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Title: Lumen output of stacked projectors
Post by: Jerry Maxey on August 23, 2007, 03:57:37 pm
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
Title: Re: Lumen output of stacked projectors
Post by: Brad Weber on August 23, 2007, 05:44:33 pm
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.
Title: Re: Lumen output of stacked projectors
Post by: Jerry Maxey on August 24, 2007, 09:01:37 am
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?
Title: Re: Lumen output of stacked projectors
Post by: Kit Hannah on September 18, 2007, 09:39:01 pm
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...
Title: Re: Lumen output of stacked projectors
Post by: E. Lee Dickinson on September 18, 2007, 09:56:52 pm
It will look like 10,000 lumens, but to our eye, doubling of lumens does not equate to doubling of brightness.
Title: Re: Lumen output of stacked projectors
Post by: Kit Hannah on September 18, 2007, 11:13:35 pm
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
Title: Re: Lumen output of stacked projectors
Post by: E. Lee Dickinson on September 18, 2007, 11:23:33 pm
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.
Title: Re: Lumen output of stacked projectors
Post by: Kit Hannah on September 19, 2007, 12:38:14 am
I'm talking about 2x 5000 projectors stacked up against a 10000, are they going to look the same?
Title: Re: Lumen output of stacked projectors
Post by: Karl P(eterson) on September 19, 2007, 12:43:05 am
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
Title: Re: Lumen output of stacked projectors
Post by: Jerry Maxey on September 21, 2007, 02:49:26 pm
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".
Title: Re: Lumen output of stacked projectors
Post by: Brian K Tennyson, CTS-D on September 24, 2007, 11:18:43 am
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.
Title: Re: Lumen output of stacked projectors
Post by: Brad Weber on September 24, 2007, 02:52:43 pm
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.
Title: Re: Lumen output of stacked projectors
Post by: Jerry Maxey on September 25, 2007, 09:38:11 am
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?
Title: Re: Lumen output of stacked projectors
Post by: Brad Weber on September 26, 2007, 12:51:37 pm
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.
Title: Re: Lumen output of stacked projectors
Post by: Jerry Maxey on September 27, 2007, 12:03:53 pm
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.

Title: Re: Lumen output of stacked projectors
Post by: Karl P(eterson) on September 27, 2007, 02:03:15 pm
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


Title: Re: Lumen output of stacked projectors
Post by: Brad Weber on September 27, 2007, 02:42:24 pm
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.
Title: Re: Lumen output of stacked projectors
Post by: Don Boone on September 27, 2007, 04:30:51 pm
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.
Title: Re: Lumen output of stacked projectors
Post by: Jerry Maxey on September 27, 2007, 05:20:36 pm
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
Title: Re: Lumen output of stacked projectors
Post by: Jerry Maxey on September 27, 2007, 05:24:12 pm
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.
Title: Re: Lumen output of stacked projectors
Post by: Brad Weber on September 27, 2007, 07:30:29 pm
Jerry Maxey wrote on Thu, 27 September 2007 17:20

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."

I agree that it is important to have a correct understanding.  However, based on the information I have, what you presented seems to be an incorrect method for determining ANSI lumens, especially as the result is ft-L not lumens, and lumens are actually a measurement of the direct output (luminous flux) and not the reflected brightness (luminance).

According to IEC 61947-1 (the original ANSI standard was actually retired in July of 2003, but the current IEC standard uses the same process), you are measuring the projector's direct light output.  According to the standard, you measure the illuminance in lux at the plane of the readings.  You do measure at 9 points and you do account for the area of the image at the measurement plane as we want lumens (a measurement of luminous flux) and lux are lumens/square meter.  So you measure the illuminance in each of the nine areas (3x3), add them and divide by 9 to get the average illuminance in lux.  Then multiply that by the plane image area in square meters to get the luminous flux in lumens.  In US units, you measure foot-candles and multiply by the area in square feet.

The important thing here is that you are measuring the light coming from the projector at the plane, not the light reflected off a surface.  Measuring the reflected light off a screen would be pointless since it would also include the characteristics of the screen rather than just measuring the projector.   I think they maybe your source just got mixed up as foot-Lamberts (fL or ft-L) is a measure of luminance and is the proper measurement for the reflected light, but it seems that it has nothing to do with determining ANSI or IEC lumens.


Edited to cover up, I mean correct, my poor typing skills.
Title: Re: Lumen output of stacked projectors
Post by: E. Lee Dickinson on September 27, 2007, 10:43:27 pm
Jerry Maxey wrote on Thu, 27 September 2007 17:20

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."


Uhh, no. Foot-lamberts are not at all equivalent to lumens. That quote is either wrong or incomplete.

Fl is a measurement of brightness on the surface. Lumens is the measurement of brightness out of the projector. A 5k lumen projector is a 5k lumen projector (same output), whether it's filling  a 6x8 or a 9x12, but the brightness on the surface is going to be significantly less on the larger screen.

We can get in to luminous flux vs. radiant flux next, if you'd like, as it is also topical to the discussion.
Title: Re: Lumen output of stacked projectors
Post by: E. Lee Dickinson on September 27, 2007, 10:49:39 pm
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?



If I have a white pixel, and I misalign another white pixel on top of it by 1/10 of a pixel, some of the white area will add not with white, but with screen-door-gray. It gets even worse if a white pixel and a blue or red pixel are added to eachother.

Pixels can only add perfectly if they are aligned perfectly. Good luck.
Title: Re: Lumen output of stacked projectors
Post by: Jerry Maxey on September 28, 2007, 10:40:16 am
Brad,
After Googling "ANSI Lumen measurement" I see that I was wrong to call you out on that.  Most agree that the measure is directly of the light source, although some describe a method of measuring reflected light then factoring in the screen size.  All claim to be the correct way to measure "ANSI Lumens".

oops,
Jerry
Title: Re: Lumen output of stacked projectors
Post by: Jerry Maxey on September 28, 2007, 11:24:49 am
The argument that keeps coming up seems to be that even though two 5k projectors may measure the same as one 10k projector, the two 5k projectors will not appear as bright as the 10k due to the likelyhood of misalignment between the 5ks.  I can't tell from your arguments whether you think that if you measured the light reflected from the screen that you would measure less light (where did the missing light go?) or whether it would only appear to be less light.

Here is an experiment.  I'm going to stack two projectors I have that I know stack extremely well.  I'm going to put some typical images up, perhaps one of the old ICIA projector shootout discs.  Then I will shift one projector over by one pixel.  Do you really think there will be any perceptible change in the image, other than some softening of the picture?
Title: Re: Lumen output of stacked projectors
Post by: E. Lee Dickinson on September 30, 2007, 08:59:59 am
I suspect you will. Again, assuming a black pixel next to a white pixel (text, for example): Instead of having white + white = twice as bright white, you have white+black = not much more light at all.

I suspect you will have, in that area, a slightly brighter (because black from a projector still has some energy) blur, instead of a twice-as-bright, higher contrast image.

I like your "Where did the light go?" question, which I will answer with an analogy: A brick wall only gets taller if you stack the bricks on top of each other. Their heights don't add up if one brick is here, and the other is over there. The brightness of the pixels will not add to create a brighter pixel unless they are aligned.

I hope you will carry out your experiment! I have several 5k projectors in my inventory; I've never tried stacking them because I assumed it would be too time consuming and difficult in a one-off situation. But if there are people out there doing it regularly and successfully....
Title: Re: Lumen output of stacked projectors
Post by: Rain Jaudon on November 19, 2007, 12:06:27 am
any new experiments lately?