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Its not terribly difficult to directly measure the lumen output of a light. You
use a calibrated photodetector (lightmeter) with filtration in front of it.
The filtration is adjusted so the spectral (color) response of
the detector*filtration approximates the (standardized) response function
of the human eye. The combination detector/filter is exposed to the light's
output and the light's output is calculated from the detector reading. The
equipment required to construct and calibrate this system is readilly
available to professionals working with optics. I don't beleive it is
necessary to try to calculate the light's output (using reflector geometry
for instance), just measure it.

However, as Richard said, From a users point of view the angle of coverage
is important. Some meaningfull standard should be defined which would
measure the light's output at several specified angles. For instance, the
meter could be used to measure the light's output at 0, 10, 25, and 45 degrees.

As Philip mentioned, battery voltage is another factor which strongly
influences the light's output. Here again, a standard should be used to
compare different lights. Battery voltage itself doesn't have any meaning
to a user. A better measure is burn time. The lights output could be
measured at burn times of 0, 10, 30, 60, 120, 240, 480, ... minutes after
inserting fresh (or fully charged) batteries. To be even more realistic,
you would conduct the test with the light imersed in a water at a
standardized temperature water (pick your favorite dive temperature).

I think it would be very interesting to see dive light output measured
using controlled, calibrated procedures. I believe that you would see large
differences between the various lights.

Oliver

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