Update: Projection Screens
Their gain, when compared to a magnesium carbonate surface, may be around 1.0 because the gray material pulls the light down, then it comes back up to nearly one by being directed forward. But when you look at the screen it will be a shade of gray compared to the magnesium carbonate surface.
Okay, why isn't directing the light forward a good idea? Who sits at the edge of the screen anyway? That has certainly been conventional thinking. Besides, high-gain screens provide isolation for the centrally located viewer from ambient light coming in from the sides of the screen, the bad viewing environment we were telling you about. The problem is, just as you see hot spots when sitting close to a rear screen set, there are hot spots in a high gain front screen at the proper viewing distance for HD images. The entire image won't be uniform in intensity. Worse yet, if you move around the position of the hot spot in the screen will travel with you. No two members of the audience will see the same image.
We found another surprise in the way gain was being measured. It turns out the lamp source being used to measure gain is incandescent, red-orange in color. It doesn't cover the entire color spectrum. The lack of covering the entire light spectrum of film or video has introduced errors in the numbers being quoted for gain. The numbers are not reliable when trying to figure out the screen size and type for a given projector.
We've seen cases where the numbers we measure for screen gain are significantly higher than the numbers provided by the manufacturer because we make our measurements from the light of the projector instead of an incandescent lamp.
The overall numbers used for gain should be derived using a lamp source that represents film or video projection. In today's world measurements are based on UHP or Xenon lamps, one of those lamps would be a better choice of a light source when measuring gain. You'll notice the unfiltered Xenon lamp is a bit shy on red, but is still a better representation of the light shown on screens from film or video projection than an incandescent lamp.