How to achieve the optimum perceived black levels - AVS Forum
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post #1 of 4 Old 12-22-2012, 11:45 PM - Thread Starter
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I came across this quote in the BenQ W7000 thread, and was going to reply there but thought I'd start a new thread to discuss the "surround" for home theatres.
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Originally Posted by tallnick View Post

Having said that, pretty much everyone one on this forum and every reviewer I've ever read seems to put black level and contrast at the top of the list of importance. I, simply, don't.

I don't either. It's a valuable characteristic of any projector, but other considerations are more important. It's an attribute that may appear to enhance the experience, but it's a bit of a fraud, especially the ridiculous numbers quoted for contrast ratio. The Holy Grail of dense black levels is an analogue of the Loudness control found on some amplifiers. "Let's boost the bass so it sounds better". The loudness control does have some technical validity (it can compensate for the "bass loss problem" when music is played at low volumes, see http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/baslos.html), but when used at high volumes the concept is a bit sus if you are really trying to experience the music as recorded. Darkening the black levels is a similar phenomenon, and what price is being paid to achieve it? Irises buzzing in and out, scenes lightening and darkening unnaturally.

Improving black levels has a long history. Kodachrome was designed to improve black levels (when projected) by seriously increasing the density of the shadows, sort of like the effect of a variable iris in a digital projector, but the effect was achieved chemically (see pages 10-12 of my PDF on scanning Kodachrome: http://www.mediafire.com/download.php?qc67n2gkdz3viyp). Scientists in the Kodak labs were compensating for the effect whereby the shadow areas of an image projected in a dark room appear less dark than they should be, not because they were actually less dark, but because of the characteristics of human vision. To make shadow areas of slides appear acceptably dark, those areas had to have their blackness increased by as much as a hundred fold as the slide was being exposed.

Some other considerations pertaining to black levels:

1. Our eyes are more sensitive to changes in brightness in dark areas than in light areas. Make the overall scene darker and suddenly what was an acceptable black in the image is now distinguisable from the deeper blacks or areas outside the image.

2. I don't know the exact figures, but contrast ratios of image-reproducing devices are really quite limited. Paper is about 100; Kodachrome under ideal projection conditions can improve on that, as can a film in a movie theatre, both of which might achieve 500; and a good LCD screen in a darkened room with dark surrounds might get close to a 1000. And yet users complain about a projector, such as the BenQ W7000, as having poor black levels when it has a native contrast ratio of 1000.

3. The human visual system operates relatively, not absolutely. Meaning: if we have nothing in view to compare an image with, we accommodate to what we see in terms of how bright it appears, the colour balance, and how much contrast we perceive. So, it is quite difficult to see the "yellowishness" of the light when you walk into a room lit by incandescents, but it becomes very obvious when you photograph such a room on a daylight setting and then view the image on a screen. Similarly, if you concentrate on nothing but the image from a digital projector, black levels, I think, are of not much importance as long as you have a certain level of contrast. You'll perceive the shadow areas as acceptably black – until you look at the black wall over to the side and begin comparing the blackness.

What interests me in all of this is: by aiming for surrounds that are as dark as possible when projecting, are we actually reducing the perceived contrast? I'm not talking about the absolute value of how black the shadow area is, but how black it is perceived. I don't know how comparable digital viewing conditions are to the way slides were projected and tested, but it is well-documented that the darker the surrounds, the lighter the appearance of the shadows will be. For a simple example, see the figure on the bottom right on p12 of my Kodachrome PDF.

I'd like to read some research about perceived black levels versus the lightness of the surroundings for digital projection. Does anyone know of such research? A good starting point is the paper by Fairchild (http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.39.6955). I imagine a room where there is a digital projector projecting stills onto a screen which is surrounded by nothing but:

1. White walls
2. Gray walls
3. Black walls which absorb as much light as possible.

Then ask a number of ordinary viewers to rate the contrast of each scenario, particularly how black the blacks appear.

As an aside: the human visual system is a strange beast. Here's a surprising phenomenon (http://www.cis.rit.edu/fairchild/PDFs/PAP13.pdf), which shows how what we think we see can change if it is viewed in isolation. Fairchild says:

One very interesting case is for the perceptions of colors such as brown and gray. These colors only exist as related colors. It is impossible to find an isolated brown or gray stimulus, as evidenced by the lack of a brown or gray light source. These lights would appear either orange or white, when viewed in isolation.

I assume there is something similar for blacks: that the darkest area of an image, under suitable conditions, will be perceived as black when the whole image is viewed in isolation. It is only when compared to a darker black that we can see that what we thought was black, could be blacker.

This black levels thing is a complicated matter, much more so than just measuring how much light is coming from the screen when the projector is projecting black. That comes into play of course, but what is more important is how we perceive that black. It wouldn't surprise me if it turned out that perceived contrast improves when the surroundings are not an all-absorbing black. Maybe the optimal perceived contrast is achieved when the surrounds have the same blackness as the blackest possible part of the image. That way, you've got nothing blacker with which to compare the shadows, and the overall viewing experience would improve.
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post #2 of 4 Old 12-23-2012, 04:19 AM
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Interesting theory. smile.gif

I have recently made up some very temporary side masks for my 2.35:1 screen as I anticipate watching quite a bit of 1.85:1 over the Christmas break on my projector. Previously the sides of my screen would be lit by the room reflections as my living room is painted in a light grey and has a white ceiling of and course the projector doesn't produce any direct light on the side areas of the screen when my lens is removed. So I would have a surround (at least on the sides) no darker than the darkest 'black' in the picture because of the room reflection so similar to what you suggest might be worth trying. However, I always felt that the contrast of this unmasked image seemed poorer than when I view 2.35:1 content which is properly framed on my 2.35:1 screen.

What I have found is that with my black velvet side panels the image looked so much better as in darker scenes the edges of the picture is now defined rather than being a bit vague blending into the unlit area of the unmasked screen. While very dark scenes show that the screen border and side masking (and my dark brown wall surrounding the screen) all look much blacker than the 'black' on the screen personally I've enjoyed viewing 1.85:1 content with this side masking over this weekend despite that I can tell that the blacks aren't perfect.

FWIW I'm using a JVC X35 at minimum zoom, minimum iris which should be giving me near maximum on/off contrast of 50,000:1 and getting 15fL on screen. A sudden fade to black, especially after a mid or high APL scene really does seem pitch black for a few seconds, so I think that is probably all I could wish for. Having used the new Chromapure/Lumagen 125 point autocalibration my gamma is set accurately so that I get the correct shadow detail (not crushed or overly bright) and I feel that being able to see the variations in black/dark greys helps distract the eyes from noticing that absolute black isn't as 'perfect' as we might wish. The room (and screen) has a large impact on this effect as poor rooms tend to washout the screen and light up these areas (as said my room isn't perfect in this regard so I'm making improvements), so IMHO having very dark walls and ceiling (at least a metre or two out from the screen) will help the viewer to be able to see these shadow details with less light pollution. I'm not convinced that putting light coloured walls near to the screen will help the overall picture quality even if it might give the impression of a better black.

What I have found is that if I use my laptop while I've got the projector on, the blacks then look very deep as the laptop's brightness even at minimum backlight must close my eyes' irises down. However, it also distracts from the film so I wouldn't recommend it (I tend to do this if my OH is watching certain TV programs on the projector that I'm not interested in biggrin.gif ).

Zooming: Been there, done that, bought the lens...
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post #3 of 4 Old 12-23-2012, 07:24 AM
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My experience is the same. Darkening the room makes as much difference as spending 2x as much on the projector, maybe more. I'd take a low to mid-range projector and a dark room over and high end projector and a light room, particularly if it has a white ceiling. The ceiling is always a killer since the screen tends to be close to it.
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post #4 of 4 Old 12-23-2012, 07:39 AM
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I used my Lux meter to measure ANSI contrast in my room, then I added various treatments (unfortunately they were quite temporary, hence my ongoing improvement program in the new year). I found that covering my ceiling even with cheap thin black cotton sheeting made the biggest difference, followed by the side walls. Covering the floor in front of the screen made very little measurable difference (though subjectively I liked not being distracted by seeing anything in the foreground) perhaps because my floor is Oak with a fairly dark rug on it already and that the screen is further from the floor than it is for the ceiling.

My plan for the new year is to add some kind of extended screen pelmet which folds open. It will content drop down black velvet side curtains and the 'ceiling' inside will be lined with the same black velvet. It will only be about 1 metre deep from the screen, but I may try to make a panel which flips out to cover more of the ceiling out to 2 metres. Judging from my previous temporary set up I think this will give a huge increase in picture quality caused by the reduction of room reflections: Put it this way, my old HD350 in this 'bat tent' set up looked better than an X70 that I saw in a less than ideal demo room. My X35 looks better already, so I'm sure it will be like another upgrade once I get this room work done. cool.gif

Zooming: Been there, done that, bought the lens...
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