Lux, Lumens, Watts

Lumens, Lux, Watts

Some LED lights can be up to three times more efficient than others so comparing them by the input Wattage does not work.  The EU now requires the light output in Lumens to be shown on packaging so that they can be compared accurately.

A LUMEN is a unit measuring the total amount of visible light being emitted by a source such as an LED or a filament.
A LUX measures the amount of light reaching a surface and it is equal to one lumen per square meter.  This can be measured by a light meter or a phone app.
Here are the recommended lux levels:
150 to 200 Lux     General lighting
300 to 500 Lux     Reading or hobbies
up to 750 Lux for finely detailed work - this really needs task lighting.

The light coming from an LED downlight is concentrated into a cone.   If the cone has a beam angle of 60 degrees, the edge of the cone has light level of 50% of the maximum.  This means that outside this cone the light levels fall rapidly from the 50% level.  The overspill allows several lights to merge without creating pools of light.
The narrower the cone the higher the Lux levels, similar to the way a torch can be very bright.

This all gets complicated so here are some rules of thumb:
  • An LED replacement for a 50 Watt halogen use an LED with 600 to 700 Lumen
  • An LED replacement for a 100 Watt downlight  use an LED with 900 to 1000 Lumen
  • If the ceiling is higher than 2.4 meters a more powerful light will be needed.