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    Ogre in the Playground
     
    Altair_the_Vexed's Avatar

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    Default Radiant intensity units VS Luminance units

    I've got a unit conversion problem that is doing my head in!

    My LED supplier says the maximum radiant intensity of their LED is 390 mW/sr (I take this to mean milliwatts / steradians).
    My safety standard for my product has a luminance safety limit of 10 000 cd/m^2 (meaning candela / metre square).

    I can't see how to convert between these two sets of units! Can anyone please help me out?
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    Colossus in the Playground
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    Default Re: Radiant intensity units VS Luminance units

    You need to know how far away the bulb is from the thing being exposed to bulb light.

    1 candela = 1/623 watts per steradian (0.001605 watts per steradian, or 1.605 milliwatts per steradian).



    https://en.wikipedia.org/wiki/Candela

    390 milliwatts per steradian = 242.97 candela. Seems awful bright for an LED - 15 candela is considered "ultra-bright".

    If your product can stand 10,000 candela per square m, then the question becomes - how far away is the product from the bulb, and what's the actual exposure?
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    Ogre in the Playground
     
    Altair_the_Vexed's Avatar

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    Default Re: Radiant intensity units VS Luminance units

    Quote Originally Posted by hamishspence View Post
    You need to know how far away the bulb is from the thing being exposed to bulb light.

    1 candela = 1/623 watts per steradian (0.001605 watts per steradian, or 1.605 milliwatts per steradian).

    https://en.wikipedia.org/wiki/Candela

    390 milliwatts per steradian = 242.97 candela. Seems awful bright for an LED - 15 candela is considered "ultra-bright".

    If your product can stand 10,000 candela per square m, then the question becomes - how far away is the product from the bulb, and what's the actual exposure?
    Ah, that makes the beginnings of sense! Thanks.

    It'll be surface area of the LED itself, BTW, rather than the target. It's about 2.5cm^2. I'll do that maths once I'm back at work.

    EDIT: back in the lab, and it looks like this LED - if they've given me the right numbers - is way too powerful to pass the safety standard.
    It's IR, so I can't tell by eye, but I think I'm going to have to go back to the client and demand more data ask for confirmation of their data.
    Last edited by Altair_the_Vexed; 2017-12-20 at 04:18 AM.
    Running the Game - the loneliness of the long distance blogger
    Quote Originally Posted by Jay R View Post
    It is overly convoluted. It adds complexity without clarity. Its effects are not clearly understood. It is intended to solve a problem that is not well defined.
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    .
    Quote Originally Posted by MrStabby View Post
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    Barbarian in the Playground
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    Default Re: Radiant intensity units VS Luminance units

    Looks like you need to know the rough frequency output of your LEDs (usually this is defined as "color temperature", but with IR I'd expect a different type of output.

    Note that IR LEDs tend to pump out a broad array of frequencies (the ones I worked with glowed visible when lit, so obviously spilling over the "IR" boundaries). So even more data from your supplier may be needed.

    Finally I'd assume that all this data is from a nominal amount of current passing through the LED. If you pump more current through, all bets are off.

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