[Michael Grant]

OK, it looks like the pressure is on for me to finish my gray screen analysis.


No, I'm not so vain or arrogant to believe that everyone wants me to do this. I've just recognized that there is a lot of misunderstanding, and a lot of misconceptions, about the theory behind gray screens and how they work in practice, and I think I've got enough of a handle on it that I can do justice to the task. In fact, this article will address some misconceptions that I've held myself in the past.


Allow me to give you a brief preview to whet your appetite. Here is a two-sentence summary of my article:
Quote:

The primary purpose of gray screens is to trade reduced brightness for increased intra-scene/ANSI contrast by suppressing secondary reflections. Gray screens cannot improve on/off contrast, and in practice are not effective for combating ambient light problems.

And here is a list of some of the common beliefs about gray screens that I intend to address:

• Using a gray screen is just like using an ND filter. False. ND filters darken the image like gray screens do, but they do nothing to suppress secondary reflections. In fact, in ambient light, an ND filter makes on/off CR worse, and gray screens do not.
• Using a gray screen is just like wearing sunglasses in your home theater room. False. This is commonly stated as a reason not to use gray screens, but in fact they actually accomplish something that sunglasses do not: improve ANSI contrast.
• Gray screens improve on/off CR. False, with one very technical exception.
• Gray screens improve absolute black levels without compromising white levels. False (this is just another way of writing the last statement).
• Gray screens improve instantaneous contrast; that is, the contrast observed within a single projected image. True. In fact, this is why letterbox bars get darker, which people often misattribute to improved absolute black levels.
• Gray screens cause color shifting. Not necessarily. Perfect gray screens do not shift colors. Gray screens can, but hey, so can imperfect white screens.
• The Firehawk has a gain of 1.35, and the StudioTek has a gain of 1.3. So the Firehawk has all the benefits of a gray screen without sacrifice. False. Unfortunately, contrast enhancement is not free. In this case, the cost is a reduced viewing cone, increased hotspotting. For good reason, some say that the Firehawk has a "useful" gain of 1.0.

Please do not hesitate to suggest what else I should include. I hope to finish by the end of the weekend.

 

[David777]

One suggestion. Someone in another thread stated that gray screens crush whites. Add that to your list please.


Thanks for putting the effort forward to do this Michael. Now I'll just have to post a link to your thread instead of clearing up misconceptions all the time. Also thanks for using an "a" when spelling gray.

 

[CalypsoCowboy]

Michael, I'm anxiously waiting as it should be a great writeup one thing to maybe add, this may be at a forum level, could we get the final thread in a FAQ or something for the Screen forum all these questions over and over again. It makes getting to the good posts hard.

 

[GreggPenn]

Please include some definitions. I'm sure I'll have some trouble following explanations that aren't placed in layman's terms. Additionally, definitions may actually clear up possible mis-conceptions.


For example, I think of secondary reflections as one type of ambient light. (As the screen lights up the room and reflects of the walls, I understand this as ambient light). With this in mind, your pre-viewed conclusion seems self-contradictory.


Also, qualify if gray screens include the combination of gray material and reflective coatings. You probably need to include discussion of the reflective material as well. fyi: I would expect the Firehawk coating on a white screen to produce brighter whites than a 1.0 matte white screen. As you indicate, the gray (subsurface) darkens the overall image. With recent statements that the Firehawk has a 1.0 "usable gain", I would interpret this to mean that whites end up about where they started. (And, since I can see that blacks are darker, I conclude there is a contrast gain).


Thanks in advance for your help.

gp

 

[Tryg]

yes I'm ready! bring it on!

 

[Michael Grant]

The problem with treating secondary reflections as just another form of ambient light is that they affect picture quality in a far different manner. For example, secondary reflections have a negligible (if any) effect on on/off contrast, while ambient light obviously has a significant effect. What's more, the way to combat secondary reflections is almost completely different from the way to combat other forms of ambient light. Painting your room black can dramatically improve the secondary reflection problem but it doesn't do nearly as much for ambient light problems.


So combining the two together conceptually really hinders our ability to explain picture quality issues.


...argh, I should get back to work


Thanks for the suggestions guys, keep 'em coming.

 

[Gray Davis]

I would very much appreciate some definitions here.


"Secondary reflections." Light reflected off the screen, strikes a flat object like a wall, and reflects back onto the screen? Is that what you mean?


"ANSI contrast". I should know what this means since it is bandied about enough, but must confess I don't.


"On/off contrast." Have no idea.


Would someone please define these for me?


And Michael, are you saying that the primary purpose of a gray screen is to cut down on secondary reflections (as defined above?) Does that mean, if I paint my walls a dark color, and have total light control, that I won't benefit from a gray screen?

 

[CalypsoCowboy]

Maybe some information on types of gray screens from the perspective of a gray screen with a negative gain (under 1, Greyhawk, HC DaMat, High Contrast Gray) vs. a gray screen with a positive gain (over 1, Firehawk, HC Cinema Vision).

 

[CalypsoCowboy]

On other thought, since you are talking about filters, sunglasses, and gray screens, from what I've read in the other post. Would using a filter be similar to wearing sunglasses while watching a movie?

 

[Michael Grant]

OK, folks, I'm delayed a bit. I worked on it quite a bit today but there's a lot of info. The good news is I'm also adding in some info about why high-gain screens can boost ANSI contrast, too. So I'm going to call it something like "Contrast Enhancement using Grey and High-Gain Screens" I think. And there will be lots of definitions...