How to Calculate Light Emitted from an LED Screen

[pixelpusher]

Hi,

I have a question how to calculate the light emitted from a LED screen. The LED display is rated at 5000 nits. Total size of the display is 64sqm. (8m x 8m touching the floor) I would like to calculate the total light emitted, the light on the floor in front 1m away and the light on the floor 10m away. The calculations don't need to factor in every detailed aspect, I am looking for a rough value how the screen would affect the lighting situation in it's vicinity at full white.

Light on the floor should be calculated in LUX, correct? Can somebody help me out with the calculation? Much appreciated, light calculations are quite confusing at times. Thanks
pp

 

[Andy Resnick]

Yikes...

"Nits" is a photometric unit of radiance, and 'lux' is a photometric unit of irradiance. So at least the conversion is straightforward. What you need to know is the L(Ω), how the radiance varies with direction. Maybe the LED is lambertian (L is constant), maybe not. Either way, it's pretty much geometry- you have an emitting plane and a receiving plane oriented at some angle to the emitter, so use the 'fundamental equation of radiative transfer' to calculate whatever you need:

https://books.google.com/books?id=i...tion of radiative transfer radiometry&f=false

 

[pixelpusher]

Thanks for your answer Andy. So if the screen and the projected surface would be parallel to each other I could ignore L(Ω)? Would 5000 nits * π = 15700 lux be sufficient as conversion? Not sure about the steradian? Why only 1*π ? What about increasing distance? Is it impossible to calculate, because the screen is a planar light source? My head hurts..

Thanks

 

[Andy Resnick]

Thanks for your answer Andy. So if the screen and the projected surface would be parallel to each other I could ignore L(Ω)? Would 5000 nits * π = 15700 lux be sufficient as conversion? Not sure about the steradian? Why only 1*π ? What about increasing distance? Is it impossible to calculate, because the screen is a planar light source? My head hurts..

Yeah, radiometry/photometry can be like that. Just remember that it's all about the transfer of energy. You are right to start with the surfaces facing each other, that simplifies things considerably. Begin by working through eqns 3-9 to 3-4 in the reference I provided above. Then section 3-3 can guide you through your problem, eqn 3-20 is particularly relevant, as are eqns 3-24 through 3-28.

Wolfe's book is excellent and worth having around for reference.