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Re: Light Meters

"Wayne Jones" <waj at mnsi_net> wrote:

> I just had a thought that if I bought that cheap light meter that George
> recomends maybe I could take measurements with the sensor both flat and
> vertical and then add them together. Would that be similar to having a
> cosine corrected light meter? If it is I think I will order one. I don't
> have even a small portion of your mathematical abilities and for me trying
> to do the math is just not any fun at all.

Ivo wrote:
I think so, if you average the readings instead of adding them together.
From what I could see with my crude measurements, you shouldn't be too
picky with the meter. Light levels change a lot from place to place in
the tank (in mine at least...) so you can get nly approximate or avergae
values anyway.

I reply:

Averaging the the readings won't do it. The actual amount of light would
always be higher than the highest reading obtained with a flat detector. For
a flat detector the amount of light the detector is able to measure is
proportional to the cosine of the angle of incidence of the light.
Unfortunately, I now see that rotating the dectector 90 degrees and adding
the readings together won't work either. This would give too high a reading.
For example if I was measuring a ray of light where the angle of incidence
were 60 degrees I would only read 50% of the light. After rotating the
detector 90 degrees the angle of incidence would be 30 degrees so I would
read 86% of the light. 50% plus 86% is more than 100% so that wouldn't work

Wayne wrote:

> BTW I am now putting together a 6 lamp fixture which has individual
> parabolic reflectors for each lamp. I think that the main problem with
> fixtures I have made is that the light in the tank is not evenly
> distributed. The top of the tank is much better lit than the bottom of the
> tank and this is probably true for a lot of other tanks. For instance, I
> don't think that many lawn type plants are particularily light demanding.
> They just happen to be placed at the bottom of the tank where there is not
> lot of light. I am hoping that a better reflector design will reduce the
> drop in light intensity at the bottom of the tank.

Ivo wrote:

What is this reflector that you can fit 6 lamps over the tank ? The only
ones I found so far that are narrow (4") are the AHS reflectors, but even
with then its hard to cram 6 over a 24" wide tank.

I reply:

The type of hood I have allows me to fit a 23" by 48" by 5" fixture. I just
divided the with by 6 and made the reflectors 3 13/16" wide. My problem is
that I don't have the means to test the fixture. I have two other fixtures I
built, one with a common reflector and one with 2 reflectors per lamp. All
the reflectors are more or less parabolic. I make them by bending a sheet of
aluminized mylar around the inside of an aluminum frame. They cost next to
nothing to make but I would really like to know if it is worth the effort.
Maybe the individual reflectors are too small compared to the lamp diameter.
I do know that they are pretty good at directing light downward. If you
shine them on the wall they will produce a distinct band of light at a
distance of 2' but without a proper light meter I don't think I am going to
be able determine if the reflectors represent a significant improvement.

Wayne wrote:

> I would also like to point out that while DIY lighting is a lot of fun for
> me, I don't think I will ever be able to build a better lighting system
> the ones that you can buy from AH Supply. The more I learn about lighting
> the more I come to appreciate what a great system they sell. I just can't
> seem to stop myself from trying to improve my setup though. Now maybe if I
> can just get a hold of some of those linear HO T5 lamps, I could go with 4
> individual reflectors, a 5' tank and ......

Ivo wrote:
Agreed 100% on the AHS system. Hard to beat by any measure. As for the HO
I did some research and gave up on using them. I quoted prices and
of the Sylvania Pentron HO system. Lamps cost $14 apiece, but the 2-lamp
ballast retails for $95 ! If you can find these ballasts at retail to begin
with. I contacted a number of distributors and none carry then on stock.
can only special order them in cases of 20.

I reply:

I think the ballasts for those lamps are the same as the ones used for 55
watt PCs. They are made by Philips/Advance or Sylvania. I never priced them
as a separate item myself but I would think that someone sells them. AH
Supply would probably sell you one of theirs. The problem is the Advance
ballast factor is fairly low so they only provide about 25% more light than
a 32 watt T8 on an HLO ballast. So I guess I would still need at least 5
lamps.The Sylvania ballast is much the better bet as it has a 100% ballast
factor which would allow me to reduce to 4 lamps. I think I will go and see
what my supplier has to say about prices and availability. Sometimes they
seem almost anxious to sell me some oddball ballast.