The CIE 1931 x,y chromaticity space, also showing the chromaticities of black-body light sources of various temperatures (Planckian locus), and lines of constant correlated color temperature. In the study of the perception of Color, one of the first mathematically defined Color spaces was the CIE 1931 XYZ color space (also known as CIE 1931 color space In Color theory, the Planckian locus is generally the path or ''locus'' that the color of a Black body would take ina particular Color space

Color temperature is a characteristic of visible light that has important applications in photography, videography, publishing and other fields. The color temperature of a light source is determined by comparing its chromaticity with a theoretical, heated black-body radiator. Chromaticity is an objective specification of the Quality of a Color regardless of its luminance that is as determined by its Colorfulness (or saturation In Physics, a black body is an object that absorbs all light that falls on it The temperature (usually measured in kelvin (K)) at which the heated black-body radiator matches the color of the light source is that source's color temperature; for a black body source, it is directly related to Planck's law. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic For a general introduction see Black body. In Physics, Planck's law describes the spectral radiance of Electromagnetic radiation

## Categorizing different lighting

TemperatureSource
1700 KMatch flame
1850 KCandle flame
2800–3300 KIncandescent light bulb
3350 KStudio "CP" light
3400 KStudio lamps, photofloods, etc.
4100 KMoonlight, xenon arc lamp
5000 KHorizon daylight
5500–6000 KTypical daylight, electronic flash
6500 KDaylight, overcast
9300 KCRT screen
Note: These temperatures are merely approximations;
considerable variation may be present.

Because it is the standard against which other light sources are compared, the color temperature of the thermal radiation from an ideal black body radiator is defined as equal to its surface temperature in kelvin, or alternatively in mired (micro-reciprocal degrees kelvin). In Physics, a black body is an object that absorbs all light that falls on it Contracted from the term micro reciprocal degree, the mired is a unit of measurement used to express Color temperature. [1] For source other than ideal black bodies, the color temperature of the thermal radiation emitted from it may differ from its actual surface temperature. Thermal radiation is Electromagnetic radiation emitted from the surface of an object which is due to the object's Temperature. In an incandescent light bulb the light is of thermal origin and is very close to that of an ideal black-body radiator. The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general

However, many other light sources, such as fluorescent lamps, emit light primarily by processes other than raising the temperature of a body. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. This means the emitted radiation does not follow the form of a black-body spectrum. For a general introduction see Black body. In Physics, Planck's law describes the spectral radiance of Electromagnetic radiation These sources are assigned what is known as a correlated color temperature (CCT). Color temperature is a characteristic of Visible light that has important applications in lighting photography videography publishing and other fields CCT is the color temperature of a black body radiator which to human color perception most closely matches the light from the lamp. Color vision is the capacity of an organism or machine to distinguish objects based on the Wavelengths (or frequencies) of the Light they reflect or emit Because such an approximation is not required for incandescent light, the CCT for an incandescent light is simply its unadjusted temperature, derived from the comparison to a black body radiator.

### The sun

As the sun crosses the sky, it may appear to be red, orange, yellow or white depending on its position. The Sun (Sol is the Star at the center of the Solar System. The changing color of the sun over the course of the day is mainly a result of refraction and, to a lesser extent, scattering of light, and is unrelated to black body radiation. Refraction is the change in direction of a Wave due to a change in its Speed. Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from The blue color of the sky is not due to black-body radiation, but rather to Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red. Rayleigh scattering (named after Lord Rayleigh) is the elastic Scattering of Light or other electromagnetic radiation by particles much smaller This phenomenon has nothing to do with the properties of a black body.

Daylight has a spectrum similar to that of a black body. In professions involving color reproduction, such as photography and publishing, daylight is often approximated using standard illuminant D50 or D65, as recommended by the CIE. A standard illuminant is a profile or Spectrum of Visible light which is published in order to allow images or colors recorded under different lighting to be compared CIE Standard Illuminant D65 (sometimes written D65) is a commonly-used Standard illuminant defined by the International Commission on Illumination

Hues of the Planckian locus, in the mired scale. Contracted from the term micro reciprocal degree, the mired is a unit of measurement used to express Color temperature.

For colors based on the black body, blue is the "hotter" color, while red is actually the "cooler" color. This is the opposite of the cultural associations that colors have taken on, with "red" as "hot", and "blue" as "cold". For the physiological perception of color see Color and Color vision. The traditional associations come from a variety of sources, such as water and ice appearing blue, while heated metal and fire are of a reddish hue. Fire is the heat and light energy released during a Chemical reaction, in particular a combustion reaction. However, the redness of these heat sources comes precisely from the fact that red is the coolest of the visible colors, the first color emitted as heat increases.

## Color temperature applications

"Color temperature" is sometimes used loosely to mean "white balance" or "white point". In Photography and Image processing, color balance is the global adjustment of the intensities of the colors (typically red green and blue Primary colors A white point (often referred to as reference white or target white in technical documents is a set of tristimulus values or Chromaticity coordinates Notice that color temperature has only one degree of freedom, whereas white balance has two, R-Y and B-Y. For information on degrees of freedom in other sciences see Degrees of freedom.

### Film photography

Film sometimes appears to exaggerate the color of the light, since it does not adapt to lighting color as our eyes do. An object that appears to the eye to be white may turn out to look very blue or orange in a photograph. The color balance may need to be corrected while shooting or while printing to achieve a neutral color print. In Photography and Image processing, color balance is the global adjustment of the intensities of the colors (typically red green and blue Primary colors

Film is made for specific light sources (most commonly daylight film and tungsten film), and used properly, will create a neutral color print. Tungsten film is designed to give accurate colors under tungsten light Matching the sensitivity of the film to the color temperature of the light source is one way to balance color. Sensitometry is the scientific study of light-sensitive materials especially Photographic film. If tungsten film is used indoors with incandescent lamps, the yellowish-orange light of the tungsten [incandescent] bulbs will appear as white (3200 K) in the photograph. Incandescence is the emission of Light (visible Electromagnetic radiation) from a hot body due to its temperature Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74

Filters on a camera lens, or color gels over the light source(s) may also be used to correct color balance. In Photography, a filter is a Camera accessory consisting of an optical filter that can be inserted in the optical path See also Photographic filter A color gel or color filter ( UK colour gel or colour filter) or a lighting gel When shooting with a bluish light (high color temperature) source such as on an overcast day, in the shade, in window light or if using tungsten film with white or blue light, a yellowish-orange filter will correct this. For shooting with daylight film (calibrated to 5600 K) under warmer (low color temperature) light sources such as sunsets, candle light or tungsten lighting, a bluish (e. g. #80A) filter may be used.

If there is more than one light source with varied color temperatures, one way to balance the color is to use daylight film and place color-correcting gel filters over each light source.

Photographers sometimes use color temperature meters. Color temperature meters are usually designed to read only two regions along the visible spectrum (red and blue); more expensive ones read three regions (red, green, and blue). However, they are ineffective with sources such as fluorescent or discharge lamps, whose light varies in color and may be harder to correct for. Because it is often greenish, a magenta filter may correct it. More sophisticated colorimetry tools can be used where such meters are lacking. Colorimetry or Colourimetry can refer to the quantitative study of Color perception

### Desktop publishing

In the desktop publishing industry, it is important to know your monitor’s color temperature. Color matching software, such as ColorSync will measure a monitor's color temperature and then adjust its settings accordingly. ColorSync is Apple Inc 's Color management API for the Mac OS and Mac OS X. This enables on-screen color to more closely match printed color. Common monitor color temperatures, along with matching standard illuminants in parentheses, are as follows:

5000 K (D50), 5500 K (D55), 6500 K (D65), 7500 K (D75), 9300 K. A standard illuminant is a profile or Spectrum of Visible light which is published in order to allow images or colors recorded under different lighting to be compared CIE Standard Illuminant D65 (sometimes written D65) is a commonly-used Standard illuminant defined by the International Commission on Illumination

Designations such as D50 are used to classify color temperatures of light tables and viewing booths. A light table is a viewing device that is used to review Photographic film or artwork placed on top of it When viewing a color slide at a light table, it is important that the light be balanced properly so that the colors are not shifted towards the red or blue. In Photography, a reversal film is a still positive image created on a transparent base using photochemical means

Digital cameras, web graphics, DVDs, etc. Many compact digital still cameras can record Sound and moving Video as well as still Photograph. DVD (also known as " Digital Versatile Disc " or " Digital Video Disc " - see Etymology)is are normally designed for a 6500 K color temperature & indeed the sRGB standard stipulates (among other things) a 6500 K display whitepoint. sRGB is a standard RGB (Red Green Blue color space created cooperatively by HP and Microsoft for use on monitors printers and the Internet

### TV, video, and digital still cameras

The NTSC and PAL TV norms call for a compliant TV screen to display an electrically "black-and-white" signal (minimal color saturation) at a color temperature of 6500 K. NTSC ( National Television System Committee) is the Analog television system used in the United States, Canada, Japan, Mexico PAL, short for Phase Alternating Line, is a colour -encoding system used in Broadcast television systems in large parts of the world On many actual sets, however, especially older or lower-quality units, there is a very noticeable deviation from this requirement.

Most video and digital still cameras can adjust for color temperature by zooming into a white or neutral colored object and setting the manual "white balance" (telling the camera that "this object is white"); the camera then shows true white as white and adjusts all the other colors accordingly. White-balancing is necessary especially when indoors under fluorescent lighting and when moving the camera from one lighting situation to another. Most cameras also have an automatic white balance function that attempts to determine the color of the light and correct accordingly. While these settings were once unreliable, they are much improved in today's digital cameras, and will produce the "correct" white balance in a wide variety of lighting situations. White balance can also be corrected in post-processing in much the same way, although extreme amounts of correction will result in a loss of image quality due to color value quantization.

### Artistic application via control of color temperature

The house above appears a light cream during the midday, but seems a bluish white here in the dim light before full sunrise. Note the different color temperature of the sunrise in the background.

Experimentation with color temperature is obvious in many Stanley Kubrick films; for instance in Eyes Wide Shut the light coming in from a window was almost always conspicuously blue, whereas the light from lamps on end tables was fairly orange. Eyes Wide Shut is a 1999 drama - mystery directed produced and co-written by Stanley Kubrick, based on the Novella Indoor lights typically give off a yellow hue; fluorescent and natural lighting tends to be more blue.

Video camera operators can also white-balance objects which aren't white, downplaying the color of the object used for white-balancing. A camera operator is a person that operates a film or Video camera for the purpose of recording motion to Film, Video, or a Computer storage For instance, they can bring more warmth into a picture by white-balancing off something light blue, such as faded blue denim; in this way white-balancing can serve in place of a filter or lighting gel when those aren't available.

Cinematographers do not "white balance" in the same way as video camera operators; they can use techniques such as filters, choice of film stock, pre-flashing, and after shooting, color grading (both by exposure at the labs, and also digitally, where digital film processes are used). In Cinematography and Photography, pre-flashing is the exposure of the Film or other photosensor to uniform Light prior to exposing it to the Color grading is the process of altering and enhancing the color of a motion picture or television image either electronically photo-chemically or digitally Cinematographers also work closely with set designers and lighting crews to achieve the desired effects.

For artists, most pigments and papers have a cool or warm cast, as the human eye can detect even a minute amount of saturation. Gray mixed with yellow, orange or red is a "warm gray". Green, blue, or purple, create "cool grays". Note that this sense of temperature is the reverse of that of real temperature; bluer is described as "cooler" even though it corresponds to a higher-temperature blackbody. In Physics, a black body is an object that absorbs all light that falls on it

 WARM GRAY COOL GRAY Mixed with 6% yellow. Mixed with 6% blue.

Lighting designers sometimes select filters by color temperature, commonly to match light that is theoretically white. The role of the lighting designer (or LD) within Theatre is to work with the director, Set designer, Costume designer, and sometimes An optical filter is a device which selectively transmits light having certain properties (often a particular range of Wavelengths that is range of Colours Since fixtures using discharge type lamps produce a light of considerably higher color temperature than tungsten lamps, using the two in conjunction could potentially produce a stark contrast, so sometimes fixtures with HID lamps, commonly producing light of 6000–7000 K, are fitted with 3200 K filters to emulate tungsten light. Metal halide lamps, a member of the High-intensity discharge (HID family of lamps produce high light output for their size making them a compact powerful and efficient The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general Fixtures with color mixing features or with multiple colors, (if including 3200 K) are also capable of producing tungsten like light. Color temperature may also be a factor when selecting lamps, since each is likely to have a different color temperature. A lamp is a replaceable component such as an Incandescent light bulb, which is designed to produce Light from Electricity. [2]

### Lighting

For lighting buildings, it is often important to take into account the color temperature of the light fittings used. For example, a warmer (i. e. , lower temperature) light is often used in public areas to promote relaxation, while a cooler, whiter light is used in offices. Due to heightened awareness of the stress that poor lighting can cause, as well as sick building syndrome, many governmental agencies have certain criteria that lighting must meet. Sick building syndrome ( SBS) is a combination of ailments (a Syndrome) associated with an individual's place of work ( Office building) or residence

The international color code is often used to denote the temperature of a lamp's light. This code is a three digit number. The first digit refers to the color rendering index: if it is 8, then the CRI is between 80 and 90, if it is 9, it lies between 90 and 100. The next two numbers are the color temperature (to the nearest hundred) divided by one hundred, thus if the temperature is 6500 K, the number is 65. [3]

## Correlated color temperature

The correlated color temperature (Tcp) is the temperature of the Planckian radiator whose perceived colour most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions

CIE/IEC 17.4:1987, International Lighting Vocabulary (ISBN 3900734070)[4]

### Motivation

Black body radiators are the reference by which the whiteness of light sources is judged. In Physics, a black body is an object that absorbs all light that falls on it A black body can be described by its color temperature, whose hues are depicted above. By analogy, nearly-Planckian light sources such as certain fluorescent or high-intensity discharge lamps can be judged by their correlated color temperature (CCT); the color temperature of the Planckian radiator that best approximates them. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. A High-intensity discharge (HID lamp is a type of electrical lamp which produces light by means of an electric arc between Tungsten Electrodes The question is: what is the relationship between the light source's relative spectral power distribution and its correlated color temperature?

### Background

Judd's (r,g) diagram. In Color science, a spectral power distribution describes the power per unit area per unit wavelength of an illumination (radiant exitance or more generally the per-wavelength The concentric curves indicate the loci of constant purity. In Colorimetry and Color theory, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity
Judd's Maxwell triangle. Planckian locus in red. Translating from trilinear co-ordinates into Cartesian co-ordinates leads to the next diagram.
Judd's uniform chromaticity space (UCS), with the Planckian locus and the isotherms from 1000 K to 10000 K, perpendicular to the locus. Judd calculated the isotherms in this space before translating them back into the (x,y) chromaticity space, as depicted in the diagram at the top of the article.
Close up of the Planckian locus in the CIE 1960 UCS, with the isotherms in mireds. Contracted from the term micro reciprocal degree, the mired is a unit of measurement used to express Color temperature. Note the even spacing of the isotherms when using the reciprocal temperature scale, and compare with the similar figure below. The even spacing of the isotherms on the locus implies that the mired scale is a better measure of perceptual color difference than the temperature scale.

The notion of using Planckian radiators as a yardstick to judge other light sources against is not a new one. [5] In 1923, writing about "grading of illuminants with reference to quality of color…the temperature of the source as an index of the quality of color", Priest essentially described CCT as we understand it today, going so far as to use the term apparent color temperature, and astutely recognized three cases:[6]

• "Those for which the spectral distribution of energy is identical with that given by the Planckian formula. "
• "Those for which the spectral distribution of energy is not identical with that given by the Planckian formula, but still is of such a form that the quality of the color evoked is the same as would be evoked by the energy from a Planckian radiator at the given color temperature. "
• "Those for which the spectral distribution of energy is such that the color can be matched only approximately by a stimulus of the Planckian form of spectral distribution. "

Several important developments occurred in 1931. In chronological order:

1. Davis published a paper on correlated color temperature (his term). Raymond Davis Jr ( October 14, 1914 &ndash May 31, 2006) was an American Chemist, Physicist, and Nobel Referring to the Planckian locus on the r-g diagram, he defined the CCT as the average of the primary component temperatures (RGB CCTs), using trilinear coordinates. In Color theory, the Planckian locus is generally the path or ''locus'' that the color of a Black body would take ina particular Color space In Geometry, the trilinear coordinates of a point relative to a given Triangle describe the relative distances from the three sides of the triangle [7]
2. The CIE announced the XYZ color space. In the study of the perception of Color, one of the first mathematically defined Color spaces was the CIE 1931 XYZ color space (also known as CIE 1931 color space
3. Judd published a paper on the nature of "least perceptible differences" with respect to chromatic stimuli. In Psychophysics, a just noticeable difference, customarily abbreviated with lowercase letters as jnd, is the smallest difference in a specified modality of sensory By empirical means he determined that the difference in sensation, which he termed ΔE for a "discriminatory step between colors…Empfindung" (German for sensation) was proportional to the distance of the colors on the chromaticity diagram. The difference or distance between two colors is a metric of interest in Color science. Referring to the (r,g) chromaticity diagram depicted aside, he hypothesized that:[8]
KΔE = | c1c2 | = max( | r1r2 | , | g1g2 | )

These developments paved the way for the development of new chromaticity spaces that are more suited to the estimation of correlated color temperatures and chromaticity differences. Bridging the concepts of color difference and color temperature, Priest made the observation that the eye is sensitive to constant differences in reciprocal temperature:[9]

A difference of one micro-reciprocal-degree (μrd) is fairly representative of the doubtfully perceptible difference under the most favorable conditions of observation. Contracted from the term micro reciprocal degree, the mired is a unit of measurement used to express Color temperature.

Priest proposed to use "the scale of temperature as a scale for arranging the chromaticities of the several illuminants in a serial order. "

Over the next few years, Judd published three more significant papers:

1. The first verified the findings of Priest,[6] Davis,[7] and Judd,[8] with a paper on sensitivity to change in color temperature. [10]
2. The second proposed a new chromaticity space, guided by a principle that has become the holy grail of color spaces: perceptual uniformity (chromaticity distance should be commensurate with perceptual difference). By means of a projective transformation, Judd found a more uniform chromaticity space (UCS) in which to find the CCT. A projective transformation is a transformation used in Projective geometry: it is the composition of a pair of Perspective projections It describes what Judd determined the nearest color temperature by simply finding the nearest point on the Planckian locus to the chromaticity of the stimulus on Maxwell's color triangle, depicted aside. In Color theory, the Planckian locus is generally the path or ''locus'' that the color of a Black body would take ina particular Color space James Clerk Maxwell (13 June 1831 &ndash 5 November 1879 was a Scottish mathematician and theoretical physicist. A color triangle is an arrangement of Colors within a Triangle, based on the additive combination of three Primary colors at its corners The transformation matrix he used to convert X,Y,Z tristimulus values to R,G,B coordinates was:[11]
$\begin{bmatrix} R \\ G \\ B \end{bmatrix} = \begin{bmatrix} 3.1956 & 2.4478 & -0.1434 \\ -2.5455 & 7.0492 & 0.9963 \\ 0.0000 & 0.0000 & 1.0000 \end{bmatrix} \begin{bmatrix} X \\ Y \\ Z \end{bmatrix}$. In Linear algebra, Linear transformations can be represented by matrices.
From this one can find these chromaticities:[12]
$u=\frac{0.4661x+0.1593y}{y-0.15735x+0.2424} \quad v=\frac{0.6581y}{y-0.15735x+0.2424}$
3. The third depicted the locus of the isothermal chromaticities on the CIE 1931 x,y chromaticity diagram. [13] Since the isothermal points formed normals on his UCS diagram, transformation back into the xy plane revealed them still to be lines, but no longer perpendicular to the locus.
MacAdam's "uniform chromaticity scale" diagram; a simplification of Judd's UCS.

### Calculation

Judd's idea of determining the nearest point to the Planckian locus on a uniform chromaticity space is current. In 1937, MacAdam suggested a "modified uniform chromaticity scale diagram", based on certain simplifying geometrical considerations:[14]

$u = \frac{4x}{-2x+12y+3}$
$v = \frac{6y}{-2x+12y+3}$

This (u,v) chromaticity space became the CIE 1960 color space, which is still used to calculate the CCT (even though MacAdam did not devise it with this purpose in mind). The CIE 1960 color space ("CIE 1960 UCS" variously expanded Uniform Color Space, Uniform Color Scale, Uniform Chromaticity Scale, Uniform [15]

Close up of the CIE 1960 UCS. The CIE 1960 color space ("CIE 1960 UCS" variously expanded Uniform Color Space, Uniform Color Scale, Uniform Chromaticity Scale, Uniform The isotherms are perpendicular to the Planckian locus, and are drawn to indicate the maximum distance from the locus that the CIE considers the correlated color temperature to be meaningful: $\Delta uv=\pm 0.05$

The distance from the locus (i. e. , degree of departure from a black body) is traditionally indicated in units of Δuv; positive for points above the locus. This concept of distance has evolved to become Delta E, which continues to be used today. The difference or distance between two colors is a metric of interest in Color science.

#### Robertson's method

Before the advent of powerful, personal computers, it was common to estimate the correlated color temperature by way of interpolation from look-up tables and charts. A personal computer ( PC) is any Computer whose original sales price size and capabilities make it useful for individuals and which is intended to be operated [16] The most famous such method is Robertson's,[17] who took advantage of the relatively even spacing of the mired scale (see above) to calculate the CCT Tc using linear interpolation of the isotherm's mired values:[18]

Computation of the CCT Tc corresponding to the chromaticity coordinate (uT,vT) in the CIE 1960 UCS. Linear interpolation is a method of Curve fitting using linear polynomials

$\frac{1}{T_c}=\frac{1}{T_i}+\frac{\theta_1}{\theta_1+\theta_2} \left( \frac{1}{T_{i+1}} - \frac{1}{T_i} \right)$

where Ti and Ti + 1 are the color temperatures of the look-up isotherms and i is chosen such that Ti < Tc < Ti + 1. (Furthermore, the test chromaticity lies between the only two adjacent lines for which di / di + 1 < 0. )

If the isotherms are tight enough, one can assume $\theta_1/\theta_2 \approx \sin \theta_1/\sin \theta_2$, leading to

$\frac{1}{T_c}=\frac{1}{T_i}+\frac{d_i}{d_i-d_{i+1}} \left( \frac{1}{T_{i+1}} - \frac{1}{T_i} \right)$

The distance of the test point to the i'th isotherm is given by

$d_i=\frac{ (v_T-v_i)-m_i (u_T-u_i) }{\sqrt {1+m_i^2}}$

where (ui,vi) is the chromaticity coordinate of the i'th isotherm on the Planckian locus and mi is the isotherm's slope. Slope is used to describe the steepness incline gradient or grade of a straight line. Since it is perpendicular to the locus, it follows that mi = − 1 / li where li is the slope of the locus at (ui,vi).

### Precautions

Although the CCT can be calculated for any chromaticity coordinate, the result is meaningful only if the light sources is nearly white. [19] The CIE recommends that "The concept of correlated color temperature should not be used if the chromaticity of the test source differs more than [$\Delta_{uv} = 5 \times 10^{-2}$] from the Planckian radiator. "[20] Beyond a certain value of Δuv, a chromaticity co-ordinate may be equidistant to two points on the locus, causing ambiguity in the CCT.

### Approximation

If a narrow range of color temperatures is considered—those encapsulating daylight being the most practical case—one can approximate the Planckian locus in order to calculate the CCT in terms of chromaticity coordinates. Following Kelly's observation that the isotherms intersect in the purple region near (x = 0. 325,y = 0. 154),[16] McCamy proposed this cubic approximation:[21]

CCT(x,y) = − 449n3 + 3525n2 − 6823. 3n + 5520. 33

where n = (xxe) / (yye) is the inverse slope line and (xe = 0. 3320,ye = 0. 1858) is the "epicenter"; quite close to the intersection point mentioned by Kelly. The maximum absolute error for color temperatures ranging from 2856 (illuminant A) to 6504 (D65) is under 2 K. CIE Standard Illuminant D65 (sometimes written D65) is a commonly-used Standard illuminant defined by the International Commission on Illumination

A more recent proposal, using exponential terms, considerably extends the applicable range by adding a second epicenter for high color temperatures:[22]

CCT(x,y) = A0 + A1exp( − n / t1) + A2exp( − n / t2) + A3exp( − n / t3)

where n is as before and the other constants are defined below:

3–50 kK50–800 kK
xe0. 33660. 3356
ye0. 17350. 1691
A0-949. 8631536284. 48953
A16253. 803380. 00228
t10. 921590. 07861
A228. 705995. 4535×10-36
t20. 200390. 01543
A30. 00004
t30. 07125

## Color Rendering Index

Main article: Color Rendering Index

The CIE Color Rendering Index (CRI) is a method to determine how well a light source's illumination of eight sample patches compares to the illumination provided by a reference source. The CIE color rendering index ( CRI) (or colour rendering index in British-style spelling sometimes called color rendition index) is a quantitative Established in 1913 and based in Vienna, Austria, the International Commission on Illumination (usually known as the CIE for its French name Commission The CIE color rendering index ( CRI) (or colour rendering index in British-style spelling sometimes called color rendition index) is a quantitative Cited together, the CRI and CCT give a numerical estimate of what reference (ideal) light source best approximates a particular artificial light, and what the difference is.

## Spectral power distribution

Here is an example of just how different an incandescent lamp's SPD graph is from that of a fluorescent lamp.

Light sources and illuminants may be characterized by their spectral power distribution (SPD). In Color science, a spectral power distribution describes the power per unit area per unit wavelength of an illumination (radiant exitance or more generally the per-wavelength The relative SPD curves provided by many manufacturers may have been produced using 10-nanometre (nm) increments or more on their spectroradiometer. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a Spectroradiometers are designed to measure the Spectral power distributions of Illuminants They operate almost like Spectrophotometers in the visible [23] The result is what would seem to be a smoother ("fuller spectrum") power distribution than the lamp actually has. Full-spectrum light is light that covers the Electromagnetic spectrum from Infrared through near- Ultraviolet, or all wavelengths that are useful to plant Owing to their spiky distribution, much finer increments are advisable for taking measurements of fluorescent lights, and this requires more expensive equipment.

## References

1. ^ Wallace Roberts Stevens (1951). Luminous efficacy is a property of Light sources which indicates what portion of the emitted Electromagnetic radiation is usable for human vision. Over-illumination is the presence of lighting intensity ( Illuminance) beyond that required for a specified activity Brightness temperature is the temperature at which a Blackbody in Thermal equilibrium with its surroundings would have to be in order to duplicate the observed Star The effective temperature of a Star is the temperature of a Black body with the same luminosity per surface area (\mathcal{F}_{Bol} Principles of Lighting. Constable.
2. ^ Color Temperature and Metal Halide Sources
3. ^ Fluorescent Light Can Be White (Osram)
4. ^ Borbély, Ákos; Sámson, Árpád; Schanda, János (December 2001). "The concept of correlated colour temperature revisited". Color Research & Application 26 (6): 450–457. doi:10.1002/col.1065. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
5. ^ Hyde, Edward P. (June 1911). "A New Determination of the Selective Radiation from Tantalum (abstract)". Physical Review (Series I) 32 (6): 632–633. The American Physical Society. doi:10.1103/PhysRevSeriesI.32.632. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. “This existence of a color match is a consequence of there being approximately the same energy distribution in the visible spectra. ”
6. ^ a b Priest, Irwin G. (1923). "The colorimetry and photometry of daylight ·and incandescent illuminants by the method of rotatory dispersion". JOSA 7 (12): 1175–1209. Josa is a municipality located in the province of Teruel, Aragon, Spain.The color temperature of a source is the temperature at which a Planckian radiator would emit radiant energy competent to evoke a color of the same quality as that evoked by the radiant energy from the source in question. The color temperature is not necessarily the same as the 'true temperature' of the source; but this circumstance has no significance whatever in the use of the color temperature as a means to the end of establishing a scale for the quality of the color of illuminants. For this purpose no knowledge of the temperature of the source nor indeed of its emissive properties is required. All that is involved in giving the color temperature of any illuminant is the affirmation that the color of the luminant is of the same quality as the color of a Planckian radiator at the given temperature. ”
7. ^ a b Davis, Raymond (1931). Raymond Davis Jr ( October 14, 1914 &ndash May 31, 2006) was an American Chemist, Physicist, and Nobel "A Correlated Color Temperature for Illuminants". National Bureau of Standards Journal of Research 7: 659–681. “The ideal correlated colour temperature of a light source is the absolute temperature at which the Planckian radiator emits radiant energy component to evoke a colour which, of all Planckian colours, most closely approximates the colour evoked by the source in question. ”
8. ^ a b Judd, Deane B. (1931). "Chromaticity sensibility to stimulus differences". JOSA 22 (2): 72–108. Josa is a municipality located in the province of Teruel, Aragon, Spain.
9. ^ Priest, Irwin G. (February 1933). "A proposed scale for use in specifying the chromaticity of incandescent illuminants and various phases of daylight". JOSA 23 (2). Josa is a municipality located in the province of Teruel, Aragon, Spain.
10. ^ Judd, Deane B. (January 1933). "Sensibility to Color-Temperature Change as a Function of Temperature". JOSA 23 (1). Josa is a municipality located in the province of Teruel, Aragon, Spain. “Regarding (Davis, 1931): This simpler statement of the spectral-centroid relation might have been deduced by combining two previous findings, one by Gibson (see footnote 10, p. 12) concerning a spectral-centroid relation between incident and transmitted light for daylight filters, the other by Langmuir and Orange (Trans. A. I. E. E. , 32, 1944–1946 (1913)) concerning a similar relation involving reciprocal temperature. The mathematical analysis on which this latter finding is based was given later by Foote, Mohler and Fairchild, J. Wash. Acad. Sci. 7, 545–549 (1917), and Gage, Trans. I. E. S. 16, 428–429 (1921) also called attention to this relation. ”
11. ^ Judd, Deane B. (January 1935). "A Maxwell Triangle Yielding Uniform Chromaticity Scales". JOSA 25 (1): 24–35. Josa is a municipality located in the province of Teruel, Aragon, Spain. “An important application of this coordinate system is its use in finding from any series of colors the one most resembling a neighboring color of the same brilliance, for example, the finding of the nearest color temperature for a neighboring non-Planckian stimulus. The method is to draw the shortest line from the point representing the non-Planckian stimulus to the Planckian locus. ”
12. ^ OSA Committee on Colorimetry (November 1944). "Quantitative data and methods for colorimetry". JOSA 34 (11): 633–688. Josa is a municipality located in the province of Teruel, Aragon, Spain.   (recommended reading)
13. ^ Judd, Deane B. (November 1936). "Estimation of Chromaticity Differences and Nearest Color Temperatures on the Standard 1931 I.C.I. Colorimetric Coordinate System". JOSA 26 (11): 421–426. Josa is a municipality located in the province of Teruel, Aragon, Spain.
14. ^ MacAdam, David L. (August 1937). "Projective transformations of I.C.I. color specifications". JOSA 27 (8): 294–299. Josa is a municipality located in the province of Teruel, Aragon, Spain.
15. ^ The CIE definition of correlated color temperature
16. ^ a b Kelly, Kenneth L. (August 1963). "Lines of Constant Correlated Color Temperature Based on MacAdam’s (u,v) Uniform Chromaticity Transformation of the CIE Diagram". JOSA 53 (8): 999–1002.
17. ^ Robertson, Alan R. (November 1968). "Computation of Correlated Color Temperature and Distribution Temperature". JOSA 58 (11): 1528–1535. Josa is a municipality located in the province of Teruel, Aragon, Spain.
18. ^ ANSI C implementation, Bruce Lindbloom
19. ^ Walter, Wolfgang (February 1992). "Determination of correlated color temperature based on a color-appearance model". Color Research & Application 17 (1): 24–30. doi:10.1002/col.5080170107. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. “The concept of correlated color temperature is only useful for lamps with chromaticity points close to the blackbody…”
20. ^ Schanda, János (2007). "3: CIE Colorimetry", Colorimetry: Understanding the CIE System. Wiley Interscience, 37–46. John Wiley & Sons Inc, also referred to as Wiley, is a global Publishing company that markets its products to professionals and consumers students and instructors DOI:10.1002/9780470175637.ch3. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. ISBN 978-0-470-04904-4.
21. ^ McCamy, Calvin S. (April 1992). "Correlated color temperature as an explicit function of chromaticity coordinates". Color Research & Application 17 (2): 142–144. doi:10.1002/col.5080170211. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.   plus erratum
22. ^ Hernández-Andrés, Javier; Lee, Raymond L. ; Romero,Javier (September 20 1999). "Calculating Correlated Color Temperatures Across the Entire Gamut of Daylight and Skylight Chromaticities". Applied Optics 38 (27): 5703–5709. doi:10.1364/AO.38.005703. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
23. ^ Gretag's SpectroLino and X-Rite's ColorMunki have an optical resolution of 10 nm.