Illuminance

Frequently asked questions

What is the difference between lumens and lux?

Where you are standing. LUMENS measure the total light a source emits in every direction — a property of the lamp, printed on the box, and unchanged by what you do with it. LUX measures how much of that light lands on a square metre of surface, which depends on the lamp's output, how far away it is, how it is aimed, how much the walls and ceiling bounce back, and how many fittings there are. One lumen spread over one square metre is one lux; the same lumen spread over ten square metres is a tenth of that. So a lamp cannot have a lux rating and a room cannot have a lumen requirement, and the common domestic question — how many lumens do I need — has no answer without the room's dimensions and surfaces. It also explains why two rooms lit with identical fittings can be very different to work in: a dark ceiling and dark walls absorb the light that a pale room would return to the working plane.

Why is the design figure lower than the level on day one?

Because the specification is a promise about the WORST day of the cycle rather than the best. Light output falls over time: lamps depreciate, dirt accumulates on the fitting and on the room's surfaces, and in a dusty or smoky environment it falls faster. Standards therefore specify a MAINTAINED illuminance — the level that must still be there immediately before cleaning and relamping — and the design calculation divides the target by a maintenance factor to work out how much light must be installed. The factor bundles several separate depreciations and is where optimistic design hides: assuming a clean office when the room is a workshop, or a cleaning interval nobody will keep, produces a scheme that meets its number for a year. It also means a newly commissioned installation should measure noticeably ABOVE its design figure, and one that measures exactly at target on handover will be below it within months.

Is 10 a safe conversion between lux and foot-candles?

It is a safe way to sanity-check a number and an unsafe way to specify one. The exact factor is the number of square feet in a square metre, about 10.764, so using ten understates lux by roughly seven percent when converting from foot-candles and overstates foot-candles by the same when going the other way. For a rough read — is this a 300 lux office or a 30 lux corridor — that is immaterial. It stops being immaterial where a figure is a compliance threshold, where a contract specifies a level to be measured on handover, or where the error is applied in the direction that loses light: a 50 foot-candle target converted at ten gives 500 lux where the honest figure is 538, and an installation designed to the lower number can fail a measured check it would otherwise pass. The safe habit is the same one that applies to gauge and to fall on this site: carry the unit with the number, and convert with the real factor rather than the memorable one.