Nearly all photographic uses of light meters are ultimately geared towards
relative measurements of one kind or another. Nobody typically cares about the
_actual_ value for W/m^2 or Lux or what-not; just that, when the meter says to
set exposure for such-and-such a combination of aperture, shutter, and ISO that
the resulting photograph is what you expected.
I happen to, for the first time I can think of, have a use for getting a truly
accurate (within typical photographic margins of error, ideally less than 1/3
stop) absolute measurement of local solar radiance. It's for an esoteric
project that might or might not pan out for making colorimetrically-accurate
photographs of the Moon and, perhaps, other Solar System objects. The
measurement would get combined with a spectrographic measurement of the Sun and
the ASTM E-490-00 extra-atmospheric solar spectrum and all sorts of other math
and what-not to, amongst other things, eliminate the color cast caused by the
atmosphere.
Can anybody suggest some sort of reliable way to get an accurate calibration in
absolute units of either a typical Sekonic meter or an i1 Pro in ambient mode
or something like that? I'm thinking, ideally, of a way of creating a light
source of dependably-known illuminance that I can measure. An unfrosted
incandescent bulb, maybe? And some way of verifying that the wattage printed on
the bulb matches what it's doing? And the math to calculate the rest?
It's both a photographic exercise and an excuse for me to do some hands-on
basic physics, so I'd welcome something, for example, suited for an high school
or college physics lab...but I _do_ want the final result to be reasonably
reliably accurate to no worse than 1/3 stop.
Thanks!
b&
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