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Author Topic: Lumen output of stacked projectors  (Read 36059 times)

Brad Weber

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Re: Lumen output of stacked projectors
« Reply #20 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.
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Brad Weber
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E. Lee Dickinson

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Re: Lumen output of stacked projectors
« Reply #21 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.
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E. Lee Dickinson
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E. Lee Dickinson

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Re: Lumen output of stacked projectors
« Reply #22 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.
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E. Lee Dickinson
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Jerry Maxey

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Re: Lumen output of stacked projectors
« Reply #23 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
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Jerry Maxey

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Re: Lumen output of stacked projectors
« Reply #24 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?
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E. Lee Dickinson

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Re: Lumen output of stacked projectors
« Reply #25 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....
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E. Lee Dickinson
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Rain Jaudon

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Re: Lumen output of stacked projectors
« Reply #26 on: November 19, 2007, 12:06:27 am »

any new experiments lately?
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