# Calculate light intensity after reflection/refraction

Hi all,

I'm playing around with an autocue type of display. So basically, there is an LCD display that reflects light on a piece of plexi, at a 45 degrees angle.

I know that the LCD dispay is horizontally polarized, so I can calculate the refraction and reflection coefficients of the horizontally polarized light using the fresnel equations to find out how much light is reflected to the viewer, and how much light is refracted and thus lost.

The brightness of the LCD display is 300 cd/m^2.

Now, I would like to express the brightness of my 'autocue display' in cd/m^2 too.

So my first question is; Do I just multiply the calculated reflection coefficient with 300 cd/m^2 to obtain the brightness of the autocue display?

Next, I would also like to express the percieved brightness of the screen depending on the viewing distance. When I'm watching the screen from 30cm distance the percieved brightness is much higher then when I'm watching it from 2m distance.

Now, I know that light intensity follows the inverse square law, so when the distance doubles, the intensity is divided by 4.

So my next question is: How can I express and calculate the percieved brightness at a certain distance? I'm having a feeling I should work with lumen instead of cd/m^2 for this part, but I'm not sure how to proceed...

Thanks a lot!
Arno