# What kind of units does the Cycles emission strength use?

I understand that the emission shader strength value is Watt/m^2. However, I haven't been able to find any documentation stating what type of light source this wattage is coming from.

Is it based on the wattage of a regular incandescent bulb? Halogen? Different light sources have different efficacy levels and a 15W LED can output the same luminous flux as a 95W tungsten incandescent bulb.

• – gandalf3 Sep 19 '16 at 1:50

The unit is Irradiance - radiant flux emitted by a surface per unit area (watt per square meter).

This is a radiometric "corresponding" unit to Illuminance (also known as Lux), which is a photometric unit measured in lumen per square meter.

Radiometric units are based on physical power, that means all wavelengths are weighted equally, while photometric units take into account the sensitivity of human eye to different wavelengths. The weighting is determined by the luminosity function (which was measured for human eye and is an agreed-upon standard).

1.0 W/m2 = 683.002 lumen/m2  # at wavelength = 555nm (green)