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SubscribeNot all home theaters are created equal. Said differently (and more accurately), not all custom installers and DIY builders bring the same experience/knowledge/performance standards to the job. Mistakes are made, some subtle, some egregious . The worst ones will be visible the moment you walk in the room, the lesser ones may require a trained eye to catch the error.
I'll probably touch on many of these in future posts, but in this space I want to discuss the one I encounter most frequently. Often, when I'm called in to do an ISF (Imaging Science Foundation) calibration on someone's system, I see it immediately.
But first some background.
The ISF, through its own research and looking at other studies, has determined an ordered list of things viewers care about when watching images on screen.
To jump to the bottom line, imagine this: 10 displays of all types—projector/screen, rear projectors, flat panels—encompassing all technologies—LCD, DLP, LCoS, OLED, Laser and Unaffordium—all in a dark room and all roughly calibrated, all showing the same clip(s). Now get a dozen people off the street who know nothing about video metrics and ask them which picture they like the best. 90% of them will pick the one with the highest contrast ratio.
Contrast ratio, you recall, is the quotient of the luminance at absolute black divided into the luminance at peak white, a.k.a., dynamic range. A large number will make the picture "pop," a low number will render the picture "washed out." Of all the video metrics, CR is clearly the most important (okay, the image has to be focused, too).
FYI, the numbers used to describe contrast ratio need to be post calibration, and they will bear little resemblance to the manufacturer's specs—due to said manufacturer's, uh, irrational exuberance!
Back to the big mistake, specific to projector/screen installations:
TOO MANY INSTALLERS OVER-SCREEN THE PROJECTOR!
For some context, here are three facts: People like big screens. People like to save money. Installers like to please clients.
The result is often a large screen with not enough light on it to produce a decent CR. At that point, everything else about the installation is lost. Once a projector is calibrated for black and white levels, a finite amount of light comes out the lens. The more you spread that fixed amount of light over more square inches of screen, the more the CR tanks. (Forget being "green," by the way— leave the lamp on High.)
Installers: Either reduce the size of the screen until you get 25 foot-lamberts (ft-L) or more off the screen, or convince the client to step-up to a projector with more light output. I like about 40 ft-L myself, but that's just me. Screens with as much as a 1.3 gain can help, but if you go that route be sure you know your screen materials. Another thing to consider is something called "Room CR." Any ambient light from any direction, such as red and green LEDs on component front panels that could splash on parts of the screen, will compromise the picture's contrast. Room CR sets the best black level you are going to get.
Mistakes will happen. But you should try hard to minimize CR errors and give an install the best shot at real success.
Throw distance doesn't matter in this case - just be sure the lens can fill the screen with light. Lumens are, of course, what comes out of the lens. Foot-lamberts are what is reflected off the screen. So screen gain is a factor. Problem is, manufacturer's specs on all the relevant metrics cannot be trusted. If they could, this formula would help.
You have a given screen size and gain and you want to know how bright your projector needs to be (post calibration)for your desired brightness off screen
PJ lumens = (Diag. x Diag. x foot-lamberts) divided by (gain x 337)
High school algebra will let you solve for any other parameter. Hope this helps.
Sorry I wasn't clear enough in my article.The numerator in the (true) calculation of contrast ratio is the amount of light coming off the screen when the pattern is full brightness (100 IRE or 100%) .... so assuming a fixed amount of light from the projector, say 20 foot-lamberts, is measured by turning "contrast" up (or down) until
the peak white is not "crushing" a linear pattern - often called a "chip chart". Now assume we change nothing except a larger screen and then adjust lens zoom and position to fill the new larger screen. We have diluted the available light to cover the larger screen area. The foot-lambert reading will be lower (less light per sq. inch). Numerator lower - CR lower. Had we made the screen smaller, CR would have gone up. Hope this helps
I'm going to stay away from projector recommendations in this column - there are plenty of good ones to fill every need. To answer your Q, you have a very large screen - and I don't know the gain, if any. And you are correct, ANSI lumen ratings are not consistent.
In the new HDR world, we want as much light on screen as possible/affordable. I would be thinking 5,000 to 7,000 REAL ANSI lumens. At the cost of those projectors, you might consider a smaller screen.
So, if you have a projector which delivers 1,200 lumens, post calibration, and you have a 60 square foot screen, you have 40 lumens per square foot, or 40 ft lamberts.
This is how most reviewers determine the actual light output from a projector post calibration. They measure the light at the screen, then multiply it by the size of the screen, and that's the projector's light output at the lens, post calibration.
HCD- So, with your 200" diagonal screen, which is FAR larger than a typical home theater setup, you have about a 120 square foot screen. Just multiply that times the desired lumens per square foot. At 40 lumens, that's a 4,800 lumen projector. At 20 lumens, it would be a 2,400 lumen projector.
Keep in mind ALL numbers are post calibration, not advertised. Many manufacturers can't come close to hitting their advertised numbers, which is why reviews and review numbers are so important.
Perhaps what you were missing was the whole story ... so here it is.
As soon as I saw that the screen was HUGE I didn't even make the calculation. I've been there before. I recommended the largest light engine I could, this side of a commercial theater. These would be laser projectors in the $35,000 to $60,000 range - hence my comment later on about considering a smaller screen. Your post caused me to do the actual calculation.
hcd threw me a bit of a curve when he described his screen in two different metrics (inches and meters), but Pythagoras and I figured it out. Turns out, that verified he has a 16x9 aspect ratio. When not provided with gain info, I always assume a 1.3 gain as that is the most commonly used, by far.
The generally accepted formula is:
FtL = (pj lumens x 337 x gain) divided by (diag. x diag.) That calculation using hcd's data and my 5000 lumen recommendation yields a desired off-screen light output of a tick under 55FtL. If he has a matte screen (1.0 gain), it would be 42 FtL. So, yes, I stand by my recommendation because POST calibration reality is that calculated peak white always goes down ... ALWAYS .. sometimes way down. In hcd's scenario , my expectation would be we'd end up in the mid 40's, even with the larger gain screen .
... and yes, you are right, the THX spec. for commercial theaters is 16 FtL off screen - but there are no THX police, so who knows what goes on in the projection room? Give the lamp about three holiday weekends and we probably are all watching 12 FtL. Clearly, that should not be the goal.
The point of this whole "contrast" thing and one of the reasons we build home theaters is - we can BEAT commercial theaters by a factor of 2 or 3 in contrast - and with today's projectors, we can do it easily.
Here's hoping hcd hits the Lotto.
Can you explain further?
Thank you.
