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Topic: Working out Xenon size
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 02-12-2011 07:39 PM
It isn't really rocket science....it is all about the screen area you are trying to light. But remember...you are going to be lighting the WHOLE imager, even if you are only using a part of it (which is the case in all formats except the elusive 1.896:1! (that is the ratio of the 2K/4K imager).
The "formula", in its most basic form is:
height x width x fL desired = lumens (remember, the H x W the FULL imager, not the format in question.
Then, you need to factor in loses in the port and such, lamp aging and screen gain.
So lets say you have a 15' x 35'-10" screen (scope) and it is a constant height screen...you know...when scope gets to be the BIG picture.
For flat, your imager is putting out a 15 x 28.44 image and you want 14 fL. So 15x28.44x14 = 5972.4 Lumens. But your port glass, even if it is really good and AR coated is going to cost you 3% (more if you don't use GOOD AR glass)...so you are up to 6152 Lumens. But wait, you say you want it color balanced correctly? You are going to give up AT LEAST 5% of your light getting it to the mecca of x=.314, y=.351. So up that to 6460, if you are lucky. But just like with taxes, you haven't stopped giving.
Xenon lamps typically loose 20% of their light in the first 100-hours. The Digital lamps (at least the Ushio/Christie ones) do quite a bit better...closer to just 10% of their light in the first 300-hours...but the point is...even being rosy...you should plan on at least 25% of your light being gone by 1000-hours...presuming a smallish lamp (not a 6KW). So add another 25% so you can keep it at 14fL (or there abouts throughout the lamp's life. We are up to 8075 lumens on a Flat, matte-white screen running 1.85:1. For scope, without an anamorphic, we are up to 12,817 lumens (quite a bit more). With an anamorphic, the figure drops to 9869 lumens.
There are other factors at play here...like the lenses. The projector manufacturers will give a lumen spec that represents their best throughput lens. Most lenses are zooms and their throughput will vary based on where it is zoomed...typically it will be at its most efficient with the largest picture it can make. How well set up is your projector? If the notch filter isn't dead on...you'll be doing more color correction. If your port or screen affects the color too much, you'll do more color correction...etc.
So what about screen gain? Yes, you can lower the lumen requirement...for the CENTER of the screen...Put in a 2.2 gain screen and you'll get that 14fL in the middle with half the light...BUT it will only be 14fL in the MIDDLE and way out of spec most everywhere else. You want to keep the screen gain as LOW as possible that lets you get the light.
So on to 3Ds...at present, the "spec" is 3.5fL to 5.5 fL with the studios really wishing it was in the 5-5.5 range for the most vibrant picture. So adjust the numbers above with a target of 5-5.5
But now you have to take into account the efficiency of the 3D. the rules of thumbs:
Dolby 3D: 10% efficient (so you need 50-55fL BEFORE you put the 3D stuff in).
Real-D: 15% efficient
Real-D XL: 28% efficient
MasterImage 3D: 18% efficient
XpanD : I don't have a handy number memorized...we need to check on that. It is likely in the 20s + though...it does not project the light through something...the eye-wear is the "loss" in the system.
Remember on those silver screens for Real-D and Master Image that while you may have a 2.3-2.4 gain screen...measure the screen all over and you will see that if you hit 5-fL in the middle you are WAY low everywhere else. I don't know about others but I'm now compromising on those screens and measuring about 1/3 in so my average is closer to 5fL...a little bright in the middle and a little dark on the sides...as opposed to dark everywhere except the middle. Curving the screen will distribute the light better but DCinema does not have a means for dealing with curved screens in the lenses, yet...all one can do is crop off pixels that spill on the masking.
-Steve
PS Manny...is that more of what you were looking for?
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