of it at the same brightness as a 100mcd LED with a viewing angle of 15 degrees, but will light a larger area, therefore is giving off more light in total.
-- No. 100 millicandelas is the total light output, so the area illuminated by the LED with the larger beamwidth will be dimmer.
front of it at the same brightness as a 100mcd LED with a viewing angle of 15 degrees, but will light a larger area, therefore is giving off more light in total.
light output, whereas luminance (in mcd/sq.m) is what I was thinking of?
-- Yes.
I'm wanting to purchase bright LEDs, but am confused by the specs.
Am I correct in the following assumption?
A 100mcd LED with a viewing angle of 30 degrees will light the surface in front of it at the same brightness as a 100mcd LED with a viewing angle of 15 degrees, but will light a larger area, therefore is giving off more light in total.
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front of it at the same brightness as a 100mcd LED with a viewing angle of 15 degrees, but will light a larger area, therefore is giving off more light in total.
I was trying to go by this:
So what you're saying is the way they quote them in catalogues (mcd) is total light output, whereas luminance (in mcd/sq.m) is what I was thinking of?
-- http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com Dan married one of a pair of identical twin girls. Less than a year later, he was in court filing for a divorce. "OK," the judge said, "Tell the court why you want a divorce." "Well, your honor," Dan started, "Every once in a while my sister in law would come over for a visit, and because she and my wife are so identical looking, every once in a while I\'d end up making love to her by mistake." "Surely there must be some difference between the two women." the judge said. "You\'d better believe there is a difference, your honor. That\'s why I want the divorce." he replied.
front of it at the same brightness as a 100mcd LED with a viewing angle of 15 degrees, but will light a larger area, therefore is giving off more light in total.
light output, whereas luminance (in mcd/sq.m) is what I was thinking of?
Thanks, just changed to buying a completely different set :-)
Why is there so much difference in specs with LEDs? Aren't they all basically the same technology?
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front of it at the same brightness as a 100mcd LED with a viewing angle of
15 degrees, but will light a larger area, therefore is giving off more light in total.Uhh... No...
Candelas aren't a measure of total light output. Candelas are a measure of lumens per steradian (a steradian being a solid angle measurement). Lumens are a measure of total light output adjusted for human eye sensitivity towards different wavelengths. Human eyeballs are vastly more sensitive to
555nm green photons than they are to say red or blue photons for example.So... A one lumen white light source should in theory be able to light up a whole room to approximately an equal amount of brightness as perceived by a human eye as a one lumen blue light source, as a one lumen green light source, as a one lumen red light source, etc. The one lumen green light source doesn't have to be as electrically efficient (in terms of total photons produced per joule input) however since the human eye doesn't need as many green photons to appear equally bright as other colors.
Unfortunately the vast majority of LED manufacturer's don't yet rate their LED product's output in lumens, instead they usually provide intensity in millicandelas and also include a "viewing angle" which is normally defined as twice the angle of half intensity (or in other words two phi one-half). Sometimes instead they might give you just the angle of half intensity instead (especially in the case of infrared LEDs). This is a most unfortunate state of affairs, because it precludes highly accurate and fair comparisons between products.
Anyway, the OP's assumption was in fact correct. A 100mcd LED with a viewing angle of 30 degrees *should* produce more total light output than a
100mcd LED with a viewing angle of 15 degrees. This doesn't absolutely always have to be true however, since it depends on the beam profile, and it also depends on how accurate and truthful the manufacturer's specifications are. By playing with the beam profile, it is possible to make a 30 degree 100mcd LED produce less total light output than a 100mcd 15 degree LED, without lying in any way on the specifications, but in most cases the 30 degree product should be superior.If you want the most total light output and highest efficiency product, go for LEDs that have high mcd ratings as well as large viewing angle ratings. Unfortunately manufacturer's and especially distributors often make mistakes on their advertising (usually because distributors don't have a clue what any of the various units mean either), further muddling the issue and making accurate direct performance comparisons even more difficult.
That's a non sequitur. Tungsten filament incandescent lamps are all basically the same technology, yet they come in many different configurations. Same is true of LEDs, which come in various colors, shapes, sizes, viewing angles, mcd ratings etc. I suppose that manufacturers decide where they think there is, or will be, a need, and make things with specs that meet the need. That would result in the many different LEDs that are available.
Ed
-- Yup.
-- Just goes to show ya, don\'t believe what anyone tells you. Try it for yourself and find out what\'s real.
The weird thing is that the ones that are 100 times brighter do not use 100 times more power.
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Oh great. Now I have two completely different answers, I'll have to buy some of each and see what they really do.
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I think you're right actually -
Well if you're right, then I've just ordered the wrong ones. I'll go order the right ones and see for myself which is really brighter.
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[grumbles at £8 extra spending]
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front of it at the same brightness as a 100mcd LED with a viewing angle of 15 degrees, but will light a larger area, therefore is giving off more light in total.
Do the manufacturer's specs actually say that the LED outputs are "total output" ?
CIE develops international specs for measuring visible LEDs. CIE
127-1997 defines measuring LED output, termed "Average LED Intensity", in (milli) candelas. I make these measurements at the day job. It requires using an extremely accurate ($) photopic detector with a 1cm dia round aperture either 100mm or 316mm in front of the LED, which is equivalent to capturing 6.5-degrees or 2-degrees of the central LED output. Obviously, this is not total output, nor is it intended to simulate that. More often than not one isn't concerned about the total output of a visible LED, but how bright it looks when seen by eye at a reasonable distance away. CIE is working on a standard for integrated spheres for use in total output measurement. I believe it is due out in a few months.Anyway... the point being you really have to be careful about LED output specs. If possible buy a few and try 'em in your application. Sad to say that I've personally seen LED manufacturers using antiquated equipment to make their measurements, use integrating spheres to make measurements in units they weren't calibrated for, and even just make up data. I wouldn't go solely by their printed specs.
Steve Noll | The Used Equipment Dealer Directory: |
I am doing. Seen as I'm about to buy a few thousand of the things, I'd rather buy a large selection of single units first to make sure.
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Correct!
- Don Klipstein ( snipped-for-privacy@misty.com)
Candela is not a unit of total light output, but of "beam candlepower".
1 candela illuminates a spot on a surface 1 meter away to the extent of 1 lux.The units of total light output are "spherical candlepower" (candela averaged over all directions, including directions into which no light is sent), and the lumen. 1 lumen is the total light in a "solid angle" of 1 steradian (1/4-pi hemisphere) where the light has an intensity uniformly of 1 candela. A "spherical candlepower" is 4-pi lumens, the output of a 1 candela omnidirectional source.
- Don Klipstein ( snipped-for-privacy@misty.com)
I purchased a range of LEDs and tested them. The mcd rating appears to be TOTAL light output.
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Have you measured total luminous flux of a 15 degree model and a 60 degree model of the same millicandela, and known to actually be so? Have you ever shone two LEDs of the same MCD, same color/wavelength and same chip chemistry but greatly different beam wionto a solar cell and measured short circuit current of the solar cell with a milliammeter? (Comparison requires matching of spectrum because solar cells have different spectral response than human vision has.)
Have you ever illuminated a dark room with two LEDs of greatly different beam width and same MCD (and same color and wavelength and spectral characteristics - night vision differs from "day vision" in a way that varies with color/wavelength)?
I have been there and done that!
An ideal 1,000 millicandela beam that is 1,000 millicandela uniformly within its specified boundary and completely lacking light outside it has has .842 lumen of light if 60 degrees wide and has only .054 lumen of light if 15 degrees wide. At least 15 degree and narrower-beam LEDs usually have enough light outside their specified beams to get total light output usually more than, sometimes by a factor of 2 more than, 2 times Pi times (1 minus cosine of half the rated beam angular diameter).
- Don Klipstein ( snipped-for-privacy@misty.com)
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