14Apr/104

When one talks about quantitative qualities of a light source, you generally subscribe to one of two schools of thought - Radiometric in SI units of Watts or Photometric in SI units of lumens, candelas or lux. In this post i am going to discuss these two different approaches.

Photometric
Photometric units are quasi-quantitative hand-wavey units; they are an attempt to quantify the qualitative aspects of a light source i.e. how bright a light source appears to the human eye. These units are useful for things like home lighting, vehicle headlights, or photography light meters.

A COTS Sylvania SoftWhite 100W incandescent light bulb advertises that it produces 122 candelas. If we were to convert that value into watts using the standard conversion factor of 1 lumen = 1/683 Watt (at 555nm as that is the accepted wavelength of peak sensitivity of the human eye), we get:

$122\left[cd\right]\cdot 4\pi\left[sr\right]=1530\left[cd\cdot sr\right]=1533\left[lm\right]\cdot\frac{1\left[W\right]}{683\left[lm\right]}=2.24\left[W\right]$

This tells us that a 100W incandescent bulb is only about 2.25% efficient in producing visible light and the rest of the power is wasted in producing NIR through LWIR (heat) photons.