Concept

Radiometry — where it appears

Measuring light in physical units of power, with no observer in the calculation. Photometry is the same measurement weighted by one agreed curve, and the difference between the two is where every luminous quantity comes from.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Four standard illuminants, and how little they have in common. Spectral power distributions for A, D50, D65, E, on one scale. Illuminant A rises steeply toward the red; the daylight illuminants carry the atmosphere's absorption structure; E is flat by definition. All four are ordinarily called white.

A spectrum is not a colour

What arrives at the eye is a function of wavelength. What the eye reports is three numbers. Keeping the two apart is the single most useful habit in the subject, and almost every confusion in applied colour comes from letting them merge.

light · Light

Luminous efficacy of a monochromatic source, under the 2° observer. Lumens per watt against wavelength. The curve is the luminous efficiency function scaled by 683, which is what luminous efficacy of radiation is — the 1979 redefinition of the candela fixed 555 nm at exactly 683 lm/W and everything else follows. Marked at 450 nm (26 lm/W), 500 nm (221 lm/W), 555 nm (683 lm/W), 600 nm (431 lm/W), 650 nm (73 lm/W). Under this observer the peak reaches 683 lm/W.

What a lamp cannot give back

Six hundred and eighty-three lumens per watt is not a measurement of anything. It is the definition of the candela, it is a ceiling no white light can approach, and the distance between a lamp and that ceiling is mostly the price of being able to see what colour things are.

light · Light

Named alongside it

The objects these essays reach for when they reach for this one.

Illuminant Planck's law Colorimetry Colour rendering LED emission Luminous efficacy of radiation Luminance Luminous efficiency Reflectance Spectral power distribution Standard observer Thermal radiation

All concepts