Film-Tech Forum ARCHIVE: question about FTL: what level should be ideal ?

This is topic question about FTL: what level should be ideal ? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=010546

Posted by David Alexander B (Member # 4862) on 07-23-2010, 05:20 AM:

SMPTE says 12 to 22FTL at center of screen
(note: we're talking 2D here as 3D requires twice that amount and i would say twice 22FTL...the upper range)
typical tv, plasma gives around 35-50FTL
would that be overkill on giant 50-70'screen (and even feasible , xenon power speaking)?
one would need a 3DLP that gives still 2000:1 on off and 500-600:1 ansi contrast with such lumens capacity...
easier to do on a smaller screen (25-30 feet wide)

Posted by Demetris Thoupis (Member # 833) on 07-23-2010, 06:12 AM:

Any screen above 40 feet is desirable to start using a gain screen varying from 1.4 until 55 feet and from there onwards one should really use a 1.8 gain screen. The benefit of course is the requirement for lower lamp wattage to be able to achieve the desirable 16Fl (which is the normal) and perhaps a bit higher. The trick when going with screens wider than 40 feet is to actually start curving the screen to begin eliminating hot spots as well as to increase the viewing area of the audience. In your case, considering using a Matt White screen with gain of 1 for the sizes you mention, for the 50' which is about 15.3 meters and considering common height (the way all cinemas should be) you will need a 5K lamp. For the 70' which is about 21.3 meters width and if you use a Matt white 1 gain screen (not recomendable for this size) you will need a 10K lamp but with a screen gain of 1.8 you will probably need something at the range of 6.5K lamp. These are for Film 35mm cinema. For D-Cinema you actually need to go a step higher in wattage.
Demetris

Posted by John Hawkinson (Member # 1135) on 07-23-2010, 06:42 AM:

Please recall that the standard for digital projectors is not the same as film projectors. The SMPTE standard for film projectors is "open gate," with no film. But if you were to project clear film (the equivalent of a white image from your D-Cinema projector), the image would not be quite as bright.

Hence the comparable standard for D-Cinema is lower. I think it is 12 footlamberts target, rather than the 16 we typically shoot for for film.

--jhawk

Posted by David Alexander B (Member # 4862) on 07-23-2010, 07:13 AM:

good pt on 35mm pj vs d cinema
as for screen gain i always found the metallic painting to be a problem
creating a haze layer
a screen should be invisible !
(in my ht i went from 1.4 gain to1.0 (not paint) and it's nite and day: invisible screen)
so i much favor powerful pj rather than screen gain trick
too many theaters have 7-10FTL way too low.
if you can "play" with 20FTL that's very nice

maybe one day there will be a multi led 3DLP with 30000lumens [Big Grin]

Posted by Demetris Thoupis (Member # 833) on 07-23-2010, 07:35 AM:

Cinemeccanica can actually achieve with their solution at screen gain 1.2 38300 Lumens and with 1.8 gain 42000 lumens using a 6.5K lamp! Now they can even go with 8K narrowing the problem of 3D light.
Demetris

Posted by Scott Norwood (Member # 30) on 07-23-2010, 08:10 AM:

I recently acquired a light meter (and had it calibrated), and measured the difference between "open gate" and "clear" film last week. It's not much--roughly 1/2 to 1 fL, with clear film. I don't remember if it was color or B&W stock, however--perhaps that meks a difference. One thing that did make a difference was how long the xenons had been lit--they get noticeably brighter after the first few minutes of operation.

Out of curiousity, I measured some computer monitors as well--a 20" CRT is about 40fL, and a 20" LCD is about 45fL. I don't know how this relates to film exhibition, though, since the light is coming directly from the monitors, but is being reflected off of the screen.

(Incidentally, everyone should have a light meter. It significantly improves the results of lamphouse alignment, while making the task considerably easier.)

Posted by Richard P. May (Member # 3598) on 07-23-2010, 09:36 AM:

Even though SMPTE standards give some flexibility in light readings, the laboratory aim is 16fl, and that is what is generally used to show prints to customers for approval.
This is with no film in the projector.

Posted by John Hawkinson (Member # 1135) on 07-23-2010, 10:05 AM:

What meter did you get, Scott? Typically the problem is they're not priced affordably.

I would have thought the effect of clear film would have been multiplicative, rather than subtractive...maybe not though. What's your baseline brightness?

Computer monitors...well, it's not as if we normally use computer monitors in darkened rooms or what the purpose of the comparison is, but it sounds about right...maybe you should check a cell phone, since that sadly seems to be visible in the same room.

I'm under the impression that the way meters integrate over time may be such that readings across multiple technologies (e.g. 48 Hz shutters versus 60 Hz raster scans with persistant phosphors) may not be directly comparable, depending on your meter. But it doesn't take much to know computer monitors are a lot brighter than movie screens...

--jhawk

Posted by David Alexander B (Member # 4862) on 07-23-2010, 10:15 AM:

of course 40-50FTL on giant screen on a black theater would be overkill, like sunburn
20-25FTL would be great but this never happens
i much prefer watching movies in theater with a 25-30ft screen as it often yield much better "punch", realism, because of higher FTL and 2K looks better too vs on a 70ft screen

Posted by Scott Norwood (Member # 30) on 07-23-2010, 10:47 AM:

Spectra SC-600. They are prohibitively expensive if purchased new, but used ones show up occasionally at more reasonable prices. Calibration is $225, however.