It’s a fair question: Do two doublestacked 5K projectors equal the brightness of one 10K projector?
The issue, it seems, has been a hot topic among projection designers, but lately it has become more prominent since the proliferation of projection, double stacking and edge blending software. Last June, when High End Systems debuted their Collage Generator, which combines up to eight DL.2 digital projectors and creates one large edge-blended image, the question became even more complicated. What is the equivalent brightness of four edgeblended 5K projectors?
The confusion about equivalent projections stems from two sources: first-hand experience with double-stacked projectors and the seeming complexity of the inverse square law. I mean, even the nomenclature sounds involved.
Some projection designers who have experience with large venue projectors are very used to using and seeing doublestacked projectors. Many are convinced that one 10K projector is brighter than two double-stacked 5K projectors. One of them is a good friend of mine, with whom I had this conversation very recently. But when I asked if he had ever compared the two side-by-side, he admitted that he had not. Without doing so, I believe it’s very difficult to assess them. Still, he believes what he believes. Fair enough.
I recently had the good fortune to witness a projector shootout between three competing manufacturers, all vying for the sale of multiple projectors in a large venue. One of the projectors was a Sony SRX-R110 projector, a 10K ANSI lumens projector, which uses two 2kW xenon lamps. The idea is that if one lamp goes down, you’ll still be able to project a usable image. The Sony factory rep told me that with only one lamp on, the brightness is the same as their 5K ANSI lumens projector. He illustrated this by first turning on both lamps, and then dousing one of them. It seemed very plausible to me.
If what he says is true, and I have no reason to doubt him, then how does that differ from double-stacking two 5K projectors? I don’t believe it does. I think it’s exactly the same scenario. I saw the difference between the two, and I would have to say that it would be very difficult to gauge the amount of difference between the two images. Is it twice as bright? Or some other multiple/fraction brighter? The only sure way to tell is with a meter because your eyes compensate by opening or closing your irises. In a controlled environment without a meter, it’s anybody’s guess.
What is for certain is the math behind it. Now just hold on — before your eyes glaze over and you lose interest, I promise not to bore you with any formulas. We’ll just intuit our way through this.
Suppose you have a projector that produces 100 nits at a throw of 25 feet. (A nit is a measure of brightness equal to one candela spread over one square meter. But Let’s not concern ourselves with the details of nits for the moment; let’s just accept that it’s some measure of brightness.) If we now measure the brightness at a throw of 50 feet — twice the original throw distance — then how many nits would be expect to see? That’s where the inverse square law comes in. Since the throw distance has doubled, we can figure that the projection area is now four times bigger and brightness is one-fourth intense; therefore we might expect to measure 25 nits.
We can now figure out what it would take to piece together four images to recreate the same brightness as a single image four times the original size. Think of the second projection — the one that’s four times as big as the original — as four individual images, each of which are 25 nits. Stitch them together and the four would equal the size and brightness of the single projector. Therefore, from a theoretical standpoint, all things being equal, four projectors with X ANSI lumens is equal in brightness to one projector with 4X ANSI lumens. Or, in English, four 5K projectors have the same punch as a single 20 ANSI lumen projector. That’s true whether you quadruple-stack them or edge blend them.
The same is true whether you’re talking about projectors or luminaires. After all, a projector is a luminaire — a very specialized one, but a luminaire nonetheless.
In this era of convergence where lighting people are learning video, and video people are learning to live with lighting people, a little confusion is understandable. Video techs are used to dealing with ANSI lumens, nits and aspect ratios, while lighting techs are used to dealing with footcandles or lux, candelas and beam or field angles. But the principles are the same when you’re dealing with light. With any given quantity, you can spread it, stack it, spend it any way you want, but you have to account for every bit, and you can’t get something for nothing.
