Silver screens, if they are well made and installed, can have minimal hot-spotting, but they still have what I call shading. I make a distinction between hot-spotting and shading. Although they may come about from the same reflective characteristics of the screen, shading has to do with an asymmetrical change in brightness across the screen and is typically dependent upon where you are sitting. Shading happens quite noticeably when sitting in the worst seat in the house, say in the front row way on the extreme left or the right. In fact, the worst seats in the house for viewing a 2-D movie on a matte screen become even worse when viewing a polarized light stereoscopic movie on a silver screen from a bad seat.
I haven’t gone to every theater that has a silver screen, and I can’t tell you for sure that they are all good screens. But I do know that in the years since Chicken Little was released the manufacturers have learned how to make better screens so when you are looking at a 2-D or a 3-D movie you will not see the seam; and I do know that the manufacturers now know how to make screens that don’t have any splotches or texturing. I have been in a number of theaters where there are good silver screens that work very well for 3-D and as well for 2-D projection. The screen’s performance for 2-D is important so it’s reasonable for studios and theater owners to demand that it function well in both modes.
A lot of work has been done on high-gain metallic-surface screens, not because of their polarization characteristics but because they are bright. The matte screen that I talked about earlier with the Lambertian surface has a gain of 1, let’s say, with light reflected more or less equally in all directions. But in a theater that’s not what is needed because there’s nobody watching the movie up on the ceiling or down on the floor or way off on the sides. A lot of the information about how to make a good high-gain screen can be found in patents that are assigned to Eastman Kodak. The idea behind a high-gain screen is to take the unwanted light that goes off to the ceiling and floor and wherever, and to send it where it is needed – to the audience.
Probably the finest example of a screen of that kind –the one I’ve used and I know about – is the Ektalite screen. The Ektalite screen, which was offered in the 1970s and into the 1980s by Kodak, is possibly the finest stereoscopic projection screen ever offered. It wasn’t designed to be a stereoscopic projection screen; it was designed to be a high-gain screen used in classrooms or conference rooms that was bright and rejected unwanted spill light from windows and overhead lights. I’ve used several of them in different sizes, and I can verify what I’ve said because I measured the performance of one of them. The Ektalite screen – on-axis, which is typically the way high-gain screens are measured – had about a 10:1 to 14:1 gain. (That’s 14 times brighter than a matte screen.) Remember, we’re talking about an era in which people didn’t have flat panel displays and they were projecting using slide projectors or 16mm or 8mm projectors; so set up in a classroom the Ektalite screen was super, because over a 60-degree angle of view, properly placed, everybody in the classroom could see a bright image.
It worked great for its intended purpose, and it achieved this by interesting technology. The surface was aluminum foil, and it was textured by placing the two pieces of aluminum foil under high pressure through rollers, with oil introduced between the two surfaces. The result was a micro-fine bark-like texture that served as the diffusing surface. The Ektalite screen combined reflection and diffusion perfectly, but it would still have had a fierce hot spot if the aluminum foil had not applied to a compound-curve screen which was shaped like the inside section of a sphere. It was made on dense foam core or some such plastic, and the foil was bonded to the surface. It had a delicate surface and you had to be careful cleaning it. But wow! what a screen.
Modern screens – and I haven’t tested one in a few years so my observations may be out of date – although they conserve polarization, are not nearly as good. An experiment I did in my days at StereoGraphics measured a standard aluminum painted vinyl screen’s polarization efficiency starting with linear polarizers that had a 5000:1 extinction ratio. That means that only 1/5000th of the unwanted image passes through crossed polarizers when measured on an optical bench. But when measuring the dynamic range reflected from the surface of the silver screen on-axis, using the same polarizer over the lens and for eyewear, the dynamic range was reduced to 200:1. Still good for stereoscopic applications, but this told me that improvement was possible because this painted vinyl screen was not as good as the Ektalite for polarization conservation. As I mentioned, Kodak wasn’t particularly interested in making polarization-conserving screens, but it turns out that they did with the Ektalite. The Ektalite screen had, on-axis, 1000:1 dynamic range using good 5000:1 linear polarizers. The polarization loss on-axis was much less than vinyl painted screens, and off at the edges of the field (admittedly narrow) the dynamic range was about 200:1.
When using circular polarizers on a bench I got numbers that were a few hundred to one for dynamic range. And the same protection measurements will show a dynamic range of maybe 20:1 dynamic range on a vinyl painted screen – which is not too good. Hence, Real D adds ghostbuster to the ZScreen system to improve the effective dynamic range.
I’ve recently seen both Real D (circular) projection and IMAX (linear) projection, and they are both good. Do I prefer one? IMAX projection with linear polarization used two gigantic pieces of 70mm film running horizontally through the projector, the size of small picture postcards, and the image looked fantastically good. If it didn’t see this image quality improvement there would be no excuse for what I paid for the family to see Cloudy With A Chance of Meatballs. I had sticker shock at the box office. With regard to the characteristics of circular versus linear and circular’s head tipping advantage, booth were fine. You can only tip your head so much before the misalignment of homologous points causes a breakdown of fusion so the circular advantage isn’t all it’s cracked up to be, especially when starting off with linear’s much higher dynamic range.
To me the Ektalite says more work can be done to improve silver screens. They may be better than ever but the Ektalite performance tells me that more is possible. True, the Ektalite had a narrow viewing angle and required a compound curve to get a super gain, but its performance points to possibilities. High-gain silver screens on-axis – which is the way they are rated – can have a 2.4:1 gain, but off-axis the gain is going to be very much less. So these screens might work out to be an average of 1.5 gain or something like that, if you make some kind of a weighted average according to some reasonable formula for the various seats in the auditorium. Same story for polarization conservation as a function of viewing angle.
If we can have a screen that has a true 2 or even 3 gain over a board viewing angle, that would be an improvement for the stereoscopic cinema. The present illumination for 3-D movies is a niggardly 4 to 5 foot-lamberts. That’s true across the boards for all DLP-based digital systems with the exception of the RealD XL system which is a twice as bright. As for the rest that’s pretty dim when you consider that the SMPTE recommendation for 2-D projection is 14 foot-lamberts. I’m of the opinion that there is really no excuse for stereoscopic images being projected any less bright than 2-D images. I don’t think something special is going on that makes it okay to have low light levels for 3-D projection. There is nothing additive going on between the two eyes in terms of illumination. There may be something going on in terms of perception of image quality, possibly a reduction in granular noise and maybe improvements in sharpness.
We really have two issues now with the silver screens, and they are (l) get the gain up, and keep it even across the seats so that wherever you are sitting you have a fairly high gain, and (2) make sure that polarization is conserved consistently for every seat so one can see a good stereoscopic image anywhere in the auditorium.
Having said this I know that there are many people, and experts counted amongst their numbers, who simply dislike silver screens and find them to be unacceptable for 2-D projection. I’ve watched many 2-D films on good silver screens and I think they look better that way, but to each his own. The way this issue is going to be resolved may be as follows: Matte screens will someday become the minority with the ubiquity of the stereoscopic cinema.