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

[Frank Angel]

We are contemplating putting in a silver screen in our large 2500 theatre when we eventually install digital. I hate seeing the inevitable hot spot on that screen type, but this is our large theatre and long throw (120ft) to a smallish screen (40ft wide CW scope), a factor that I was told lessens the hot spot much more so than a larger screen and a short throw. Is this a myth or is it a geometric/optical fact?

Is it false hope to think I can get away with a silver screen and still have very little or a nearly unobjectionable hot spot? Or should I just tell them to forget about polarized 3D since they will be running 95% 2D content anyway; why compromise the majority of the presentations for 5% of the engagements?

[Sam Chavez]

Long throw, small screen would have nothing to do with how bad the hot spot is. if anything it would look worse than short throw, wide screen. In any case you are not going to like it, the spot will follow you around like a bad penny.

[Jim Cassedy]

A theater owner here in SF I sometimes work with had put in a big, new silver screen around a decade ago when Technicolor™ (film) 3D came out. It was a large screen in an auditorium with a short throw and a steep projection angle and from day one he HATED the 'hot spot'. The auditorium ( in a former 70mm house) was also wide, and the light difference between sitting in the center and anywhere in any of the side aisles made me gag. On bright scenes, there was also a noticeable (to me) difference in light between the top and bottom of the screen.

I had originally advised him this was going to be an issue, and that the auditorium dimensions were outside of Technicolor's recommended published specifications. But when both he and I questioned the vendor, who was obviously interested in selling a new screen & frame system, about this, he insisted that Technicolor's specs were just meant as "guidelines" & insisted that everything would be " just fine".

It wasn't.

In fact, it really sucked.

(There was another auditorium available hat I personally feel would have worked much better, as it was not only narrow, but had an almost 0° projection angle)

So, during the year-long pandemically mandated theater closure, the owner took the opportunity to rip out the silver screen and replace it with a new 'standard' one. So for now, there is no 3D at that location, and, quite frankly, 3D titles NEVER did enough business in that location to justify the expense of the sliver screen installation and 'trouble' of having to change to a larger bulb for 3D shows once the theater put in the digitalcienmacrap. So he's perfectly happy with no 3D, and if, for some reason, in the future, he does decide to re-install a 3D system (digital) he'll get one that does not require a silver screen.

(☞ anybody need a used Master Image™ 3D system? )

Trivia-DejáVu: In the 1950's, when silver screens (and I assume hot-spots) became an issue, RCA marketed something called the "Evenlite" screen, which varied the size (and I think the density) of the perforations, putting more in the center and less at the sides, to theoretically minimize the effects of a "hot spot". There's a short discussion on the topic, and an RCA Advertisement I posted for one in Y2k14 on the FT Archive
HERE> http://www.film-tech.com/ubb/f1/t011613.html

[Mark Gulbrandsen]

Yes, there is a way to tame the hotspot provided the entire auditorium and projection system is designed around a center weighted neutral density filter. I honestly can't believe that after all these years and all the active hot spots in the world, that one of them hasn't thought of it. ,I have several of these filters for me extreme wide angle camera lenses because those lenses can give uneven exposure when the aperture is at the widest openings. It tends to disappear past about F8... So in theory you'd adjust the lamp house focus for even light with the filter on the front of the lens, for even light. Then adjust lamp current for the proper brightness. Again, this would have to be an totally engineered system, but it would work... Also these filters can cost from $500 up to $800 each...

[Jim Cassedy]

I'm curious, Mark, but would such filters withstand the heat at the front of the lens? (Or is heat at the front element not that significant? ~ I've never thought to take a heat measurement of a front lens element.) Would the filter 'fade' over time from the extreme light level being pumped thorough it, which would be thousands of times brighter than light levels experienced in camera use. Or are there actually some filters one designed to withstand prolonged heat/light without any effect?

[Mark Gulbrandsen]

These camera filters MIGHT NOT, but a filter that is purpose designed for this use would. Moving it further out in front of the lens would also help, as would a small blower aimed at it. I did a stacked 3-D system at Zion Canyon with large glass polarizing filters to spread tje heat out and I used air cooling on those. Adaquate heat filtration in the projector may also be enough. I have one center weighted filter for my 6x17 panoramic camera that is just under 4 inches O.D.

[Steve Guttag]

It wouldn't really work anyway. It would help, if you were seated in the middle but make things uneven as you go off center. For a camera, you have a fixed aperture that is the "observer" in a theatre, it is reversed...the observer could be anywhere.

If you want to even the light out some, you have to curve the screen. Curve it in one direction, it spreads the light, curve it in the other and you'll have just about even light...but at the cost of a near impossible screen to put up/maintain or have decent sound.

Getting the curve right is key though. This is one that is a little too much curve:



Here is the PSA of it
Screen Shot 2021-07-14 at 7.09.21 PM.png

Pretty even across the middle and even distribution.

Off access, it shows that it needed to be just a little more shallow:

Screen Shot 2021-07-14 at 7.10.08 PM.png

Normally, a hotspot follows the observer...here, it is on the opposite side because of the curve focusing the far side to the observer and the near side away but still, it isn't too bad and better than a flat screen (when silver. The people seated in the center, as shown above, get a reasonably evenly distributed light.

[Marcel Birgelen]

If you MUST have 3D capability, why not go for an active 3D system as not to ruin the presentation quality of 95% of all other presentations? No matter what you'll do, a silver screen will always be troublesome. Not only does it come with hotspotting issues, a silver screen is also almost impossible to clean and will require re-calibration of the projector to get the colors back into the realm of the acceptable.

Long throw, small screen would have nothing to do with how bad the hot spot is. if anything it would look worse than short throw, wide screen. In any case you are not going to like it, the spot will follow you around like a bad penny.

Shouldn't a long throw on a "smallish" shallow the angle at which the light hits the screen in the corners and therefore, theoretically, should result in less hotspotting than a short throw setup with a rather large screen, at least for those sitting in the middle of the screen?

[Sam Chavez]

I'm waiting for Guttag to post up and correct us all. I theorize the long throw, small screen would make the hot spot even more concentrated so the fall off would be even worse as you walk side to side. Just guessing as I've never done the experiment.

[Mark Gulbrandsen]

If you MUST have 3D capability, why not go for an active 3D system as not to ruin the presentation quality of 95% of all other presentations? No matter what you'll do, a silver screen will always be troublesome. Not only does it come with hotspotting issues, a silver screen is also almost impossible to clean and will require re-calibration of the projector to get the colors back into the realm of the acceptable.



Shouldn't a long throw on a "smallish" shallow the angle at which the light hits the screen in the corners and therefore, theoretically, should result in less hotspotting than a short throw setup with a rather large screen, at least for those sitting in the middle of the screen?

Sam and Marcel, This is exactly my experience to a point... In most places I installed systems that had small screens and long throws, you still had some unusable seats in about the first ten rows. The fall off sitting close up was often really bad. I installed (by customer request) a lot of Real-D systems because they did not have to buy the adapter, nor did they have to pay for rental if it was not being used. I also did some Master Image spinning wheel systems... had one wheel explode during a show! And a bunch of Dolby 3-D systems. IMHO, the Master Image looked the best of them all, in spite of a hot spot. The image was bright and the 3-D was superb. Customers never complained about any hot spot, so I think my 3-D customers were already on to that from doing film 3-D in past years. If I had my druthers I would have installed the active French system... what ever it was called... Saw a movie on that system in Los Angeles once and it was literally perfect, did not require a silver screen. But it is also a high maintenance system Like Dolby. With Dolby you clean and sterilize the glasses, with the active system you are stuck buying a gazillion batteries for the life span of the product. The glasses are also expensive if someone walks off with them.

[Steve Guttag]

I'm waiting for Guttag to post up and correct us all. I theorize the long throw, small screen would make the hot spot even more concentrated so the fall off would be even worse as you walk side to side. Just guessing as I've never done the experiment.

Did you not see my post above? In any event, for silver screens, long throw/smaller screen will have a LESS pronounced hot spot but a very sharp cut-off angle. It is all in the geometry. Long throw, narrow angle has very collimated lines so you are either in the sweet spot, or you are not. Short throw wide screen you are guaranteed to have a pronounced hot spot and nobody has a chance of a good seat.

The realities are, anyone that puts in a silver screen couldn't care less about their picture. Not only does it ruin the light and completely alter the contrast/dynamic range of the image, it also alters the color representation too. If you just have to run 3D in this venue, Frank, consider either active glasses or flying a silver screen JUST for 3D events (or if you have obscene money, get a 6P laser system). 3D continues to fade back into the fad it has always been. I realize that there 3D lovers out there and more power to you. That said, a sliver screen is such a compromise such that they should be avoided at all costs if you care about the image for the other 99.9% of the movies and moviegoers. Note, Real-D does have some white screen alternatives now but you will still get a hot spot on a flat screen.

[Carsten Kurz]

Not every screen material has the same tendency for hot spot. First thing would be to choose the right screen material. There are new improved 'silver screens' with a gain in the 1.6 ball park. That will of course need consideration for the 3D System and projector/bulb selection.

[Steve Guttag]

While this is certainly true (not all screen material reflects the same)...there is a reasonably direct correlation between gain and reflectivity, which results in more or less hot spotting. My experiments have shown that about as high as you can go on screen gain without a noticeable hotspot is 1.3. And by noticeable, I mean while a movie is running. With a light meter, it is still there but spread out. Ideally, a gain of 1.0 is used. S2K projectors, particularly with xenon lamps, have such horrible contrast, they can't use matte-white screens without a very visible contrast problem. The higher gain screens hide some of their deficiencies. However, you trade that in for hot spotting. Then again, what screen manufacturers call matte-white seams to vary from manufacturer to manufacturer. One of the popular manufacturers seems to have a matte-white screen with the same scatter as their 1.3 gain. You just lose 30% of your light without the evenness to show for it. Also, most screen manufacturers list their screens as having a "nominal" gain, so you are already in a window.

In the lower screen gain for polarized 3D, there are "white" screen versions now in the lower-gain. Real-D's "Precision White." Strong-MDI markets it under the "Highwhite" They have a 2.0 and a 1.4 gain screen with this technology. Note the half-gain angles (which means half light...so if you have 14fL (48cd/m²) in the middle, just 30 degrees off center, you're down to 7 for the 2.0 gain. The lower gain gets you 33% more seating area before the same fall off.

Screen Shot 2021-07-15 at 8.34.16 AM.png

Note, as you go beyond 40-degrees off center, the 1.4-gain is even brighter than the higher-gain 2.0 (not just as a percentage but actual light level). Harkness has similar results from their PWT/PWT+ screens, also made with Real-D's technology.

Note, much of this data applies to straight on projection. If you have a substantial down angle (more than 5-degrees), well that just moves the hot spot down too and lights up the front rows better than the rear ones....well except the top of the screen.

[Steve Guttag]

Interesting, on post #7, the straight-on screen shot of the curved silver screen seems to have become broken and the time to edit has expired. Here is a repost of that screen shot:

Screen Shot 2021-07-15 at 8.46.29 AM.png

[Sam Chavez]

Most of us would settle for a hot spot that looks as good as that shown here. A more typical example of hideous spotting is the Castro, SF. Balcony house, silver screen, two 35/70mm projectors, one Christie 2230, all pointed downward about 10 degrees and somewhat sideways as well . The hot spot actually seems to move in real time, but certainly between reel changes.