SHADES OF GRAY
Mar 1, 2003 12:00 PM,
PETER H. PUTMAN, CTS
We love numbers — the bigger, the better. From horsepower to megabytes, from square feet to miles per hour, we use all kinds of numbers to convey superiority of one product or system over another. Sometimes those numbers are based on facts and measured performance, and sometimes they are based on marketing hype.
It should be no surprise that the electronic display industries are subject to the same numbermongering that pervades the automobile, real estate, and computer sectors. Now that projectors are small enough, are bright enough (in most cases), and have sufficient resolution for about 90 percent of their end-users, the latest craze is to play up the contrast — hence, figures such as 2,000:1 and 3,000:1.
This sort of creative number massaging isn’t limited to projectors. For better or worse, the plasma and LCD manufacturers have gotten into the act, too. Where will it stop?
Although there’s no question that contrast is certainly an important display attribute, it can be a misleading number if used incorrectly. Remember the ANSI lumens versus white lumens versus peak lumens debates among the projector crowd a few years back? The truth is gray scale is the single most important attribute of any electronic display. Without shades of gray, there isn’t any contrast. Without shades of gray, you can’t create wide color palettes. Gray scales are where it all begins when a projector or monitor first comes to life on the drawing board.
YOU WANT COLOR WITH THAT?
Those who evaluate and write about projectors and monitors are drawn to those displays that provide the most lifelike images. That means the widest possible gray scale with a virtually unlimited number of color combinations created by an equal-energy light source, such as the sun. Anything else represents a compromise, but some of those compromises look pretty good.
Short of using a portable nuclear fusion system to power projectors, the next best thing is to employ short-arc lamps that ionize mixtures of gases and metal halides to produce blinding shafts of light. You then force these shafts through condensers and integrators, refract out the primary colors, use those colors to create red, green, and blue images from monochrome light modulators, and finish up by precisely overlaying the RGB images to create full-color pictures.
With flat-panel monitors, light can be forced from a cold-cathode light source (such as a fluorescent lamp) through a light shutter (AM-LCDs) made up of pixels coated with tiny precision filters. Or electricity can be discharged through pixels filled with a rare gas mixture (plasma) as color phosphors are stimulated to produce RGB color imaging.
In the old days, color imaging was accomplished by tickling phosphors with an electron gun. Surprisingly, this system produced (and continues to produce) the most lifelike images of all, which is why CRT front projectors are still preferred by a small number of high-end customers for home-theater applications.
That’s because CRTs are capable of a wide gray scale and can show images with very low luminance levels (shadow detail) as well as very high luminance levels (highlights) in the same scene. More importantly, when a CRT is idling, it is essentially shut off. I mean really shut off, as in black, not a deep gray, as you’ll see with LCD, DLP, and LCoS projectors and AM-LCD and plasma monitors.
Although there have been tremendous advances in color imaging with flat-panel displays, one stumbling block still remains: the ability (or inability) to show a gray scale with the widest possible dynamic range. In some systems, the resolution of the imaging device is limited by brightness (CRTs). In others, it’s limited by scattered or refracted light (DLP, LCoS, LCD).
YOU MEAN THERE’S A BASEMENT?
Black levels are also problematic. (The term black level is really an oxymoron, for there can be only one level of black, and that’s black or zero luminance. A better choice of words would be shadow detail or low gray levels.) When viewing content with relatively high luminance levels — say, 20 percent of white or higher — you won’t see any problems with the display.
But movies and TV programs shown with high-key lighting are a different story. If the monitor or display can’t resolve luminance values below a certain level (say, 10 percent of white), then any detail in the program content with luminance values at or below that level won’t be visible. If you raise the black levels by adjusting the brightness control, then you also elevate the luminance levels at the high end and wind up compressing the gray scale at some point. Granted, you’ll see more of the detail in the image, but not as the cinematographer or videographer intended.
Some funky things are now happening with the subtle shades of light that approach 100 percent gray or white. They begin to blend together or “crush” into the ceiling of 100 percent gray. The display no longer has wide dynamic range, and you’ve also clipped the gray scale, reducing the number of shades of color that can be rendered.
If the gray-scale capability of a CRT-based display could be likened to the number of floors in a house, that house would have a full-size basement and a walk-around attic. LCD, DLP, and LCoS projectors will reduce that basement to a crawl space or eliminate it altogether, and the attic becomes a tight crawl space, too. You have fewer floors to work with and less space overall.
MORE FUN WITH NUMBERS
To better understand this concept, I selected a basic 16-step gray-scale ramp from the DisplayMate test pattern series for illustration. All 16 steps are clearly seen in Fig. 1, and that is how the gray scale would appear on a correctly calibrated CRT display. Setting the step above black to about 6 percent of white results in a contrast ratio of about 440:1 on my Princeton CRT monitor. However, with a Samsung 42-inch plasma, I measured only 60:1 contrast.
The difference? Black on the CRT monitor registered about 0.2 nits, whereas on the Samsung plasma, black registered as 3.6 nits, or 18 times as bright. With a little playing around, I could expand the contrast ratio on the Samsung panel to 107:1, but black now measured 1.8 nits. Because my lower black level was limited by not having a “basement” to speak of, the Samsung’s 16-level gray scale resembled that of Fig. 2.
You can still see the 15th and 16th steps (barely) at the high end of the gray scale, but there’s no difference between steps 1 and 2 at the low end. That is a typical gray-scale rendering for plasma and LCD monitors. Keep in mind plasma and CRT displays have some degree of current limiting to minimize image burn-in and premature phosphor aging, and these circuits will limit contrast with images having high overall luminance values.
In the case of projectors that shutter or reflect light (that also includes LCD monitors), the value of white can be substantially higher than that of the combined steps 1 and 2. That’s because the resolution of the projector is not affected by brightness levels, and neither is the stability of the color dichroics as sensitive to luminance values. The result is high contrast levels (great for marketing) but a loss of shadow detail (not great for viewing).
Last fall I measured some projectors with exceptionally high contrast. Several of them exhibited peak contrast ratios much higher than the 440:1 measured on my Princeton CRT. But none could come close to the value of black that I measured on the Princeton set, and consequently, the gray-scale images they displayed didn’t have as wide a dynamic range below about 8 to 10 percent of white.
Fig. 3 shows an approximation of the typical LCD, DLP, and LCoS projector gray scale. Of the plasma panels I have tested, only those made by Panasonic (also used in Fujitsu’s 50-inch product) can produce black levels that approach that of a CRT and subsequently display a gray scale with CRT-like shadow detail performance. The Panasonic panels typically produce a black level of 0.2 nits, equivalent to my Princeton CRT monitor.
As a result, these panels create wide gray scales with nice color palettes. But they also do well in the contrast numbers game, although I’ve never measured the 3,000:1 contrast that Panasonic has claimed in the past. Instead, my numbers (taken after the panel was calibrated for best gray scale) were in the 600:1 to 800:1 range.
CAVEAT EMPTOR
So just how much contrast do you need to see in an image? Empirical data suggests the human eye is limited to a dynamic range of 100:1 at any given instant. That means if you look at a scene with objects of different luminance values, you won’t be able to discern more than a 100:1 difference between the darkest and lightest objects. Of course, the instant your eye moves, its built-in auto iris function raises and lowers the gray-scale boundaries. That’s what allows you to perceive shadow detail and also pick out a white cat scurrying along in a field of snow.
If you are watching a movie on a plasma or an LCD monitor or with a front LCD/DLP/LCoS projector, you’ll probably be satisfied with the displayed images as long as there is not a preponderance of dark gray and black objects. But switch to a nighttime scene with high contrast lighting, and your eyes will strain to pick out any shadow details.
Obviously, there’s a long way to go to improve the rendering of low gray levels on projectors and monitors, but there has been progress. In addition to Panasonic’s work with plasma, Texas Instruments has made enhancements to its digital micromirror devices to reduce light scattering and refraction. That drops the value of black and improves both gray-scale rendering and contrast.
Unfortunately, polysilicon LCD technology seems to be limited in this area. Although projectors have become brighter and contrast has improved, black levels are still higher than those measured on DLP projectors by 100 percent or more. LCoS imaging isn’t any improvement — the black levels I measured on a D-ILA projector were equivalent to several polysilicon models in the review.
Numbers are great for impressing people and can sustain a good argument for several hours. But peak contrast claims don’t tell you everything about performance of a projector or monitor when it comes to rendering images with lifelike gray scales. They can only tell you how much brighter the white levels can be than the black levels. Caveat emptor.
Peter H. Putman owns PHP Communications, in Doylestown, Pennsylvania. The author of The Toastmasters Guide to Audio/Visual Presentations, Putman is also a regular columnist in S&VC.
