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LED intensity/angle mcd?

Discussion in 'Electronic Design' started by [email protected], May 18, 2006.

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  1. Guest

    Looking at the specs for 3 red LEDs, I'm wondering which would be the
    better choice for maximum brightness.

    Allied part number 670-1257 which claims to be a 400mcd device with a
    viewing angle of 60 degrees at 2x theta. Cost is 72 cents.

    A second part 670-1224 claims to be a 1800mcd device with a viewing
    angle of 30 degrees at 2x theta. Cost is 50 cents.

    A third device is 670-1226 claims to be a 1200mcd device with viewing
    angle of 30 degrees at 2x theta. Cost is 31 cents.

    I'm not sure what 2x theta means, but I think if the angle is twice as
    wide, the intensity should be 1/4 as much, so that a 1800mcd device at
    30 degrees would be the same as a 450mcd device at 60 degrees?

    Why would the the 400mcd device sell for 50% more than the 1800 mcd
    device which has more total output?

    Would the more expensive device be the better choice?

    -Bill
     
  2. Pooh Bear

    Pooh Bear Guest

    It means 2x the 'view angle' theta e.g. 30 degrees, so it's talking about a cone
    of light with a sixty degree included angle ( for theta = 30 ).
    For the same led 'chip' inside the led you got it.
    No idea.
    What do you actually want ? A narrow angle led or a wider one ? Whichever's more
    suitable for your application is better.

    Graham
     
  3. I suggest you view all the data sheets side by side and try to
    understand all the differences in detail.

    The first one (670-1257) is a 10mm diameter LED with a wide viewing
    angle. The wide angle means that the light is spread over a wider area
    so it appears not to be as bright on-axis, but can be seen over a
    wider angle. Too far off-axis and the others may be difficult to tell
    whether they are on or off, depending on ambient lighting conditions.
    It also has a red *diffused* lens, which means that the light appears
    to come from the surface of the dome rather than through the 'water
    clear' lens that the others have. It's also tinted red, which affects
    how ambient light is reflected or absorbed. Chances are good it has
    the exact same type of 660nm color die in it as 670-1226, just a
    different molding.

    The main difference between the second two is the color of the emitted
    light. They are both 5mm diameter LEDs. The 670-1224 emits 636nm
    wavelength light, which is more of an orange-red. The 670-1226 emits
    longer wavelength 660nm light. Both are water-clear epoxy lens. They
    are narrow angle (30°), which means they look much brighter when you
    look almost directly into them, as opposed to much off the axis. Most
    manufacturers provide a graph of brightness versus viewing angle. The
    different dies have slightly different forward voltages as well
    (higher typical forward voltage by 300mV for the 670-1224, which emits
    shorter wavelength light).

    BTW, please get into the habit of referring to such parts by the
    manufacturer's (Lumex, in this case) part number rather than the
    distributor catalog number. You will find it infinitely easier to deal
    with the manufacturer's data sheets and when you change distributors
    or shop around you won't have to translate everything if you stay with
    the same manufacturer.

    That said, why not buy all three, power them up at a sensible current
    (in series is good for side-by-side comparisons) and look at the
    differences for yourself?


    Best regards,
    Spehro Pefhany
     
  4. RHRRC

    RHRRC Guest


    The ratio of the intesities for a given led die with different theta's
    is the inverse of the ratio of the square of the tan of each theta.
    Since tan15=0.268 and tan30=0.577 we would expect the narrow angled
    device to be
    (0.577*0.577)/(0.268*0.268) =4.64 times 'brighter' than the wider
    angled device.
     
  5. Pooh Bear

    Pooh Bear Guest

    Which indeed seems to be the case here.

    Graham
     
  6. It's diffused too, which should make it even worse.


    Best regards,
    Spehro Pefhany
     
  7. Guest

    Spehro,

    Thanks for the reply. As I understand it, the (670-1257) part with the
    red diffused lens emits a closer pure red light than the other options,
    but also has less total power output due to the red colored lens which
    filters out some of the light?

    The 670-1224 emits more of an orange-red color, but also appears
    brighter compared to the 670-1226 which is closer to pure red but rated
    at a lower mcd?

    How does one compare mcd rating to color and apparent brightness?
    I'm guessing orange LEDs of the same power output will appear brighter
    than pure red LEDs of the same power output?

    -Bill
     
  8. It's the same color as 670-1226, just with a huge 10mm diameter lens
    that is also tinted and diffuse (like a regular frosted incandescent
    bulb as opposed to a clear glass candelabra bulb). It's less bright
    looking right on axis because the light goes all over the place, and
    is emitted from a large surface.
    Yes, although the difference in brightness is not much.
    The number is already corrected for eye response AFAIUI, so compare
    directly. But the eye has a log response so a 2:1 difference in
    brightness doesn't look huge.




    Best regards,
    Spehro Pefhany
     
  9. James Waldby

    James Waldby Guest

    All else being equal (eg illumination pattern and viewing angle, besides
    power output, LED efficiency, "ordinary" eye) that might be so, since
    as mentioned before and as shown in some of the following webpages,
    the eye's sensitivity is dropping off as wavelength increases from
    orange to red. http://www.yorku.ca/eye/specsens.htm
    http://www.yorku.ca/eye/lambdas.htm
    http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/light/u12l2b.html

    However, I don't recall that you've said yet what it is you are
    trying to do (other than having blue and red LED's with about the
    same apparent brightness). Will the LED's be panel indicators, or
    do they illuminate something? From how far are they being viewed?
    Under what lighting conditions? Proper answers depend on knowing
    your requirements.

    I don't know what you are trying to do, and don't know your
    requirements, but will mention a few things anyway. First, based on
    a nonscientific look at some ebay prices, it's substantially easier
    to get red/green/white LED's that are bright, vs. yellow/orange, so
    much so that your concerns about eye sensitivity probably are moot.
    (See ebay examples below.) Second, most of these LED's are bright
    enough that they are dangerous to look at, making them unsuitable
    for use as panel indicators if driven at full power. Third, several
    of these will produce visible (ie, noticeable but not very bright)
    light at currents of a few hundred microamps, and could be used as
    indicators with currents of a milliamp or two.

    Auction# $Each Item and pertinent specs from auction pages
    7619669955 .23 100x 5mm Red LED 25ha 15000 mcd typ. at 20mA
    7620754821 .32 50x 5mm Red LED 20pa 20000 mcd min at 20mA
    7619480649 .60 20x 10mm Red Led 30va 80000 mcd claimed at 60mA
    7620924300 .17 30x 3mm Yellow 20va 4000 mcd typ. 5000mcd claimed
    7620916574 .12 100x 5mm Yellow 20va 4000 mcd typ. 5000mcd claimed
    7619668700 .26 50x 10mm Green 13ta 65000 mcd typ. at 20mA
    7619668531 .36 50X 10MM White 13ta 110000mcd typ. at 20mA
    7619670585 .26 50x 10mm Orange 13ta 45000 mcd typ. at 20mA
    7620929777 .24 50x 10mm Yellow 12va 60000 mcd typ. at 20mA

    $Each= (Price and shipping total dollars) divided by (number of LED's).
    The angle (in degrees) of the LED light is listed 4 different ways in
    the above auctions, which I've indicated by ha, pa, va, and ta, as
    follows: ha, half angle; va, viewing angle; pa, "50% Power Angle";
    ta, "1/2 Viewing Angle". Last auction also has a "Radiation Diagram".
    Note, where typical mcd ratings were given I listed those rather
    than the max values, or if typ not given, min instead of max.

    -jiw
     
  10. RHRRC

    RHRRC Guest

    Just to clear up any misunderstanding about photopic units:

    An Xmcd led of any colour has *exactly* the same apparent 'rightness'
    as a led with the same mcd of any other colour.
    That is what a Candela (milli candela or whatever) is by definition.
    It is not a measure of the power emitted by a light source it is a
    measure of the human response to the radiated power.
    (An infra-red led obviously emits power (in the infra-red region) else
    my TV remote control would not work but it emits none in the visible
    region ad thus emits zero milli candela's.)
     
  11. RHRRC

    RHRRC Guest

    could even be the same 'brightness' - iff eye kood spel propurly
     
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