Light Measurement Glossary

Outlined below are some of the most widely known light measurement metrics utilized to characterize light.

Spectral Power Distribution (SPD)

The shape of the spectrum measured from a light source

The power per unit area per unit wavelength of a light source

Lux (lx)

The brightness of a light as perceived by humans

The intensity of light reaching a surface as it would be perceived by humans (in relation to the luminosity function). 1 lux = 1 lumen/square metre

Correlated Colour Temperature (CCT)

The inverse of a conventional description of “warmth” of a light – high CCT is white/blue, low CCT is yellow/orange

“The temperature of a 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)

Image showing the planckian locus and correlated colour temperature lines (CCT)

Planckian locus and lines associated with various colour temperatures

E.g.  The CCT of a light is the temperature corresponding to the point on the Planckian locus closest to the light’s CIE xy coordinates.

Colour Rendering Index (CRI)

A quantitative measure of how a light source affects colour of objects compared to an ideal or natural light source

CRI is calculated from the difference in chromaticity of eight standard colour samples when illuminated by the light source to be tested and a reference light source with the same CCT. The more similar the colours are under each light source, the higher the CRI, with a maximum value of 100 if the object looks identical to how it does under the ideal light source.

R1-R8

Difference in chromaticities from each of the 8 standard colour samples

Ra

Average of R1-R8

R9-R15

Difference in chromaticities from each of the 7 additional (more saturated) colour samples

IES TM30-15 (TM30)

A newer quantitative measure of how much a light source affects the colour of objects compared to an ideal or natural light source

TM30 is a newer colour rendering measure, developed by the IES. It is calculated by comparing to 99 colour samples (rather than CRI’s 8). The colour samples are chosen to represent real world objects such as skin colour, plants, textiles etc.

Fidelity Index (Rf)

A measure of the chromaticity difference, similar to CRI’s Ra but the average of 99 different colours. A value between 0 and 100 is possible.

Gamut Index (Rg)

A comparison of the chroma of a light source to that of daylight. E.g. A measure of the intensity/saturation of colours under a particular light source. Below 100 indicates a decrease in saturation compared to sunlight, above 100 indicates an increase.

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