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Author Topic: Testing for an already "calibrated" fabric for use as a projection screen  (Read 1237 times)
guyburns
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« on: September 29, 2012, 01:48:45 AM »
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I would like some guidance on how best to determine the whiteness, brightness, and uniformity (if those are the correct terms) of various surfaces for use as a projection screen. What is the connection with photography? Well, I may be using a digital camera to determine the characteristics of the various fabrics, and ultimately I will be projecting my digital photos on the screen. So basically, I want to test fabrics to find one that is already "calibrated", so to speak. And I thought that a photographic forum specialising in colour management would be the best place to ask questions.

See Background at the end of this post for more details why I am doing this.

Test Procedure
I will be placing 600 x 600 mm test samples on a large board, projecting light onto them from a digital projector, and photographing the samples with either a Nikon D700 or a Lumix GH2. Then I'll open the images in Photoshop, and using histograms or other techniques I'll compare the whiteness, brightness, and uniformity of the samples. The results will end up in a PDF with side-by-side comparisons of the histograms and anything else from Photoshop that would show useful information about how the fabrics reflect light.

These technical objective tests would be followed by subjective viewing tests to see if the differences highlighted by the technical tests can actually be seen in practice.

What I would like to know from people more knowledgeable than I am, is how best to do the technical tests. I don't really need absolute figures for most of what I'm doing, only relative figures. So I doubt whether I need a spectrometer or calibration equipment. I'm hoping that I can use a digital camera to achieve what I want. But maybe I'm wrong about that, in which case I'm all ears.

And just in case someone feels it has to be mentioned: this is not the place to get into discussions about the merits of grey screens versus white screens, or high-gain screens versus low gain screens. At the moment, I'm in search of a white material of high colour purity, that has a gain of not much more than 1.0, and is not projector screen material.

The Scenario
Imagine a darkened room, a projector throwing light onto a 600 x 600 sample, and a digital camera on a tripod, probably behind the projector. After photos are taken, the images will be opened in Photoshop to determine certain characteristics. Here are my questions:

Q1: What should be the colour-balance setting of the camera?Just set it to daylight and stick with that setting for all photos, or automatically set the colour balance from one of the samples, and stick with that?

Q2: How should the manual exposure be set? From a grey scale card, or would set exposing from one of the white samples be just as good?

Q3: Brightness. Photos will be taken at angles from 0º to 60º (measured from the perpendicular at the centre of the screen) in increments of 10º to determine the variation in brightness with angle. Will "L" values from Photoshop will be a suitable measure of brightness?

Q4: White purity. In Photoshop, the histogram for each image should indicate the colour purity. i.e. how closely the R, G and B histograms line up under each other. The histograms will be included in the PDF, but is there a way to distill one number from a histogram that represents how white is the light reflected from the sample?

Q5: Uniformity. The histogram should also reveal the uniformity of the light reflected from the sample. If I want a measure of how the brightness varies across each image (due to hotspots or defects in the material) what is the best way to portray that variation? The standard deviation?


Any suggestions as how best to evaluate different screen materials in terms of brightness, whiteness, purity, and uniformity, using a digital camera or other equipment, would be most appreciated.


Background
Given that I'm wanting a projection screen, I suppose I could just buy a $150 budget model and endure all its problems…

(http://www.amazon.com/Elite-Screens-M120UWH2-120-Inch-Projection/product-reviews/B000PHLB88/ref=cm_cr_pr_hist_1?ie=UTF8&filterBy=addOneStar&showViewpoints=0)

… or pay 20 times as much for a Stewart Studiotek 130 screen and never have a problem…

(http://www.stewartfilmscreen.com/residential/materials/front_projection_screen_materials/studiotek_130_g3/studiotek_130_g3_residential.html)

… but I think there will a happy medium somewhere in between where I can obtain nearly the same image quality as the Studiotek 130, but at a tenth of the price. So I've obtained samples of screen materials and intend comparing them against white bed sheets, roller blinds fabrics, paper, and anything else that is white and flat, including painted surfaces. You might laugh at the latter, but surprisingly, a $100, well-painted, flat surface using the right paint, can give an image very close to that obtained from an upmarket screen material such as the Studiotek 130…

projectorcentral.com/paint_perfect_screen_$100.htm?page=Finding-the-Perfect-Paint

Basically, I want to find out if there really is something special about projector screen material that other materials cannot duplicate. If there is some special characteristic, I want to know what it is. But if another fabric has similar light-reflecting properties, I may decide to use it, combined with a good-sized roller mechanism from either Benthine (http://www.benthin.info/blinds/roller-1.aspx), or Rollease (http://www.rollease.com/skylinegalaxy.html).
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