numerical description of the chromatic response of the observer (described above). They can be thought of as the spectral sensitivity curves of three linear light detectors yielding the CIE tristimulus values X , Y and Z . Collectively, these three functions describe the CIE standard observer. Table lookup can become impractical for some computational tasks. Instead of referring to the published table,

2745-420: The physiological meaning of these values are known only much later. The Hunt-Pointer-Estevez matrix from the 1980s relates XYZ with LMS. When inverted, it shows how the three cone responses add up to XYZ functions: In other words, the Z value is solely made up of the S cone response, the Y value a mix of L and M responses, and X value a mix of all three. This fact makes XYZ values analogous to, but different from,

2806-624: The primaries to the monochromatic test color, the test color can be brought into the RGB gamut, allowing a match to be made. Adding a primary to the monochromatic test color is effectively the same as subtracting it from the adjustable color, which of course cannot be done since it is impossible to have a negative intensity for any of the primaries. For wavelengths between the blue and green primaries, some red primary must be added to allow matching, resulting in negative values of r ¯ ( λ ) {\displaystyle {\bar {r}}(\lambda )} . Likewise, between

2867-468: The radiance spectrum L e,Ω,λ is bounded. The reflective and transmissive cases are very similar to the emissive case, with a few differences. The spectral radiance L e,Ω,λ is replaced by the spectral reflectance (or transmittance ) S(λ) of the object being measured, multiplied by the spectral power distribution of the illuminant I(λ) . where K is a scaling factor (usually 1 or 100), and λ {\displaystyle \lambda }

2928-452: The remaining two color matching functions will be positive. It can be seen that the deviation of the RGB gamut from the complete gamut is rather small except between the blue and green primaries at 435.8 and 546.1 nm. In this wavelength band, rather large amounts of the red primary needed to be added to the test color, and it is in this band that the red color matching function has rather large negative values. In their regions of negative values,

2989-481: The responses of these cone cells are called the Tristimulus values , and the combination of their responses is processed into the psychological effect of color vision. RGB use in color space definitions employ primaries (and often a white point) based on the RGB color model, to map to real world color. Applying Grassmann's law of light additivity, the range of colors that can be produced are those enclosed within

3050-437: The resulting tristimulus values, in which they are denoted by "X", "Y", and "Z". In XYZ space, all combinations of non-negative coordinates are meaningful, but many, such as the primary locations [1, 0, 0], [0, 1, 0], and [0, 0, 1], correspond to imaginary colors outside the space of possible LMS coordinates; imaginary colors do not correspond to any spectral distribution of wavelengths and therefore have no physical reality. In

3111-434: The results. The color matching functions and primaries were settled upon by a CIE special commission after considerable deliberation. The cut-offs at the short- and long-wavelength side of the diagram are chosen somewhat arbitrarily; the human eye can actually see light with wavelengths up to about 810 nm , but with a sensitivity that is many thousand times lower than for green light. These color matching functions define what

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3172-473: The right of the mean. With the wavelength λ measured in nanometers , we then approximate the 1931 color matching functions: The squared differences between the above approximation and the measured CIE xyz color matching functions is less than the within-observer variance encountered in the experimental measurements used to form the CIE standards. It is also possible to use fewer gaussian functions, with one gaussian for each "lobe". CIE 1964 fits well with

3233-490: The same tristimulus values, regardless of the spectral power distributions of the sources. Most wavelengths stimulate two or all three kinds of cone cell because the spectral sensitivity curves of the three kinds overlap. Certain tristimulus values are thus physically impossible: e.g. LMS tristimulus values that are non-zero for the M component and zero for both the L and S components. Furthermore pure spectral colors would, in any normal trichromatic additive color space, e.g.,

3294-420: The sense that they cannot be generated in any light spectrum. The CIE XYZ color space encompasses all color sensations that are visible to a person with average eyesight. That is why CIE XYZ tristimulus values are a device-invariant representation of color. It serves as a standard reference against which many other color spaces are defined. A set of color-matching functions, like the spectral sensitivity curves of

3355-480: The singular term "RGB color space" can be misleading, since a chosen color space or signalled colour can be described by any appropriate color model. However the singular can be seen in specifications where storage signalled as RGB is its intended use. The normal human eye contains three types of color-sensitive cone cells . Each cell is responsive to light of either long, medium, or short wavelengths, which we generally categorize as red, green, and blue. Taken together,

3416-404: The specifications in this category use the RGB color model to describe their space, it is not mandatory to use that model to signal pixel color values. Broadcast TV color spaces like NTSC, PAL, Rec. 709, Rec. 2020 additionally describe a translation from RGB to YCbCr and that is how they are usually signalled for transmission, but an image can be stored as either RGB or YCbCr. This demonstrates using

3477-461: The test color is one of the primaries. The primaries with wavelengths 546.1 nm and 435.8 nm were chosen because they are easily reproducible monochromatic lines of a mercury vapor discharge. The 700 nm wavelength, which in 1931 was difficult to reproduce as a monochromatic beam, was chosen because the eye's perception of color is rather unchanging at this wavelength, and therefore small errors in wavelength of this primary would have little effect on

3538-466: The triangle on the chromaticity diagram defined using the primaries as vertices . The primary colors are usually mapped to xyY chromaticity coordinates, though the uʹ,vʹ coordinates from the UCS chromaticity diagram may be used. Both xyY and uʹ,vʹ are derived from the CIE 1931 color space , a device independent space also known as XYZ which covers the full gamut of human-perceptible colors visible to

3599-442: The true color matching functions. By proposing that the primaries be standardized, the CIE established an international system of objective color notation. Given these scaled color matching functions, the RGB tristimulus values for a color with a spectral power distribution S ( λ ) {\displaystyle S(\lambda )} would then be given by: These are all inner products and can be thought of as

3660-400: The value z can be deduced by knowing x and y , and consequently the latter two values are sufficient for describing the chromaticity of any color. The derived color space specified by x , y , and Y is known as the CIE xyY color space and is widely used to specify colors in practice. The X and Z tristimulus values can be calculated back from the chromaticity values x and y and

3721-481: Was encoded from RGB components to a composite signal such as YIQ , and decoded back by the receiver into RGB signals for display. HDTV uses the BT.709 color space, later repurposed for computer monitors as sRGB . Both use the same color primaries and white point, but different transfer functions, as HDTV is intended for a dark living room while sRGB is intended for a brighter office environment. The gamut of these spaces

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