How Ambient Light Rejection (ALR) works: clearing the confusion
March 21, 2018
Ambient light rejection (ALR) projection screen technology has been around for a while now. Despite this, there is some confusion in the AV marketplace as to how ALR works. Some of this confusion is caused by manufacturers making inaccurate claims or promoting non-ALR surfaces as a solution. To help clear the confusion, here are some things to remember about ambient light rejection:
All screens have varying reflective characteristics
Every screen has a certain percentage of diffuse reflection and specular reflection. Diffuse reflection is when light hits a surface and is reflected at all angles. Specular reflection is when light hits a surface and is reflected away at the opposite angle. The amount of specular reflection is the indicator of ALR performance.
For example, Draper’s Matt White XT1000E has 80% diffuse reflection and 20% specular reflection characteristics. This is definitely not the solution for ambient light since it is mostly a diffuser. Most ALR screen surfaces have a high percentage of specular reflection and reject 60% or more of ambient light while some—such as TecVision CS1200X ALR—has a high percentage of specular reflection at 82%, and a percentage of diffuse reflection at 18%.
Just because a manufacturer assigns an ALR value to a screen material doesn’t mean that solution is right for ambient light. Draper only publishes ALR information for screens designed to be an ALR solution.
A grey tint doesn’t make it ALR
Most grey screens have a grey tint, but are still mostly diffuse reflective surfaces. The grey tint only helps to improve contrast in moderate ambient light conditions. But that does not make it an ALR screen surface.
How an ALR screen works
ALR screen surfaces work to keep non-projector light from reflecting back to viewers. Most do this by using specular reflective elements in the screen formulation. These elements reflect off axis light away from viewers. As with everything else, there can be trade-offs. The more specular reflective a surface is, the more ambient light it will reject, but the narrower the optimal viewing cone will typically be.
The real numbers Matter
Draper frequently tests our own projection materials to make sure they are within our reported specifications. We also test those of our competitors, and unfortunately, as noted in a previous post, they often fail to verify the numbers they publicly use. This includes ALR performance. While differences in testing conditions and equipment could have some effect on the numbers, they are usually too far off for that to be the only factor.
There are two ways ambient light rejection is communicated: by reporting how much ambient light gets through or how much is rejected. Draper uses the amount of ambient light being rejected to communicate ALR performance. Whatever number is used, though, one thing you can be certain of: our numbers reflect what we actually make. Or you don’t see that product.
For more information on choosing the ambient light-rejecting solution that’s right for you, click here.
Company: Draper, Inc.
Product: Projection Screens
Of: Terry Coffey
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