Digging Deeper into Color Temperature

Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics,horticulture, and other fields. The color temperature of a light source is the temperature of an ideal black body radiator that radiates light of comparable hue to that of the light source.

In practice, color temperature is only meaningful for light sources that do in fact correspond somewhat closely to the radiation of some black body, i.e. those on a line from reddish/orange via yellow and more or less white to blueish white; it does not make sense to speak of the color temperature of e.g. a green or a purple light. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K.

Color temperatures over 5,000K are called cool colors (blueish white), while lower color temperatures (2,700–3,000 K) are called warm colors (yellowish white through red). This relation, however, is a psychological one in contrast to the physical relation implied by Wien’s displacement law, according to which the spectral peak is shifted towards shorter wavelengths (resulting in a more blueish white) for higher temperatures.

The color temperature of the electromagnetic radiation emitted from an ideal black body is defined as its surface temperature in kelvins. This permits the definition of a standard by which light sources are compared.

To the extent that a hot surface emits thermal radiation but is not an ideal black body radiator, the color temperature of the light is not the actual temperature of the surface. An incandescent lamp’s light is thermal radiation and the bulb approximates an ideal black body radiator, so its color temperature is essentially the temperature of the filament.

Many other light sources, such as fluorescent lamps, emit light primarily by processes other than thermal radiation. This means the emitted radiation does not follow the form of a black body spectrum. These sources are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black body radiator which to human color perception 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

The Sun closely approximates a black body radiator. The effective temperature, defined by the total radiative power per square unit, is about 5,780 K. The color temperature of sunlight above the atmosphere is about 5,900 K.

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 scattering of light, and is not due to changes in black body radiation. The blue color of the sky is caused by Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red light.

The color temperature of direct sunlight is greatly affected by the atmosphere, position in the sky, and the lattitude of the observers. Values can range from  2800K-3200K at Sunrise/Sunset, 5600K at midday, and 6500K on an overcast day.

History of Color Temperature

Planck and the beginnings of Quantum Theory:

http://youtu.be/i1TVZIBj7UA

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

Color Rendering Index

The color rendering index (CRI), sometimes called color rendition index, is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such asphotography and cinematography.  It is defined by the International Commission on Illumination as follows:

Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant

History

Researchers use daylight as the benchmark to which to compare color rendering of electric lights. In 1948, Bouma described daylight as the ideal source of illumination for good color rendering because “it (daylight) displays (1) a great variety of colours, (2) makes it easy to distinguish slight shades of colour, and (3) the colours of objects around us obviously look natural.

Around the middle of the 20th century, color scientists took an interest in assessing the ability of artificial lights to accurately reproduce colors. European researchers attempted to describe illuminants by measuring the spectral power distribution(SPD) in “representative” spectral bands, whereas their North American counterparts studied the colorimetric effect of the illuminants on reference objects.

The CIE assembled a committee to study the matter and accepted the proposal to use the latter approach of using reference object.  Eight samples of varying hue would be alternately lit with two illuminants, and the color appearance compared. Since no color appearance model existed at the time, it was decided to base the evaluation on color differences in a suitable color space, CIEUVW. In 1931, the CIE adopted the first formal system of colorimetry, which is based the trichromatic nature of the human visual system. CRI is based upon this system of colorimetry.

To deal with the problem of having to compare light sources of different correlated color temperatures (CCT), the CIE settled on using a reference black body with the same color temperature for lamps with a CCT of under 5000 K, or a phase of CIE standard illuminant D (daylight) otherwise.

Testing Method

 Determining CRI is done by comparing the appearance of a number of known color swatches under the light in question to their appearance under either daylight (if the correlated color temperature is above 5000K) or the equivalent light produced by a black body (if the correlated color temperature is below 5000K). The values are derived using equations developed by the CIE.

 Most Recent Color Swatch comparison

TCS01* TCS02* TCS03* TCS04* TCS05* TCS06* TCS07* TCS08* TCS09* TCS10*
L* 40.9 61.1 81.6 72.0 55.7 51.7 30.0 51.0 68.7 63.9
a* 51.0 28.8 ?4.2 ?29.4 ?43.4 ?26.4 23.2 47.3 14.2 11.7
b* 26.3 57.9 80.3 58.9 35.6 ?24.6 ?49.6 ?13.8 17.4 17.3

This method does not take into consideration the spectral output of the lighting fixture itself.

Criticism of CRI

Ohno (2006) and others have criticized CRI for not always correlating well with subjective color rendering quality in practice, particularly for light sources with spiky emission spectra such as fluorescent lamps or white LEDs.

 Another problem is that the CRI is discontinuous at 5000 K, because the chromaticity of the reference moves from the Planckian locus to the CIE daylight locus. Davis & Ohno (2006) identify several other issues, which they address in their Color Quality Scale (CQS):

  • Calculating the arithmetic mean of the errors diminishes the contribution of any single large deviation. Two light sources with similar CRI may perform significantly differently if one has a particularly low special CRI in a spectral band that is important for the application. Use theroot mean square deviation instead.

  • The metric is not perceptual; all errors are equally weighted, whereas humans favor certain errors over others. A color can be more saturated or less saturated without a change in the numerical value of ?Ei, while in general a saturated color is experienced as being more attractive.

  • The CRI can not be calculated for light sources that do not have a CCT (non-white light).

  • Eight samples are not enough since manufacturers can optimize the emission spectra of their lamps to reproduce them faithfully, but otherwise perform poorly. Use more samples (they suggest fifteen for CQS).

  • The samples are not saturated enough to pose difficulty for reproduction.

  • CRI merely measures the faithfulness of any illuminant to an ideal source with the same CCT, but the ideal source itself may not render colors well if it has an extreme color temperature, due to a lack of energy at either short or long wavelengths (i.e., it may be excessively blue or red). Weight the result by the ratio of the gamut area of the polygon formed by the fifteen samples in CIELAB for 6500 K to the gamut area for the test source. 6500 K is chosen for reference since it has a relatively even distribution of energy over the visible spectrum and hence high gamut area. This normalizes the multiplication factor.


Quizzes

Digging Deeper into Color Temperature Quiz