--- wouter meeussen <wouter.meeussen@pandora.be> wrote:
Bill,
how many photons in a (cd s)?
How to deal with the proper units such as : candela (cd) as photons/s [generated by the whole source over 4 pi sr] and lumen (lm) as photons/s [as captured by the receiver's solid angle of observation (sr)]
Optical radiation measurements are based on three distinct systems of notation. The radiometric system is based upon true power, whose basic unit is the watt. The quantum system is based upon the number of quanta, the corresponding unit is photons per second. The quantum system of measurement is useful in dealing with detectors such as photovoltaic, photoconductive, or photoemissive, whose output is proportional to the number of absorbed photons. As long as the photon is sufficient to raise an electron through the energy gap, there will be a response, and the response doesn't depend on how much excess energy the photon provides. The quantum and radiometric units are related by Planck's law, that the energy of a photon of frequency f is hf, h=6.6e-34 Joule second being Planck's constant. The photometric system applies only to visible light and is a measure of visual brightness according to an international standard human. The basic unit is the lumen. At the peak of visual sensitivity of 555 nm wavelength, the conversion is 1 watt = 683 lumens. Corresponding to the radiometric unit of irradiance, watt per square meter, is the photometric unit of illuminance (for an illuminated object) or of luminance (for an emissive object), the lumen per square meter, or lux. Corresponding to the radiometric unit of intensity, watt per steradian, is the photometric unit of lumenous intensity, the lumen per steradian, or candela. __________________________________ Do you Yahoo!? Exclusive Video Premiere - Britney Spears http://launch.yahoo.com/promos/britneyspears/