Front projection screen materials have evolved over the years. In the days of filmstrip and 16MM projectors, we had glass beaded screens. For a long time, we have had matte white screens with both unity and higher gain, (to make up for lower light output projectors). Recently, while matte white screens are certainly still around, we have entered into the era of gray screens and ambient light rejection screens.
To pay proper respect, a traditional matte white screen like the Stewart Filmscreen SnoMatte 100 is a near perfect diffuser of light (a Lambertian surface), and is considered a reference standard of reflectance. This is the property that defines an ideal "matte" or diffusely reflecting surface. The reflected brightness of a Lambertian surface to an observer is the same regardless of the observer's angle of view. In basic terms this means that the screen reflects nearly identically across a 180° viewing angle, as produced by the projector. In a totally dark environment this type of reference screen is like a studio monitor in audio. It brings no added “coloration” to the equation and takes into consideration all angles of view. The illustration below shows the diffusion properties.
A Lambertian surface in the form of a matte white screen is great, but when ambient light falls on the surface, the image is degraded in terms of both contrast and color saturation. One answer is to totally control the ambient light in a room and/or use a brighter projector to overcome that factor… or a gray screen can be used. A gray screen is designed to resist some of the elements of ambient light, and significantly improve contrast and the appearance of color saturation as a result. The key issue here is that gray screens are intended to be used in low level ambient light situations, with general illumination. While they do have a degree of ambient light rejection capability, they are not able to reject a significant amount of ambient light coming in from all angles such as windows, open spaces, and areas with little to no lighting control. This begs the question… what to do in those cases?
With limitations being accepted, and most lighting out of the control of the AV designer, the choice should be an ambient light rejection screen (ALR for short). An ALR is a screen engineered specifically to reject light coming in from obtuse angles. To understand better, there are two basic type of reflection; specular reflection (where the angle of incidence equals the angle of reflection) and diffuse reflection. See illustration below:
ALR screens use a combination of optical dispersion techniques to accomplish their task. Each ALR screen has characteristics that define it. There is both a horizontal and vertical viewing angle that is designed as the “sweet spot” for the viewer. Once again, we can’t break the laws of physics, so the techniques of rejecting ambient light will reduce the viewing angle for the audience to a varying degree. As with gain screens, going beyond the specified viewing angle will reduce the quality of the image on the screen and the ambient light rejection capabilities… but within the specified zone, the ambient light is directed (reflected) outside of the viewer’s eyesight. In this process, brightness, contrast, and color saturation are maintained in the presence of higher ambient light and in some cases even enhanced.
The point here is that there are really three general types of screens, and they should not be confused. A matte white screen is a near perfect Lambertian surface, ideal as a reference standard in dark environments. A gray screen is designed to enhance contrast and color saturation in the presence of a small amount of general ambient light. Finally, an ALR screen, designed to maintain image integrity for applications where there is a great deal of ambient light on the screen from numerous angles. Knowing when to use what can save the day for an AV designer.
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