One of these days I will get a Light 101 post up here somewhere, until there, here are some links dealing Radiometry and Photometry.
http://physics.nist.gov/cuu/Units/units.html US National Institute of Standards and Technology (NIST) pages on the Internation System of Units (SI). The units in which we are interested (at least in this post) are the watt, candela, lumen, lux, and watt/meter^2
A National Measurement System for Radiometry, Photometry, and Pyrometry Based upon Absolute Detectors (nist.gov site)
Radiometry and photometry FAQ http://en.wikipedia.org/wiki/Photometry_%28optics%29 http://en.wikipedia.org/wiki/Candela http://en.wikipedia.org/wiki/Lumen_%28unit%29
http://en.wikipedia.org/wiki/Luminosity_function http://en.wikipedia.org/wiki/Chlorophyll (compare absorption spectra with luminosity function, above)
http://en.wikipedia.org/wiki/Foot-candle (caveat: horticulturists do not discuss artificial light for plants in terms of foot-candles).
http://en.wikipedia.org/wiki/Inverse_square_law http://en.wikipedia.org/wiki/Lambert%27s_cosine_law
Hyperphysics Light and Vision entry point:
http://hyperphysics.phy-astr.gsu.edu...ligcon.html#c1 Black Body Radiation Applet http://en.wikipedia.org/wiki/Blackbody http://en.wikipedia.org/wiki/Color_t...or_temperature
All gas discharge lamps, including both fluorescent and HID, emit line spectra as illustrated
http://en.wikipedia.org/wiki/Fluores..._emitted_light How are Spectra Produced? Fluorescents and the Kelvin Myth: Changing the Calibration for Color
Edit: Almost forgot PAR (radiance measured in watts, irradiance measured in watts per square meter) and Photosynthetic Photon Flux Density (measured in microEinsteins)
http://en.wikipedia.org/wiki/Photosy...tive_radiation http://en.wikipedia.org/wiki/MicroEinstein Watts, Lumens, Photons and Lux