Uniformity refers to not only how the screen displays an image, but also how well the projector is projecting the image. Traditionally, screen measurements are made by pointing a light metering device at the center of the screen. This is good information, but it only tells how bright a screen's center will appear from different viewing angles ranging from 0 to 180 degrees. Information and images on the edges may appear dimmer.
Projector manufactures also do a type of light reading of their projector called lumens. The American National Standards Institute (ANSI) has created a way to measure brightness of an image. The image is divided into 9 rectangles each of which measures 1/3 of the screen Width. A brightness reading is taken at the center of each rectangle and then the average of the nine readings in lux (lux = lumen/square meter) shall be multiplied by the number of square meters of the image at the plane of the meter reading. The result is the light output of the projector in lumens.
Optical coating may also help in creating light uniformity. Optical coating scatters light rays producing a uniform image.
Resolution† is defined as “the fineness of detail that can be distinguished in an image, as on television." There are two major projectors, which either deal with bandwidth (CRT Projectors) or with pixels (LCD, DLP, D-ILA Projectors).
When we look at an image, we are actually seeing the product of the visual chain. Image data is combined from software and hardware, and then electrically divided up into an exact number of bits. Projectors convert the bits into beams of light, which are called pixels. One pixel is the smallest image-forming unit of video display.
If a projector has a resolution of 640 by 480, we can then calculate it will cast an image with 307,200 pixels. Pixels are organized in horizontally and vertically, completely filling in or not filling in each pixel with color creates every image. A good way to compare an image made by pixels is to a large puzzle with tiny pieces.
Contrast is just as important as uniformity and resolution. Contrast simply is the difference between light and dark areas. Contrast depends on the ratio between the maximum and minimum light levels within any image. It is calculated by dividing the peak white levels by the light level at the dark part of the picture. Measurements are taken at the peak white and an area of black near the white. Blacks are never true black in color; they are shades of dark gray. The reason is that projectors do not illuminate the color black, black is the absent of light. If you turned on the lights during a presentation, the screen once filled with vibrant colors and contrasts are now faded and dull.
Half Gain Viewing Angle
Half gain is the standard that the projection screen industry uses to measure the brightness performance of a projection screen when the viewer is observing the screen from am extreme angle or "off to the side".
A projection screen's peak brightness is when the viewer is directly in front and perpendicular to the center of the screen. This is referred to as Peak Gain at Zero Degrees Viewing Axis. As the viewer moves out to the side of the center of the screen axis, the brightness of the projected image will drop off. When the brightness drop off angle reaches 50 percent of peak gain, which determines the screen's half gain (or half brightness) viewing angle specification.
Example: Let us say a projector screen has a peak gain of 4 and its output is 22 Foot Lamberts when directly in front of the screen at zero axis. Let us say the screen's half gain specification is at 52 degrees. That means that when a viewer is seated at 52 degrees of center screen, the viewer would observe half the brightness as a viewer that is seated directly in front of the screen - 11 Foot Lamberts - Gain of 2.
† Defined by The American Heritage Dictionary