Lighting correlated color temperature

Typical values of correlated color temperature (CCT) and color rendering index (CRl) for some common electric light sources. 

The International Commission on Illumination (CIE) International Commission on Illumination (CIE) has defined a set of standard illuminants to be used for colorimetry (International Commission on Illumination 1996). Figure 3.21 shows the CIE illuminants A, C, Z>5o, Z>55, Z>65, and D15. Illuminant A represents the power spectrum of light from a black-body radiator at approximately 2856 K. If this type of light is required for experiments, a gas-filled tungsten filament lamp that operates at a temperature of 2856 K is to be used. Illuminant Z>65 represents a phase of daylight with a correlated color temperature of approximately 6500 K. The CIE recommends to use this illuminant wherever possible. 

The actual color appearance of light that comes from a "black body" source is called its color temperature or chromaticity. Correlated color temperature (CCT) is the terminology used when referring to the color of an artificial source. Because the color temperature of an artificial source will most likely not fall on the normal black body curve, it is the accepted practice to refer to their temperature as the temperature of the closest CIE daylight (D) temperature value. Hence, a CCT of 6500 K means that its color temperature is closest to that of CIE D6500 daylight standard. 

Fluorescent lamps are rated in catalogs by two indices, their correlated color temperature (CCT) and color-rendering index (CRI). The CCT is the temperature of a black body whose chromaticity most nearly matches that of the subject light source. Because a fluorescent lamp only approximates a "black body," it is called correlated to distinguish it from actual. The CRI on the other hand is a subjective method developed by the CIE in which eight test colors are viewed under the test and a reference lamp(s) and its ability to reproduce the test colors numerically rated. Both numbers are not absolute but useful in selecting lamps for color rendering applications. 

The color of a light source is typacally characterized in terms of its color temperature. If the x,y coordinates of an illumination source do not exactly sit on the blackbody locus, the color of a light source is characterized in terms of its CCT. The CCT is the temperature of a blackbody radiator that has a colour that most closely matches the emission from a non-blackbody radiator. For high quality white light illumination the CCT should between 2500K and 6500 K. There is an accepted method (Wyszelki et al 1982) to determine lines of constant correlated color temperature in x, y space. CIE, CCT and CRI for common white light sources are given in Table 1 for comparison purpose (Misra et al 2006). 

Correlated color temperature Term used to describe the color of white light sources. Specifically, it is the temperature of the Planckian (black body) radiator, which produces the chromaticity most similar to that produced by the light source in question. The temperature is expressed in degrees on the absolute or Kelvin scale, or in mireds (micro-reciprocal degrees), 10 /T. See color temperature. 

Kelvin temperature scale An absolute temperature scale, in which the unit is the Kelvin (K), defined as 1/273.16 of the temperature difference between absolute zero (OK) and the triple point of water. K = °C -h 273. This scale is used to describe the correlated color temperature of light sources and illuminants in color designations and color rendition. Symbol K. See correlated color temperature and Kelvin. Kelvin is used extensively in gas laws and calculation for expansion of gases. Theoretically, all molecular motions cease at OK or absolute zero temperature. Paint pigment, drying oils, polymers, resins, naval stores, cellulosics esters, and ink vehicles, vol 3. American Society for Testing and Material, Conshohocken, PA, 2001. 

Artificial Daylight n Term loosely applied to light sources, frequently equipped with filters, which are claimed to reproduce the color and spectral distribution of daylight. A more specific definition of the light source is to be preferred. See Correlated Color Temperature,... 

Daylight da- llt n (13c) Natural illumination that is the result of various mixtures of direct sunlight covering a wide range of correlated color temperatures. For precision in definitions, see North Light, Artificial Daylight, and Daylight Illuminants. 

Light Source An object which emits light or radiant energy to which the human eye is sensitive. The emission of a light source can be described by the relative amount of energy emitted at each wavelength in the visible spectrum, thus defining the source as an illumi-nant, or the emission may be described in terms of its correlated color temperature. (Johnson SF (2001)... 

History of light and colour measurement A science in the shadows. Taylor Francis, UK Saleh BEA, Teich MC (1991) Fundamentals of photonics. Wiley, New York) See llluminant and Correlated Color Temperature. 

The correlated color temperature (CCT). CCT is the temperature of a black-body radiator that emits the same color of the light sovuce [13]. In general, CCT values ranging from 2500 to 6500 K are required for lightings... 

Color Temperature - A measure of the quality of a light source by expressing the color appearance correlated with a black body. 

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