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RE: Light meters

<< Photographic ligth meters trade range for resolution. They are capable of
 measuring very brigth and very dim ligth, but cannot tell the difference
 between ligth levels that are too close apart. A difference of a 1/4 f/stop
 corresponds to a factor about 20%, and its' hard to get from a regular
 photographic meter. I believe meters designed specifically for aquarium
 use would work on a narrower range of illumination, but would provide
 resolution. >>

<<< Yes Ivo and they are called One Degree digital spotmeters. I've been
one for years with my large format work and  for double checking the
calibration of the metering of my F4 and  other systems. Not cheap these
days. A  decent one will cost anywhere from $200.00 on up. ANd a good one
will meter such a small increment as you cited  but B & W film latitude is
rather forgiving. Color Transparency film is not. Define what you mean by
resolution please. In traditional terms,  resolution is defined by resolving
lines or whatever per X. Then there's actutance. As far as defining a one
point difference, a good digital spot meter will do that easily. remember we
can be talking about one inch at 100 yards. If you can hold the meter
sufficiently steady you can read the shadow values, or highlight values and
adjust your expose so that the desired reading falls where you want it on
curve of the film. >>>

I think that David might have misunderstood Ivo's comments. I have used many
types of photographic light meters over the years and even the newer digital
spot meters are poor at measuring the intensity differences we want to
measure as aquarists (they are great at measuring exposure times for film
though). Most "digital spot meters" still (usually) only give readings in
1/3 of an f-stop increments (the finest increment that you can manually
adjust most lenses). The "resolution" David refers to sounds more like the
definition used for the resolving power of a lens or a piece of photographic
film. That isn't applicable here.

The closest photographic equivalent which might be useful would be a digital
Color Temperature meter (Minolta makes a very good one), but they are
expensive (they can be rented by the day in most major cities). Using one of
these, you could get an idea of how much light of different wavelengths
(colors) is being emitted by your light source. But I still think that you
would only get a close approximation, not a bang on exact amount (if that
would really matter).

The "angle of view" of the metering cell in a light meter might be important
to a photograher, but it isn't (or shouldn't be) important to an aquarist or
the plants we want to grow. A plant doesn't care _where_ the light is coming
from, it just wants the photons.

Finally, we are more concerned with PAR and I've yet to see a photographic
light meter that is capable of measuring that.

James Purchase