What causes less of a hot spot on a silver screen?

[Frank Angel]

Then there is the "lenticular" silver screen that was part of Fox's CInemaScope package. I remember seeing old Box Office issues in a booth I work in; it had the issues around 1951 and 2 that had Fox ads when it was pushing CInemaScope. I was intrigued that the description of the screen which Fox called their "Silver Mirror" or "Miracle Mirror" curved screen (or something close to that is what I remember). The thing is, it WAS a silver screen that they needed to maintain brightness given that they were spreading light over twice the area. The screen also had a mild curve as part of the spec, which I assume helped. The ads claimed you HAD to purchase the whole package, the lens, the screen, the penthouse. And it did use the word "lenticular" referring to the screen but did not explaining that any further. But evidently however that lenticular surface was constructed, it was a way to reduce hotspoting.

I asked a tech at MDI years ago before they were Strong about this lenticular thing; he knew about it but said today it would be much too expensive to manufacture and it hasn't be made by anyone for years. (I can't imaging Stewart not making a product because it cost too much.) But then the question also is, while that surface design might have given scope the needed gain while also reducing the hotspot, it might not have worked to hold polarization either -- in other words, not usable for polarized 3D.

BTW, there is no question what the hotspot moves with the patron. I walked into the back of a theatre that was using a silver screen so he could use a 1600w bulb rather than the higher wattage that should have been used. I walked across the rear of the house and watched this vicious hotspot follow me from one side of the screen to the other, so i doubt manipulating the hole size or neutral gradation filters will do anything but correct for one position in the theatre; everyone else will still see weird uneven lighting.

[Martin McCaffery]

From the special Cinemascope edition (printed in scope) of 20th Century-Fox Dynamo, Dec 1953
Miracle Mirror Screen or Magniglow Astrolite Screen:

with their carefully designed optical units, are not simply sheets of fabric sprayed with aluminum or other reflecting materials, but they have been embossed with tiny elements, each one of which is so shaped as to reflect light toward the seats in a theatre and to keep it out of waste spaces. The result is a screen substantially uniform in brightness from side to side when viewed from any seat in the theatre, which is not the case with an ordinary high reflection screen with pictures the size of CinemaScope.

Keep it out of waste spaces sounds like a euphemism for hot spot.

[Steve Guttag]

Technikote tried to do a lenticular type screen (looked a bit like a corduroy type pattern), their "Hilux." It was cotton backed to help with bellying on curved screens. They put one in the Uptown in DC when they removed the original Cinerama ribbon screen. What you got, instead of a singular hot spot (the screen had some gain to it was several hot spot. One hotspot for each section of the screen (which was seamed). Since the material was thick and cotton backed, the seaming was a bit more noticeable than on conventional vinyl screens. It should be noted that Cinerama strips were not just flat vinyl...they too had linticulations. They had both vertical and horizontal so it was a crosshatch pattern.

[Stefan Scholz]

I seem to remember the SMPTE RP on installation of gain screens, starting with the words @Installing gain screens is extremely bad practice, to be avoided under any circumstances. And then the hints that gain screens must be installed curved to minimize hot spotting.
So why did 3D actually change this, I never understood.
IMG_20210807_093125_edit_32825239872073[1].jpg
Going through the East German textbook used in educating film technicians, I had to learn, that an absolutely even reflecting white screen (use of ceramic or anorganic white pigments) has a reflection index of 0.7 to 0.8 at the best, being "very light inefficient to in itself inefficient film projection system". He claims, even a white PVC foil screen with a gain of 1.0 has a noticeable bright spot, as a plastic film is predominantly glossy in the initial manufacture. So that was, why screens needed a surface treatment to get them more or less diffused, loosing reflection below 1.
IMG_20210807_093417_edit_32939864207473[1].jpg
Steve Guttag mentioned the Technicote screen, "Hi-Lux". That seemed to have been a popular approach in the Sovjet Union, a fabric backing (cotton) with an embossed lenticular structure and a metallic coating, to give better light distribution on very large screens IMG_20210807_093324_edit_32881524105398[1].jpg
The problem has always been, film projection is very inefficient, not even 2% of the initial lamp light stream reaches the screen. Trying to get the best possible edge illumination even counteracted with the goal of bright images. 14/ 16 fl is on the edge of color recognition, in the range of twilight vision, far away from good color recognition.
Understanding these basics, simply means screens can't be as big as you'd like to install them, together with the limiting contrast issue of larger screens, staying with moderate room sizes and screens helps in maintaining perfect projection. This has always been known, was reflected in room geometry guidelines, but seemed to have been forgotten in recent times. OK, modern hi power semiconductor light sources make it easier to achieve high power output projectors, but these feature a lot of other problems to be overcome.

[Steve Guttag]

As one of the caretakers of RP95 ("Installation of Gain Screens"}, no the document did not start with

@Installing gain screens is extremely bad practice, to be avoided under any circumstances

The actual start was:

1 Scope

This practice specifies the optimum installation parameters for gains screens used in motion-picture theaters.

It does discuss considerations and gives a rule-of-thumb formula on how to curve the screen plus screen tilt. The Annex does discuss further considerations.

So why did 3D actually change this, I never understood.

It didn't. And it never does. I've found over my career that when people can't get around physical fact they take one of two approaches:

1. "If you can't fix it, feature it."
2. Dismiss the problem and claim that your work around is "just about as good."

Item 2 can be anything from outright lies to wishful thinking. When the Torus (aka "Suck") screens came out, the screen speakers had to move above/below the screen. Did it sound good? No but that didn't stop the proponents from claiming that their technique was just about as good. When the emissive LED screens came out a few years ago...again, the proponents (now owning a sound company) made claims that their form of putting speakers above/below and now to the side of the screen addressed the issue. Did they? No, it just got more complicated and every person got a different sound quality depending on where they were seated.

At the end of the day, for projection screens, Matte-White (real matte-white with a nearly 180° dispersion is going to be the best surface if the room has a controlled ambient lighting situation. As the image is gimmicked, one has to compensate for the gimmick by altering it some other fashion. I liked curved screens (always have) but they do present challenges as well as geometric distortions. Lenses are not perfect and what should be a nice rectangular picture has bends in it. Projection angles, screen frame construction/boarders/masking all play into how that image looks. It has to be viewed as a "system" rather than in its individual parts.

I think the greatest hope for images is in some form of emissive screen that addresses the sound issue. Ambient light, color, even curvature all become addressed. Focus is a none issue, convergence is a non-issue, corner-to-corner brightness/evenness all become non-issues. Even Exit signs become non-issues because the screen isn't designed to reflect...just emit so no Green or Red splash of color from the signs. Curved screens wouldn't self-illuminate (cross-reflect, lower contrast).

[Gordon McLeod]

There was for a while a screen called protolite that was a cloth back with a embossed surface
Never heard what happened to them but they were around for a while in the 80's

[Steve Guttag]

I never knew anyone that put one in. it was a silver screen type material.

[Mark Gulbrandsen]

D-150 screens were also lenticular, but they had variable lenticles from the center to the sides to provide more even illumination. The last lenticular screen I can remember being available here was made by Technikote. When Big Sky bought them out, they quit making those. Hurley, which I sold for many years right up till they closed, did not produce one in my working cinema days. I don't remember what happened to Hurley, but they made nice screens with nearly invisible seams right up till the end.

[Steve Guttag]

Hurley made the Cinerama screen strips. The would roll the vinyl through these rolling pins to put a crosshatch pattern on the vinyl.