I would like some guidance on how best to determine the whiteness, brightness, and uniformity of various surfaces for use as a projection screen. 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. 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.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: 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?
Q3: I assume that "whiteness" is not too important a characteristic because the project-screen combination should be calibrated anyway, thereby removing any "non-whiteness" of the fabric. Is that a correct assumption, assuming the fabric is near-white and not coloured.
Q4: My initial testing indicates that some of the whitest fabrics are probably fluorescing (http://en.wikipedia.org/wiki/Optical_brightener
) i.e. made to appear whiter than they otherwise would be. Anyone know if professional screen fabrics use optical brightners? Are they likely to cause image problems?
Q5: Uniformity. The Photoshop 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…
… or pay 20 times as much for a Stewart Studiotek 130 screen and never have a problem…
… 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…
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 Benthin (http://www.benthin.info/blinds/roller-1.aspx
), or Rollease (http://www.rollease.com/skylinegalaxy.html