Ah the silver screen: searchlights scanning the Hollywood sky, glamorous premiers, gorgeous actresses…. The silver screen is a term that has denoted the glamour and excitement of Hollywood since Chaplin twirled his cane. While to some it is the most visible sign of hope for the cinema for others it is a dreaded surface upon which to project those old standby 2D movies. But there’s so much more to it than glamour – there’s dreadful science. It’s a technology that ought to command the industry’s keenest minds, because, after all, that’s where a hundred and fifty million bucks wind up as a vibrating veneer of a hundred billion photons reflected into the eyes of tens of millions of photon consumers. That’s one big point in favor of the film industry – they have not dehumanized the customer to the point where he or she is called a consumer. The customers are still the audience, people with feelings rather than human maws born to consume piles of chazarai made in China.
In Hollywood, as elsewhere, technology decision are made based on incomplete information and herd instinct rather than on common sense, and science and engineering are basically common sense. It’s the scientific method that is the hope of mankind, not emotion and prejudice. There are many in the industry who reject projecting on a silver screen, and they have good reasons. But how substantive are these reasons?
The silver screen, actually an aluminum surfaced screen, is not only an enabler for the best method of projecting 3D images but it is the weakest link in the optical system for that method: polarized-light-image-selection. While the Dolby system produces good images and does not require a silver screen it is not bright enough for the biggest screens (unless two projectors are used) and it uses dauntingly expensive eyewear that must be cleaned in the theater. (Ditto XPand.) Most theaters projecting 3D use the polarization selection method whether of the circular variety (RealD and Master Image) or the linear variety (IMAX and most theme parks).
In the past four years or so silver projection screens have improved. When the first screens were installed for Chicken Little, the studio and exhibitor complaints centered on two issues: visibility of seams and mottling and a textured appearance on the surface of the screen. People in the industry were also greatly concerned about how well these screens performed for 2D projection, not just because of the defects mentioned, but because their colorimetric characteristics are different from matte screens. And in this town technical and creative people will go bonkers if the images they have so painstakingly created aren’t accurately reproduced. They can go as bonkers as they want but outside of LA the projection of movies is sometimes a smack in the face to every DP, art director, and colorist who cares about the movies. (And let’s not get started on what happened to that image quality on LCD TVs.)
Which stumps me becasue the Dobly system to my eyeballs does not meet spec. One image is more highly saturated than the other, and you can see that oh so clearly with the violet-blue laden Avatar.
Silver screens are designed to conserve polarized light and have gain. When polarized light is projected onto a nonmetallic surface, a matte screen, it is depolarized. The reflected rays no longer have the orderly orientation of the electric vector (a component of the electro-magnetic wave construct that describes the physics of light) that defines polarized light. When polarized light is reflected by a nonmetallic (a dielectric surface) it is depolarized and no longer useful for image selection. A dielectric is a material that has closely-bound electrons. It is a poor conductor of heat and electricity. It’s an insulator. So the same kind of material that has difficulty conducting electricity and heat will not preserve the characteristics of polarized light. Matte screens, usually made of vinyl, are dielectrics.
Dielectrics, which do not preserve polarization characteristics, have reflection characteristics explained by Brewster’s Law that says that light rays that are at glancing angle to the surface undergo some degree of polarization. But the material that is most interesting for making silver screens is a metal (or a conductor). Silver screens are manufactured by painting or coating matte screens with an aluminum pigment mixed into some kind of a binder or medium to be coated or sprayed onto the screen surface. Motion picture screens are usually made of vinyl plastic 54 inches wide. These sheets are welded together in vertical sections. The vinyl is welded together and then painted with aluminum pigment. The weld is accomplished in a different ways but however it is done the weld has to be invisible because nobody wants to look at them. They are easily as distracting as the guy sitting in front of you wearing a Dodgers baseball cap.
Before we dig deeper into the characteristics of the silver screen a word or two about matte screens: Matte screens are perfectly fine for 2D if the projector has a bright enough source of illumination, and they have a more or less “Lambertian surface” so that the incoming light is reflected or distributed evenly in space with pretty much the same brightness for any seat in the house.
In addition they have very little shading. As you look across the surface of the screen from your seat you will see very brightness change from corner to corner. Much of the shading you will see is a result of the projector optical system and that’s called vignetting, but you can’t see it unless you are looking for it and the movie happens to be shots of the sky or a close up of a sheet. So when you’re off in the corner in the worst seats in the house –in the front row, way over on the left or the right side – illumination holds up pretty well across the screen. The closest part of the screen and the furthest part of the screen are pretty equal in brightness. Of course you are looking at a distorted image, but that’s another problem. Some people seem to like sitting in the front row and some people like pickle parfait pie or chicken mint ice-cream. Do you really subscribe to the seemingly enlightened old saw that says there’s no accounting for the other guy’s taste? In your heart of hearts you know that if other guy doesn’t agree with you he’s warped.
Burt we are interested in the silver screen. With the surface of the screen coated with metal it now has the ability to conserve polarization. If polarized light is projected on it polarized light will be reflected because the metallic atoms of the surface have free electrons, and when light shines on its surface those electrons are able to vibrate in any which way to reflect polarized light.
These screens ought to get pretty good conservation of polarization even at steep angles for the worst seats in the house but the ability to conserve polarization falls off with at the side seats. Why does it happen? I think it probably has to do with the binder which may be a dielectric or even (heaven forbid) birefingent. I have heard it described in terms of diffusion, but I don’t think that’s right because a diffused metallic surface is still a metallic surface that has free electrons.
Motion picture screens are a tradeoff between reflection and diffusion. If you were to look into a mirror that’s being used as a projection screen you will see a bright glob of light where the projection lens is. If you add some diffusion to the surface you are able to form an image on the surface of the screen and you get rid of the so-called hot spot. A silver screens has to be the right balance between reflection and diffusion and just like a mirror it can also have a hot spot. But if the screen is curved the hot spot can be spread out over its surface so it cease to be visible. Silver screens are typically curved into a section of a cylinder and this tends to mitigate the hot spot.
Hot-spotting depends on the geometry. For a long narrow auditorium with a long throw – in other words, the distance from the lens to the screen is great – when sitting in the middle of the auditorium, you will see very little hot-spotting. But that’s not the way modern auditoria are designed. They are designed to be more square-ish and have large screens. A theater in a modern multiplex is the toughest geometry for a silver screen.