Standard illuminant
Encyclopedia
Spectral power distribution
In color science and radiometry, a spectral power distribution describes the power per unit area per unit wavelength of an illumination , or more generally, the per-wavelength contribution to any radiometric quantity .Mathematically, for the spectral...
) which is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting.
CIE illuminants
The International Commission on IlluminationInternational Commission on Illumination
The International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
(usually abbreviated CIE for its French name) is the body responsible for publishing all of the well-known standard illuminants. Each of these is known by a letter or by a letter-number combination.
Illuminants A, B, and C were introduced in 1931, with the intention of respectively representing average incandescent light, direct sunlight, and average daylight. Illuminants D represent phases of daylight, Illuminant E is the equal-energy illuminant, while Illuminants F represent fluorescent lamps of various composition.
There are instructions on how to experimentally produce light sources ("standard sources") corresponding to the older illuminants. For the relatively newer ones (such as series D), experimenters are left to measure to profiles of their sources and compare them to the published spectra:
Nevertheless, they do provide a measure, called the Metamerism Index, to assess the quality of daylight simulators. The Metamerism
Metamerism (color)
In colorimetry, metamerism is the matching of apparent color of objects with different spectral power distributions. Colors that match this way are called metamers....
Index tests how well five sets of metameric samples match under the test and reference illuminant. In a manner similar to the Color Rendering Index, the average difference between the metamers is calculated.
Illuminant A
The CIE defines illuminant A in these terms:The spectral radiance of a black body
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
follows Planck's law:
At the time of standardizing illuminant A, both
(which does not affect the relative SPD) and 
were different. In 1968, the estimate of c2 was revised from 0.01438 m·K to 0.014388 m·K (and before that, it was 0.01435 m·K when illuminant A was standardized). This difference shifted the Planckian locusPlanckian locus
In physics and color science, the Planckian locus or black body locus is the path or locus that the color of an incandescent black body would take in a particular chromaticity space as the blackbody temperature changes...
, changing the color temperature of the illuminant from its nominal 2848 K to 2856 K:
In order to avoid further possible changes in the color temperature, the CIE now specifies the SPD directly, based on the original (1931) value of c2:
The coefficients have been selected to achieve a peak SPD of 100 at 560 nm. The tristimulus values are (X,Y,Z) = (109.85,100.00,35.58), and the chromaticity coordinates using the standard observer are (x,y)=(0.44758, 0.40745).
Illuminants B and C
Illuminants B and C are daylight simulators. They are derived from Illuminant A by using a liquid filters. B served as a representative of noon sunlight, with a correlated color temperature (CCT) of 4874 K, while C represented average day light with a CCT of 6774 K. They are poor approximations of any common light source and deprecated in favor of the D series:The liquid filters, designed by Raymond Davis, Jr. and Kasson S. Gibson in 1931, have a relatively high absorbance at the red end of the spectrum, effectively increasing the CCT of the gas lamp to daylight levels. This is similar in function to a CTO color gel
Color gel
A color gel or color filter , also known as lighting gel or simply gel, is a transparent colored material that is used in theatre, event production, photography, videography and cinematography to color light and for color correction...
that photographers and cinematographers use today, albeit much less convenient.
Each filter uses a pair of solutions, comprising specific amounts of distilled water, copper sulfate, mannite, pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
, sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, and ammonium sulfate
Ammonium sulfate
Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...
. The solutions are separated by a sheet of uncolored glass. The amounts of the ingredients are carefully chosen so that their combination yields a color temperature conversion filter; that is, the filtered light is still white.
Illuminant series D
H. W. Budde of the National Research Council of Canada
National Research Council of Canada
The National Research Council is an agency of the Government of Canada which conducts scientific research and development.- History :...
in Ottawa
Ottawa
Ottawa is the capital of Canada, the second largest city in the Province of Ontario, and the fourth largest city in the country. The city is located on the south bank of the Ottawa River in the eastern portion of Southern Ontario...
, H. R. Condit and F. Grum of the Eastman Kodak Company in Rochester, New York
Rochester, New York
Rochester is a city in Monroe County, New York, south of Lake Ontario in the United States. Known as The World's Image Centre, it was also once known as The Flour City, and more recently as The Flower City...
, and S. T. Henderson and D. Hodgkiss of Thorn Electrical Industries
THORN Electrical Industries
Thorn Electrical Industries, Limited was an electrical engineering business. It was listed on the London Stock Exchange but it merged with EMI Group to form Thorn EMI in 1979...
in Enfield
Enfield Town
Enfield Town is the historic town centre of Enfield, formerly in the county of Middlesex and now in the London Borough of Enfield. It is north north-east of Charing Cross...
had independently measured the spectral power distribution (SPD) of daylight from 330 to 700 nm, totaling among them 622 samples. Judd et al. analyzed these samples and found that the (x,y) chromaticity coordinates had a simple, quadratic
Quadratic function
A quadratic function, in mathematics, is a polynomial function of the formf=ax^2+bx+c,\quad a \ne 0.The graph of a quadratic function is a parabola whose axis of symmetry is parallel to the y-axis....
relation:
.Simonds supervised the characteristic vector analysis of the SPDs. Application of his method revealed that the SPDs could be satisfactorily approximated by using the mean (S0) and first two characteristic vectors (S1 and S2):
In simpler terms, the SPD of the studied daylight samples can be expressed as the linear combination
Linear combination
In mathematics, a linear combination is an expression constructed from a set of terms by multiplying each term by a constant and adding the results...
of three, fixed SPDs. The first vector (S0) is the mean of all the SPD samples, which is the best reconstituted SPD that can be formed with only a fixed vector. The second vector (S1) corresponds to yellow–blue variation, accounting for changes in the correlated color temperature due to presence or absence of clouds or direct sunlight. The third vector (S2) corresponds to pink–green variation caused by the presence of water in the form of vapor and haze.
To construct a daylight simulator of a particular correlated color temperature one merely needs to know the coefficients M1 and M2 of the characteristic vectors S1 and S2.
and making use of known tristimulus values for the mean vectors, they were able to express M1 and M2 as follows:
The only problem is that this left unsolved the computation of the coordinate
for a particular phase of daylight. Judd et al. simply tabulated the values of certain chromaticity coordinates, corresponding to commonly-used correlated color temperatures, such as 5500 K, 6500 K, and 7500 K. For other color temperatures, one could consult figures made by Kelly. This problem was addressed in the CIE report that formalized illuminant D, with an approximation of the x coordinate in terms of the reciprocal color temperature, valid from 4000 K to 25,000 K. The y coordinate trivially followed from Judd's quadratic relation.Judd et al. then extended the reconstituted SPDs to 300–330 nm and 700–830 nm by using Moon's spectral absorbance data of the Earth's atmosphere.
The tabulated SPDs presented by the CIE today are derived by linear interpolation
Linear interpolation
Linear interpolation is a method of curve fitting using linear polynomials. Lerp is an abbreviation for linear interpolation, which can also be used as a verb .-Linear interpolation between two known points:...
of the 10 nm data set down to 5 nm. The limited nature of the photometric data is not an impediment to the calculation of the CIEXYZ tristimulus values since the CIE standard colorimetric observer's color matching functions are only tabulated from 380 to 780 nm in increments of 5 nm.
Similar studies have been undertaken in other parts of the world, or repeating Judd et al.s analysis with modern computational methods. In several of these studies, the daylight locus is notably closer to the Planckian locus than in Judd et al.
Computation:
The relative spectral power distribution
Spectral power distribution
In color science and radiometry, a spectral power distribution describes the power per unit area per unit wavelength of an illumination , or more generally, the per-wavelength contribution to any radiometric quantity .Mathematically, for the spectral...
(SPD)
of a D series illuminant can be derived from its chromaticity coordinates in the CIE 1931 color spaceCIE 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....
,
:
where
are the mean and first two eigenvector SPDs, depicted above. The characteristic vectors both have a zero at 560 nm, since all the relative SPDs have been normalized about this point.The CCTs of the canonical illuminants, D50, D55, D65, and D75, differ slightly from what their names suggest. For example, D50 has a CCT of 5003 K ("horizon" light), while D65 has a CCT of 6504 K (noon light). As explained in a previous section, this is because the value of the constants in Planck's law have been slightly changed since the definition of these canonical illuminants, whose SPDs are based on the original values in Planck's law.
Illuminant E
Illuminant E is an equal-energy radiator; it has a constant SPD inside the visible spectrumVisible 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...
. It is useful as a theoretical reference; an illuminant that gives equal weight to all wavelengths, presenting an even color. It also has equal CIE XYZ tristimulus values, thus its chromaticity coordinates are (x,y)=(1/3,1/3). This is by design; the XYZ color matching functions are normalized such that their integrals over the visible spectrum are the same.
Illuminant series F
The F series of illuminants represent various types of fluorescent lightingFluorescent lamp
A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful...
.
F1–F6 "standard" fluorescent lamps consist of two semi-broadband emissions of antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
and manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
activations in calcium halophosphate phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
. F4 is of particular interest since it was used for calibrating the CIE Color Rendering Index (the CRI formula was chosen such that F4 would have a CRI of 51). F7–F9 are "broadband" (full-spectrum light) fluorescent lamps with multiple phosphors, and higher CRIs. Finally, F10–F12 are narrow triband illuminants consisting of three "narrowband" emissions (caused by ternary compositions of rare-earth phosphors) in the R,G,B regions of the visible spectrum. The phosphor weights can be tuned to achieve the desired CCT.
The spectra of these illuminants are published in Publication 15:2004.
White point
The spectrum of a standard illuminant, like any other profile of light, can be converted into tristimulus values. The set of three tristimulus coordinates of an illuminant is called a white point. If the profile is normalisedNormalizing constant
The concept of a normalizing constant arises in probability theory and a variety of other areas of mathematics.-Definition and examples:In probability theory, a normalizing constant is a constant by which an everywhere non-negative function must be multiplied so the area under its graph is 1, e.g.,...
, then the white point can equivalently be expressed as a pair of chromaticity coordinates.
If an image is recorded in tristimulus coordinates (or in values which can be converted to and from them), then the white point of the illuminant used gives the maximum value of the tristimulus coordinates that will be recorded at any point in the image, in the absence of fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
. It is called the white point of the image.
The process of calculating the white point discards a great deal of information about the profile of the illuminant, and so although it is true that for every illuminant the exact white point can be calculated, it is not the case that knowing the white point of an image alone tells you a great deal about the illuminant that was used to record it.
White points of standard illuminants
A list of standardized illuminants, their CIE chromaticity coordinates (x,y) of a perfect reflecting (or transmitting) diffuser, and their correlated color temperatures (CCTs) are given below. The CIE chromaticity coordinates are given for both the 2 degree field of view (1931) and the 10 degree field of view (1964). The color swatches represent the hueHue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
of each white point, calculated with luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
Y=0.54 and the standard observer, assuming correct sRGB
SRGB color space
sRGB is a standard RGB color space created cooperatively by HP and Microsoft in 1996 for use on monitors, printers, and the Internet.sRGB uses the ITU-R BT.709 primaries, the same as are used in studio monitors and HDTV, and a transfer function typical of CRTs...
display calibration.
| Name | CIE 1931 2° | CIE 1964 10° | CCT (K Kelvin The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all... ) |
Hue Hue Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"... |
Note | ||
|---|---|---|---|---|---|---|---|
| x2 | y2 | x10 | y10 | ||||
| A | 0.44757 | 0.40745 | 0.45117 | 0.40594 | 2856 | Incandescent / Tungsten | |
| B | 0.34842 | 0.35161 | 0.34980 | 0.35270 | 4874 | {obsolete} Direct sunlight at noon | |
| C | 0.31006 | 0.31616 | 0.31039 | 0.31905 | 6774 | {obsolete} Average / North sky Daylight | |
| D50 | 0.34567 | 0.35850 | 0.34773 | 0.35952 | 5003 | Horizon Light. ICC profile ICC profile In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium... PCS |
|
| D55 | 0.33242 | 0.34743 | 0.33411 | 0.34877 | 5503 | Mid-morning / Mid-afternoon Daylight | |
| D65 | 0.31271 | 0.32902 | 0.31382 | 0.33100 | 6504 | Noon Daylight: Television Television Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound... , sRGB color space SRGB color space sRGB is a standard RGB color space created cooperatively by HP and Microsoft in 1996 for use on monitors, printers, and the Internet.sRGB uses the ITU-R BT.709 primaries, the same as are used in studio monitors and HDTV, and a transfer function typical of CRTs... |
|
| D75 | 0.29902 | 0.31485 | 0.29968 | 0.31740 | 7504 | North sky Daylight | |
| E | 1/3 | 1/3 | 1/3 | 1/3 | 5454 | Equal energy | |
| F1 | 0.31310 | 0.33727 | 0.31811 | 0.33559 | 6430 | Daylight Fluorescent | |
| F2 | 0.37208 | 0.37529 | 0.37925 | 0.36733 | 4230 | Cool White Fluorescent | |
| F3 | 0.40910 | 0.39430 | 0.41761 | 0.38324 | 3450 | White Fluorescent | |
| F4 | 0.44018 | 0.40329 | 0.44920 | 0.39074 | 2940 | Warm White Fluorescent | |
| F5 | 0.31379 | 0.34531 | 0.31975 | 0.34246 | 6350 | Daylight Fluorescent | |
| F6 | 0.37790 | 0.38835 | 0.38660 | 0.37847 | 4150 | Lite White Fluorescent | |
| F7 | 0.31292 | 0.32933 | 0.31569 | 0.32960 | 6500 | D65 simulator, Daylight simulator | |
| F8 | 0.34588 | 0.35875 | 0.34902 | 0.35939 | 5000 | D50 simulator, Sylvania F40 Design 50 | |
| F9 | 0.37417 | 0.37281 | 0.37829 | 0.37045 | 4150 | Cool White Deluxe Fluorescent | |
| F10 | 0.34609 | 0.35986 | 0.35090 | 0.35444 | 5000 | Philips TL85, Ultralume 50 | |
| F11 | 0.38052 | 0.37713 | 0.38541 | 0.37123 | 4000 | Philips TL84, Ultralume 40 | |
| F12 | 0.43695 | 0.40441 | 0.44256 | 0.39717 | 3000 | Philips TL83, Ultralume 30 | |
External links
- Selected colorimetric tables in Excel, as published in CIE 15:2004