HOME > Review: Ambient Light Rejections Screens
Ambient Light Rejection Screens
March 3, 2016,
Contrast as viewed from the center position is only one of several factors that contribute to image quality and screen functionality. A second vital factor to consider is the viewing angle -- what does the picture look like when viewed from a position other than dead center zero degree viewing axis? These eleven screens behave quite differently in this regard. Some quickly get dim the moment you move away from a center viewing position and others remain remarkably bright even when viewed from an extreme side angle.
The statistical measurement used to quantify this phenomenon is the half gain angle. This is the angle at which the center of the picture appears to be half as bright as it does when viewed along the zero degree viewing axis. A screen with a small, narrow half gain angle will look bright at dead center, then quickly become dim as you move to the side. Conversely a screen with a large, wide half gain angle will continue to look reasonably bright as you move away from center.
On ambient light rejection screens in particular, not only does the screen begin to dim when you move to the side away from center, but the brightness uniformity degrades as well. When you view from the position of the half gain angle, the center of the screen is half as bright, but the far side of the screen is much dimmer than the near side. This is about as bad as you'd want it to get, so viewing at an angle greater than the half gain angle is considered to be unacceptable from an image quality perspective.
On spec sheets and in promotional literature, vendors often use the technical term viewing angle instead of half gain angle. The viewing angle is double the half gain angle. Why? If a screen has a 30 degree half gain angle it will look half as bright as it does when viewed from the center position if you move either 30 degrees to the left or 30 degrees to the right. So the total "viewing angle" is considered to be 60 degrees.
There is nothing inherently good or bad about wide or narrow viewing angles, but it means that you need to choose a screen that will meet the needs of your anticipated audience -- If you have a small viewing room with two or three seats placed in front of the screen near the zero degree viewing axis, a screen with a narrow viewing angle will be just fine. On the other hand, if you expect people to be viewing from very wide angles, you will need a screen that will give them a bright and relatively uniform picture when seen from the sides--that is, a screen with a very large, wide viewing angle.
In our ambient light mid-day set up, we put our spot meter on the center of each screen when viewed at zero degree axis, and then moved to the side toward the direction of the incoming ambient light to find the angle at which the meter was reading half the amount of light it was at dead center. To avoid confusion, we give you both the half gain angle and viewing angle for each of the screens in this review:
** For comparative purposes only. These ratios show how the screens responded to this particular ambient light condition with this particular projector. They may or may not match official screen specifications.
Not only do screens begin to lose brightness and uniformity when you move sideways from the center viewing axis, they typically lose brightness and uniformity if you move up or down and look at them from either above or below the center neutral position. This phenomenon is measured by the concept of the vertical half gain angle, which is the angle above or below the center neutral viewing axis at which the picture looks half as bright as it does at the center position.
A screen with a narrow vertical half gain angle will require precise placement of the projector at an angle exactly complementary to the vertical viewing angle of the audience. Slight deviations from this angle may introduce vignetting, uniformity, and brightness issues. A screen with a wider vertical half gain angle will allow more latitude in the vertical positioning of the projector relative to the audience.
The screens in this review measured the following vertical half gain angles under the ambient light conditions used for the Contrast Test 1:
** For comparative purposes only. These ratios show how the screens responded to this particular ambient light condition with this particular projector. They are not official screen specifications.
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