Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Brian K Tennyson, CTS-D]

Finding a 5000 ANSI lumen projector that actually puts out 5000 ANSI the day its delivered (let alone 3 months later) is quite a feat.

Perceived brightness has a lot more to do with contrast (inter image and environmental) than actual Lumen output.

[Brad Weber]

If you are looking at full white field brightness, then the alignment may not have that much effect, but in real use where not every pixel is the same, then the alignment can affect the resulting image brightness, contrast and sharpness.

Getting two projectors perfectly aligned pixel-by-pixel with the pixels exactly matching and overlaying is very difficult in large scale presentation applications, especially in rental situations where the time and conditions required for such an alignment can become an issue.  Even if you did get the projectors aligned initially, that alignment would likely start to drift over time and with any changes in temperature and humidity.  The projectors would also have to have the same brightness individually, meaning the same reduction due to lamp life, etc., to end up with twice the brightness together.  Having two projectors with the same brightness, the conditions and time to perform an 'ideal' alignment and a controlled physical environment are rarely practical in rental and staging applications and that should be factored in.

[Jerry Maxey]

I agree that stacked projectors are not likely to look as good as a single projector, mainly because of a softening of the image due to less than perfect alignment.  But one thing the white paper from DaLite goes to great pains to assert is that if you measure the light on the screen, 10k (or whatever number) lumens is 10k lumens whether they come from stacked projectors or from a single projector.  Are you trying to say that even if they measure the same brightness, that the stacked pair will appear less bright?  And what would be the mechanism that could cause this?

[Brad Weber]

So what happens if you stack a 5,000 lumen projector showing full white with a 5,000 lumen projector showing full black?  It is still two 5,000 lumen projectors, but I doubt that the result is the same as a 10,000 lumen projector showing full white.  The ANSI lumen rating is the potential output of the projector, not the image brightness.

The way I see it is that if the images from two identical, stacked projectors are full field white (which is what is used for rating projector output) then the brightness is the same everywhere on the image so even if two projectors are not aligned you still have two full brightness sources at every point on the image and they add to be the same as a projector of twice the brightness.  But with real world images the brightness varies over the image and if the projectors aren't precisely aligned then what each projector is displaying at a point can differ and the result could be darker and lighter areas from the two projectors overlapping.  Typically this means effectively less than doubling the brightness and a reduced contrast.  In effect, when stacking two projectors the potential brightness is the same as using a single projector with twice the output, but with typical media the stacked projectors would likely have slightly less brightness and contrast due to anomalies in the alignment and between projectors.

[Jerry Maxey]

Obviously, we're not talking about projectors displaying different images.  I think you are promoting the very "misconception prevalent in our industry" referred to in the paper you quoted above.  When the paper you, yourself quoted says "1 + 1 does = 2; every time", do you think that the author was really only talking about full white screens?

If some (or all) the pixels from one projector do not overlap the same pixel from the other projector, then they must overlap other pixels adding to their brightness and preserving the total brightness of the image.  Why do you think it would be less bright? This would look soft, but not less bright.

I started aligning projectors back in the CRT age when each color had to be aligned individually.  Stacking projectors is quite simple compared with that process.  With even a moderate amount of care, high quality projectors with good lenses can be very closely aligned.  Even lower quality projectors, if they have lens shift, can be closely aligned in the critical central area of the image, with the alignment falling off near the edges and especially in the corners.  If the projectors are solidly mounted I have not noticed any perceptible drift of the alignment of LCD projectors.  In other words, the pixels stay on top of each other and brightness is preserved.

I find stacking projectors to be a very useful tool in some situations.  I want to feel confident that when I tell my client that stacking projectors will give twice the light output, the same as if we brought in a projector twice as bright, that I am correct.  (Of course, twice as bright still only appears about 50% brighter to the human eye.)  So far the only strong evidence presented is in the DaLite paper quoted above.

[Karl P(eterson)]

I think the combined experience of many of us here is strong evidence in and of itself.


The gist of all of this is that under absolutely perfect conditions you will get a doubling of light output, but many of us have never seen such perfect conditions and in fact doubt that it can be reliably and repeatedly done.

Nonetheless I think we can all agree that if you have found it to work reliably for your clients, that is all that matters and if nothing else you are proving that as projection technology moves along it is getting easier to obtain consistent output calibration.

Karl P

[Brad Weber]

Jerry Maxey wrote on Thu, 27 September 2007 12:03

Obviously, we're not talking about projectors displaying different images.  I think you are promoting the very "misconception prevalent in our industry" referred to in the paper you quoted above.  When the paper you, yourself quoted says "1 + 1 does = 2; every time", do you think that the author was really only talking about full white screens?

In many ways, yes.  ANSI Lumens is the maximum light output of the projector, it is not the image brightness, in fact lumens is not even a measurement of image brightness.

What is being said is that two identical projectors stacked would have the same light output as a single projector of twice the brightness.  That does not mean the stacked projectors provide the same image brightness on the screen.  You have to separate light output from image brightness, they are related but are not the same.  An extreme example would be two projectors displaying alternating black and white pixels.  Regardless of image alignment on the screen, the light output of the two projectors is indeed equal to that of a single projector with twice the output.  But when considering the image brightness, if the projectors are one pixel out of alignment on the screen then you have overlaying black and white pixels and the resulting image brightness is not the same as if you had a single projector.

An audio analogy would be two speakers versus one with twice the output.  If the two speakers are placed so they are in phase at the listener for all frequencies involved, then the two situations are the same.  But if the two speakers are out of phase at some frequencies, then the result is not the same.  While projectors are not a wavelength and phase issue, the same idea applies in that they have to be producing the same thing at the point in interest in order to add directly, if they are not the same the result is different.  So even of the projectors are showing the same image, is what each projector is providing exactly the same at every point on the screen?  Only if they are perfectly aligned or it is a flat field image that is the same everywhere.  However, the light output is indeed doubled even if the images do not align.

[Don Boone]

I think in a perfect world 1+1 would = 2. But when you use lens shift to overlay 2 images in the real world you start using the part of the lens away from the center and the light losses increase thus Karl's experience that 1+1 = more like 1.75.

Don

There are some distant synapses firing in my head, something about A, B & C regions of lenses in order of decreasing quality.

[Jerry Maxey]

Brad:
I'm sorry to keep beating this somewhat dead horse, but I think what you are saying is quite simply wrong, and that we as professionals need to have our facts straight.  

First of all ANSI Lumens are not a direct measure of the light output of the projector, but rather a measure of the light reflected from the screen.  From Hiretec "Choosing an LCD Projector": "ANSI lumens is an average Lumen measurement around the screen. This is accomplished by dividing the screen into 9 rectangles  (three vertical  coulombs and three rows). Take a measurement of light using a good spot light meter 1 foot away from the screen  pointing at the center of each of the nine areas. Then you take the average of the nine areas and that is your average (ANSI) Fl (foot-Lambert) light output."

Let's consider your extreme example of alternating light and dark pixels.  I agree the ANSI Lumen measurement (as described above) would be the same whether the projectors were aligned or not since the same amount of light is falling on the screen so one would expect the same amount of light to be reflected back to the measuring device.  In fact, at any reasonable distance back from the screen, I submit that the screen would look to the eye almost exactly the same, a smooth bright gray, whether the projectors were aligned or misaligned.  As you get closer to the perfectly aligned screen it would begin to look a little grainer than the misaligned screen, but no brighter.


Karl,
Just because a lot of people believe an urban legend doesn't make it any more true.  A major point of the DaLite paper Brad so kindly directed me to is that there is "misconception prevalent in our industry" that stacking identical projectors produces less that twice the light output, but that is what you and Brad have both been trying to say in spite of the evidence put forth in the DaLite paper.  Or, in Brad's case, he's saying even though they may measure twice as bright, stacked projectors won't look as bright as a single projector that measures the same.

I would like to say one more thing.  I agree that stacked projectors do not tend look as good as a single projector of similar brightness and quality and I will always prefer one bright projector if that is easily available.  The reason is that it does take longer to stack projectors and there is always a risk of reduced sharpness of the image, especially around the edges and in the corners.  Brightness is not an issue.  

And I want to thank Brad for directing me to the paper that had the answer to the question I was asking, even if he doesn't want to believe it.

Jerry

[Jerry Maxey]

Yes, here is where projector/screen geometry comes in.  Try to place the projectors where the least amount of lens shift is required.  This is just good practice anyway.