Well, since you asked...please find below my questions/thoughts about viewing conce and viewing angle.
I hear these terms thrown out very often on this and other forums, and want to make sure that I understand the true meaning of them.
So, let's discuss the viewing angle and viewing cone for the following scenario.
If I want at least 50% brightness at every seat, do I need a screen with a viewing cone 53.2 degrees (2 x 26.6)? The thought is that I can calculate the needed viewing cone based on the audience position, and then directly compare that to the viewing cone of candidate screens when I am selecting the screen for my application.
For viewing angle, since 90 degrees is "ideal", is his/her viewing angle 62.4 degrees (90 - 26.6), even though his angle to the left part of the screen is 45 degrees and to the right side of the screen is 90 degrees? Viewing angle could also, I suppose, be defined as ideal at 0 degrees (meaning no deviation from perpendicular), which would shift these values accordingly (in this case, to 26.6 degrees).
If the viewing angle is defined how I think it is (relative to the axis through the center of the screen), it seems like a misleading statistic to me. For the room layout above, I can be 26.6 degrees off of the center axis, but my angle to the full extents of the screen can vary significantly. In the sketch below, I've added more viewing positions to the same room.
Position #1 is the same as in the sketch above...26.6 degrees off center axis, and a 45 degree angle to the left side of the screen. Position #2 is still 26.6 degrees off-axis, but the angle now is much less than 45 degrees to the left edge of the screen. The opposite is true for position #3. Still 26.6 degrees off center axis, but now an angle of much greater than 45 degrees. So, all 3 seats can have the same viewing angle, yet the picture from Position #2 may look wonderful at the same time view from Position #3 may be unwatchable (due to the left portion of the screen being very dark at an angle of significantly more than 45 degrees).
For the sake of argument, let's ignore the fact that Position #3 is too close to the screen to be a realistic seating position. The point remains, that the viewing experience from different seats that are the same angle off-axis can be very different. The further away from the screen that you are, the better off you will be (as far as a uniform image across the screen).
For this reason, I think viewing angle should me defined as the angle between the viewer and the vector that is perpendicular to the far edge of the screen (edge on the opposite side of the viewer). That definition would give the worst angle from a given seat location to any portion of the screen, and in the illustrated case would be 45 degrees from seat #1, < 45 from seat #2, and > 45 from seat #3.
The bottom line is that I recognize that viewing cone is typically listed relative to a percentage drop off in viewing brightness...e.g. a 50% viewing cone of 60 degrees, meaning that the gain is half on the on-axis gain when someone is 30 degrees off-axis. But, since in the process of selecting of a screen, the customer would typically uses the viewing cone spec in conjunction (comparing to) the viewing angle of the audience, that comparison can be flawed (for the reasons that I outline above). A number that only represents the number of degrees off of screen-axis (which is how I believe viewing angle is defined) simply does not capture the effect of angle to the full width of the screen on the viewing experience.
Hopefully, you can follow my logic, Bill...my intent was to make this clear with the illustrations, but I'm not sure that I did so.