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LED brightness?

Discussion in 'Electronic Basics' started by Peter Hucker, Nov 19, 2005.

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  1. Peter Hucker

    Peter Hucker Guest

    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.
  2. John Fields

    John Fields Guest

  3. Peter Hucker

    Peter Hucker Guest

    I was trying to go by this:
    But there are too many different units!

    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?


    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.
  4. John Fields

    John Fields Guest

  5. Peter Hucker

    Peter Hucker Guest

    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?
  6. ehsjr

    ehsjr Guest

    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.

  7. 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.
  8. John Fields

    John Fields Guest

  9. Peter Hucker

    Peter Hucker Guest

    The weird thing is that the ones that are 100 times brighter do not use 100 times more power.


    A blind man was standing on the corner with his dog when the dog raised his leg and went on the man's trouser leg.
    The man reached in his pocket and took out a doggie biscuit.
    A busybody who had been watching ran up to him and said, "You shouldn't do that. He'll never learn anything if you reward him when he does something like that!"
    The blind man retorted, "I'm not rewarding him. I'm just trying to find his mouth so that I can kick him in the ass."
  10. Peter Hucker

    Peter Hucker Guest

    Oh great. Now I have two completely different answers, I'll have to buy some of each and see what they really do.
  11. Peter Hucker

    Peter Hucker Guest

    I think you're right actually - tells us that candelas are perceived power in one direction. Hmmm it made sense the other way too - there are too many bloomin units!

    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.
  12. Peter Hucker

    Peter Hucker Guest

    [grumbles at £8 extra spending]
  13. John Fields

    John Fields Guest

  14. Steve Noll

    Steve Noll Guest

    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:
    | Peltier Information Directory:
  15. Peter Hucker

    Peter Hucker Guest

    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.
  16. Correct!

    - Don Klipstein ()
  17. 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

    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 ()
  18. Peter Hucker

    Peter Hucker Guest

    I purchased a range of LEDs and tested them. The mcd rating appears to be TOTAL light output.


    The teacher in Johnny's school asked the class what their parents did for a living. One little girl said her father was a doctor, another said her mother was an engineer. When it was Little Johnny's turn, he stood up and said "My mom's a whore."
    Naturally, after that remark, he got sent off to the principal'soffice. Then, 15 minutes later, he returned.
    So the teacher asked "Did you tell the principal what you said in class?"
    Johnny said "Yes"
    "Well, what did the principal say?"
    "He said that every job is important in our economy, gave me an apple and asked for my phone number."
  19. 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 ()
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