# Thread: Compute irradiance over an array with values from leds datasheets

1. ## Compute irradiance over an array with values from leds datasheets

Hello,

Here is my problem.

I want to compute the irradiance over a flat area at distance d from the light source (LS).

The light source is a led. From all the Cree, Lumileds, Osram or Led Engin datasheets, I can extract these values:

Radiant Flux express in Фe (W)
FWHM or viewing angle express in °

With these value, I try to compute the radiant intensity Ie (W/sr) and after the irradiance Ee (W/m˛) of a surface at d from the LS and view with an angle θ theta relative to the normal to the surface.

So, some leds have a radiation pattern which is close to a lambertian pattern Iθ = I0.cos(θ) but other leds and lenses have dissimilar patterns.
All these radiation patterns are relative which means that the radiant intensity vary from 1 to 0 for 0< θ < 90°.

As I know the Radiant Flux how can I compute the radiant intensity I0 for θ = 0° ?

I try different methods, the last one:
I assume that the radiant flux ФE is contain in a 2π sr angle which is an hemisphere.
I divide ФE by 2π = W/sr and look at this value as the radiant intensity I0 for θ = 0°.
But it is not the good method.
So, what is the good ?

I compute the area under the radiation pattern chart. This area is equal to 100% of the radiant flux Фe. but with this method at θ = 0° the radiant flux = 0 and of course, θ = 0° means solid angle equal to 0 !
Perhaps but….
So I take a relative radiation pattern, extract values with “Datathief” soft and work on the value.
Using Excel, I compute the area under the line. Assuming that at the half angle θ1/2 value the radiant intensity is half the peak intensity, I compute the relative part in % of the area from 0° to θ1/2 value and from θ1/2 to the include angle.
So for example using the datasheet of the Osram LH-CP7P, I have :
Фe = 0.728W
θ1/2 ± 40°
Values from the relative radiation pattern => Compute array
From 0 to θ1/2, S0 to θ1/2 = 79.29 % of the total surface
From θ1/2 to 90°, Sθ1/2 to 90° = 20.71 % of the total surface
So, If I think properly, 79,29% of Фe is contained in the solid angle describeb by the θ1/2 angle.
As θ1/2 => 2π . (1-cos(θ1/2)) sr , 40° => 1.46999 sr
Therefore, as I have 79.29% of Фe in 1.46999 sr , can I compute the radiant intensity at 40° as I40° = 0.728 * 0.7929 / 1.46999 = 0.393 W/sr ????
As I40° = 0.5 I => I = 2 . I40° = 2 . 0.393 = 0.785 W/sr ???????

Does any of the forum member do this sort of computation ?

2. ## Re: Compute irradiance over an array with values from leds datasheets

So,

After an other morning to "cogito", I find a method which seems to be good.

In this .zip file I found some files. Two of them are interesting.
The first one is a pdf with two chart:
- Near Field Irradiance in W/m˛
The first chart allow me to know the on axis Radiant Intensity at a fixed Radiant flux.
So : 1.912E-1 W/sr for 3.1923E-1 W

Therefore, for other radiant flux value, as the peak of radiant intensity is proportional to the radiant flux the calcul is simple.

But these charts are not available for other led from other brand.
IES files are available.

If we open this .ies file with a soft "IESViewer), it display the radiation pattern with values.
Values are :
- lumens
- candela
As I want W and W/sr and as Candela = Lumens/sr I think a can use a proportionality rule to compute Radiant Intensity.

For example:
If I use the value from the .pdf file.
- 1.912E-1 W/sr for 3.1923E-1 W
From .IES file
- 14.1 candelas for 23.7635 lumens
So let's go:
14.1 / 23.7635 * 3.1923E-1 = 1.89422E-1
This value is close to the one find in the pdf.
I explain the difference between this two value because the radiation pattern in the .IES file is not as accurate as the real measurement use for the .pdf file chart.

3. ## Re: Compute irradiance over an array with values from leds datasheets

Anyone do these calculations ?

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