The tricky thing in this matter is that the screen is 'more effective' at reflecting light, not 'more efficient': the canvas cannot reflect more light that the amount of light that falls onto it. What you win in one location, you lose in another. To make matters worse: traditional high-gain silver screens have an overall efficiency - of all the light falling onto it, how much is sent to the audience - of less than 100%. It eats light up, and that's exactly why it has a greyish color. This is improved in newer generation silver screens. That is exactly why you only measure the expected higher luminance when measuring 'straight-on': when you would measure the luminance under an angle, it would not be as high compared to the matt white reference. For angles above a certain value, it can even be lower than the reference. Said otherwise: only the audience in the center location of the auditorium benefits from the higher gain. Moviegoers sitting more to the left, right, front or back will see the screen under a certain angle and perceive less than this target value. The further you are from the center, the dimmer the image is. To make matters even worse, even the person in the ideal seat does not see all positions of the screen under the same angle. The center will be brighter than the sides and corners; an effect called ‘hotspotting’.

Half gain angles

This effect can be quantified by the so-called half-gain angle of the screen: under what angle does a viewer see half the luminance, compared to the straight-on position? The curves on the right - provided to us by Harkness screens - gives some examples of how the perceived gain varies across angle. The half gain angle is the position on the horizontal axis where the curves reach 50% of its maximum value on the vertical axis.

For a 1.4 gain screen, this is 50°. For a 1.8 gain screen, this is 34° (in the screen samples as used in the above measurements). This effect is less known with exhibitors and installers aiming for improved luminance values. If you play with higher screen gain to meet your target; you might be only meeting it for a fraction of the seats in the auditorium. A big part of your audience might be seeing less than half your luminance target. This is not different than taking into account light drop over time like we discussed earlier in this article. It’s a matter of scaling for the best case or worst case scenario. Do you want to offer good image quality to all audience members all of the time, or are you willing to compromise on that?