Your question is a good one and one that is asked many times.
You see at Da-Lite we list our viewing angles by the industry standard. That standard is to list them as the 1/2 angle. For example, when you are talking about our 1.1 gain Da-Mat material we list a viewing angle of 50 degrees. What that means is that your viewing cone is double that or 100 degrees. That also states that at 50 degrees from the center you will have 1/2 the brightness that you have at screen center (or on axis).
Now, that does not mean that after 50 degrees you lose the picture. On the contrary, after 50 degrees in most applications you still have a very acceptable picture. We are just trying to adhere to what has been considered to be the industry standard for many years. We do not embellish our numbers as others may.
This is not unlike the way that different projector manufacturers list their brightness and contrast measurements. You may see one with a listing of 1500 peak lumens but another with 800 ANSI lumens. If you were to put the two side by side you may find that they are either very similar or in fact the 800 ANSI projector appears brighter.
I hope this helps. The basic thing to remember when comparing screen surfaces is that if the gain is the same then the viewing angle must be very similar. The reason is because, unless you introduce things such as lenses or other alterations, there is a direct relationship between the gain and the viewing angle. That relationship states that as you increase the gain you decrease the viewing angle. After all, a screen cannot magically create more energy, it can only reflect, scatter or refract what light you provide it with.
Blake Brubaker, CTS
Vice President of Sales
Da-Lite Screen Company, Inc.