Color_temperature

The CIE 1931 x,y chromaticity space, also showing the chromaticities of black-body light sources of various temperatures, and lines of constant correlated color temperature

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. The temperature (in kelvin) 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.

Contents 1 Categorizing different lighting 1.1 The sun 2 Color temperature applications 2.1 Film photography 2.2 Desktop publishing 2.3 TV, video, and digital still cameras 2.4 Artistic application via control of color temperature 3 Correlated color temperature 3.1 Motivation 3.2 Background 3.3 Calculation 3.4 Precautions 3.5 Approximation 4 Color Rendering Index 5 Spectral power distribution 6 See also 7 References 8 Further reading 9 External links

Categorizing different lighting

Temperature	Source
1700 K	Match flame
1850 K	Candle flame
2800–3300 K	Incandescent light bulb
3350 K	Studio "CP" light
3400 K	Studio lamps, photofloods, etc.
4100 K	Moonlight, Xenon arc lamp
5000 K	Horizon daylight
5500–6000 K	Typical daylight, electronic flash
6500 K	Noon daylight
9300 K	CRT screen

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).Wallace Roberts Stevens (1951). Principles of Lighting. Constable.  For bodies other than ideal black bodies, the color temperature of the thermal radiation emitted from it may differ from its actual surface temperature. In an incandescent light bulb the light is of thermal origin and is very close to that of an ideal black-body radiator.

However, many other light sources, such as fluorescent lamps, do not primarily emit light because of the temperature of the source and the emitted radiation do not follow the form of a black-body spectrum, and are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black body radiator which in the perception of the human eye most closely matches the light from the lamp. 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 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. 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. This phenomenon has nothing to do with the properties of a black body.

Increasing hues of the Planckian locus.

The colors of 5000K and 6500K black bodies are close to the colors of the standard illuminants called respectively D50 and D65, which are used in professions involving color reproduction, such as photography and publishing.

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". 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. 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"^{[attribution needed]}. Notice that color temperature has only one degree of freedom, whereas white balance has two, R-Y and B-Y.

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.

Film is made for specific light sources (most commonly daylight film and tungsten film), and used properly, will create a neutral color print. Matching the sensitivity of the film to the color temperature of the light source is one way to balance color. 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.

Filters on a camera lens, or color gels over the light source(s) may also be used to correct color balance. 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.

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. 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.

Designations such as D50 are used to classify color temperatures of light tables and viewing booths. 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.

Digital cameras, web graphics, DVDs, etc. are normally designed for a 6500 K color temperature & indeed the sRGB standard stipulates (among other things) a 6500 K display whitepoint.

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 6500K. On many actual sets however, especially older and/or cheaper ones, 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. 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. 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). Cinematographers also work closely with set designers and lighting crews to achieve their 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 temperature in kelvin; bluer is described as "cooler" even though it corresponds to a higher-temperature blackbody.

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. 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 6000-7000K, are fitted with 3200K filters to emulate tungsten light. Fixtures with color mixing features or with multiple colors, (if including 3200K) 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.http://www.highend.com/support/training/colortemp.asp

Correlated color temperature

The correlated color temperature (T_cp) 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)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. 

Motivation

Black body radiators are the reference by which the whiteness of light sources is judged. 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. The question is: what is relationship between the light source\'s relative spectral power distribution and its correlated color temperature?

Background

The notion of using Planckian radiators as a yardstick to judge other light sources against is not a new one.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. “This existence of a color match is a consequence of there being approximately the same energy distribution in the visible spectra.”  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:Priest, Irwin G. (1923). "The colorimetry and photometry of daylight ·and incandescent illuminants by the method of rotatory dispersion". JOSA 7 (12): 1175-1209. “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.” 

• "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:

CIE (1931) xy chromaticity diagram including the Planckian locus, with temperatures in Kelvin indicated. Wavelengths of monochromatic light are shown in blue. The lines crossing the Planckian locus are lines of constant correlated color temperature.

1. Davis published a paper on correlated color temperature (his term). 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.Davis, Raymond (1931). "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.” 
2. The CIE announced the XYZ color space.
3. Judd published a paper on the nature of "least perceptible differences" with respect to chromatic stimuli. 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. Using the (r,g,b) trilinear coordinates of Maxwell\'s color triangle (an early chromaticity space), he hypothesized that:Judd, Deane B. (1931). "Chromaticity sensibility to stimulus differences". JOSA 22 (2): 72-108. 

$K\backslash Delta\; E=|\; c\_1\; -\; c\_2\; |=\backslash max(|r\_1-r\_2|,|g\_1-g\_2|)$

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: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). 

A difference of one micro-reciprocal-degree ($\backslash mu\; rd$) is fairly representative of the doubtfully perceptible difference under the most favorable conditions of observation.

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, Davis, and Judd, with a paper on sensitivity to change in color temperature.Judd, Deane B. (January 1933). "Sensibility to Color-Temperature Change as a Function of Temperature". JOSA 23 (1). “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.” 
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 color space (UCS) in which to find the CCT. Judd determined the nearest color temperature by simply finding the nearest point on the Planckian locus to the chromaticity of the stimulus. The transformation matrix he used to convert x,y,z trilinear coordinates to r,g,b coordinates was:Judd, Deane B. (January 1935). "A Maxwell Triangle Yielding Uniform Chromaticity Scales". JOSA 25 (1): 24-35. “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.” 
3. The third depicted the locus of the isothermal chromaticities on the CIE 1931 xy chromaticity diagram.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.  Since the isothermal points formed normals on the UCS diagram, transformation back into the xy plane revealed them still to be lines, but no longer perpendicular to the locus.

Calculation

Judd\'s idea of determining the nearest point to the Planckian on a uniform color space is current. In 1937, MacAdam suggested a "modified uniform chromaticity scale diagram", based on certain geometrical considerations:MacAdam, David L. (August 1937). "Projective transformations of I.C.I. color specifications". JOSA 27 (8): 294-299. 

$u\; =\; \backslash frac\{4x\}\{-2x+12y+3\}$

$v\; =\; \backslash frac\{6y\}\{-2x+12y+3\}$

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 definition of correlated color temperatureRobertson, Alan R. (November 1968). "Computation of Correlated Color Temperature and Distribution Temperature". JOSA 58 (11): 1528-1535. 

The distance from the locus (i.e., degree of departure from a black body) is traditionally indicated in units of $\backslash Delta\; uv$; positive for points above the locus. This concept of distance has evolved to become Delta E, which continues to be used today.

Precautions

Although the CCT can be calculated for any chromaticity coordinate, the result is meaningful only if the light sources is nearly white.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. “The concept of correlated color temperature is only useful for lamps with chromaticity points close to the blackbody…”  The CIE recommends that "The concept of correlated color temperature should not be used if the chromaticity of the test source differs more than [$\backslash Delta\_\{uv\}\; =\; 5\; \backslash times\; 10^\{-2\}$] from the Planckian radiator."Schanda, János (2007). "3: CIE Colorimetry", Colorimetry: Understanding the CIE System. Wiley Interscience, 37-46. ISBN 978-0-470-04904-4. 

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. McCamy uses a cubic spline: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. 

$CCT(x,y)=an^3\; +\; bn^2\; +\; cn\; +\; d$ (a,b,c,d: constant)

with $n=(x-x\_e)/(y-y\_e)$ as the inverse slope line and $(x\_e,y\_e)$ as the intersection point of isothermal lines for a particular spline ("epicenter").

A more recent proposal uses exponential terms, but a single epicenter:Hernández-Andrés, Javier; Lee, Raymond L.; Romero,Javier. "Calculating Correlated Color Temperatures Across the Entire Gamut of Daylight and Skylight Chromaticities". Applied Optics 38 (27): 5703-5709. 

$CCT(x,y)=A\_0+A\_1\; \backslash exp(-n/t\_1)+A\_2\; \backslash exp(-n/t\_2)+A\_3\; \backslash exp(-n/t\_3)$

where A and t are new constants, with n is as before.

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. 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 compared to a fluorescent lamp\'s.

Light sources and illuminants may be characterized by their spectral power distribution (SPD). The relative SPD curves provided by many manufacturers may have been produced using 10 nanometre increments or more on their spectroradiometer.Gretag\'s popular SpectroLino has a physical resolution of 10nm. The result is what would seem to be a smoother ("fuller spectrum") power distribution than the lamp actually has. Owing to their spiky distribution, much finer increments are advisable for taking measurements of fluorescent lights, and this requires more expensive equipment.

See also

• Luminous efficacy
• Over-illumination
• Brightness temperature, Effective temperature

References

Further reading

This article or section includes a list of references or external links, but its sources remain unclear because it lacks in-text citations. You can improve this article by introducing more precise citations.

• Berns, Roy S. (2000). Billmeyer and Saltzman\'s Principles of Color Technology, 3rd edition, New York: Wiley. ISBN 0-471-19459-X. 
• Stroebel, Leslie; John Compton; Ira Current; Richard Zakia (2000). Basic Photographic Materials and Processes, 2nd edition, Boston: Focal Press. ISBN 0-240-80405-8. 
• Wyszecki, Günther; W. S. Stiles (1982). Color Science Concept and Methods, Quantitative Data and Formulae. New York: Wiley. ISBN 0-471-02106-7. 

External links

• Charles Poynton\'s Color FAQ for the basics.
• Frequently asked questions about Color Physics - also includes history of the CIE color specification
• What color is a blackbody? - Colorimetrically-calculated blackbody RGB color values and comment
• Color-temperature: visualising blackbody radiation - Blackbody curves for typical lightsources, and colorimetrically-calculated blackbody color strip
• Specifying Color GE\'s "Learn About Light" pages

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