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Re: Light Meters

"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
> use one in my tank without lifting the hood. The light fixtures are
> 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
> 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

Ivo wrote:

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
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.

Wayne wrote:

> of the tank. In addition, I have a feeling that if you put a flat object
> 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
> 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
> idea to get a cosine corrected light meter. The light from all angles
> be treated equally.

Ivo wrote:

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.

I reply:

I can appreciate that a rough surface will radiate a ray of light striking
it in all directions but I still think the shape of the object and the angle
you view it at matters. If you placed a flat object on the bottom of a tank
I think it's effective area for collecting low angle light would be would be
much smaller than for light shining perpendicular to it's surface. Low angle
light has a much smaller target to hit so a proportionately smaller amount
of light will be reflected. I think that is why light sensors are covered
with a hemispheric lenses for cosine correction if you want to measure light
at all angles.

The reason why I am concerned about this effect is that I suspect that a
non-cosine corrected sensor might make it falsely appear that a light
fixture with a parabolic specular reflector is much better than a fixture
that just uses flat white paint for a reflector. If a specular reflector
directs a higher proportion of light directly downward then I wonder if the
sensor would read unnaturally high in the case of hi-tech reflector. The
flat white reflector may produce a lot of low angle light at the bottom of
the tank but most of it will go unmeasured. In any case, I suppose that the
effect can't be all that significant as I doubt that there would be a lot of
light striking the object at low angles.

I think you could also obtain different readings for the same object
depending on the angle you view it at through the glass. Also the
orientation of the object seems to me should matter as well. Wouldn't a
vertical flat object collect less light than a than a horizontal object if
there was more light entering the tank at a high angle. I find it hard to
believe that changing the orientation of something flat would not change the
reading that you get. Changing the orientation of a sphere though obviously
wouldn't do anything.