Lux:

The degree of image brightness required in order to perceive the image clearly (the amount of light coming from the projection screen) depends on the level of ambient light, the gain characteristics of the fabric type and the projector’s light output. Optimum results and legibility are achieved when the projector’s light output, the size of the projection screen and the gain characteristic of the screen fabric are all appropriately matched to each other. The following guideline helps here:

LUX = (Projector ANSI Lumen/ Screen surface area in m2) X gain

LUX = 100 low ambient light levels
LUX = 400 normal ambient light levels

The perceived amount of light that is reflected by the projection screen is expressed in LUX. Optimum image quality is achieved when the ratio between the LUX value of the projected image and the wall behind it is at least 5:1. As a general guideline, one can say that in normal ambient light conditions, the background light output is approximately 80 LUX meaning that the quantity of light that comes from the projection screen needs to be about 400 LUX. In conditions of low ambient light, the background LUX value is about 20, meaning that the LUX value of the screen needs to be about 100.