# How many candela?

#### beav

##### Newly Enlightened
I need to make a light that has a minimum of 30 candela output (omni-directional) and I have no reference of how much that really is.

Can anyone give me some kind of reference to help me visualize how much light this is??

For instance, how many candela would a white 1W luxeon lambertian LED (with no optics or reflector) put out? I am familiar with how much light those put out, so I can use those as a reference... I looked through the datasheets and they only list luminous flux - and I don't know how to compare that to candela (or if they are comparable...)

Thanks for answering a noob's question...

#### beav

##### Newly Enlightened
no helpers? someone's gotta have some ideas...

#### nerdgineer

##### Flashlight Enthusiast
This is all abstract as I have no practical experience making lights, but:

True omni-directional light seems difficult. No single LED provides a beam wider than 70 degrees or so, so it would take some number of them arranged regularly (i.e. with 3D symmetry) to shine light in all directions, and it would probably require something like a frosted glass globe to diffuse their light evenly as each LED would not have even lighting across its beamwidth.

Geometrically, there are 5 kinds or regular 3D shapes (polyhedra) as noted here. They have 4, 6, 8, 12, and 20 sides, and that's it.

You could use one of those polyhedra as a guide for pointing your LEDs inside the diffusing globe. Using such an arrangement with more LEDs would of course provide more even light sources for your diffusing globe. I would use low dome Luxeon type LEDs with no optics to get the widest beam possible.

As for 30 cd of brightness - the equivalent cd for a diffuse source is harder to calculate than for a point source, but you could estimate what you needed based on the lumens required. A 30 cd omnidirectional source is the equivalent of 30 theoretical "candles" burning. Each theoretical candle puts out 4 x pi or about 14 lumens. Therefore, a 30 cd omni source would require you to generate about 420 lumens with the total number of Leds you use. I'd make that 600 lumens to account for light lost going through the diffusing globe.

So you need to drive the total number of LEDs you used in your LED arrangement to generate about 600 lumens of light.

If you use Lux IIIs (no optics) driven at 50 lumens each (which is close to 1 amp, I think), then 12 of them arranged symmetrically (technically, in a dodecahedron arrangement) would put you in the ball park.

Of course, this is just theoretic. 12 Lux III's driven at 1 amp each will require lots of cooling which is not suitable for air cooling in a more or less closed sphere, so you'll need to do something about that. I don't know, liquid cooling?

All that said - and I con't know what your constraints are, but - I think a regular 60 watt bulb will put out about 850 lumens, more or less omnidirectionally except for the shadow of the base/socket (says here). So could a 60 watt soft white bulb in a frosted glass globe also do? If so, it would be a lot easier...

#### nerdgineer

##### Flashlight Enthusiast
I just read your post again. Did you really mean that you needed to make a light that puts out 30 cd brightness omnidirectionally? In that case, see my post above. If you only meant that you needed a light which puts out 30 cd brightness in a given direction, then any of the review sites can show you that, I think a Dorcy 1xAAA puts about about 25-30 cd brightness at its beam peak.

If you need a wide beam light of 30 cd brightness, I think (someone correct me if this is wrong) that a 1W low dome Luxeon with no optics puts out about a peak of 25 cd over about a 70 degree beam when fully driven.

#### SilverFox

##### Flashaholic
Hello Beav,

Welcome to CPF.

Doug (Quickbeam) has a table of Output vs Throw in the reviews section. If you look on the throw chart, 30 candela falls between 5 and 6 on his chart. You can see what lights are in that range for a comparison.

To find that value, I took the square root of 30 = 5.48.

Tom

#### beav

##### Newly Enlightened
Thanks for the replies. Nerdgineer, your name is appropriate! hehe. That first reply was very thorough and analytical. Your boss must be proud. I should have been more specific when I said 'omni-directional'. You're right, I don't need 'true' omni - just a wide, unfocused beam... So you're second post is more on the money, but I enjoyed reading your first post nonetheless.

Silverfox - thanks, that's a great reference. After comparing some known lights, it appears that 30 cd isn't very bright...

#### nerdgineer

##### Flashlight Enthusiast
Thanks, beav, it was fun. And by the way, welcome to CPF! /ubbthreads/images/graemlins/grin.gif

#### SilverFox

##### Flashaholic
Hello Beav,

Correct. Not very bright.

To expand the explanation a bit... Doug records lux at 1 meter and reports that in his reviews. Lux at 1 meter = candela. Taking the square root of his measurement gives the distance in meters for the light level to fall to 1 lux. That is why his chart is labeled throw.

Tom