SMD LED: How to calculate optical power (in mW) from luminous intensity (in mcd)?

Thread Starter

Patrick19

Joined May 22, 2019
11
I chose an LED for my application but I need optical power, in mW.

In the [data sheet][1], luminous intensity is shown in mcd but I did not find formulas for calculation OPTICAL POWER [mW] although there are graphs, etc.
Where luminous intensity is 900 mcd and viewing angle 120°, INTENSITY 3000 mlm.

Please provide any necessary formulae for my case.

I could not find the exact formula on this website.
data sheet of SMD LED
:https://ro.mouser.com/ProductDetail/OSRAM-Opto-Semiconductors/LT-A6SG-V2AB-35?qs=sGAEpiMZZMseGfSY3csMkdgyOOAg6kv27McJjvoUqH/vTOfP8fK0Yw==
 

oz93666

Joined Sep 7, 2010
737
I need optical power, in mW.
You cannot get optical power in mW .... mW is a measure of the electrical power fed in .... light output will vary with different leds for same input ....

It will also vary (up to 50%) with the same led .... driving an led at high currents will give low lumens/Watt ... At low currents they are much more efficient.

As a rough figure output is usually about 100 lumens / Watt

A lumen is a measure of total light output ... a Candella ( or mcd ... millicandella) is a measure of INTENSITY of light .... if focused to a narrow angle the same lumen output will create a higher candella reading ....

A bare led emitter will have a wide range of candella (intensity) readings .. highest 0 degrees , lowest at 90 degrees

This type of led incorporates a dome shaped lens , focusing the light fairly tightly

 
Last edited:

Sensacell

Joined Jun 19, 2012
2,567
Here's an idea, or more should I say an interesting waste of time...

Take a high power LED, mounted on a minimal heat sink with a temperature sensor.
create a closed loop servo that controls the temperature of the system, using PMW.

Under controlled airflow and ambient temperature conditions, measure the power consumed to maintain a given temperature.

Now, take the identical setup and apply a tiny amount of black tape, or paint on the LED to totally absorb the light.
Run the setup until the temperature stabilizes at the same point, measure the input power.

The difference in power input will be exactly the amount of light energy normally radiated into space by the LED.
 

Thread Starter

Patrick19

Joined May 22, 2019
11
Dear fellows, I put this question in other forum , for similar case of SMD LED and this îs answer- bellow -please comment about steradian result and even formula for optic power seems strange for me:
Parameters: peak wavelength 527 nm, Luminous Intensity 0.18 cd at 5 mA and 2.9 V, emission angle 120 °.

From Wikipedia article "Candela" :

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

So 1cd = 1/683 W/sr = 1.464 mW per steradian.

Luminous intensity measures (lm and cd) are linked to human eye sensitivity curve, which complicates re-calculations into optical power. The maximum in light frequency in the above definition corresponds to wavelength of 555 nm, which is pretty close to the green LED emission, so we can ignore the difference along the "Photopic Spectral Luminous Efficiency Curve".

The LED emits into 120°, but if we want to calculate total emission, we need to integrate over all angles, so the "effective" (ballpark estimate) angle for "flat emission" will be about +- 50°. One steradian is +-33°, so the led illuminates an area of about (50/33)^2 = ~2.3 sr. The narrower is viewing angle, the brighter is LED appearance, but the emitted power is obviously the same.

Therefore, the emitted power is 1.464 mW/sr * 0.18 cd = 0.263 mW/sr, times 2.3 sr, or about 0.6 mW total.

The consumed power is 5 mA * 2.9 V = 14.5 mW, and therefore "luminous efficacy" of this LED is about 4%.
___________________________
I could not contact this person ți explain his answer. So please help with explanation of his formula.
 

Picbuster

Joined Dec 2, 2013
998
I chose an LED for my application but I need optical power, in mW.

In the [data sheet][1], luminous intensity is shown in mcd but I did not find formulas for calculation OPTICAL POWER [mW] although there are graphs, etc.
Where luminous intensity is 900 mcd and viewing angle 120°, INTENSITY 3000 mlm.

Please provide any necessary formulae for my case.

I could not find the exact formula on this website.
data sheet of SMD LED
:https://ro.mouser.com/ProductDetail/OSRAM-Opto-Semiconductors/LT-A6SG-V2AB-35?qs=sGAEpiMZZMseGfSY3csMkdgyOOAg6kv27McJjvoUqH/vTOfP8fK0Yw==
This is not possible.
Reason: the producer can only measure the light output.
Every production batch is producing slight different results due to small changes in used materials resulting in less light and more heat or a shift in light spectrum.
Please consult the spec sheet.

Picbuster
 
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