Return to the Lighting Forum | Post a Follow-Up

calculating number of fixtures needed

Posted by calumin (My Page) on
Tue, May 14, 13 at 2:31

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?

Follow-Up Postings:

RE: calculating number of fixtures needed

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.

RE: calculating number of fixtures needed

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

RE: calculating number of fixtures needed

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%.

 o Post a Follow-Up

Please Note: Only registered members are able to post messages to this forum.

    If you are a member, please log in.

    If you aren't yet a member, join now!

Return to the Lighting Forum

Information about Posting

  • You must be logged in to post a message. Once you are logged in, a posting window will appear at the bottom of the messages. If you are not a member, please register for an account.
  • Please review our Rules of Play before posting.
  • Posting is a two-step process. Once you have composed your message, you will be taken to the preview page. You will then have a chance to review your post, make changes and upload photos.
  • After posting your message, you may need to refresh the forum page in order to see it.
  • Before posting copyrighted material, please read about Copyright and Fair Use.
  • We have a strict no-advertising policy!
  • If you would like to practice posting or uploading photos, please visit our Test forum.
  • If you need assistance, please Contact Us and we will be happy to help.

Learn more about in-text links on this page here