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# Watts & Lux's

#1
09-29-08, 05:43 PM
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Watts & Lux's

I need help figuring out the lux output of a lamp. This is something requested by my doctor. I need a lamp that produces 5,000 to 10,000 lux at 18". I have been looking at a lamp with a 27w flourescent bulb (equal to a 150w incandescent). But I can't find what the lux production would be at 18". Any assistance would be greatly appreciated. Thank you. rmcl1214

#2
09-29-08, 06:06 PM
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There's no way that I know of to do a direct conversion of watts to lux or lumens.

Watt is measure of power consumption....the others are measurements of actual illumination(?) or visible light output(?).

You are in for a search...from what I have in my cabinet a 150W incandescent only puts out about 2200 lumens. I think a call to a manufacturer may be in order.

Your Doc needs to use a measurement that is an industry standard. Maybe the lux at 18" is, not sure. Time for dinner right now. Maybe someone else will add more.

#3
09-30-08, 03:16 AM
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Watts & Lux's

Thanks Gunguy. I knew this was going to be a difficult task and I appreciate your feedback. Enjoy your day.

#4
09-30-08, 05:06 AM
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Generally, a single-lamp 40-watt T8 fluorescent fixture will produce 3,000 lumens at 30 inches.

Lux is the amount of light hitting a one square meter space, generally at one meter from the source. Fortunately, at that same distance, lumens and lux are the same.

The inverse square law applies, which means that every doubling of distance results in a drop to 1/4th of the lux hitting that same one-meter space. Every halving of distance results in an increase of 4 times the lux.

It sounds like your doctor wants you to spend time under a grow light, but check with him/her whether the UV will be a problem.

Last edited by Rick Johnston; 09-30-08 at 05:22 AM.
#5
09-30-08, 10:17 AM
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Watts & Lux's

Hey Rick, I appreciate the feedback. What I need to know is: What is the lux at 18" for a 27w flourescent full spectrum bulb? Much appreciated rmcl1214

#6
09-30-08, 10:23 AM
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Tell us the lumens (should be on the package) and we can help you compute the lux.

#7
09-30-08, 11:52 AM
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Watts & Lux's

Hey Guys! I appreciate the feedback. I haven't purchased yet. The lights I'm looking for don't give me the details. I'm trying to find this specific light and don't want to buy the wrong thing. The lamps I'm looking for don't specify what the lux are (is). I guess whay I'm looking for is: How can I go by the info given on the box to determine if I have the right light? Thanks again, very much appreciated.

#8
09-30-08, 04:41 PM
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Most bulbs do list lumens on the box.

#9
09-30-08, 05:12 PM
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Well, some of the imported fluorescents don't list lumens. You may want to hit a specialty lighting store, or do some web searching.

I just looked at 3 different types in my bulb cabinet and for the same wattage, they vary from 1120 to 1400 lumens. But I had some cheaper ones, subsidized by the power company, that had no info other than "equivalent to 100 watt incandescent"

#10
09-30-08, 05:21 PM
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Watts & Lux's

Hey Guys, Thanks for the feedback. I know the lux's aren't listed on the boxes. I was hoping to get a cheaper light at the bargain store than have to get one at a specialty store and pay premium buck for a similar item. Once again, your feedback is appreciated.

#11
10-01-08, 05:00 AM
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A quick search for 27-watt compact fluorescents (not T8) showed a general range of 1,100 to 1,700 lumens. However, one claimed only 280.

If they are spec'd at the standard 30" (and that's a big "if" knowing how some companies fudge their numbers to sell products), remember the Inverse Square Law: Cut the distance in half to increase the lux by a factor of four.

The 1,100-lumens bulb will produce 4,400 lux at 15 inches, and 9,900 at 10 inches. The 1,700-lumens bulb will produce 6,800 lux at 15 inches. Both are within the doctor's recommendation of 5,000 to 10,000.

#12
10-01-08, 11:25 AM
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Watts & Lux's

Hi Rick, Thank you soo much for explaining that to me in such detail. Now I have a much clearer idea of exactly what I'm searching for. I really appreciate your time.

#13
10-01-08, 04:11 PM
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One more thing:

When the lumens of a lamp are specified by a manufacturer, supposedly it is tested as a bare lamp with no shade or reflector. This means it radiates in all directions (360 degrees in a globe).

If you put a flat reflector next to it, the energy that was once radiating at 360 degrees is now radiating into 180 degrees, or half the globe. This increases the lux by a factor of close to two times the original. (It's "close to two" because not all of the energy is reflected. Some is absorbed by the reflector.)

Parabolic reflectors can increase the lux even more, because they focus the light into a smaller section of the "globe".

#14
10-01-08, 05:18 PM
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Watts & Lux's

Wow Rick! Sounds like you really know your lux and lighting! I really appreciate you're assisting me with this pursuit. You've been an extreme source of info and a lot of help. Thank you very much. Now I can search with confidence! Sincerely, rmcl1214