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Please check my math re beam spread from CR6?

Posted by la_koala (My Page) on
Sat, Oct 15, 11 at 13:54


The final number comes out so large, that I have to imagine that I did something wrong in this calculation.

The specs on the Cree CR6 say is has an approximately 80 degree beam angle. My ceilings are going to be 9 feet tall.

I'm trying to get an idea of the distance of the light spread on the floor.

The geometry formula looks to me like the radius of the light circle on the floor (R) is equal to the tangent of half the beam angle (tan 40) times the ceiling height (9 feet). (Because if I recall my high school geometry correctly, the tangent of the angle is the opposite side over the near side).

If I plug in the numbers, I get tan 40 = 0.839 times 9 feet = 7.55 feet (!)

Which would indicate a light circle on the floor of 15 feet in diameter (twice the 7.55 feet), which seems awfully huge.

Any mistakes in my math? Or set up of the geometry?


Follow-Up Postings:

RE: Please check my math re beam spread from CR6?

This math stuff is way over my head, but looking at the output of my CR6's, there's no sharply defined light circle, or even really a vaguely defined circle. It's more like a simple surface-mounted overhead light - it's brighter underneath than far away, but the brightness is relatively even for at least a 15 foot circle from my less-than-9-foot ceilings, fading slightly as you get further away as it should (since presumably there's either another recessed light starting to take over, or a wall). The light's less focused than with a 55-degree PAR38 reflector bulb, though perhaps less than a cheap blown-glass R40 incandescent. The CR6 does an excellent job of not having any inordinately bright spots under the bulb though - it gives off nice even light.

Anyway you can always go to Home Depot and buy one, plug it into a small table lamp or other movable socket, and try it out for yourself (or put it in a can if you already have a 6" recessed fixture somewhere) and see how it looks yourself, and return it if you don't like it.

RE: Please check my math re beam spread from CR6?

"there's no sharply defined light circle, or even really a vaguely defined circle."

The angle is were the light has fallen off by 50%.

It may not be sharp, but it is getting dimmer.

RE: Please check my math re beam spread from CR6?

Hi lee676, thank you so much for posting from your real-world experience! It really helps to hear from someone who actually has them in the field. You've described the light quality in such detail, that I get a good sense for how it might work in my space.

You mention getting one and plugging it into a movable socket--does the CR6 just plug into a regular (screw-in) socket like that? If so, then that's a great idea--thanks for that!

Hi brickeyee,
Thanks for the clarification about what goes into the "beam angle". I have to say that you do provide solid technical info in all of your postings, and I really appreciate that!

Please correct me if I've misunderstood: when you say the angle is where the light has fallen off by 50%, you mean that that is the technical definition of "beam angle"? That I should think of the 80 degree beam angle as the angle in which the light has fallen off by 50%?


RE: Please check my math re beam spread from CR6?

40 degrees off the center axis of the fixture the light intensity should be half what it is in center.

RE: Please check my math re beam spread from CR6?


- yes you plug a CR6 into a standard light bulb socket (unless you have the GU24 version which is mostly sold in California and other states that have regulations pushing for new lamp fixtures to require LED or fluorescent bulbs).

Hardware stores also sell an inexpensive adapter that converts a light bulb socket to a 120V electrical plug, so you can plug the CR6 (or other light bulb) into the adapter, and the adapter into a standard electrical extension cord, and stand on a ladder and try different placements and experitment before you cut holes in the ceiling.

RE: Please check my math re beam spread from CR6?

Also there's a beam spread chart on the Cree website, link below

Here is a link that might be useful: CR6 beam spread (2nd page)

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