You Light Up My Screen....

Back to the old screen brightness thing again. This time, the question is, “How do I pick a projector that will light up my screen to 16fl?” I wish this were easy. Actually it is. You call Platinum Home Theaters, and we design your theater for you. We take care of site lines and sound, seating and acoustics, screen size, shape, material, projector, projector position and lens, control systems, décor, the whole works, or any part of it. You get what you really want, you don’t take a chance and spend your hard earned money for something that doesn’t work, and we both walk away smiling. You get our knowledge and expertise; we get you as a client and friend. But so much for the easy way, I know some still want to duke it out with physics, so here we go again. Got your slide rule ready?

In an earlier post, we noted that projector light output is measured in luminous flux, with the Lumen as the unit, actually a unit measurement of energy (actually is 1/683 joules per second, for those who care). ANSI Lumens are figures obtained with a defined test procedure that makes projector comparison more valid, and Home Theater ANSI Lumens defines the test procedure more tightly, and more realistically, as projectors are only tested after they are ISF calibrated to the D6500 color standard. Lower numbers result due in part to the fact that a metal-halide lamp emits a native white light that is a warmer color than D6500, and calibrating a projector can only be done by inserting precise amounts of light loss in controlled areas of the spectrum. Note I said, “light loss” – you can only reduce the intensity of a particular color, you can’t add it in magically if it isn’t strong enough. To increase the strength of one color, you actually have to lower the other two. The result is proper color rendition, but lower overall screen brightness. Sadly, not many projector manufacturers want lower Lumen numbers, so very few use Home Theater ANSI lumens.

As you look at projector data, do you find your eyes glaze over? They should. A look at any projector data base reveals tons of specs that are all hard to digest. But lets try. The biggie is Brightness, usually in ANSI Lumens. Remember, this is a measure of luminous flux, or all the light coming out of the projector. All that luminous flux has to hit a screen before it makes a picture, so we’re only part-way there with ANSI lumen figures. But it’s a start. Ignoring, for the moment, that you are dealing with an uncalibrated projector operating in “torch” mode (full uncalibrated white output) for that big beautiful ANSI lumen figure, here’s how you calculate the resulting screen brightness in Foot Lamberts:

Brightness (fl) = Projector Light Output (ANSI Lumens) / Screen Area (W x H)

So if we had a 1300 ANSI lumen projector and a 10 foot wide 16x9 screen…
Screen area is 10 x 5.625= 56.25 sq ft
1300 / 56.25 = 23.11 fl

I have no idea why water analogies work so well, perhaps it’s the fact that everyone has used a hose. So here’s what’s happening from a wet point of view. Lets say your projector is really a hose with a nozzle pointed downward, and a bucket below is the screen. Your hose sprays out exactly one gallon of water into that bucket. But now you raise your hose, or widen out the spray (take your pick) to cover, evenly, 16 buckets. You still spray out only one gallon, but it now disperses over all those buckets. How much lands in one of them? 1/16th of a gallon, right? The water in the buckets is the light on your screen. The more screen you have to fill, the less light hits any given area. It’s called the “Inverse Square Law” and is pervasive in light, sound, and all other forms of energy. And that’s what we are working with when figuring out how bright your screen will be with a given water output of your projector…or should I say light? If you get water out of your projector, write and tell us at once!

How much lower will the calibrated maximum be? Depends on the projector, but it’s always lower. Remember, you loose light in calibration to D6500. As you can see, though, a 1300 ANSI lumen projector might be a little dim once calibrated.

But then there’s contrast. Your eye likes contrast, and is indeed capable of a huge contrast ratio. Projectors, while getting better, are not as capable. But it may not matter anyway. Take a look at your projection screen with the projector off and the lights on. What color is it? White, right? Or light grey? What you’ve just seen is actually black, or at least as black as black would be on that screen with the room lights up. How do you get that huge contrast ratio of 2000:1? Simple. You gotta make the room dark…very dark. The reason is that your projector has a bright light limit that we just talked about. Even when it’s turned off, or lens capped, the blackest black will be defined primarily by the ambient light falling on the white screen. If you get that to nearly zero, you will start to realize that contrast ratio.

Here’s an example. Say your projector has a specified 2000:1 contrast ratio, you have a white screen with a gain of 1, and a 1300 ANSI lumen projector. With a little soft incandescent light in the room of only 1 foot candle (geezopete, another measurment unit!) your 2000:1 gets bumped down to 34:1. Lets cut that light on the screen down to a measly .1 footcandle. Bingo, you now have 286:1. Still not great. In fact, to get even close to that 2000:1 spec, you’ll have to get screen ambient light down to .001 footcandle.

Now lets work it from the other side. We can’t cut screen ambient light, for some reason, like windows (!). Lets say screen ambient light sticks at 3 footcandles, and that’s all the better we can get it. If we could get a bright enough projector, we could still get your 2000:1 contrast ratio back, right? Um….sort of. We could, if we only had a projector that could spit out 100,000 ANSI lumens! And then we’d land at around a 600:1 contrast ratio, which might almost be acceptable. Too bad we can’t get a projector like than in through the front door. Or plug it into a standard wall outlet. Or pay the bill for it.

The secret: you have to keep light off the screen to save projector lumens and dollars.

We’ll deal with plasma and LCD screens another time, but keep this in mind…look at a plasma screen turned off in a lit room. What color is it? Black. And that’s the black you’ll get in that kind of room light. Hmmmm!!