Scottish Government

lumen

-A unit of light (luminous flux) emitted from a point source of one candela intensity, usually expressed in kilolumens - (kLm).

candela

A unit of luminous intensity radiated in a particular direction.

illuminance

-The quantity of luminous flux incident upon a unit area, expressed as lumens per square metre or lux.

luminance

-The luminous intensity (or brightness) of a surface or source expressed in terms of surface area, i.e. candelas per square metre.

reflectance

The reflection factor (or index) of a surface or material.

inter-reflection

The result of various reflections.

efficacy

-In lighting terms - the value of light obtained per unit of electrical energy input, i.e. lumens per watt.

wattage

-The nominal load rating of a lamp (excludes any allowances for associated operating gear losses).

luminaire

-The total package of lantern, lamp and all associated integral items of operating control gear.

projector

-A special luminaire designed to provide a concentrated pattern of light.

skylight

-The variable brightness value of daytime sky caused by sunlight scattered by particles of dust and vapour in the earth's atmosphere (skylight can reach values in excess of 2000 candelas per square metre).

moonlight

-The luminous flux emitted by the moon received at the earth's surface at an average value of between 0.2 and 0.4 lux (a rural surface under moonlight conditions will have an average brightness of about 0.002 candelas per square metre i.e. 1/500 cd/m ²).

sky glow

-The variable brightness value of night-time sky caused by upward components of light from direct and inter-reflected light off the earth's surface (the brightness of sky glow is dependent on the amount of upward light and the presence and density of atmospheric particles and their distance above ground level).

aura

-The hemisphere of light rising up from ground level encircling a light source or lighting array caused by low level mist and fog particles.