Half gain angles, hot spots and other huge benefits of low gain screens
Posted by Tom Bert
Laser illumination is picking up quickly as the preferred technology for projection in digital cinema. Since the first products were introduced approximately two years ago, exhibitors have rapidly adopted the technology. All-laser complexes are opening at a fast pace. The reasons for adoption are very diverse: improved image quality, reduced TCO, easy maintenance, etc.
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.
Brightness vs. luminance
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,…
What's the luminance target?
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.
As said, this temporal effect is known to most exhibitors and installers. What is less known is the spatial variation: the fact that not all audience members will see the same luminance. This topic is becoming more and more relevant as the higher brightness capabilities of laser projectors are being looked at to achieve 'brighter 3D': going beyond the common levels of 3-5fL to 6-8fL. Some people even dream about going to 14fL 3D, to bring it on par with 2D. Not only laser technology is being regarded to meet this ambition, also screen technology is added to the mix. As explained above: since the screen is a parameter that defines how you translate brightness into luminance, it has an impact on the net result. Screen gain is the parameter that is key here: screen gain is a metric to express how much more effective the screen is at reflecting light towards the audience, compared to a reference matt white surface. Typical screen gain values vary between 1.4 to 2.4; meaning: when you put this screen next to a matt white one, project the same brightness onto it and measure the luminance straight-on, the value will be 1.4 to 2.4 times bigger. This is sometimes taken into account when designing an auditorium and its components: getting to twice the luminance value (e.g. 6fL 3D instead of 3fL) can be achieved by going from a matt white screen to a gain 2.0 screen.
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
Screen gain graphThis 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?
What does this mean for my auditorium?
In order to know this, you need to know where the audience will be in your auditorium: under which angles (horizontal and vertical) will they be observing the screen? The horizontal angles are defined by how long the auditorium is versus the width. Assuming a wall-to-wall screen with a 2:1 aspect ratio: someone sitting to the side of the auditorium, 1 screenheight away, sees the screen center at an angle of 45°. On a screen with 1.8 gain or higher, this person will see less than half the luminance you dimensioned for (he/she is sitting beyond the half gain angle). Note that everyone sitting in front this person, to the side, will see the even lower luminance values. Someone sitting to the side of the auditorium, 2 screenheights away, sees the screen center at an angle of 26°. On a 2.0 gain screen, this person will see less than half the luminance you dimensioned for. This is visualized in the image below in a seating map that shows the variation across the auditorium seats for a 2.2 gain screen.
Picture courtesy of Harkness screens
The numbers above can be slightly improved by curving the screen, thereby reducing the worst case angles under which the screen is observed. Also the impact of dust and screen aging cannot be underestimated: it not only has an impact on perceived brightness, but also on 3D quality. Both these topics, screen curving and aging, will be the focus point of a next article in this series.
Low gain with the risk of lower luminance or high gain with the risk of hotspotting? From an overall image quality viewpoint, low gain screens are always to be preferred above the high gain variant: not only because of hotspotting and uniformity, but also because the highest gain screens are more prone to speckle. Does that mean that it’s not possible to achieve higher luminance ambitions? No: the introduction of RGB laser projectors, has brought the brightness performance for cinema to 60klms, twice what was possible with lamps. In reality, a common sense mix of screen gain and projector brightness will bring the most viable solution. Boundary conditions on how far you should go depend on you auditorium layout: curved screens have more margin, as do long and narrow auditoriums.
About the author
Tom Bert is a senior member of the Product Management team in Barco’s Entertainment division; he is responsible for the digital cinema servers and projectors. Tom joined Barco in 2006 as Research Engineer for Barco’s Technology Center. In 2009, he joined the Product Management team in the Digital Cinema division. Since 2015, Tom has been actively working on digital cinema servers and projectors and he has been promoted to Sr. Product Manager. Based in Belgium, Tom has international experience in display technology. He holds a PhD degree in Engineering from Ghent University.