Do 2 bulbs with 500 lumens provide the same illumination as one with 1000 lumens?

Question

When calculating the illumination in a room, do I simply add up the lumen value of the individual bulbs, or is the equation more complicated?

Answer 1

I agree with the other answers if they are both located in the exact same place. Of course this is physically impossible so they would have to be right next to each other in the same fixture and you will percieve about the same amount of light.

However, if you place the two lamps apart from each other their overlapping light will eliminate shadows that you will perceive as more light or at least a better lit space. IMHO more lights of lower lumens will light a space better than a single lamp of the total of all their lumens. Ask the theater people.

Answer 2

Do 2 bulbs with 500 lumens provide the same illumination as one with 1000 lumens?

I agree with our local Retired Master Electrician's answer but if you want to put on your physicist's hat, the answer is no.

You need to understand (at least in a basic way) the difference between

• Illuminance (Lux)
• Luminous Intensity (Candela)
• Luminous Flux (Lumens)

Lumens are a measure of luminous flux not of illuminance (which is closest to what you probably mean by illumination).

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Two 500-lumen bulbs might produce very different values of Illuminance as one 1000-lumen bulb.

Two 500-lumen bulbs produce the same luminous flux as one 1000-lumen bulb.

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If you were specifying some office space and needed to meet some regulations, you'd need to understand these distinctions.

If you are a home-owner choosing between light fittings or bulbs made 8,000 miles away, it is likely that this sort of distinction is unimportant to you.

You can buy light-meters that measure illuminance. I suppose an old-school photographer's light-meter makes a reasonable substitute.

The marketing departments of different manufacturers/distributors lie in different ways and different light-fittings are likely to make a difference to illuminance.

Answer 3

2 bulbs of 500 lumen will produce the same quantity of light as a 1000 lumen bulb.

But you will perceive it differently.

A collection of smaller light bulbs will be perceived as more luminous than a single source.

To calculate the illumination of a room you need to take into account the room shape, fixture position and height, light source angle, CRI, light temperature, wall coverings, etc..

Answer 4

Don't get too confused with all that candela/lux witchcraft, that requires breaking out the sharp pencil and that's only useful if you're suing a lighthouse. Lumens are simply visible light output at the bulb.

If they're reasonably aimable, then yes. Two 2-tube fluorescent fixtures next to each other will be almost exactly as bright as one 4-tube. If they're not aimable, then each light is putting the other light in shadow, so you're losing some to that.

It really helps to use LEDs which can be well-aimed.

Answer 5

The simple answer is yes. Of course there are several other things you need to consider. One brought up by Ed Beal. Then there is a Room Cavity Ratio, Lamp Lumen Depreciation, Reflectance, and Maintenance Factor.

Answer 6

I was asked this question by someone who works with large projection screens and I found this thread researching the answer. This thread caused my thinking to change so I have different conclusion. This because it is the combination of Lumens and Reflectance that gives us the perceived brightness.

The question given to me is specific to projecting images in large spaces, indoor or outdoors, and whether two lamps of 4500 lumens projecting on the same screen would be equivalent to a single 9000 lumens lamp. The simple answer seems to be that the intensity of the lamp would be the same, but the absorption by the reflecting surface (in this case a projection screen). Reflectance is the measure of light reflected by a surface.

Every surface absorbs some of the light that is projected on it, this is measured as Reflectance. So if the degree of reflectance is a constant, then it will be added to each separate light source. So two projectors of 4500 lumens pointing at the same surface will produce 9000 lumens, but then each lamp beam will encounter the same absorption inefficiency of the surface, and if there is a absorption of 2% then the light from two lamps will have a total 4% absorption rate and the single 9000 lumen lamp will have 2% absorption rate. This will be more noticeable the lower the frequency of the projected lightwave is on the screen.

Answer 7

I'd have thought this sufficiently answered, however I do see a few points that may spawn confusion or even misinformation. In writing this my goal is to further (re-)clarify that "Yes, they are equal, BASICALLY."
Some of these answers mention additional variables that could be used to argue away from this verdict. They can not lead you toward another verdict; Only astray into ambiguity. Ambiguity can easily be forced into or pulled forth from any argument. It would behoove those of us who value intellect to remain vigilant towards that as well as to point them out (or correct for them) to others when we come across them. That being said, the question as asked implies one's ability to commensurate these two possibilities.

Possibility A: 1000Lm Possibility B: 500Lm • 2

This further implicates an equality of variables that MUST be required for this question (as well as general reason) to make any sense whatsoever. (E.q. both take places in the same reality, same fundamental laws, same light fixtures, same reflective materials, same room, illumination method, etc. etc.) The color of the room doesn't matter, because it's the same room and before you ask, the appeared temperature of the bulbs also does not attribute itself to this discussion either. A possibility of response would suggest that they share the same colors. Additional factors of light (e.g. lux) should be assumed either equal or to be generated by a realistically similar sources. Yes, All Things Being Equal,*(=¹as was implicated)*they are equivalent.

That being said, aside from theoretical, there is no such thing as perfection in any domain, equivalency included. This means that in practice the two possibilities WILL differ. However, 'equality of variables' ensures us a less than significant difference between the two. This is due to their having less than significant non-equivalents.

Thanks for reading. 🤙🏽