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?

  • I'm not gonna say this is out of bounds on DIY, but you might consider asking on physics or photography if you don't get satisfactory answers here. – Sidney Aug 23 '17 at 15:48
  • @Sidney, I considered asking this in physics, but this has more to do with human perception. If you open the other half of your blinds too, the room would not seem doubly bright. – András Aug 23 '17 at 15:50
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    It depends on the fixture and lamp type some fixtures reflect the light like metal halide but led in most cases have the light directed in the needed direction. – Ed Beal Aug 23 '17 at 15:53
  • The whole point of the lumen unit is that it is based on human perception, and represents the amount of light being cast into the fixture or room. How the fixture or room uses it is a different matter. The classic being the wallpack that spends most of its lumens lighting the wall instead of the parking lot. – Harper - Reinstate Monica Aug 24 '17 at 4:36

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.


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).

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.

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.

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    Made 25,000 miles away? As in "made in the house next door"? – Jim Garrison Aug 24 '17 at 6:29
  • @Jim, oops! fixed. – RedGrittyBrick Aug 24 '17 at 9:03

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.


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.


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..

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