Calculate foot-lambert luminance on a screen

In the projection business, there is a unit for ‘how much light is coming from that screen, actually?’. This is called the foot-Lambert or ftL

A cinema screen requires between 10 and 15 ftL If it’s lower (e.g. the projector lamp is not strong enough or the screen is too large), the image is too dark, and the white is rather grey (remember the Long Night episode of Game of Thrones?).

If the footLambert is too high (over dimensioned lamp) , the dark scenes become too bright, and it hurts the eyes.

Calculate the foot-Lambert like this: