Illuminance, measured in lux, is the density of light landing on a surface. It is the primary number lighting designers size a room around, since too little strains the eyes and too much causes glare and fatigue.
Required lux varies sharply by task — a napping room needs near-darkness while a science lab needs strong, even illuminance for detailed work.
The Science
E = I / d² × cos(θ)
E = illuminance (lux), I = candela, d = distance from the source, θ = angle of incidence — drop the cosine term when the light hits straight-on.
Illuminance is what a surface actually receives, while candela describes how tightly a fixture focuses that output in one direction — the same total lumens can read very differently depending on beam spread. Because illuminance falls off with the square of the distance from a point source, small changes in mounting height change perceived brightness far faster than small changes in fixture output. The eye's own sensitivity also isn't flat across wavelengths — it peaks around 555nm, which is what the lux unit is weighted against.
Bright vertical light at the eye in the morning (roughly 250–500 lux) helps anchor circadian rhythm and daytime alertness; very low lux (50–100) forces the pupils to dilate, which reduces depth of field.
Adequate task lighting lowers cognitive effort and reads as secure and energising; low overall lux paired with targeted accents does the opposite on purpose — creating quieter, more private-feeling spaces.
See this parameter in action, across every stage and environment, in the full interactive simulator.
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