Propagation 2011 - How to measure light
Lumens
For many years houses used filament light bulbs which were rated in watts e.g. a 40W bulb would be used in a bedroom. With the change to CFL bulbs, the Watts required is much less e.g. a 9W CFL is equivalent to a 40W incandescent bulb. However how much light is produced? A 9W CFL typically provides 450 Lumens. Given CFLs have electronics inside they are available in different 'colours'. A warm light is typically 3000K, whereas a daylight one is typically 6400K. So back to light basics to understand all this!
Lumens to lux
The first thing to understand is the difference between the light output from a source of light and the light received on an surface .
The light output from a source, radiant flux, is measured in Watts.
The measure of the power of light perceived by the human eye, adjusted to take into account the sensitivity of the eye to different wavelengths, is luminous flux - the SI unit of which is the lumen. If the light from our 9W CFL is concentrated into an area of 1 square metre, then it lights up that square metre with an illuminance of 450 lux. If it is used to light an area of 10sq m then the illuminance is 45 lux. The closer you are to a light, the brighter it appears.
The relationship between lumens and lux is:
lux = lumen / m2.
Sensitivity of the human eye
The average visual sensitivity of the human eye to light of different wavelengths is the luminosity function curve. The photopic curve is for everyday light levels. This is normalised to a peak at 555nm, seen as green to the eye, as seen in the picture below.
With it so far?
I now understand how the human eye responds to light differently to plants & chlorophyll activity. Given that lights are measured wrt human eye, I need to adjust the lumens of lamp light to light energy for plants. To get from one to the other, I need to understand another term - luminous efficacy. Hopefully this is where the mole links in.
