LED Vs Distance

I'm new to this group. I hope this group would be the best one for design innovation and support. Following are my first queries,
How do I measure the distance supported by LED? if the LED provided with following spec :
a)Light Intensity : 15mcd, viewing angle of 60 deg @ forward current 10mA
b)Emiting color : Green
c)Peak wave length l: 565nm, dominan wave length ld : 568nm

Please assume that the receptor is of normal human or equivalent. Please give me the information for scotipic and photopic domain

Can I carry forward the answers for LED-Displays also (which contains collection of LEDs)


Thanks in advance

You need an NIST calibrated eyeball. Come back after you get that and we will go from there.

If you cannot obtain a calibrated eyeball. Do what I would do make a mock-up and field test under various lighting conditions with different test observers.

Hello Munichand,

Welcome to CPF.

Light generally falls off with the square of distance. I believe green light does the same.

To find the distance down to 1 foot candle, you take the square root of the intensity.

In your case, you have 0.015 candella which is similar to 0.015 foot candles or 0.015 lux. In the case of foot candles, the square root of 0.015 is about 0.122. That means that at 0.122 feet (or about 1.47 inches) you will have 1 foot candle. In the case of lux, the square root of 0.015 is still about 0.122. That means that at 0.122 meters you will have 1 lux.

1 lux is about 0.0929 foot candles and 1 foot candle is about 10.764 lux.

With multiple light sources I believe you can add the values, as long as they are positioned so the do not interfere with each other.

Tom

Hi SilverFox,
Seems , Your answer is very interesting. But, based on the feedback i could not able to measure the lux. How can I compute the lux at distance of 10 meter. Also my datasheet mentioned with 60 deg viewing angle. Shouldn't i consider this. Also based on the observer the viewing distance may increase or decrease. But my query is what best it can reaches in photopic and scotopic region to the ideal human.

Eventhough the query is complex to answer, I feel you expertise can instantly answer this.

Hi Double_A,
I will try to get Eyeball, But i need some theoratical calculation right now.

Thanks

Hello Munichand,

Your 0.015 candela light will illuminate an object at 10 meters with 0.00015 lux of light. Your initial value is for on axis, you would have to take measurements within the 60 degree viewing angle and do some math to figure out the distances involved to come up with additional values. At 30 degrees off axis, I don't think you still have 0.015 candela.

The transition from scotopic to photopic vision occurs at around 3 candela (I think), plus or minus, depending on how sensitive your eyes are. It looks like you would need a lot of those LED's to illuminate an object at 10 meters...

Tom

If you want to minimize the number of LEDs then use ones with more lumens and with narrower beam angles. Is there any good reason you need a 60° beam angle as opposed to 15° or 20°? And the specs for that LED seem horrible by today's standards. I've seen 60° LEDs with maximum brightness in the 1500 to 3000 mcd area. Even keeping with your 60° beam angle you would only need about 100 to 200 of these brighter LEDs to illuminate a target at 10 meters to a brightness of 3 candela (=3,000 mcd) as opposed to 20,000 of the LEDs with the spec you gave. If you go with narrower beam angles, one Jeled 50,000 mcd white according to my tests will give about 41,000 mcd at 1 meter, or 410 mcd at 10 meters, when running at 20 mA. You would only need about 8 of these to get the necessary light intensity. Narrow beam highly intense colored LEDs also exist if for some reason you have to have a specific color.