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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 TSL235r photodiodes 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.. (so two calculations to give me a 'spec' to search for). 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. I also don't know what a 'sensible' number is and I was getting lux in the 6 figure range. Is that normal?

The required information:

Required power at surface: 1mW/mm^2 (so what's the required LED strength to achieve this). 7200mcd (so what is the theoretical power at the surface)

Viewing angle of LED: 23 degrees.

Diameter of surface: 11.5mm.

Distance from surface to LED: 20mm.

(Apex Angle: 33.67 degrees)

Wavelength of LED: 470nm.

Luminous Efficacy (nv): 78.

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

Datasheet for LED: http://www.farnell.com/datasheets/573417.pdf