The candela *is* photometrically related. Its definition is absolute only at one frequency (corresponding to ~555 nm in free space). For other frequencies, the luminous efficiency function is used. The CIE standard luminous efficiency function is photonic (daylight).
There are actually two of these. One, V(lambda) was adopted by the CIE in 1924 (based on data published by Gibson and Tyndall, 1923) and it is the function adopted by the CIPM. One particular, modified function was published in 1988, V_M(lambda), which has slightly higher values at the blue end, allowing for the response of young males below 410 nm. It was based on work by Judd (1951) and later modified by Vos (1978) I will have to check my notes, but I believe the 1924 curve was also based on the response of young human males, also. Whereas the first curve looks fairly gaussian, the latter looks like there was a mudslide on the left and that a flat, upsloping stretch of mud is at the bottom. There is also a scotopic function,Vprime(lambda) representing night vision. This looks similar to V(lambda) but it peaks closer to the blue end of the spectrum. It is based on measurements by Wald (1945) and by Crawford (1949) and was adopted by the CIE in 1951. I'll try to remember to post those curves on my college web page someday if you think it would be useful. By contrast to the candela per meter squared (cd/m2) [luminance], the unit watt per meter squared (W/m2) [energy density] is radiometric (based on energy) and not photometric (based on vision). Jim Jim Elwell wrote: > > Engineers who work with human-readable displays are familiar with the unit > "nit", which is one candela per square meter (cd/m^2). It is used to > specify how bright a display is (either directly for some types, or via > backlighting for others). For example, a 150-nit display will be readable > in roomlight, but washed out in sunlight, whereas a 600-nit display will be > readable in sunlight, but too bright for viewing in a darkened room. > > The unit "nit" is used for two reasons: displays are a distributed light > source rather than a point source (so lux is not appropriate), and, used > properly, it is a photonic unit, which means it takes into account the > response of the human eye. > > There clearly is no directly equivalent SI unit here, due to the photonic > nature. But I think engineers use it for another reason: it is a short, one > syllable word. It is a heck of a lot easier than saying "photonic-weighted > candela per square meter." > > I bring this all up to make two points: (a) SI does *not* cover all > necessary uses of measurement systems, so there are going to be new units > "invented," although we hope they are based on SI (as nit is) and (b) > people inherently like short words, which I think is one thing that turns > people off about the metric system (e.g., inch vs. centimeter). > > I doubt anything can be done about (b), but I bet that words like "klick" > (for kilometer) are not going to be eradicated even in all-metric countries. > > Jim Elwell -- Metric Methods(SM) "Don't be late to metricate!" James R. Frysinger, CAMS http://www.metricmethods.com/ 10 Captiva Row e-mail: [EMAIL PROTECTED] Charleston, SC 29407 phone/FAX: 843.225.6789
