Standards for measuring viewing angle + brightness

Hello Everyone,

My good friend Doug Owen is letting me borrow his light meter to run some tests. I've been searching like crazy and can't find any info on standards on how to make measurements. Here are my questions:

1) How do I properly measure the viewing angle of an LED? Do I do so from the die or from the tip of the casing?

2) On LEDs that have multiple areas of brightness in the "pattern" (dark in the middle, then bright, then dark, then bright) where do I start / stop my viewing angle measurement?

3) What is the most efficient and then the easiest (but relatively accurate) way to measure viewing angle? Using a protractor lying on a flat surface, or with a throw against a wall and measuring distances and doing some geometry?

4) When measuring brightness how far away from the meter / wall should the LED be? 12 Inches? 12 Feet?

5) My LEDs all have an MCD rating on them (I'm pretty sure that is the average rating within the viewing angle?). This meter has LUX and FC on it. How should I convert between the two, and also a regular candela?

I'm guessing this has all been asked before, but I can't find it anywhere. I would love to compile any and all answers to this on my website for others to read and learn.

Thanks in advance!

Hi, Rob,

Well...first of all, there is a link to a light measurement handbook on my Web site. It's wonderful.

(1) Typically a polar plot of the brighness vs. angle would be the most useful and that's what you'll see (actually in cartesian form) on the Lumileds Luxeon data sheets, for example. I prefer polar plots, but then I'm an old audio guy and that's what they do for microphones.

(2) See (1)

(3) Protractor and light meter with multiple points.

(4) I generally measure at 2 feet and 10 feet, although with really big lights (like spotlights) I'll go out to 30 feet.

Conversion factors from fc to candela 4, 100, 900 respectively.

Others measure at one meter and the conversion factor from lux to candela at that distance is 1.0.

(5) MCD/1000 = cd
fc/distance(feet)^2 = cd
lux/distance(meter)^2 = cd

But check the light handbook as I'm in a rush.

Cheers,

Richard

Hello Rob,

Let me expand on what Richard presented.

I do not know what the standard for beam angle measurement is. I do know that as you move away from the center of the beam, the light gets less intense. This is why a polar plot is so useful. In battery run time measurements, it is usually thought that the time to replace the batteries is when the light output drops to half of the original output. You might measure beam angle as where the light drops to half. You will have to do some checking on this. By the way, the light measurement handbook on Richard's site is excellent.

Doug (Quickbeam) uses a beam shot against a calibrated target. Craig (LED Museum) uses a graphical approach demonstrating beam angle. A protractor or a beam shot will both work. What are you most comfortable with?

If you state a light measurement it footcandles, the assumption is that you took the reading with a footcandle meter at a distance of 1 foot from the source.

If you state a light measurement in lux, the assumption is that you took the reading with a lux meter at a distance of 1 meter from the source.

1 fc = 10.764 lux
1 lux = 0.0929 fc
and I believe 1 fc (at 1 foot) = 1 lux (at 1 meter)

If you take readings at other distances, then you need to either state the distance that the reading was taken, or better to use the conversion equation to convert to a standard measurment.

Lumens is the total output of the lamp and does not consider the effect of lenses or reflectors (unless they are included in the measurement as SureFire and ARC do). Fc and lux measure the center hot spot of the beam. Beam angle would give an indication of spill light. Doug has a chart of throw vs output. He took lux readings (at 1 meter), then took the square root of the reading. This gives the distance (in meters) for the light to drop to 1 lux. His total output is taken by a home made integrating sphere (actually a box in his case). This gives a relative measurment of the spill light of the light. He has a good description in the reviews forum and on his web site. Check it out.

Quickbeam

Richard got the equation a little backward in his haste.

Candella = footcandles times distance squared (in feet)
Candella = lux times distance squared (in meters)

Richards conversion factors should be clear now. If you take a fc measurment at 10 feet, you have to multiply the measurment by 100 (10 squared) to get candella.

Finally as Richard pointed out, an LED that has 16000 mCD is rated at 16 candella.

Hope this helps.

Tom

EXCELLENT!

Thanks guys.

This is some good stuff! I'm going to compile this on a page and put some Excel sheets together to make this as easy as possible.

If there is any other information that others believe would be usefull, please let us know.

BTW:

That is a great article / book, but OUCH... it's making my brain hurt!

What to look for in a meter

if one wants to start doing these type of comparison tests?

Duane