# Need Help Calculating mcd/m^2/lx



## tf8252 (Jan 21, 2013)

Guys,

I am NOT an engineer so forgive me...

My company is developing an LED road marker that outputs light using Cree's 15 degree round LED.
This lamp is rated at 16800-35000 mcd when driven with 20mA.

We are driving it with quite a bit _less than 20mA_.

I have an Amprobe LM-200LED light meter.

What I'm trying to do is calculate the the output of our LED relative to the
unit of measure in the tile of this post (http://www.amazon.com/dp/B005E0Y1KG/?tag=cpf0b6-20). This is the unit of measure
the Department of Transportation uses to specify how bright a road marker should
be....






Can anyone explain to me how to use this meter to measure the LED's output
in terms of mcd/m^2/lx?

(For example, I'm not sure how close to hold the meter to the light source.)

Thanks in advance for any help.


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## Optical Inferno (Jan 24, 2013)

That particular unit mixes units of luminance and illuminance. What they are wanting to know is how much luminance (mcd/m^2) you get per illuminance (lx).

Your problem is that you are not relying on a retro-reflector and instead are using an LED source. You may want to confirm that you are using the correct standard for your particular device.

If you are seriously considering creating a road marker that is LED based I would suggest not using that lightmeter and instead use an integrating sphere or (better yet) have a third party do the testing for you. Contact Calcoast ITL. in California and they may be able to point you in a better direction.

Finally, tell your lazy engineer to research this stuff himself. LOL.


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## SemiMan (Jan 25, 2013)

I could answer you, but normally that charge would be $125/hour and that's with follow on work ... that and my answer would not get you any closer to where you need to be.

If you guys don't know how to do this basic stuff, I suggest hiring someone who does. This is a safety item and not something to be taken lightly. There is far more to this than simple unit conversions.

Let me guess, solar too?

Semiman


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