Lumens, Lux and Watts – What is it all about?3rd July 2012
The introduction of LED lighting has LED to a variety of terms being used which for most is a source of confusion rather than illumination.
We will attempt here to understand the terms in a more meaningful way – so here goes.
A light source such as a bulb may be specified in terms of its LUMENS output. This is a measure of the total amount of light emitted by the bulb as interpreted by the human eye. A typical value for an LED replacement spotlight in a kitchen would be 200 lumens.
LUX is a measure of the amount Lumens falling on a certain amount of space as perceived by the human eye, generally 1 square metre of surface area. As light spreads out from a bulb its intensity will reduce with distance. So a lux value can tell you how many lumens are needed over a particular area, for instance at desk level in a room, 1 metre from the ground. 1000 lumens from a light bulb spread out of an area of 10 square metres produces a less intense light of 100 lux. The same light over an area of 1 square meter produces an intensity of 1000 lux.
The table below provides some recommended values for domestic lighting.
|Areas – activities||Type of Work||Recommended LUX – (Min. Lux)|
|Reception areas (living rooms)||(General)
(Localized: reading, writing, etc)
1000 – (500)
(Localized: sink, table)
500 – (250)
(Localized: mirrors, children beds)
200 – (70)
|Corridors – staircases– garage||(General)
500 – (250)
To compare different lights on the market very often lumens/watt is quoted. This tells you how much light is produced for every watt of mains power. A bulb producing 50 lumens/watt is more efficient than one producing 40 lumens/watt. Here are typical measures of efficiency for LED lighting, CFLs and filament type bulbs:
LED 90 Lm/W
CFL 50–70 Lm/W
Filament – Incandescent and halogen 10–20 Lm/W
It is important to note that LED lights are highly directional and they are designed with optical diffusers that spread the light in a cone. Other lighting types emit light in all directions and require reflectors to make them more efficient. LED light can therefore be highly efficient in its ability to brighten our lives.