Lumen (lm, unit of light output)

What a lumen is

A lumen is the unit used to measure how much light a source gives out. With incandescent bulbs we got used to talking about watts because they were all similar: more watts, more light. With LEDs that no longer works, because an LED bulb gives the same light using far less power. So today you compare by lumens.

Equivalents with old bulbs

An LED giving about 800 lm replaces a 60 W incandescent using only a fraction of the power. The ratio between light and consumption is called efficacy and is given in lumens per watt: the higher, the better.

Lumen and lux

The lumen measures the light leaving the bulb; the lux measures the light reaching a surface. One lux is one lumen spread over one square metre. The same 800 lm bulb gives many lux on a small nearby table and few if it lights a large living room from a high ceiling. To size a room, you start from the lux it needs and work out the lumens.

How many lumens each room needs

There is no single figure: it depends on the size of the room, the ceiling height, the colour of walls and floor and what you do there. A kitchen or study needs much more light than a bedroom or hallway. The rigorous way is a lighting calculation, for example with Dialux; a lighting study costs roughly 150 to 600 euros.

Who to ask

If you are switching to LED or fitting recessed spotlights, we work out the lumens room by room. See LED upgrade and lighting design in Seville, Spain.