According to BRE’s The essential guide to retail lighting:
Illuminance is ‘The level of light on a surface; measured in lux (lx). Previously called the illumination value. It can be used as a reference measurement of the performance of a lighting system as related to the activity.'
It is measured in lux (lx), the ‘...SI unit (the International System of Units) of illuminance or amount of light on a surface. One lux is equal to one lumen per square metre (lm/m^2)’ where a lumen (lm) is the ‘...SI unit of luminous flux, describing the quantity of light emitted by a lamp or received at a surface.’
Average illuminance is ‘The illuminance averaged over a specific area. In practice, this may be derived either from an average of the illuminances at a representative number fo points on the surface, or from the total luminous flux falling on the surface divided by the total area of the surface’
Maintained illuminance is ‘Illuminance at the time when maintenance is expected to take place. Most values of illuminance that are quoted as applicable to a store, eg 500-1000 lx, refer to this value. The time taken to reach the maintained illuminance level would depend on the lamp types and application’.
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