Struggling with calculating light intensity / power / conversions

Thread Starter


Joined Jan 13, 2014
Hi guys, not sure if this is the right place but its to do with an electronic project I've been working on. However please point me somewhere else if you know anywhere better suited! (It is quite long sorry)

Basically, I have created an LED array used for stimulating cells with light and I need to calculate the output power from the LEDs incident on a surface a certain distance away from the LEDs. I have grabbed some TSL235rs for this and just measure the frequency at the specific distance away and then convert that frequency to mW/m^2 and then to mw/mm^2 (which is what I need my results in).

The bit I am struggling with is the theoretical calculations. I know the LEDs strength (7200mcd, their luminous efficacy etc from the datasheet.) and I need to somehow convert this into a theoretical power at a surface a specific distance away and also calculate a theoretical required mcd to give a specific power at the surface.. I've been looking online for a while but sites/guides seem to use different terms for the same thing (flux/lux/lumens/iridescence/intensity etc) and its very confusing trying to get my head around it.

It would be great if somebody could walk me through the steps for this so I actually understand what I am doing (if I don't understand it, how can I be confident in the results??)

Am I correct in thinking I can go from a required mW/mm^2 -> lux -> luminous flux -> required mcd?
And I can go from a given mcd (with angle, distance, luminous efficacy) -> luminous flux -> lux -> mW/mm^2?

I have tried these calculations but I just don't believe in the steps since some sources don't seem to allow you to convert from one to another yet others do.

The required information:
Required power at surface: 1mW/mm^2 for one calculation, and 7200mcd for the other. (opposite theoretical calculations).

Apex Angle: 33.67 degrees. Viewing angle of LED: 23 degrees. Diameter of surface point: 11.5mm. Distance from surface to LED: 20mm.
Wavelength of LED: 470nm. Luminous Efficacy (nv): 78.

That should be enough to do the calculations I think .. but I am very stuck and confused. Thanks in advance!

Datasheet for LED:


Joined Sep 9, 2010
I don't think you can rely on a calculated value if intensity is important. You'll need to measure it. The calculation is fairly complex anyway. You have to essentially build a model to convert from mcd (what the LEDs put out) to lux (the intensity hitting a surface) and that model is complicated by multiple LEDs, angles off-axis and so on. Better to just measure it.

Thread Starter


Joined Jan 13, 2014
I am calculating it on a per LED basis sorry - not the whole thing. Each LED is treated as its own individual light source that does not interact with the others. (they all provide stimulation to cells independently within enclosed wells).

I am measuring it however I need to just create a rough estimate of some requirements.

So I can say: A power of 1mW/mm^2 is required at the surface 20mm away. Therefore an LED of 'minimum strength' is required.

And then I can choose my LEDs and say 'These LEDs are 7200mcd in strength, therefore they should output roughly xxmW/mm^2 at a surface 20mm away which fulfills the requirements etc.

This is then actually tested via measurements.

I have actually already been on both of those links! Its just a rather confusing subject (or complex as wayneh said). I'll have another play around though, thanks :D