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If, instead, you have a 1.3 gain screen, that’s simple too: 21.6 x 1.3 = 28.08. So if we take 4440.3/6145.8 we get 0.72, which means the JoeBob will be about 21.6 ft-L on your screen. If you have a 120 inch screen, this means your screen is 58.8-inches tall by 104.6-inches wide, for a screen area of 6145.8 in^2. Simple ah-rith-mah-tic shows us that this means my screen has an area of 4440.3 in^2 (Height x Width = Area). My screen is 50-inches tall, and 88.9 inches wide (technically it’s a 2.35:1 screen that I use the center portion, but let’s not confuse the issue, as that doesn’t matter here). Figuring out the screen area when you know the diagonal and the angles isn’t difficult, but let’s make it easy. Keep in mind we’re dealing with area not diagonal, and I think this is what’s tripping up most emailers. So an image that’s twice as big is going to be half as bright. However, the same amount of energy is thrown at the screen. Moving the projector further from the screen, or zooming out, makes the image larger. Most projectors project a rectangular image. They’re still useful, though, as even a rough estimate is still probably more accurate than any lumen measurement from a manufacturer.īeyond screen gain (itself a rough estimate), the math is less simple, but not badly. As I said, brightness comparisons are rough estimates. I hesitate to give an exact margin of error, but 10% is probably optimistic.
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If I measure 30 ft-L will it be 10 ft-L or 50 ft-L on your similar size, similar gain, screen? No. All of these errors are cumulative, so all brightness measurements are going to be rough. For that matter, we didn’t even address the obvious factors like your screen being slightly smaller (more on that in a second), and standard production variances in the projector. I say roughly as there are many factors we can’t account for, like the angle of the projector the screen, the angle of the screen to your eyes (as in, your seating height), the color and reflectiveness of the room, or the fact that no screen is going to be perfectly any gain. If you have a 100-inch 0.9 gain screen, it’s safe to assume the JoeBob will be roughly 27 footamberts. Let’s say I measure the JoeBob 2000 projector and I get 30 ft-L off my screen. I measure projectors on a 102-inch, neutral gain (1.0), 16x9 screen. I’d certainly entertain luminance and illuminance arguments, but for my main numbers, I’m sticking with ft-L.
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Measuring in footlamberts also allows us to compare brightness to televisions, something I think most people know a lot better. First of all, all manufacturer specs are lies anyway, so what’s the point there? Also, illuminance measurements are less able to tell you how bright the projector will actually be in your home. While this would give us a chance to give you a direct “lumens” comparison to the projector specs, I find it to be a significantly less useful metric. directly measuring the light from the lens). Some would argue we should measure illuminance (i.e.
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OK, being an American publication, we use footlamberts, a measure of luminance (i.e. This is easy to do, but it involves the maths. I feel this size is the best way to judge the performance of a projector, while at the same time offering you, our fair readers, a way to judge how bright the projector will be on your own screen. Every once and a while I get an email questioning our choice of using a 100-inch screen to measure projectors.