calculating number of fixtures needed

caluminMay 14, 2013

I am putting together my kitchen lighting layout and have seen posts on this forum that talk about ~35 lumens per square foot as a guide to calculate how many fixtures to use.

The question I have is do people just use the advertised lumens on the light, or do they factor in the coefficient of utilization (CU) to determine effective lumens?

For example, I have read that the best coefficient of utilization to use for the Cree LR6 is 0.77. Essentially that means that about 77% of the luminous output of the light effectively hits its surface.

When I factor in the CU, it increases the number of lights I need by about 30%.

Or are there free tools out there which let you model your room layout and and fully calculate fixtures based on the Lumen Method?

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David

The best tools are still paper and pen/ pencil if you just want to quickly sketch up a layout without having to learn the tools.

May 18, 2013 at 11:04AM
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calumin

Thanks davidtay, the main question I have is do you use advertised lumens on the package when calculating lumens / sf, or do you multiply it down by a coefficient of utilization factor (e.g. which I've heard to be 0.77 for LR6 and CR6).

That one calculation changes the number of lights needed by about 30%. Most calculations I've seen on the Internet do include this factor but I haven't seen it discussed here.

I guess the other option would be to just increase the lumens / sq ft target value (which might be what's happening, because I've seen 25-30 lumens / sq ft in other places but have heard 35 lumens / sq ft on this forum).

This post was edited by calumin on Sat, May 18, 13 at 11:19

May 18, 2013 at 11:16AM
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David

I use the advertised lumens. The formula works very well with the CR6/ 4 and RT6 type lights since that is the actual output.

If you have dimmers, the max output is typically reduced by 5%.

May 18, 2013 at 7:27PM
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