The Screen Does Make a Difference

A commentary by Joe Kane

Until the beginning of this decade, video projectors occupied a “niche” in the corporate and industrial market. The primary goal was to present a large, bright video image. The quality of that image was far less important than obtaining the highest possible light output from the screen. Emphasis shifted towards image quality in the early 1990’s with the coming of home cinema. Audio quality could already rival that of most neighborhood movie houses. Similar expectations were being placed o­n the video, within the limits of available software.

As a display standard consultant, I was brought in at the beginning of this emphasis shift in an attempt to make it happen faster. My first efforts centered o­n making projectors conform to accepted industry display standards. That was and is such a full time task that it took me nearly a year to understand the role being played by the screen in image quality.

One problem with high gain video screens is “hot spotting”. The image artifact comes from focusing the light in particular direction. From any viewing angle there will appear to be an area of the screen that is brighter than any other. The position of the hot spot will depend o­n your viewing angle. It will change as you move around the screen. A high gain video increases the difficulty of obtaining a high quality video image. Reducing the size gain, from the popular flat screen values of 2.0 and 1.5, diminishes the amount of light fall off as you move around the screen. But that isn’t the o­nly change needed to improve picture quality. Ideally, screen gain should be at unity, but some projectors still require help from the screen in obtaining a usable amount of light. Screens with gain factors above o­ne will have some degree of hot spotting. What I realize is that attention is being drawn to the hot spot, primarily because of the color shift as you move around the screen, and not the light intensity difference.

Screen reflectivity of red, green and blue light o­n most high gain screens is not equal, green being the highest, followed by red and blue. Your attention is drawn to hot spotting primarily because of the color shift you see as you move around the screen, not the luminance or light level shift.