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Need to produce fake sunlight: power question

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Posted by Paul Ciszek on August 16, 2007, 4:57 pm
 
I need to produce fake sunlight over a small area--maybe a square 20cm
on a side--on a low budget.  I have been told that ELH projector bulbs
make a good first approximation to sunlight on the cheap.  I am trying
to make an estimate of how many of them I need to achieve sunlight
intesnsity.  I tried to work it out based on the following numbers:

Sunlight = 100,000 lumens/m^2
Halogen bulbs yield 16 to 24 lumens/W
That's 6250 to 4167 W to produce 1m^2 of sunlight
250 to 167 W to illuminate my 20cm square

That just seems awfully low; am I missing something?

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Posted by R.H. Allen on August 16, 2007, 6:27 pm
 
Paul Ciszek wrote:

The *numbers* seem fine to me, but they do suggest that you can get away
with using a single ELH lamp (which runs about 300 W). That would be
fine if you were covering a much smaller area -- many laboratory solar
cell I-V measurement setups do exactly that for square-ish cells smaller
than about 40-50 cm^2 -- but your 400 cm^2 area is far too large for a
single lamp. Even if it gives you the right number of photons, the
inverse-square law will ensure they don't fall uniformly over your area
(at least not until the lamp is so far away that your surface is only
dimly lit).

For 100 cm^2 solar cells, most low-cost I-V testers I've seen use 4 ELH
lamps arranged in a square pattern. You might have to fiddle with exact
positioning a bit to get good uniformity, and for a 20 x 20 cm area you
might need more than 4 lamps. Figuring out exactly how many you need
will depend on exactly how you set things up and might require some
trial-and-error. Using a diffuser in front of the lamps will probably
increase uniformity and reduce (or eliminate) the amount of fiddling you
need to do. You might also consider boxing in as many sides of the test
area as possible, using walls that are diffusely reflective (e.g., a
rough surface painted white).

If your light is too bright or not bright enough, there are two things
you can do about it: Adjust the voltage to the lamps, or adjust the
height of the lamps above the surface. Strictly speaking, the latter is
the "correct" thing to do since changing the voltage changes the
filament temperature and can mess with the lamp's spectrum. However,
adjusting the voltage is generally a lot cheaper and easier than rigging
up a height adjustment system, particularly if you want to be able to
adjust it while the lamps are hot. Every single ELH-based I-V tester
I've ever seen uses voltage adjustment to get the right intensity,
probably because ELH-based measurements are generally used only
in-house, with public claims based on more sophisticated testing by an
independent laboratory.

Hope that helps.

Posted by Usenet2007@THE-DOMAIN-IN.SIG on August 17, 2007, 8:28 pm
 says...

What is the "fake sunlight" for?  Have you checked the prices on
those special bulbs for growing plants indoors?


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Posted by never on August 17, 2007, 9:28 pm
 On Fri, 17 Aug 2007 13:28:02 -0700, Usenet2007@THE-DOMAIN-IN.SIG


I just somehow figured someone would get around to the essential
questions.

Enjoy the answer.

Donn

Posted by Morris Dovey on August 17, 2007, 9:36 pm
 Paul Ciszek wrote:
| I need to produce fake sunlight over a small area--maybe a square
| 20cm on a side--on a low budget.  I have been told that ELH
| projector bulbs make a good first approximation to sunlight on the
| cheap.  I am trying to make an estimate of how many of them I need
| to achieve sunlight intesnsity.  I tried to work it out based on
| the following numbers:
|
| Sunlight = 100,000 lumens/m^2
| Halogen bulbs yield 16 to 24 lumens/W
| That's 6250 to 4167 W to produce 1m^2 of sunlight
| 250 to 167 W to illuminate my 20cm square
|
| That just seems awfully low; am I missing something?

Maybe. Why not use the real thing?

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/



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