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[Jerry Maxey]

I've been in the audio/video rental and staging arena for many years now and I have always assumed that when you stacked two identical projectors you got twice the lumen output at the screen.  The image is not "twice as bright" but it is noticeably brighter.  My point is that if you stacked say, two 5K projectors you would get the same results as if you used one 10k projector of the same quality.

But recently I've had two people who have been in the business for awhile tell me that that is not true, that stacking, for example, two 5k projectors would only be the equivalent of one 7k or 7.5k projector.  My experience and common sense tell me that this is wrong, but I don't know the math.  Can anyone enlighten me on this subject?  Perhaps direct me to a paper or article that deals with this?

If I'm wrong, I want to know and if I'm right I'd love something in writing to back me up.

Thanks to all

[Brad Weber]

From Da-lite's "Angles Of View", Vol. IV, 11 titled "Light Reading":

Quote:

A misconception prevalent in our industry and erroneously promulgated by the author of these articles has been that double stacking projectors produces less than twice as much light. This assumption, we are now pleased to understand, is not true. 1 + 1 does = 2; every time. In an important sense, however, this lineally additive quality of lumens is true only if the device measuring them is other than the human eye. Our eyes, you see, do not respond to changes in brightness in a straight, linear fashion. Instead, they react logarithmically to changes in their input and thus, for instance, we are able to find our way out of a darkened movie theater, yet continue to see when we emerge onto the brightly lit street outside it. If we array a human audience before some screen and first illuminate it with a 1,000 lumen projector and then turn on a second 1,000 lumen projector, will the audience see twice as bright an image? No, it will not. The perceived brightness will increase only by about 50%. It will not double.

This has always been my understanding.  In theory, double stacking projectors provides twice the brightness on the screen.  The reality is that stacking is often not perfect and unless the projectors are exactly the same and aligned perfectly, it may be a little less.

However, the perceived brightness does not double, it increases by 50% or less.  While the same factors apply when going from an X Lumen projector to a 2X Lumen projector, it may be the perceived change in the viewed image brightness, and not the physical brightness on the screen, that they were relating when noting it being a 50% increase.

[Jerry Maxey]

Just so I'm clear, if I understand correctly, stacking two identical 5k projectors is the same as one 10k projector of the same quality, but in either case the perceived brightness is only 50% greater that one 5k projector.  Yes?

[Kit Hannah]

I think what it is saying is that 2x 5000 lumen projectors is indeed the same as a 10,000 lumen of the same quality right next to each other, but watching one then adding the other will not look like 10000 lumens.....hrmmm...

I always thought that it doubled...

[E. Lee Dickinson]

It will look like 10,000 lumens, but to our eye, doubling of lumens does not equate to doubling of brightness.

[Kit Hannah]

so then, would the 2 different ways next to each other look different to our eye, but register the same on a light meter? that would be mind boggling

[E. Lee Dickinson]

Well, no. A 10k lumen projector does not look twice as bright as a 5k to anything but a light meter.

I have a bunch of 2.5k and a bunch of 5k, and I'd estimate that the 5k projectors appear about 30 to 50% brighter, to my eye.

Our perceptions of light and sound are logarithmic. This is why we use dB and why more expensive lighting desks don't use linear fades.

[Kit Hannah]

I'm talking about 2x 5000 projectors stacked up against a 10000, are they going to look the same?

[Karl P(eterson)]

If you could align them perfectly, then yes, they would.... But you can't. If you were really on your ball game dual 5000's could look like 9000, but more often than not dual 5000's will only get you comparable to 7500 or 8000.

Karl P

[Jerry Maxey]

Why would projectors not perfectly stacked appear less bright?  I know from experience that you lose resolution when stacking projectors unless you are using high quality projectors with excellent glass.  I can stack my old Sony FX200s pixel on pixel corner to corner and you cannot tell there is more than one on the screen.  Lower quality projectors, not so much.  According to the dalite white paper, stacked projectors will measure twice as bright "every time".