# Struggling with light calculations (incident power output)

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Hi guys, not sure if this is the right place. 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 a long post 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 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 whats 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

## Answers and Replies

After having a little look around I think I probably should have posted this in the homework help forum? (It's not homework but that forum seems to have more similar type questions)..

Very sorry!

Tom.G
Science Advisor