In this article, we will zoom in on one specific topic: the ambition to offer higher luminance to the audience. Typically brighter 3D content, but potentially also brighter 2D. We will discuss all the aspects coming into play when looking into achieving this ambition; zooming in how screen selection has an impact. This is important because, if you’re not careful, you might be underperforming for the majority of the audience, by overachieving for a few lucky spectators.
Allow me to start with some tech-talk to set the scene right. When we talk about 'brightness', this is a metric of - all - the light coming out of the projection lens at the back of the auditorium. It is measured in lumens. 'Luminance' is a metric of the light you see on the screen at the front of the auditorium (coming out of the lens and bouncing off the screen towards the viewer). It is measured in nits, candela per square meter or footlambert. When an exhibitor aims for a certain '3D brightness' or '2D brightness', he is actually targetting a certain luminance: 14fL is typical for 2D, 3-5fL was the industry norm for 3D but is being looked at to bring to higher values. Once you have the luminance target, you translate this for the selection of your projector via room parameters such as screen size, screen gain, porthole losses,…
A first important thing to take into account is whether the luminance target is for day 1 (a 'freshly' installed projector) or for the end-of-lifetime spec of the light source. Xenon lamps as used in cinema projectors, drop to 50% of their light output at the end of their lifetime (500-3,000 hours). Lasers provide a completely different performance: Barco’s RGB laser models only lose 20% of their light after 30,000 hours of typical operations. If you scale for 14fL luminance on day1, your audience will only be presented with 7fL by the time you change your lamp. Exhibitors are very aware of this behavior and the fact that it has an impact on projector selection. Lamp projectors’ luminance fluctuates between 100% and 50% during the lamp replacement cycle; laser projectors bring a much more stable (and on average, higher) light output. When you scale to never drop below 14fL, then you’ll have to scale your lamp-based projector to bring 28fL on day 1; while you only have to dimension your Flagship laser projector 20% extra.
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’.
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?
