Does one 200 watt bulb put out as much or more light than ten 20 watt bulbs?

August 14, 2009 2:26 AM

Does one 200 watt bulb put out as much or more light than ten 20 watt bulbs?

We were sitting in a bar the other day and there was a lamp on the table with a 200 watt bulb. Above us, were lots of lamps, each one considerably less bright than the lamp on our table but it got us to thinking: would the collection of lamps above us put out the same amount of light as the one lamp on our table? I

posted by leemajors to Science & Nature (23 answers total)

Probably more light, but it depends on the actual bulbs used.
posted by effbot at 2:30 AM on August 14, 2009

(And yes, that was an attempt to answer your last question, not your first one. The answer to that is "probably less light, but it depends ...".)
posted by effbot at 3:04 AM on August 14, 2009

Ditto effbot. If they use similar technology then larger, higher wattage bulbs are generally more efficient.
posted by jon1270 at 3:06 AM on August 14, 2009

Eh, clarification fail. I obviously cannot count to first and last today. Or maybe it was a parsing fail. Need more coffee. The folllowing quote should be unambiguous enough, though:

"Note that one 100 watt [General Electric Standard] bulb puts out more lumens than two 60 watt bulbs and more than three 40 watt bulbs."
posted by effbot at 3:29 AM on August 14, 2009

There should be efficiencies of scale, but I'm at a loss to tell you why...
posted by phrontist at 3:29 AM on August 14, 2009

You have to go by the lumens rating of the bulb, not the wattage. Lower wattage bulbs tend to run the filament at a lower temperature which puts it at a less efficient region of operation.
posted by Rhomboid at 3:35 AM on August 14, 2009

The Wikipedia article on light bulbs has an unsourced table that confirms the general impression that higher wattage bulbs are more efficient.

I presume it is because they heat the filament to higher temperatures, thus moving their blackbody spectrum "more into the visible". Wolfram Alpha visualizes this neatly for 2200 K and 3000 K (the upper/lower ends of the temperature range given on Wikipedia).
posted by themel at 3:45 AM on August 14, 2009

But I think you could stretch 20 bulbs way further than the light of one bulb would ever hope to reach?
posted by sully75 at 4:18 AM on August 14, 2009

Is your question about the photons *leaving the bulbs* or the photons *reaching your table*? I would think that multiple bulbs would provide more even lighting, which could give the impression of more light overall.
posted by DU at 4:29 AM on August 14, 2009

Response by poster: so is the amount of light the same but the light from the single more powerful light source more intense?
posted by leemajors at 5:15 AM on August 14, 2009

Consider that a 100W tungsten light puts out the same amount of light as a 23W CFL. Suffice it to say, wattage is not a measure of light.
posted by JJ86 at 6:02 AM on August 14, 2009

Watts are a measure of electric demand. Lumens are a measure of the perceived power of light. You can solve this problem by determining the lumens of 20 of the lower-watt bulbs and comparing it to the lumens of the higher watt bulb.
posted by Pants! at 6:43 AM on August 14, 2009 [1 favorite]

You mention how much the lights "put out" but I don't think you actually care about that. You want to know about the brightness at the table.

Light intensity falls off as the square of distance. Let's say you have a light that puts out N units and you are distance D away from it. Halve your distance to it, so you are now D/2 away. Now the light is (apparently) 4 times brighter. So cut the wattage in half. Now it's only 2 times brighter. But add a second light that's also D/2 away. Back to 4 times brighter.

So an array of low-to-medium power lights can *give you* more light than a single high-power one even though they *put out* less. However, your array of low-power lights in the bar is actually farther away. So you are getting far, far less from those bulbs.
posted by DU at 7:02 AM on August 14, 2009

You can't judge based on wattage. 2 lightbulbs could give off different amounts of light at the same wattage. Just look at CFL, a 20W buld can give off the the same about as something with a wattage rating 5x higher.
posted by blue_beetle at 9:16 AM on August 14, 2009

Optical Engineer here.

There should be efficiencies of scale, but I'm at a loss to tell you why...

phrontist, I'll admit this seems reasonable, but it is actually irrelevant, because as Rhomboid notes:

You have to go by the lumens rating of the bulb, not the wattage. Lower wattage bulbs tend to run the filament at a lower temperature which puts it at a less efficient region of operation.

Lower temperatures = cooler filaments = peak light output at "redder" wavelengths (possibly even peaking in the infrared, thus wasting the vast majority of light) = dimmer bulb.

All of this assumes a "blackbody radiator" light source, such as an incandescent bulb. Sodium ion & argon ion bulbs have different light spectra, and are inherently far more efficient. Ditto for CFLs; the winner in this game are LEDs, whose output spectra can actually be custom-shaped (adding some manufacturing cost).

sully75 asks:
But I think you could stretch 20 bulbs way further than the light of one bulb would ever hope to reach?

Light from a bic lighter will eventually reach another galaxy, so the answer to your question is "no". I think you meant to ask, "Wouldn't several lights make a large space more evenly lit?", which may or may not be useful. For instance, you don't want a reading light spread out; you want it shining tightly on your favorite reading space.

And, DU, whatever you are trying to describe... I don't know. Yes, a tiny bulb up close is brighter than a high-watt bulb far away, but that's a distortion of the original question. There's no reason to insist many small bulbs must be located farther away. For apples-to-apples comparison, an X-watt incandescent is (generally*) brighter than n (X/n)-watt incandescents, at the same distance.
posted by IAmBroom at 11:40 AM on August 14, 2009 [1 favorite]

What we are discussing, by the way, is ultimately the difference between photometry (measurement of light energy) and radiometry (measurement of the perceived intensity of light by a "normal" human eye).

An X-ray machine may have high photometric brightness, but is still completely dark as a radiometric light source.

Light bulb wattage is photometric. Light bulb brightness is radiometric, and is measured in lumens (or candles, previously).
posted by IAmBroom at 11:45 AM on August 14, 2009 [1 favorite]

One practical complication here is that often more powerful lights are used for indirect lighting (e.g, bounced off the ceiling), while less powerful ones are shone directly downwards. You lose between a tenth and a half of your light each time you bounce it off white paint.

This can easily offset any greater efficiency you gain by having a single hotter filament.
posted by aubilenon at 12:06 PM on August 14, 2009

And, DU, whatever you are trying to describe... I don't know. Yes, a tiny bulb up close is brighter than a high-watt bulb far away, but that's a distortion of the original question.

I don't think the original question is very clear about this.

Above us, were lots of lamps, each one considerably less bright than the lamp on our table but it got us to thinking: would the collection of lamps above us put out the same amount of light as the one lamp on our table?

On the one hand, we have apparent brightness and on the other the words "put out". Are we talking about production or consumption?
posted by DU at 6:59 PM on August 14, 2009

Response by poster: I'm sorry, i know my original question wasn't as clear as it should have been. I'm not a scientist and so what seems reasonable for me to ask may seem stupid to some of you.

Really we wanted to know if each source was as "light" as each other. On the one hand, the 200W incandescent bulb at our table was quite intense, and, while each individual incandescent bulb above us was clearly dimmer, we wondered if grouped together they made up something that was as "light" as the one single source.
posted by leemajors at 7:14 PM on August 14, 2009

"Lumens" is the unit of measure used to describe the quantity of light produced by a source, without respect to what direction it's traveling in.

"Candlepower" is a way of talking about the intensity of light emerging from source and aimed in a particular direction. So, you might have a cone-shaped reflector bulb (like those used for spotlights) that has 8000 candlepower in the center of its beam, and zero candlepower directly behind the bulb.

"Footcandles" refers to the density of light that actually reaches a surface some distance from the light source. Light spreads out as it travels (unless we're talking about lasers), so footcandle measurements drop as you get further from a given light bulb.

Larger wattage bulbs typically produce more lumens per watt than smaller wattage bulbs using the same sort of technology, but it's not clear that that's what you're asking about. You won't find a satisfactory answer until you decide which sort of measurement matters to you.
posted by jon1270 at 8:09 AM on August 15, 2009

Response by poster: well, lumens seems about what we were after.
posted by leemajors at 4:47 PM on August 15, 2009

Best answer: Incandescent bulb brightness is a function of the temperature of the filament, which is more closely related to the current than the wattage. For a given voltage, a group of lower wattage lamps will produce less light than a single lamp of the same wattage.

However compare the light output of a 20W 120V bulb with a 20W 12V bulb. The 12V lamp runs at a higher current, and is consequently much brighter.

So 10 x 20W bulbs would be dimmer than a 200W bulb operating at the same voltage. 10 x 20W 12V bulbs would be considerably brighter.
posted by HiroProtagonist at 8:25 PM on August 16, 2009

Thanks, DU; I get your point now.

And, leemajors, I don't think you need to get insecure about your question. Questions about light get complicated in language really quickly. I always have to look the units & terms up, myself, before using them in technical contexts.
posted by IAmBroom at 11:56 AM on August 18, 2009