I'm trying to determine how bright my projection image will be on a projected surface. I'm struggling to find a formula for this. I'm using (for example) the Canon Rialis projector which is 4500 lumens. I would be using the zoom lens with maximum zoom so the throw ratio would be 7:1. At a distance of 100 feet from the projected surface the projection would have a width of 14.28 feet. Also we are assuming the ambient light will be around.5 foot candles. (it will be after dark but outdoors in an urban environment). Any help is much appreciated.