Light level examples

Michael Brill and Bob Hunt agree that R, G, and B tristimulus values have no units. See Hunt, R.W.G. (1997),“The heights of the CIE colour-matching functions,” in Color Research and Application, 22 (5): 337 (Oct.).

Image science

Absolute luminance has units of cd·m-2, colloquially called nits [nt]. In image science, luminance is usually normalized to a range of 100 (or as I prefer, 1) with respect to a specified or implied white reference, and expressed without units. So normalized, its symbol is Y. The term luminance is often used as shorthand to refer to this pure quantity; however, it is properly called relative luminance. The term luminance factor should be avoided for this quantity, since the latter term refers to a property of a surface or material: Luminance factor is the ratio of luminance of a surface, under specified conditions of light source, incidence, and observation, to the luminance of a perfectly diffusing (“Lambertian”) surface, under the same conditions.

Relative luminance (Y) is one of three distinguished tristimulus values standardized by the CIE; the other two distinguished tristimulus values are X and Z. Other tristimulus values such as [R, G, B] are related to CIE

[X, Y, Z] values by a 3× 3 linear matrix product. Relative luminance (Y) and other tristimulus values such as X, Z, R, G, or B are pure numbers.

It was inconsistent mixture of US customary (Imperial!) units and SI units that led to the 1998 crash of NASA’s Mars Climate Orbiter. American cinema experts often express luminance in footlamberts; why then do they refer to cinema film as

35 mm instead of 13/8-inch?

Units

Many bizarre units have been used for illuminance and luminance. I urge you to abandon these, and to adopt the standard SI units. Radiometry and photometry are sufficiently difficult without having to deal with

a plethora of arcane units. If radiometry and photometry are new to you, I believe that your understanding will come more rapidly if you ignore the traditional units – which were deprecated by the scientific community 40 years ago – and adopt the SI units. If you are

a practitioner who learned the science and the craft using the traditional Imperial units, please don't stubbornly stick to them: According to the CIA World Factbook, only three countries – Burma, Liberia, and the United States – have not adopted International System of Units (SI, or metric system) as their official system of weights and measures. I urge you to use SI units. Simply learn to multiply footlamberts by 3.4 to get candelas per meter squared. A movie screen has a typical white luminance of about 14 fL; call this 48 nits. A studio reference display in North America typically has a reference white luminance of about 100 nits (about 33 fL).

To convert illuminance into lux [lx], use Table B.3:

Table B.3 Conversion of illuminance into lux

To convert luminance into candelas per meter squared, use Table B.4:

Table B.4 Conversion of luminance into cd·m–2