Making the circut to power an LED is a breeze. However, LEDs are rated in Mcd(Millicandelas) and I need to get a 660nm red LED such that when powered, will produce 6mw of power at the surface of the LED(I assume measured with a power meter).
The question that's not clear is what MCD LED do I use? How many mcd do I need?
-- Yours, Tom
When you are looking for specific powers, it is often because the application requires some calibrated value for an optical need. Why is it that you need 6mW of optical power? How precisely does this need to be maintained? Could it be 7mW? And your uncertainty about where that power is to be measured makes me wonder if you really need to know the power at all. What is the application, here?
Jon
I don't mean to be skanky, but that's the power I need to replicate a device I have.
What MCD translates to 6mw of output power measured at surface of LED?
-- Yours, Tom
You don't have enough information. Candelas measure the intensity of the luminous flux. To get the total flux you must integrate this value over a sphere around the led. Then to get the power you must apply the human eye sensitivity factor 1/683.
So assuming a uniform intensity i cd (definately not the case for a LED) and a solid angle a, the power would be i*a/683.
vic
Most of the really good high intensity 660 nm LEDs have a "luminous efficacy of emitted light" around 60-75 lumens per radiated watt, as opposed to the 683 for 555 nm yellow-green. So divide by 60-75 instead of
683.-----------
In my experience, most 660 nm LEDs rated 3500 mcd and of the 5 mm diameter I have found to be good ones, producing maybe 4 mW at 20 mA, and this gives hope for 6 mW at the 30 mA maximum they are rated for. 3500 mcd 660 nm 5 mm ones tend to have beams as narrow as possible, usually a roughly 6-8 degree hot spot with a 15-18 degree ring around the hotspot. Wider beam LEDs with lower mcd could put out the same amount of lumens and the same amount of milliwatts.
- Don Klipstein ( snipped-for-privacy@misty.com)
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