i have this projector setup with a light source having about 500 lumens as its light output.I know the distance to the fresnel lens which will be at the focal distance so that parallel light goes through the lens. I want to know how much of this light would be falling on the fresnel lens. I think this should be the illuminance or the brightness. Now i need to find out how much of the light is required for the eye to see the image clearly. Since in this case when some lumens is coming out of the light source this shld be converted to candela/steradian and hence i need to determine the angle formed by the light source to the fresnel.This is needed so that i can design the reflector which will be my next in the task. considering that the contrast ratio is abt 20:1 . i dont have much idea about this contrast ratio and would like to know how this could be related to how much brightness the eye can see. The brightness and the illuminance at the fresnel need to be found out which can be used with the magnification of the image known to say what the luminance on my final image would be.