Impacts of Gain Screen on 35mm Presentation

[Whitney Miller-Bird]

I'm responsible for overseeing the upcoming upgrade and installation of three new series 4 Christie 4K RGB laser projectors, along with replacement of 10+ year old screens, and wanted to get some input or discussion on the potential effects of transitioning to 1.0 or 1.5 gain screen at that same time. In one of our three houses, we still run 35mm reel-to-reel, and have gotten quotes on new systems to include a NorthView 1.5 gain micro-perf Pearlescent White screen in this house (along with a CP4420-RGB with a 1.45-2.17:1 UHC lens), but I'm curious about the potential effects, drawbacks, and/or implications of this decision on our 35mm presentations.

This is by far the largest project I've ever overseen and I'm learning a lot about the factors that need to be considered, and figured I'd do my due diligence to ask some of you fine folks. I tried to browse the forum to see if there's been a similar discussion on this topic already but didn't come across anything.

[Ryan Gallagher]

What was the gain on the original screens that they are used to with the existing 35mm systems? Are they so filthy as to be under-performing? What lamps/lamp-houses? What were they metering at prior?

Not an integrator... but In general the lower gain you can get away with the better, especially with laser, unless you have specific special format needs for a silver or high gain screen. Someone here will correct me if I've mis-stated the general principles.

[Ed Gordon]

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For 3D movies, a silver screen is ideal for passive 3D projection, as it maintains polarization and enhances brightness. For active 3D systems, a high-gain white or grey screen is recommended to ensure a bright image.

The statements above cover the basics. I see you already have quotes for screens. Did the vendor consult you regarding your specific requirements?

[Marco Giustini]

In short, with PROJECTION (doesn't matter which type) 1.0 gain is the best that you can get, period. It'll give you amazing light uniformity.

Higher gains will not make light out of thin air. They will simply reflect it at a narrower angle, concentrating the light towards the audience. In theory, you get more light. However, the higher the gain, the narrower the angle. That means that eventually you will get a brighter spot (the hotspot) somewhere on the screen (it depends on the geometry of the room, where you are sitting etc) and the rest will be noticeably darker. It's physics, you cannot avoid that.

I consider acceptable gains up to 1.4 on a properly curved screen (to "spread the hotspot") but if it was MY cinema, it would be 1.0 gain screen on a FLAT frame - I mean not curved.

Let me comment on Ed's last point "Gain Screens require lower brightness (from the projector)". This is very unlikely. Properly installed high-gain screens are rare. Most of the times you end up with a very obvious hotspot. Yes, if you measure the hotspot you have 14fL with less power from the projector but the rest of the screen will be WAY BELOW DCI requirements.

Two obvious examples, you can spot which is 1.0 and which one is silver screen ? (The cross effect must be an effect introduced by my phone)

[Paul H. Rayton]

Excellent demonstration photos, Marco. A fine illustration of "hotspotting". I've done some similar pix, walking around to various locations on the floor of my cinema (back when I was working). Your photos distinctly illustrates the asymmetrical light distribution resulting from silver (and also, esp. the higher ratio "gain") screens; kudos for illustrating the phenomenon so clearly!

[Marco Giustini]

Thanks Paul

My favourite pics are the below: Matte white 1.0 screen rolled on top of Silver screen. Same projector. It tells you exactly why Silver screens should never be considered and why the "you get more light" tale is often BS (nothing personal, Ed).

silver2_1.jpg

silver1_1.jpg

[Steve Guttag]

Yeah, gain screens are the worst scam foisted upon the industry...again and again. They want to sell you something with the ruse of getting something for nothing. That is, putting a gain screen gets you more light. They get you less light. They cause you to lower the output of the projector until the very center of the screen hits the prescribed peak brightness (14fL/48Nits)...with the rest of the screen being progressively lower.

SMPTE standard 431-1 for "Theatre" (the easiest category), the sides are to be 75%-90% of center...if you have a gain above 1.3 and you have a flat screen, you'll never hit it. Note, ideally, the sides should be 85% or better of center...so matte-white or a slight gain, like 1.1.

In your theatre with film projection. The issue with gain screens is that you have three projectors trying to point at that screen. You have at least two different projection angles for the film projectors and likely three different ones with film projectors on the outside of the digital one but, perhaps the film machines kept their original positions and digital is off-center. Gain screens accentuate the projection angles by creating different hotspots based on projection angle. So, lower gains with matte-white (1.0) are the most ideal. Then lamp up and choose your light source for digital appropriately. The picture will look substantially better. Put that UHC (premium if available in your lens size...trust me) on a matte white screen and behold a picture that you will think is incredible and very bright.