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Clarifications for led intensity measurements...

Discussion in 'Electronic Basics' started by KILOWATT, Jan 27, 2005.

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  1. KILOWATT

    KILOWATT Guest

    Hi fellow hobbyists (and professionals!) ;-).
    I'm on the net since may 2000 and just started to discover Ebay a few days
    ago...serious! :) There's some interesting components i can't find in
    stores in Montréal...espacially good deals on leds. Because of the newer
    high-powered white leds availability, it's more and more interesting to
    consider them for lighting requirements. To go to the point, some are
    specified in microcandelas while most high powered ones are specified in
    lumens (here's an nice high power one)
    http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&category=66952&item=3868725654&
    rd=1

    Anyone here have an interesting link where i can find conversion/equivalence
    charts to allow me to better understand those units im not very familiar
    with. TIA for any replies

    --
    Alain(alias:Kilowatt)
    Montréal Québec
    PS: 1000 excuses for errors or omissions,
    i'm a "pure" french canadian! :)
    Come to visit me at: http://kilowatt.camarades.com
    (If replying also by e-mail, remove
    "no spam" from the adress.)
     

  2. The very bottom of this part of Don Klipstein's website talks a little about
    this:

    http://members.misty.com/don/led.html#lc

    Candelas are a measure of luminous intensity, while lumens are a measure of
    luminous flux. They are not equivalent, but they are related. If you stare
    into a laser pointer beam you will find that it has extremely high luminous
    intensity (candelas). On the other hand if you try to light up a whole room
    with the laser pointer you will find that it does a very poor job (total
    lumens or total light output is very low). If you stare straight at a
    standard bare 60W incandescent lamp you will find that it has a lower
    luminous intensity (candela rating) than the laser pointer. On the other
    hand, the 60W lamp produces vastly more lumens, so it will light up a whole
    room much more effectively than the laser pointer.

    If you want to make a very intense spotlight you will want LEDs with high
    luminous intensity (high candela ratings). If you don't really care about a
    bright spot, but want overall bright floodlighting of a whole room you will
    want LEDs with very high lumen ratings.

    Lumens are an interesting unit because they at least try to take into
    account the effects of human eye sensitivity. Human eyes are vastly more
    sensitive to green light than to blue and red light. A 100 lumen green
    light should in theory light up a room to approximately the same overall
    effective brightness (as perceived by a human) as a 100 lumen blue or red
    light.
     
  3. KILOWATT

    KILOWATT Guest

    Thanks a lot Fritz for your very informative reply...was appreciated.
    I discovered that i was wrong with units... millicandelas stand
    for"mcd"...not microcandelas! ;-) Since my OP, i've dicovered this
    interesting site that many are (you?) aware of:
    http://ledmuseum.home.att.net
    Thanks again and for any future replies

    --
    Alain(alias:Kilowatt)
    Montréal Québec
    PS: 1000 excuses for errors or omissions,
    i'm a "pure" french canadian! :)
    Come to visit me at: http://kilowatt.camarades.com
    (If replying also by e-mail, remove
    "no spam" from the adress.)
     
  4. Here's what your auction says about the LEDs:
    My experience with the cheap Hong Kong LEDs that I've been running 24/7
    is that, depending on the current, they last less than 1/10 that figure.
    If I run the LEDs at 20 mA, they might last for 6 months before the
    light decreases to less than half.
    extremes that rapidly incapacitate fragile incandescent lamps

    There's nothing hotter than a tungsten filament, so this is just
    advertising hype.
    operating costs by up to 90%.

    LEDs are little, if any, more efficient than incandescent lamps. Read
    Don Klipstein's LED pages for the truth.
    http://members.misty.com/don/ledx.html

     
  5. Even at 10,000 hours that is still dramatically better than a comparably
    sized incandescant lamp. Many incandescant flashlight lamps have rated
    lifetimes of something on the order of 15 hours, or 40 hours, or maybe a
    couple of hundred (it varies allot). Light output of incandescant lamps
    also decrease significantly with usage as well.
    So a tungsten filament runs hotter. Yeah, so. LEDs are far more immune to
    physical shock than incandescant lamps, so the manufacturer isn't in any way
    lieing about their product here. It is true.


    Claiming 10-50 times energy efficiency is of course most fanciful (except of
    course in the case of colored light sources). The particular LEDs the OP
    linked to have a luminous efficacy of around 25 lumens/watt. Small
    incandescent flashlight lamps have bad luminous efficacy, perhaps in the
    neighborhood of 6 lumens/watt. This does represent over four times the
    efficiency when used as a flashlight replacement. Combined with the LED's
    other advantages such as dramatically longer life, built in optics/reflector
    (external ones optional, but for incandescent they are required and further
    reduce the already low luminous efficacy), ability to be rapidly modulated,
    dimming ability (without major loss of efficiency), etc., LEDs are really
    much more versatile and superior to incandescent lamps in a vast number of
    applications.

    Comparing a little 0.5W LED's luminous efficacy to a large 120V 500W halogen
    isn't really very fair. Even so the LED's 25 lumens/watt is still a bit
    better than the 21 lumens/watt or so for a typical large halogen. Compared
    to a typical 60W non-halogen long life lamp (long meaning 1000 hours)
    operated at 120V the LEDs excel even more since such an incandescent might
    only achieve 14 lumens/watt. In high voltage (240V) countries the luminous
    efficacy and durability/mechanical shock resistance of the incandescent is
    even worse.
     
  6. Some submini incandescents are rated for over ten thousand hours.
    It is _not_ true that LEDs will handle heat better than an incandescent,
    as the ad implied.
     
  7. Unfortunately at less than something like three lumens per watt.



    Oh right. Yeah I glossed over that part of their statement. Yeah LEDs
    don't handle temperature extremes better than incandescent. In fact they
    handle high temperatures much worse, while low temperature extremes are
    pretty immaterial since neither have any problems with any reasonably low
    temperature.
     
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