The brightness of a video system gives the image a powerful vividness that strengthens its impact, its believability. Without enough brightness, the image appears weak, pale, wan, and generally unconvincing and unbelievable. High lumen (brightness) output capability is especially important when those lumens must be allocated over a bigger screen area in a projection system, and/or when the ambient light in a room cannot be reduced to near zero.
The maximum achievable contrast ratio of a video system is an interesting subject. Technoids speak of contrast being important for portraying black areas in scenes, and for revealing details within dark areas, so they spend hours scouring films with dark and black patches, searching endlessly for that better black. But contrast has a much more important role than the mere reproduction of black patches for technoid film scrutinizers. This more important role affects you and me as we simply enjoy video and want to believe in what we see on screen. How? Two important ways.
First, the maximum achievable contrast of a video system effectively determines its dynamic range. A wide dynamic range gives an image pop and punch and three dimensional palpability, so it can reach out and grab you and involve you. If the dynamic range is compressed by an inadequate video system, the same scene appears flat, two dimensional, unbelievable, and uninvolving, more like looking at a rectangular paper photograph on the wall (paper reproductions do indeed suffer from low contrast/dynamic range) and less like looking at the real world through a rectangular window of the same size on your wall.
Second, a bigger contrast ratio with its dynamic range means that the incremental intermediate gray steps of the luminance scale each can become larger (with suitable calibration), and thus become more easily discriminated, both by the video system and by your eye/brain. We discussed above how believability depends crucially on the video system's ability to portray manifold subtle variations across a human face or a textured object, variations in both color and luminance. These variations involve subtle incremental steps on the luminance scale and on the color wheel. If those incremental steps are too small for the video circuitry or the eye/brain to discriminate, then you won't be able to see them, and so the image will be less three dimensional, less palpable, less believable. If the video system's luminance dynamic range as a whole is compressed, then the incremental steps within that range also might often get compressed to be smaller (depending on calibration), perhaps too small to be readily discriminated or appreciated. But a wider luminance dynamic range can (and should) produce larger incremental steps, which are more readily perceived and appreciated. This will make all the variations across human faces and textured objects more apparent, thereby making the whole image more three dimensional, more real, more believable.
We noted above that luminance variations within a given color can produce the perception of color variations. This means that incremental luminance variations across an object such as a face can become an important contributing component of the perceived color variations across that face, those color variations that make the face look real, three dimensional, palpably solid, and believable.
Thus, a high contrast ratio can have important implications for the vast midrange of the luminance scale, not just for the extremes of bright and black. The midrange is where most of the image lives, and is where we see most of the reality and believability of an image. So don't get preoccupied with staring at the black patches of movies, as some folks do. Instead, when you want to evaluate and compare video systems, take in and enjoy the whole picture. And you don't need to get overly technical in your visual evaluation, because even the technical factors such as contrast still have, as their most important consequence, the simple believability (or not) of the image. So you can simply ask yourself one question when evaluating a video system. Do I believe?
Brightness and contrast are largely determined by a video system's mechanical, physical, and optical components (its exciting energy source, modulation device, and image surface), which have various capabilities and limitations. Then, even after the luminance dynamic range has been established, some video systems might still be better than others at portraying subtle incremental gradations in the midrange of the luminance scale. This is largely determined by the sophistication and accuracy of the electronic video circuitry.
The electronic video circuitry must meet stringent requirements in some aspects, if it is to be able to handle a video signal (especially in any analog stages) without degrading its fidelity. The signal to noise requirements for video circuitry are not as demanding as for audio circuitry, but the bandwidth and even phase requirements are much more stringent (phase is often virtually ignored in audio circuitry because human hearing is relatively insensitive to it, but phase in video circuitry can play a crucial role in color accuracy). The video circuitry in every component, from source to display, affects video quality, including the portrayal of incremental steps in the midrange of the luminance scale. There are many factors in all this video circuitry which can affect video quality, and there are many tasks (e.g. gamma correction) that the video circuitry must perform which bear upon video quality in general and upon the discrimination in the midrange of the luminance scale in particular. So, if for example you ever ask yourself why you should spend $2500 on a high end DVD player, instead of just $100 on a WalMart special, the answer is in part that the video circuitry in the high end player is much more expensive, and will do a much better job of preserving the subtle aspects of the video signal, the subtle aspects that are so important to the believability of the final image you see.
-- Image Size
Most video displays are too small. Too small to be believable.
Of course, way back in the late 40s, our parents looked at a 5" round black and white CRT, on this wonderful new tele-vision device that brought moving pictures from afar into their living room, and thought that was believable. Just as back in the 20s, in a demonstration test of a primitive acoustical recording (no microphones yet), listeners said they could not tell the difference between a full live orchestra and a wind up acoustical phonograph. So you might think it's all relative.
But there are some standards which relevantly apply.
For the video experience to be the most believable, the display width should be just large enough so that your visual field cannot quite take it all in at one glance, at the viewing distance you have chosen for your room. When the display is this large, you get sucked in. You forget to be constantly aware of the rectangle framing and containing the portrayed scene, since you can't see the entire rectangular frame boundary at one glance. And so you get sucked into the scene being portrayed within the rectangle. You believe.
On the other hand, with the smaller displays common today in video systems, you can easily see the outer frame of the entire image rectangle at one glance. And you can see in that same glance your room's walls beyond the portrayed scene. So the portrayed scene becomes merely a picture in your room, a flat object trapped within your room, just like you and various other objects also trapped in your room. You never get to escape your room. You never get transported to another world, the world taking place on screen. You never believe.
We all already instinctively follow this same guideline, without even thinking about it. We can sit in our room, looking at a rectangular window far across the room on the wall opposite where we are. We can see the real world outside this real window. But we also can easily at a glance see the complete frame of the rectangle that is our window within the opposite wall. And so we're not really involved in that real world outside our window. Now, here's the interesting part. If we do get fascinated by something going on in the world outside our window, if we want to get involved in that other world, what do we do? We instinctively move closer to the window. How close? At least close enough so that we can no longer see the entire rectangular frame outline of the whole window at one glance. Close enough so we are no longer distracted from our goal of being involved in the outside world, distracted by a constant visual awareness that this window is merely an object within our room (as a video display screen is merely an object within our room). Only by getting close enough to that window, so that we can't see the whole window portrayal at once, and don't get distracted by the window as an object, do we then subliminally feel satisfied that we are involved in that other world outside our window.
How does this guideline translate into video screen size? Humans vary slightly in their visual field and brain processing algorithms. But for most of us, we can't quite take in at one glance and mentally process the entire width of an image if it is 60 degrees wide. So our screen image should ideally be wide enough so that it subtends a 60 degree portion of our visual field. This means that the screen width should be 1.16 times your viewing distance to the screen. That's much bigger than most home video systems are, and indeed is bigger than most other home theater people recommend. For example, if you choose a viewing distance of 10 feet in your room, then your screen should be 11.6 feet wide (I'd round this up rather than down, to make it 12 feet wide).
You'll find others recommending that screen width be less than your viewing distance, perhaps as little as half your chosen viewing distance (whereby the subtended angle would be merely about 30 degrees, rather than our recommended 60 degrees). Some folks would argue that, if you can't see the whole screen width at once, it becomes difficult to follow fast action or multiple action sequences. That may be true, but that's also part of what contributes to reality and believability. When real life action swirls around you, you can't see it all in one glance either, and if you get this same feeling from home theater then you'll feel as if you're in the center of the action swirling around you, rather than just watching a flat screen image up on a wall within your small room. Of course, we don't want to take the screen width so far that it becomes confusing or fatiguing to follow the action, which is why we recommend that the screen width be only just slightly beyond what you can take in at a single glance with your visual field.
You can easily experiment for yourself to find out what subtended angle works best for your field of vision and your taste. Take a couple of 8x11 sheets of cardboard or even 4x6 index cards to the movie theater. Go see a movie that is unpopular, so you can easily move about the theater. Start by sitting where in the theater you instinctively would like to watch the movie from. Try to sit as close to the left-right middle of the row as possible. Lay one piece of cardboard in your lap, with one of the 90 degree corners pointing at you. Have a pen or pencil also laid out and handy. Now, hold your two hands together as if you were clapping, then butterfly them out into a V, still joined at the wrists. Arrange the spread of this V so that each hand points at the left and right edges of the movie screen, respectively. You can hold your flying V in front of your nose and sight along each hand to make sure each is pointing at an edge of the screen. Then, rigidly holding this same V spread, lower your hands to the cardboard in your lap, so that the apex of your V is at the 90 degree corner of the cardboard. If you're right handed, align your right hand with the right edge of the cardboard, so that your left hand points somewhere in the middle of the cardboard (if you're left handed, align your left hand with the left edge, and reverse the handedness in the following instructions). Now, keeping your left hand aligned where it is above the cardboard, free your right hand from the V to reach for your pen or pencil, and mark where the tip of your left hand is aligned above the cardboard. The angle between the right edge of the cardboard and the mark, measured from the cardboard corner pointing toward you in your lap, will be your instinctively preferred subtended angle of viewing, some portion of the 90 degree cardboard corner sitting in your lap (you can measure the actual angle with a protractor when you get home). At this point, also make a hatch mark along the right edge of the cardboard, so you can tell later which edge was the right edge from which you are to measure the subtended angle.
That's step one, where you discover what your initial instinct is. Step two is to explore what happens as you expand your horizons. For step two, try moving forward in the theater, two or three rows each time, and spending at least five minutes in each progressively closer position row to acclimate yourself to that field of view. Try to sit as close to the left-right middle of each row as possible for each position (that's why it's important to pick an unpopular movie). Keep moving forward until the screen image becomes just slightly wider than your field of view that you can take in at one glance. Check if you can get psychologically comfortable with watching an image this large (disregard the neck pain from tilting your head backward, since your home display won't be as high up as the movie theater screen is). If you can't acclimate to it, then start progressively backing off, two or three rows at a time, until you can feel comfortable. This new position of comfort will almost surely be a row somewhat forward of your initial instinctive row selection, because you have now become acclimated to a wider screen image. Repeat the V measurement from this new position of comfort, and record it on a fresh piece of cardboard. This is the subtended angle you should try for, if you want your home theater to be involving and believable.
At home, you can achieve the target subtended angle by buying a large screen display, or by the simple expedient of sitting closer to the display, or some combination of the two. So long as your subtended angle is 45 degrees or greater, you're in good shape (don't worry about achieving our 60 degree recommendation). At 45 degrees, you will be able to see beyond the frame outline at a single glance, but not by much; thus the display image will command the attention of most of your field of vision, and your wall and other objects outside the image frame won't distract your vision much, so they won't detract much from overall believability. We do believe, however, that your subtended angle should be significantly higher than the 30 degrees recommended by some other home theater folks, in order for your video experience to be truly involving and believable. That's partly because home video suffers many limitations compared to true movie theater or professional video (limitations of brightness, contrast, resolution, compression, artifacts, etc.), and these limitations detract from the believability of what you see. We can partially offset these limitations, and re-kindle the believability and involvement factors, by employing a large subtended angle, so that you are visually sucked out of your room and into the portrayed scene, rather than merely watching an entire small rectangular picture trapped in the same small room that you are.
We've spoken of only screen width so far, not screen height, because our home screens are wider than they are high (especially the 16:9 wide screen format). It seems sufficient that the frame boundaries disappear for merely one dimension (i.e. width), in order for us to get sucked into the portrayed scene and suspend disbelief. So if the screen image is wide enough so that you just can't quite see both side edges in one glance, then the screen image is big enough to meet this criterion of believability, regardless of its height.
But adequacy of height does enter the picture (sorry) in another way. With wide screen aspect ratios of 16:9 or more, the height of the screen becomes a small fraction of the advertised screen measurement. In a standard 4:3 TV set, the height is approximately 60% of the advertised diagonal screen measurement. But with a 16:9 screen image, the height is merely 49% of the advertised diagonal screen measurement. That difference, 49% vs. 60%, might not look like a big numerical difference on paper, but it does make a big difference visually when viewing actual screen images. A 34" screen puts forth an image with impressive size and height when it is a 4:3 CRT TV, but a 16:9 widescreen TV with the same 34" spec puts forth images with visually puny height, indeed the same height as the 27" kiddie TV you allocated to your child's bedroom. And the height of the images seems even punier than this to our visual perception, precisely because of the dramatically larger width dimension of a 16:9 display, which perceptually makes the images' heights look even shorter in relative comparison to the large width dimension of the scene. It's great watching widescreen films in their true aspect ratio, but the experience loses credibility when John Wayne has puny height when he tries to stand tall. John Wayne's height is actually somewhat punier on a 34" 16:9 screen than on a 34" 4:3 screen, but on the 16:9 he visually appears to be even far punier than this, relative to the scene he's in, precisely because the expanse of the scene portrayed by a 16:9 is so much vaster.
This puny height issue is not a strike against the 16:9 format. We're strongly supportive of wide screen video. But wide aspect ratios do pose additional requirements, if you want to maintain that all important believability of the video experience. We suggest that you do employ a 16:9 display. And if you do, there is now an additional reason to make sure that you have an adequately very large display screen. We already saw how it must have enough width so you can't take the entire width in at one glance, so you can be sucked into the scene and believe in the scene. Now we realize that it must also have enough height so that you can believe in those heroes standing tall, so that they don't look puny.
We're emphasizing here these two reasons for employing a very large display, even larger than you'll see in most demo systems. But we also realize that you may face some temptations to downsize from our ideal. First, large displays are very costly. Direct view displays (CRT or plasma) are priced approximately by their area, which of course goes up as the square of the advertised diagonal or width dimension. Second, there might be other tradeoffs. With any given video projector, achievable screen brightness decreases as you increase the screen size, so if you want both large screen size and high brightness, you'll have to buy a different projector that's more expensive, probably noisier (to cool the heat of a more powerful light source), and might have lower achievable contrast. Thirdly, a screen width occupying 60 degrees (or even 45 degrees) of your visual field is initially intimidating to some viewers, who prefer the security of being able to easily see everything at once, or who perhaps simply enjoy sitting at the rear of even real movie theaters.
Of course, you could get your cake and eat it too, via the simple expedient of sitting up closer to a screen display of moderate or even modest size, to achieve your own target subtended angle in the 45 degree to 60 degree range. For example, Revox has a new 32" plasma display, in 16:9 format, with an exceptional 1024 x 852 pixel resolution. This modest sized Revox display provides a stunning picture, and also a wonderfully involving and believable video experience provided you sit close enough (its target market is the confined quarters of yachts and motor homes, though you are
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