High contrast ratios don’t make the display—grayscales do.



Grayscale more accurately reflects a display product’s ability to present lifelike images than contrast ratio.
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 the superiority of one product or system over another. Sometimes those numbers are based on facts and measured performance. Sometimes they are based on marketing hype.

It should be no surprise that the electronic display industries are subject to the same number mongering that pervades the automobile, real estate, and computer sectors. Now that projectors are small enough, bright enough (in most cases), and have sufficient resolution for about 90% of their users, the latest craze is to play up their contrast performance. “2000:1!” we hear. “3000:1!” we read.

And this sort of creative number bloating isn't limited to projectors. For better or worse, the plasma and LCD manufacturers have gotten into the act. “3000:1!” “4000:1!” Where will it all stop?

There's no question that contrast is certainly an important display attribute, but it can be a very 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, grayscale is the single most important attribute of any electronic display. Without shades of gray, we don't have useful image contrast. Without shades of gray, we can't create wide color palettes. Grayscales are where it all begins when a projector or monitor first comes to life on the drawing board.

Want color with that?

Those of us who evaluate and write about projectors and monitors are drawn to those displays that provide the most lifelike images. That means the widest possible grayscale 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 darn 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. We then force these shafts through condensers and integrators, refract the primary colors out of 'em, 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, we can force light 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 and get our color images that way. Or, we can discharge electricity through pixels filled with a rare gas mixture (plasma) and watch 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 a small number of high-end customers still prefer CRT front projectors for home-theater applications.

That's because CRTs are capable of a wide grayscale 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 and not a deep gray, as you'll see with LCD, DLP, and LCoS projectors, and AM LCD and plasma monitors.

While there have been tremendous advances in color imaging with flat-panel displays, one stumbling block still remains. And that's the ability (or inability) to show a grayscale with the widest possible dynamic range. In some systems, brightness limits the resolution of the imaging device (CRTs). In others, it's limited by scattered or refracted light (DLP, LCoS, LCD).

There's a basement?

Black levels are also problematic. (The term “black level” is really an oxymoron, for there can only be 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 shadow detail with relatively high luminance levels — say, 20% of white or more — then we won't notice any problems with the display.

But movies and TV programs with high-key lighting are a different story. If the monitor or display can't resolve luminance values below a certain level (say, 10% of white), then any detail in the program content with luminance values at or below that level simply won't be visible.

If we raise the black levels (sorry, low gray levels) by adjusting the brightness control, then we also elevate the luminance levels at the high end and wind up compressing the grayscale at some point. Granted, we see more of the detail in the image, but not as the cinematographer or videographer intended. This type of signal processing is often referred to as “black stretch” on consumer TV sets.

To make matters worse, some funky things are now happening with the subtle shades of light that approach 100% gray, or white. They are beginning to blend together or “crush” into the ceiling of 100% gray. Our display no longer has wide dynamic range, and we've also clipped our grayscale, reducing the number of shades of color that can be rendered.

If the grayscale capability of a CRT-based display could be compared to the number of floors in a house, that house would have a full-sized 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. We have fewer floors to work with and less space overall.

More fun with numbers

Figure 1: Grayscale on a correctly calibrated CRT.

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 Figure 1, and this is how the grayscale would appear on a correctly calibrated CRT. Setting the step above black to about 6% of full white results in a contrast ratio of about 440:1 on my Princeton CRT monitor. However, with a Samsung 42in. plasma, I measured only 60:1 contrast.



Figure 2: 16-level grayscale for Samsung plasma.

The difference? “Black” on the CRT monitor registered around .2 nits, while 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 as “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 grayscale resembled that of Figure 2.

You can still see the 15th and 16th steps at the high end of the gray-scale, but there's no difference between steps 1 and 2 at the low end. This is a typical grayscale rendering for plasma and LCD monitors. Keep in mind plasma and CRT displays have current limiting to prevent image burn-in and premature phosphor aging, and these circuits will limit also contrast in images having high overall luminance values.

With projectors that shutter or reflect light (this also includes LCD monitors), the value of white can be many multiples of the summed grayscale steps 1 and 2. That's because the resolution of the projector is not affected by brightness levels, nor are 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 so good for viewing).

Last year, 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 grayscale images they displayed didn't have as wide a dynamic range below 8% to 10% of white.



Figure 3: Approximation of typical LCoS, LCD, and DLP projector grayscale.

Figure 3 shows an approximation of the typical LCD, DLP, and LCoS projector grayscale. Of the plasma panels I have tested, only those manufactured by Panasonic (also used in Fujitsu's 50in. product) can produce “black” levels that approach that of a CRT, and subsequently display a grayscale with CRT-like shadow detail performance. The Panasonic panels typically create a black level of .1 to .2 nits, equivalent to my Princeton CRT monitor.

As a result, these panels display wide grayscales with nice color palettes. But they also do well in the contrast numbers game, although I've never measured the 3000:1 contrast that Panasonic has claimed in the past. Instead, my numbers (taken after the panel was calibrated for best grayscale) typically fell in the 600:1 to 800:1 range.

Conclusions

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 that 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 grayscale 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 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 (DMDs) to reduce light scattering and refraction. This, in turn, is dropping the value of “black” and improving both grayscale rendering and contrast.

Unfortunately, polysilicon LCD technology seems to be limited in this area. While projectors have become brighter and contrast has improved, black levels are still higher than those measured on DLP projectors by 100% or more. And 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.

Remember: 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 grayscales, only how much brighter the “whites” can be than the “blacks”. Caveat emptor.


Pete Putman, senior contributing editor and president of Roam Consulting, Doylestown, Pa., can be reached at roam@projectorexpert.com.