I've used an instrument to measure the power from a Xenon lamp that comes
down a piece of fiber optic cable. The instrument has a black disk with a
slot cut around it maybe 3/4 of the way out. It is joined to the center of
he disk by several "spokes" of the same material. There's a thermocouple
across each of the spokes. They are put in series and drive a meter. The
thermocouple voltage is proportional to the difference in temperature
between the hot and cold junctions, and that difference in temperature is
proportional to the power delivered to the center of the disk (and
inversely proportional to the thermal resistance across which we are
measuring the temperature difference). This was a dental curing light (to
cure composite fillings in teeth). The xenon lamp went through a filter
that passed only blue light, so with a 300W lamp, we got about 2W of light
out the end. We got somewhere between 1 and 2 watts with a blue LED driven
with 10W.

This, of course, is a measure of broad band power. Often, though, we are
interested in knowing how bright the light looks, taking in to
consideration the varying sensitivity of the human eye to various
wavelengths. The most accurate method of measuring this that I know of is
to use a spectrometer to gather a power measurement every 10nm or so.
Multiply these by coefficients of a photopic filter that emphasizes the
wavelengths around green. Add the results. The instrument is typically
calibrated with a $1,000 light bulb (an "A-illuminant" as I recall). They
do this stuff where I work, but I don't directly work on it. So, this is
just what I've overheard.

Harold


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