The effect of a black GOM screen wall on percieved black level

It starts with a simple example. If you take a dark gray piece of paper, and hold it up in front of white backdrop, the dark gray piece of paper looks black. Conversely, if you hold that same dark gray piece of paper up to a black backdrop, it looks much brighter by comparison.

Many of us own digital projectors that project a less than optimum black level (read: the dark gray piece of paper), and yet we choose to project this image against a black GOM screen wall (read: the black backdrop).

I've seen people mention this reality from time to time, often in the form of a question regarding its relevance or lack thereof. The most common response is that someone will mention the benefits of reducing light scatter in the room and the improvement that this can make to ANSI contrast (not on/off) and black levels. But I've never seen one single person substantiate the very real effect that the black GOM backdrop has in terms of perceived black levels.

So let me be the first.

I've had the opportunity to watch my Sony Pearl FP in action in a variety of configurations. I project onto a Carada Brilliant White screen, which I have seen mounted to a blue wall, an unfinished false wall with blue and yellow insulation behind it, and now a finished false wall with solid black GOM.

Prior to installing the GOM, I was truly delighted with my black levels. But once I covered that wall in black GOM, the charcoal grey "black level" of my projector looked more like light gray than dark gray, and certainly looked nothing like black.

The funny thing is that the contrast ratio of my projector hasn't changed. In fact, the ANSI contrast may have hedged upward by a few notches due to there being less light scatter in the room. But my blacks just don't appear as black as they did previously, and there's no getting around that.

Make no mistake, the solid black GOM wall looks fantastic when the lights are on. The screen has the illusion of floating in mid air. And when the lights are off, high APL scenes (a sporting event, for example) look even better than before the GOM went up, due to the fact that the screen almost appears to be a window in this otherwise black wall. But in low APL scenes, the lack of a true black is more apparent now than ever before because a perfect reference black is sitting right there next to it, in the form of a black GOM wall.

If I had this to do over again, I might go with a slightly more reflective color behind the screen (a gray perhaps) and make the walls darker.

Guys, you may find this hard to believe, but the presence of the perfect reference black basically makes any adjustment to the projector's black level an excercise in futility. For example, switching from high bulb mode to low bulb mode has the illusion of making the picture less bright without actually lowering the black level. As another example, turning the projector's brightness control all the way down appears to do absolutely nothing to lower the black level -- it simply crushes shadow detail and dulls the image. The same can be said of all the Pearl's fancy iris adjustments. I can see wide variances when it comes to the actual brightness of the image, but that black level just floats around at a shade of gray that is perceived as being fairly constant, no matter what I do, because of the GOM reference black. Prior to installing the GOM, all of these controls clearly had a visual impact on the black level, however slight it might have been.

In fact, I now find that I have the best image with the iris turned off. Brightness is now my best friend. A bright 3 chip DLP would look great against this wall, I'm sure. Of course, a nice CRT projector with a more ideal black level would probably look fantastic as well.

Just for reference, my ceiling is flat black, my walls are a mid-gray, my carpet is black, my seating is burgundy. The room is very dark, and the Pearl has an excellent contrast ratio and good black levels for a digital. The effect I'm seeing is clearly due to the black GOM, since it is the only element in the room that has changed.

I think the majority opinion will differ from mine, just based on what I've seen posted elsewhere. But there is no question about what I am seeing.

The black velvet border around the screen itself has never caused me any such negative experience, since it is such a small area compared to the screen and the surrounding wall.

But when the wall is completely black, the digital projector's black level just looks anemic in comparison.

One of the benefits of bias lighting behind RPTVs is that it improves the black level perception (in addition to reducing eye strain). So brightening the wall behind the screen creates the illusion of a deeper black, while darkening the wall creates the illusion of a lighter black.

In a front projection system, things are more complicated because the entire room is much moreso a part of the system, so darkening all of the surfaces of the room does indeed have a positive impact on the real, measured ANSI contrast and the real, measured black floor of the image.

But in my case, perception is more powerful than the actual measurements.

One more quick thought:

I never really understood the "brighter is better" mentality among projector owners until now. I mean, I could understand that mentality with regard to projectors used in rooms with high ambient light, but in darkly colored, light controlled spaces, that mentality has always been a puzzlement to me.

Not anymore. Since the dark reference black of the GOM wall makes the projector's BL look anemic no matter what, the only percieved improvement must come via brightness increases.

Digital projectors are going to have to get much better at producing a true black with high dynamic range before they can compete with the black wall.

With an RPTV, that is a good solution for more than one reason. 1 reason is eye fatigue. Another is perceived BL.

In an FP system, it's more complicated. A giant screen at a marginal ft.-L doesn't really pose the same issue with regard to eye strain. Plus, You really don't want to add any extra light to the room, and a rope light on the screen wall might be a bit much. And thirdly, unless that rope light is installed very carefully, it might cast reflections on the back of the screen, and it might also color the images on the screen. This would not be my first choice.

I think something as simple as using a gray fabric screen wall instead of a black one might do the trick, providing that the room was "black enough" elsewhere to tame light scatter. In general, darker is better in a dedicated HT.

Wow, I didn't realize that. Very interesting indeed. In the picture, it actually looks like a plasma.

I think if you're going to do something like this, it would make sense to use real bias lighting -- they have bulbs specifically designed to be the right color temperature for this sort of application. Of course, that would be more difficult to install than a rope light, and perhaps too bright as well.

I'm not sure. I thought most of the proper-temp bias lights were flourescent, and of the style that cannot be dimmed. I could be wrong.

As a quick follow up, I just finished watching the movie Slither on HD DVD. Man, what a fun and totally gross horror movie. This movie is very low APL for the most part. Unfortunately, I am sad to report that my black levels have never looked so terrible.

This black GOM wall has made my 4K projector look sort of like my friend's cheap 1K projector. It's that bad.

My room is so dark now that I'm really seeing the contrast limits of the projector, especially with that black GOM wall standing as a reference for true black. I think I'm going to try some bias lighting to see if that helps. Unfortunately, the aesthetic of a rope light casting light onto a black GOM wall doesn't sound all that appealing to me.

I'll let you guys know how it goes.

But I have to say, after seeing the effect of this firsthand, I am surprised that the group think regarding screen walls is to make them ultra-light-absorbent, considering that most of us are using digitals with less than optimum black levels. And this group think has persisted for how long? Largely unchallenged?

I feel like a voice in the wilderness here.

I also feel like if I yell loud enough, someone professing to be an expert is certainly going to show up in this thread with some scientific, quasi-mathematical explanation for why what I'm seeing is impossible. But I'm hear to tell ya that putting a reference black right next to your projector's charcoal gray is inviting a comparison that your eyes and brain will not be able to ignore, whether or not you are consciously aware of what's going on.

I might re-do my wall in a mid-gray GOM fabric to match my walls, as that will undoubtedly help. I hate to do this, since it was a royal PITA and about 10 hours worth of work trying to cut, stretch and velcro the fabric the first time around. So I'll try the bias lighting first. I guess a theater owner's work is never done.

After seeing how good my projector can look, I just can't leave it this way.

I agree that a CRT projector would likely have zero problems, since these things have MUCH higher on/off contrast, and MUCH deeper blacks.

While it is true, according to some well respected reviewers, that many digitals have long surpassed CRTs in terms of sharpness, ANSI contrast, and other key performance indicators, nothing beats the on/off contrast or black level of a CRT.

P.S.

My projector is LCOS, not LCD or DLP. It tends to produce deeper blacks than LCD, but nowhere near the CRT range for sure.

Unfortunately, my room cannot handle the bulk. Every inch of this room is accounted for.

Thanks Chris.

Your suggestion is an excellent one. I could probably recover at least 50% of my cost if I don't damage the gray roll while I'm doing my test. If the bias light doesn't work, I'll give it a shot when the tax money comes in.

I could not sell the black. It's small pieces, stapled at one end, velcroed at the other. Not saleable IMHO.

I was thinking about taking some pictures. I will try. But I do not think I will be able to capture the effect I am seeing, since my camera is nowhere near as sensitive as my eye. In fact, my camera has a tendency to crush blacks, so it might end up being pointless.

After making my best attempts to install a bias light tonight, it is clear to me that it will be impossible without a complete redesign of my false wall, which isn't going to happen.

I tried what I thought would be the easiest approach -- a rope light. The first and most important issue is that even the brightest rope light cannot illuminate a wall of light-absorbing black GOM. Ain't gonna happen. That stuff sucks up light, so the best you can do is create an illuminated border about 2 inches wide. It's not enough.

All of the other obstacles I ran into (and there were several) are incidental in light of the first.