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Re: Light Meters

"Wayne Jones" <waj at mnsi_net> wrote:

> 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.

But the detectors we are ultimately interested in, the plant's leaves, are 
not cosine-corrected. They are, on first approximation, flat surfaces, that
can orient themselves at random angles. So taking a set of readings with a
flat detector oriented at random angles and averaging them, gives IMO a good
estimate of what level of ligth the plants actually "see". 

> 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.

Ah ! Thanks for the explanation. I'm trying to avoid going into the reflector
manufacturing business <g> so I didn't consider this option. As for the
ligth enhnacing effect, IME if you can see a distinct bright band of light, 
the reflectors are doing a good job. Our eyes are pretty bad photometers, in
the sense that they respond logarithmicaly and non-linearly to changes in
ligth level. A small perceived effect is often associated with a quite large
change in actual ligth level. I remember seeing on a web site of a commercial
light fixture manufacturer an explanation on how they exploit these non-linear
effects when designing energy-saving ligth fixtures. They put out less light
than a regular fixture, but with careful exploitation of the non-linearities,
users don't notice.

> 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.

Yes, they seem to be the same. But as I said, the Sylvania model is way too
expensive, and I didn't consider the Advance because of the low ballast 

- Ivo Busko
  Baltimore, MD