Possible for a screen to be both brighter and darker than another?

[guyburns]

I'm looking into different surfaces for use as a projector screen. One of the most highly-regarded screen surfaces is the Stewart StudioTek 100 (http://www.stewartfilmscreen.com/residential/materials/front_projection_screen_materials/studiotek_100/studiotek_100_residential.html ). In a comparison test…

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

… a certain type of paint exceeded the performance of the StudioTek 100. The article states:

Setting up the test board with the ProClassic Smooth Enamel Satin against the Studiotek 100, we discovered that we had not only arrived at our objective, but surpassed it. Color balance was dead on, just as with the Duration. But this paint actually delivered a slightly brighter image with deeper blacks. With a checkerboard test pattern, the black and white squares that fell on the test board were visibly higher in contrast than those that fell on the screen. A spot meter confirmed what we could already see -- white highlights were brighter by about 10%, and blacks were blacker by about 10%.​

What intrigues me is the last sentence. Let's assume the two screens are tested separately in a dark room (no wall reflections) with identical light falling upon them. I can understand why one of the screens might be brighter than the other for highlights ("white highlights were brighter by about 10%"), but I can't work out why that brighter screen becomes dimmer ("blacks were blacker by about 10%") when the lighting is reduced.

What might be the physics behind that phenomenon? If it is not physically possible, one explanation might be that the screens were tested side-by-side in a non-darkened room, and reflections from the walls back to the screens caused inaccurate light readings at the darker levels.

An off-topic question: how come the projectorcentral link above is not accepted as a link by this website?

 

[davenn]

Hi there

A spot meter confirmed what we could already see -- white highlights were brighter by about 10%, and blacks were blacker by about 10%.

Thats just telling you the screen/projector has much better contrast than some other model

An off-topic question: how come the projectorcentral link above is not accepted as a link by this website?

Possible it was detected as a commercial www site and disallowed it as possible spam ?


Dave

 

[guyburns]

The same projector would have been used for throwing test images on both screens, so projector contrast-ratio can't explain the difference. Something about the paint surface was making it brighter and darker at the same time, depending on the light intensity. Doesn't sound right to me.

 

[Khashishi]

It's not physically impossible, although it is physically questionable. I can think of a few ways it could be true.
1. The bidirectional reflectance distribution functions are different; one reflects more light near a normal direction and less from the sides. This means the projector image will be more visible and stray ambient light less visible.
2. One has more internal scattering which causes some light from the light region to bleed to the dark region.
3. The spectral or polarization content of the highlights and lowlights are different. In particular, an LCD projector uses liquid crystals to polarize light causing it to be blocked. This effect is wavelength dependent so perhaps the screen could reflect less light of wavelengths which are more likely to leak through the LCD filters. Also, I think the lowlights are less polarized than the highlights, but I don't see how the screen could take advantage of that.
4. Some nonlinear optical effect is going on where the screen becomes more reflective at higher intensity.

1 is the most likely, in my book.

 

[davenn]

yes I realize that was probably the case with the projector

but I stand by what I said ... one screen is producing a better contrast than the other
how they do i I have no idea

Dave

 

[sophiecentaur]

Review articles are always suspect, unless you have experience of the publication being reliable in the past. Always take with a pinch of salt.

I agree that a possible (most probable) explanation for the different contrast ratio could be down to the projector (possibly two different projectors?). Another possibility is that the meter was subject to flare or that the two surfaces had different directivities. A light meter placed in front of a more directive screen could give better blacks because there would be less light getting to it from the white areas of the screen.
There is always the possibility of the light meter being non-linear and, if the two screens had significantly different reflectivities, the meter would be operating in a different region of its sensitivity scale.
And how about different levels of ambient light or backscatter from the room?
You'd have to be there to see what they actually did.

If there's not a lot of difference then go for the cheaper alternative!

 

[guyburns]

Thanks for the responses. Khashishi's comment "It is physically questionable" probably sums up the situation.

1. The bidirectional reflectance distribution functions are different; one reflects more light near a normal direction and less from the sides. This means the projector image will be more visible and stray ambient light less visible.

This is very likely what is happening. Screen manufacturers talk about their screen "gain". The StudioTek has been designed for a gain of 1, meaning it reflects light pretty much in all directions, whereas the paint has a slightly higher gain and so is more directional in it's reflections. So the StudioTek will spread light all around the room, with the result that some will be reflected back (if the room is not perfectly black) and thus wash out the shadows.

Stewart make a screen called a "FireHawk" which goes to extremes in its directionality. It is a grey screen which is highly directional. Almost no light from anywhere but directly ahead will be reflected to the audience.

2. One has more internal scattering which causes some light from the light region to bleed to the dark region.

Unlikely to have happened for the two surfaces in question because both don't have internal structure that light can penetrate into and scatter from. But I have seen the effect you mention – when I tested a cotton bed-sheet as a screen. The light scattering from the open weave seriously washed out the shadows. The highlights were bright, but the shadows were dull.

4. Some nonlinear optical effect is going on where the screen becomes more reflective at higher intensity.

This is what my question boils down to. Sounds unlikely, doesn't it, for a ordinary type of screen surface.

I reckon the light meter was not in a suitable position for measuring the light, and/or, there were reflections from walls which degraded the shadow performance of the StudioTek 100 as compared to the slightly more-directional paint surface.

 

[sophiecentaur]

Quote by Khashishi View Post
4. Some nonlinear optical effect is going on where the screen becomes more reflective at higher intensity.

Reply from guyburns
This is what my question boils down to. Sounds unlikely, doesn't it, for a ordinary type of screen surface.

Sounds unlikely under non-lethal levels of light intensity, too!

When you get a strange result, the first thing to question is the measurement technique, I think. Flare and a directional screen reflectivity seem to account for the phenomenon.

 

[rcgldr]

One possiblity is that this particular screen is natively much darker (more black) a low light intensities than other screens, but also more reflective than other screens.

 

[sophiecentaur]

One possiblity is that this particular screen is natively much darker (more black) a low light intensities than other screens, but also more reflective than other screens.

I think your argument is not correct.
The reflectivity is the same for all intensities of incident light so you would expect the same contrast as in the source image. However, if this is diluted by any ambient light, the darkest parts of the black image levels would not be as dark. The difference between the screens could be that some surfaces throw back the direct light more than the light from the sides so the dilution would be less when you sit in the right place for viewing. The screen image would look dimmer from the side, of course - and possibly have even less contrast than the 'poorer' screen.
It is true that black coated TV screens (CRT) can produce more contrast but that is a different situation.