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Re: Light Meters

FocaIPoint at aol_com wrote:

> rather forgiving. Color Transparency film is not. Define what you mean by
> resolution please. In traditional terms,  resolution is defined by resolving

I meant the resolution of the measuring device. We are measuring ligth levels, 
not angles, so resolution is the ability to discern in between closely spaced
ligth levels.

"James Purchase" <jpurch at interlog_com> wrote:

> 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.

The angle of view is important when we use the white-target-in-the-tank
technique. The white surface must cover the entire angle of view of the
photometer. That's why I use a tele lens in my camera, which has a 
center-weighted photometer, not a spot one. Apart from that, the angle 
shouldn't be important.

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

If we know the factor that converts from lux to PAR, we can measure lux and
get PAR automatically. In http://www.aquabotanic.com/lightcompare.htm you can
get lumen/PAR factors for all listed bulbs. This would not account for 
differential water absorption with wavelength though, but the effect migth 
not be important if we are interested only in ballpark values.

"Wayne Jones" <waj at mnsi_net> wrote:

> I have a few problems with using a spot meter for measuring light
> intensities at the bottom of the tank. For one thing, I don't see how I can
> use one in my tank without lifting the hood. The light fixtures are attached
> to the hood and are very close to the water. The other problem is that the
> light that I would be measuring would have travelled from the lights through
> the water surface down to the object on the bottom of the tank and back up
> to the spot meter. That is twice far as from the lamp fixture to the bottom

I think David was suggesting to use the white reflector technique. The same
I used. In that case the ligth travels once from the ligths to the target 
down into the tank, then sidewise out from the front glass pane. Put the target
close to the front pane and you minimize the effect of absorption at the 
second leg. Or you can guesstimate the absorption and correct for the effect
at the second leg using a simple exponential decay law.

> of the tank. In addition, I have a feeling that if you put a flat object on
> the bottom of the tank it would not gather light from diffuse low angle
> light very well even though that type of light is perfectly usable to
> aquatic plants. Maybe if the object were a hemisphere and the spot meter and
> the hemisphere filled the spot meters field of view then you would get a
> correct reading for diffuse light. That is why I thought it might be a good
> idea to get a cosine corrected light meter. The light from all angles would
> be treated equally.

If the white object has a rough, non-glossy surface, it will act close to
a perfect Lambertian reflector, no matter what its shape is. That is, it 
will gather ligth from all directions and reflect back into all directions 
as well. But of course, a photometer head with a hemispheric integrating 
window placed at the very positions where the plants are, would be the 
ideal choice. It's just a matter of cost.

FocaIPoint at aol_com wrote:

> OK. I give up. <g> What in the WORLD is a difference in lighting less than
> 1/4 an f-stop mean in relation to aquarium lighting please ... in practical
> terms that is as it would affect plant growth in an otherwise *well light*
> aquarium?

I guess Wayne (as myself) is not so concerned with the actual ligth level
at the plants, but with how that ligth level changes when his reflector
setup changes. In this case, different types of reflectors may give 
approximately equal efficiencies, and it becomes important to tell apart
differences in the range of 10% or so. That's pretty hard to get from a
photographic photometer. If a reflector that costs twice gives a 5%
increase in ligth level, you want to assure yourself that the difference
you see in readings is cause by a real difference in ligth level, and not by
a lack of precision in your measuring apparatus. 

Sorry for the long post...

- Ivo Busko
  Baltimore, MD