Display Technology Shoot-Out: Part I

By Channel Insider Staff  |  Posted 2004-12-01 Email Print this article Print
 
 
 
 
 
 
 

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This is Part I in a four-part series, by DisplayMate author Dr. Raymond Soneira, exploring the four most popular display technologies. Here we analyze and compare black level, color temperature, peak brightness, dynamic range, and display contrast.

This is the first in a four-part series by DisplayMate author Dr. Raymond Soneira, exploring the four primary display technologies. This first installment analyzes and compares black-level, color temperature, peak brightness, dynamic range, and display contrast for each technology. DisplayMate has worked closely with ExtremeTech, PC Magazine and many other Ziff-Davis publications for over ten years to develop our display testing procedures.

Part 1: The Primary Specs
We are in a renaissance of display technologies. Ten years ago the CRT (cathode ray tube) was the single prevailing technology. Today CRT, LCD (Liquid Crystal Display), Plasma, DLP (Digital Light Processing using Texas Instrument's Digital Micromirror Device DMD), and LCoS (Liquid Crystal on Silicon) are mature and mainstream, while many other technologies are trying to emerge from the development lab and grab a significant market share. All of these choices raise the questions: How does one technology differ from another and which one is best for your needs?

To provide some substantive answers, DisplayMate Technologies performed an in-depth comparison between these technologies to analyze their relative strengths and weaknesses. The DisplayMate products generate proprietary test patterns for setting up, tuning up, testing, and evaluating displays and projectors. They're used widely throughout the computer and video industries worldwide, including ExtremeTech and PC Magazine, and have been part of the InfoComm Projection ShootOut event and software since 1997, so they are accepted by most manufacturers as reference standards. Direct-view and rear-projection units were included in our comparison, but front projectors were excluded because they require different measurement and evaluation criteria.

We used a combination of high-end laboratory instrumentation, advanced diagnostic test patterns, and old-fashioned viewing tests to compare them simultaneously in a side-by-side Shoot-Out. We chose the top performer for each technology from the 2004 DisplayMate Best Video Hardware Guide, which includes selections for the best video hardware in 40 categories. The candidates included a 40" direct-view LCD (NEC LCD4000), a 61" plasma (NEC 61XM2), a 50" DLP Rear Projection (Optoma RD-50), and a much smaller 19" CRT professional high-definition studio monitor (Sony PVM-20L5) used as the reference standard for color and gray-scale accuracy. Only the Optoma RD-50 is marketed as a home-theater display quality, although the NEC Plasma 61XM2 is carried as an OEM unit by some of the very best home-theater specialty manufacturers (in some cases with additional front-end electronics). The NEC LCD4000 is marketed as a commercial computer display, but it's an outstanding large-screen LCD panel and will perform extremely well with video when interfaced with the appropriate front-end electronics. The Sony PVM-20L5 is a standard-size CRT studio monitor and was selected as the reference display because it delivers virtually perfect performance.

 

LCoS is another upcoming display technology that works as a reflective mode LCD. While there are many variations on LCoS technology, only JVC's D-ILA can be classified as mature and mainstream. JVC's front projectors have used it since 1998. JVC recently announced rear-projection versions of its D-ILA that meet our selection criteria, but these aren't included since they weren't available until after testing was completed. Most other attempts at LCoS have been unsuccessful, with Philips and Intel pulling out most recently, in October 2004. On the flip side, Sony has recently announced very promising LCoS products. Sony's SXRD is now shipping as a front projector, and a rear projection version will be available in early 2005.

It's important to emphasize that this article is designed as a comparison of four different display technologies and not as an editorial review of the above models. By comparing a top-performing product in each technology, we are effectively examining the state of the art for that technology. We will be looking at fundamental image and picture quality performance issues—not the implementation details or idiosyncrasies of any particular model.

The central concept for this article was to carefully set up, test, and evaluate all four display technologies at the same time under identical conditions and procedures, using advanced instrumentation where appropriate. All of the displays were set up side-by-side for simultaneous comparative viewing in a completely dark lab treated with black felt to eliminate reflections. The simultaneous viewing allowed us to detect subtle differences between the displays. We used computer and video-based test patterns, plus DVD, television, and computer applications. We used a wide selection of test patterns at the HD resolutions of 1920x1080 and 1280x720 from our own DisplayMate for Windows, Multimedia Edition. This product generates a large set of advanced diagnostic test patterns from scale-free mathematical equations, so they work for any resolution and aspect ratio up to 4096x4096. We also used a pre-release version of the DisplayMate Professional DVD, which includes DisplayMate's proprietary test patterns on DVD (available mid-2005).

For DVI and component video HD signals, we used an ATI Radeon 9800 Pro with an ATI HDTV Component Video Adapter, which provides high-quality computer-generated 720p and 1080i component video outputs YPBPR. This allowed us to generate HD DisplayMate test patterns for the television video inputs. In order to do simultaneous display testing, we used distribution amplifiers and switchers from Kramer Electronics. Our reference standard was the Sony Professional Multi-format High Definition studio monitor PVM-20L5, which was carefully calibrated for testing. Each display was compared with this monitor for color and gray-scale accuracy and overall image quality.

Instrumentation for Parts I and II:
All of the photometry and colorimetry measurements were made with a Konica Minolta CS-1000—a high-end laboratory Spectroradiometer with a narrow 1-degree acceptance angle for light emitted by the display. This advanced instrument costs $25,000. Most of the photometers and color analyzers used for display measurements are actually accurate only for CRTs, because they rely on filters calibrated to the light spectrum of a CRT. Some filter-based instruments are designed and calibrated for other display technologies, but because of the wide variation in light spectra for non-CRTs, these instruments need to be used with caution and qualified for each application with a high-end Spectroradiometer. (While in principle filter-based instruments can be very accurate, in practice, manufacturing filters that accurately match the CIE functions over a broad range of wavelengths is extraordinarily difficult and expensive, so reasonably priced instruments have compromises that can affect their accuracy when used with some display technologies. High-end filter-based instruments can be as accurate as Spectroradiometers.)

Most instruments also have broad acceptance angles that are not accurate for many flat panel technologies because the display's light distribution can vary with both viewing angle and intensity. The NIST (National Institute of Standards and Technology—the new name for the National Bureau of Standards) and VESA (Video Electronics Standards Association) specify a maximum acceptance angle of 2 degrees for measuring flat panels (in the Flat Panel Display Measurements Standard). The Spectroradiometer measures the light spectrum directly and was crucial for making precise comparisons between the different display technologies. The Spectroradiometer and all of the displays (except for the Sony) were generously provided on a long-term loan basis by their manufacturers. We offer special thanks to all of them for agreeing to participate. It was especially challenging to get all of this high-end hardware together at the same time.

We start off the comparison with an item that doesn't get all the attention it deserves: the display's ability to produce black. This capability of suppressing light output turns out to be a major challenge for all of the display technologies. It's important because a poor black-level lifts the bottom end of the display's intensity-scale and introduces errors in both intensity and color throughout the entire lower end of the scale, not just at the very bottom. All displays produce some light in the form of a very dark-gray when asked to produce a black. This background light is added to all of the colors and intensities that the display is asked to produce. This washes out the dark grays and also the dark colors. For example, dark reds will instead appear as shades of pink. What's more, if the display isn't properly adjusted, the dark background glow will have a color tint instead of appearing neutral gray, and this will add a color cast to the entire lower end of the intensity scale—particularly noticeable in dark images.

No display can produce a true black, so it's important to know just how close it can actually get. CRTs do extremely well, but the flat panels all struggle with black, though they do pretty well with peak brightness, so black level can be a great differentiator. The actual black level produced by a display is almost never reported in manufacturers' spec sheets or published reviews, yet for most applications it's actually much more important than peak white brightness, which seems to get most of the attention. Black-level should be the single most important spec after screen size if you're working in multimedia, imaging, photography, home theater, or in any environment with controlled or subdued lighting.

Black-Level Control
All displays should have a black-level control to allow the black level to be accurately adjusted. If it's too low then the darkest portions of the gray-scale will be lost; if it's too high then the display's precious minimum background light level is needlessly increased. In either case the gray scale is distorted. The default factory value will almost certainly be inaccurate, because the proper setting varies with the make and model of the graphics board, DVD player, set-top box, or whatever signal source you're using. The black-level will also vary with the operating mode you select, such as the color-depth for a computer graphics board or progressive/interlaced scanning for a DVD player. Many analog and digital video modes also have a non-zero black-level offset that needs to be taken into account. The only way to properly adjust the black-level is with specialized test patterns, and we used the set in our own DisplayMate for Windows (all editions). One subtle point to bear in mind: In some cases it's necessary to intentionally misadjust the black-level control in order to compensate for some other display parameter or to improve the visibility of the gray scale under bright ambient lighting conditions. We'll discuss this further in Parts II and III.

For most displays the black-level is adjusted using a control inappropriately labeled "Brightness." Further confusing this issue is that many LCDs now have a control labeled "Brightness" that instead varies the intensity of the backlight—at least that's closer to the true meaning of brightness. Unfortunately, most LCDs lack any form of black-level control—they're fixed at a factory-set value. The NEC LCD4000 in this article is one of a small number of LCDs that actually provides a real black-level control and even labels it "Black-Level." However, they left the control out for the DVI input. It's unfortunate that the DVI inputs for many displays (of all technologies) are often missing some essential color and gray-scale controls, making it impossible to properly adjust them. This is due [[what is due, the missing controls or the inability to adjust?]] to the complexity of digital signal processing, which we'll discuss in Part III. This situation should improve with each subsequent generation of components.

Here are the black levels measured with the Konica Minolta CS-1000 Spectroradiometer. It measures brightness by matching the eye's own spectral sensitivity to light of different wavelengths. The photometric term for this form of brightness is called Luminance. (We will be informally referring to Luminance as brightness and will use the two terms interchangeably throughout the article.) The measurements are shown for cd/m2 (candelas per square meter), a luminance unit that used to be called "nits," but has been deprecated in favor of cd/m2. To convert to another common luminance unit, foot-Lamberts, fL, divide by 3.43. The screen was set to the proper black level with a very sensitive full-field black-level test pattern. The black level must be set precisely so that it's neither too high nor too low. The measurements were made in a completely dark lab, so there was no contamination from ambient room lighting.

Product: Sony PVM-20L5 NEC LCD4000 NEC 61XM2 Optoma RD-50
Measurement: 0.01 cd/m2 0.72 cd/m2 Max Backlight
0.27 cd/m2 Min Backlight
0.42 cd/m2 0.26 cd/m2

The CRT wins by a huge factor of about 25. It barely produces any detectable light when set to black. The flat panels all produce a noticeable dark-gray glow for black. The CRT's enormous black-level advantage is the major reason why it remains the technology of choice for home theater perfectionists. (Note that the black-level luminance of a CRT can be reduced even more by turning the Black-Level Control further down into a "blacker-than-black" regime, but this will cause a loss of the lower end of the gray-scale.)

There are two values listed for the LCD—one when the backlight is set to maximum brightness and the other for minimum brightness. So for an LCD equipped with a backlight control, you can get a darker black if you are willing to accept a lower peak white brightness. In many instances that's a fabulous trade-off. Some projection units include an iris aperture control that can reduce the light output from the projection lamp or lens for a similar effect. A related technique is the use of dark glass or dark screen material. When the image comes from the rear this can also substantially increase image contrast because reflected ambient light originating in the room has to travel through the screen layer twice but light from the display goes through only once. CRTs have always taken advantage of this technique, and it's also one reason for the Optoma's relatively dark black level. We'll discuss this further in Part IV.

It's important to emphasize that these measurements were all made in a completely dark lab. Any ambient room lighting will reflect off the screens and add to the black levels listed above. How much will depend on the quality of the anti-reflection coatings, surface treatments, and other light absorbing techniques that each display uses (see Part IV). It will also depend on the particulars of the lighting distribution in the room. The end result is that ambient lighting tends to equalize the differences between black-levels in displays.

In particular, ambient light will quickly erode the CRT's huge black-level advantage. So the more important a dark black-level is to your application, the more you'll need to control ambient lighting. The reflectivity and color of the walls in the room can also have a major impact, particularly with front projectors. Completely black walls will eliminate spurious light reflections (a neutral dark gray will work almost as well). That's why in brightly lit stores, evaluating the relative black-levels between different models is virtually impossible. Another reason is that the displays are almost never properly adjusted.

Black-Level Interpretation
There are two major issues for black-level luminance: How low does it really need to be and what steps can be taken to reduce its visibility. Home-theater perfectionists insist on a completely dark viewing environment, because that's how movie theaters operate. Under these conditions any noticeable black-level luminance adversely affects image quality and can also be an annoying distraction. The real problem is that the eye's sensitivity varies over an incredibly wide luminance range (6 orders of magnitude or 1 million to 1 for color vision at indoor lighting levels). This is a result of several light-adaptation mechanisms, so the threshold for detecting black-level luminance will vary with the average scene brightness over a period that can extend from several seconds up to several minutes for color vision (and half an hour for complete dark adaptation, which adds an additional 4 orders of magnitude or 10,000 to 1 in sensitivity for black-and-white night vision).

For typical movie content with varying scene brightness the eye will be operating at reduced sensitivity and is less likely to notice the black-level luminance in dark scenes, but the odds go up considerably with a perpetually dark movie like Dark City. Note that if you sit in a pitch black room with a display showing a completely black image or test pattern, you will eventually see the black-level luminance on even the best CRT display or projector, because the eye's sensitivity will progressively increase with time as the result of dark adaptation.

But the real question is how does a video display or projector compare with a movie theater. Kodak motion picture film has maximum densities of roughly 4.0 (for the standard Vision Color Print Film 2383) and 5.0 (for the high-end Vision Premier Color Print Film 2393). For film the density is defined so that the light transmission equals 10-density so a density of 4.0 corresponds to a transmission of 1/10,000 or to a dynamic range of 10,000. This corresponds to Dynamic Range values of 10,000 and 100,000, though production movie prints will not reach these maximum values. With typical exposure and development, movie prints can be expected to deliver roughly a factor of 10 less than their spec maximum. A specialist with Eastman Kodak provided Minority Report as an example of a motion picture that delivers the darkest state-of-the-art blacks, with actual print densities reaching 4.0, equivalent to a Dynamic Range of 10,000.

As we'll see below, CRTs typically fall in the range of 10,000 to 30,000 for Dynamic Range, so they can actually perform significantly better than any motion picture film (if they're carefully set up). The best that non-CRT displays and projectors can do now is about 3,000, so their performance is better than standard-grade motion picture film but well below what the best films deliver. The higher the Dynamic Range, the darker the black level for a given peak brightness. Motion picture theaters typically operate between 41 to 75 cd/m2 (SMPTE 196M), which is comparable to front projectors but much lower than the direct view and rear projection displays considered here (see below), so different eye adaptation levels apply.

The final issue is what to do when the black-level luminance becomes noticeable. For front projectors, you can get some help by switching to a screen with below unity gain like the Stewart Filmscreen GrayHawk. If you've exhausted all of the options discussed previously, then lighting the area behind the display or surrounding the projection screen will activate the eye's light adaptation mechanism and reduce the visibility of the black-level luminance. Make sure that none of the light falls on the screen itself, and use the lowest lighting level needed. The walls should be a neutral white or gray so as not to upset the overall color balance. The light should also have the same color temperature as the white point of the display, which is discussed next.

Most people are aware that white is not a single color—there's no such thing as "pure white." Instead a whole range of colors can accurately be referred to as white. But if we are to have accurate color reproduction, it's necessary to define one or more standard whites, which can then serve as a point of reference for generating all of the other colors. One way to do this is by applying laboratory physics using a specially defined black body raised to a specified temperature, which is referred to as a color temperature. (A black body is a specially prepared perfect thermal radiator with a light spectrum that depends only on temperature.) The temperature is based on an absolute scale expressed in degrees Kelvin, or K. Each temperature produces a known spectrum that yields a unique color with specific chromaticity coordinates (a quantitative measure of color that we'll discuss further in Part II). As the temperature increases, the changing chromaticity coordinates trace out a black-body curve. Whites typically fall in the range from 5,000 K (a reddish-white) to 10,000 K (a bluish-white).

Most computer and television displays come from the factory set to a relatively high color temperature, which produces a white with a bit of a blue cast, similar to "cool white" fluorescent bulbs. This is done because most displays produce a brighter image at higher color temperatures. The standard cool white is 9300 K, but many displays come set even higher. For multimedia, photography and television the standard color temperature is 6500 K, roughly the color of natural daylight. For optimum color accuracy, a display for these applications needs to be set to a white point of 6500 K. More precisely to the chromaticity coordinates of CIE Illuminant D65 or D6500, which corresponds to average natural daylight for an overcast sky at noon and includes a blue sky component added to a blackbody spectrum.

For many non-imaging computer applications, particularly under typical office fluorescent lighting, 9300 K is a better choice. Note that there are other color-temperature standards, for example, 5000 K is used in graphic arts because it corresponds to typical indoor lighting consisting of a mixture of incandescent lighting and sunlight. Note also that if an image is designed or color-balanced at one color temperature and then viewed at a different color temperature, all of the colors in the image will be shifted by varying amounts. For example, reds need to be overemphasized in TVs operated at 9300 K in order to counteract the blue cast imparted to flesh tones, particularly facial complexions. This so called "red push" introduces other color errors. We'll discuss this further in Part II.

For all of our tests, the white point for each display was set as close to D6500 as possible without resorting to any internal service modes. Many displays have a color temperature control, but often it isn't very accurate. Colors that lie close to, but not exactly on the black-body curve can be assigned a color temperature value that produces the closest color match to a black body. This is referred to as a correlated color temperature. Below are correlated color temperature values measured with the Konica Minolta CS-1000 Spectroradiometer and a window test pattern set to peak white.

Product: Sony PVM-20L5 NEC LCD4000 NEC 61XM2 Optoma RD-50
Measurement: 6480 K 6,580 K Computer Inputs
10,250 K Video Inputs
6626 K 6786 K

The results were all relatively close to D6500, except for the video inputs on the NEC LCD4000, which did not provide any adjustments for the white point (Color Temperature or RGB Drive), so it was stuck at a high value. The color temperature (and chromaticity coordinates) shouldn't change as the gray-scale intensity changes, but it always does to some degree because of slight differences between the primary red, green, and blue channels. The variation of color with intensity is called color tracking (because the primary color intensities need to track each other accurately) or gray-scale tracking (because gray-scale variations are tracked with intensity) and one benchmark of a good display is a small variation. All of the displays did quite well with color tracking, but it's nice to see end-user controls that allow you to easily correct for it. Only the CRT and plasma models included end-user RGB Drive and Bias controls needed to make these adjustments.

One serious problem with color temperature measurements and specifications is that they don't actually specify a unique color, only the closest match to a black-body radiator. So there can be a considerable variation in color (chromaticity coordinates) when color temperature alone is used to measure a gray scale. As a result, color temperature measurements and specifications can be quite misleading and should be used together with chromaticity coordinates.

For most typical viewing conditions, these display technologies all deliver more than enough light for comfortable viewing, so a higher peak brightness isn't necessarily better. In fact, for most of the viewing tests, we turned down the brightness somewhat for each display. On the other hand, if you have bright ambient lighting conditions (that cannot be reduced) then high brightness may be an important requirement. Using a display or projector with more peak brightness than what you need often results in a higher black-level luminance, which is undesirable, but phosphor and lamp aging will reduce brightness over time, so some reserve is a good thing.

Despite all this, brightness is still the number at the top of just about every spec sheet and published review. There are NIST/VESA, ANSI and ITU-R standards for measuring the brightness of peak white, but they all have some "wiggle room" that allow the numbers to be exaggerated. Worse, many manufacturers' spec sheets don't reference any standard, so they're free to choose their own procedures. Frequently, what happens is that every single control is turned up to maximum, including brightness, contrast, RGB drive, and any other control that can increase the light output. Under these conditions, essentially all displays produce horrendous image quality, are completely uncalibrated, and effectively unusable. For these reasons, you shouldn't place too much weight on brightness and contrast specifications or make buying decisions based on them. They can be off by a factor of two or more from objective measurements. If brightness matters to you, then only pay attention to values measured under identical, standard conditions. Press reviews are generally the best source.

Peak Brightness Control
The contrast control is the primary means for adjusting peak brightness and the top end of the intensity scale. (It's also inappropriately named, because it affects the display's brightness and not its contrast. We'll discuss this in Part II.) If it's set too high, then two or more of the top-end steps in a gray-scale test pattern will reach peak brightness and merge together. This loss of gray-scale is called either white saturation (a soft limit for CRTs and LCDs) or clipping (a hard limit for plasmas and DLPs). The only way to properly adjust the contrast control is with a specialized white saturation or extreme gray-scale test pattern, provided in all editions of DisplayMate for Windows.

In many applications a display doesn't need to be operated at peak brightness. In fact, some displays are now so bright that they may bother your eyes under typical indoor lighting conditions, so you'll feel compelled to dim them. To reduce peak brightness, turn down the contrast control, or in the case of an LCD, a backlight "brightness control." Note that when you lower the contrast control the black-level control may need some adjustment, because the two interact.

Some technologies, particularly LCD, also suffer from white compression, where the gray-scale steps get closer and closer together near peak white. Although this was not the case with the NEC LCD4000, it's a severe problem on some LCDs. If you experience this problem, lower the contrast control rather than the backlight control. This will move peak white below the problem "S" region of the LCD's transfer characteristic (this will be discussed in Parts II and IV). If that doesn't correct the problem then it's most likely signal clipping in the input electronics rather than compression or saturation. Many displays lack sufficient headroom near peak white. To correct that, reduce the input signal level using external electronics.


For our tests, all of the monitors were set up identically: First the white point was set as close to D6500 as possible, the black levels were carefully adjusted as discussed above, and then the contrast control was set with a DisplayMate White Saturation test pattern so that no more than 2% of the gray scale was lost near peak white. Of course it's better not to lose any gray scale, but for some technologies, such as LCD, this is often impossible, so 2% is a well-defined "red-line" for a precise specification. (Dr. Edward F. Kelley of the NIST (http://www.nist.gov/), National Institute of Standards and Technology, and I are working on a standard for measuring peak brightness that has no wiggle room.)

For CRTs there are additional requirements on focus and screen regulation, but they did not affect the Sony monitor. This specification is more stringent than any of the above standards. The values obtained with this procedure will generally be lower, and sometimes much lower, than what you'll see listed on a spec sheet. Here are the brightness levels measured with the Konica Minolta CS-1000 Spectroradiometer and a window test pattern set to peak white:

Product: Sony PVM-20L5 NEC LCD4000 NEC 61XM2 Optoma RD-50
Measurement: 176 cd/m2 428 cd/m2 Max Backlight
160 cd/m2 Min Backlight
212 cd/m2 5% APL
133 cd/m2 25% APL
81 cd/m2 50% APL
53 cd/m2 100% APL
359 cd/m2

The LCD has two entries, depending on the backlight intensity setting. At its highest available color temperature setting of 9023 degrees K the LCD produced 471 cd/m2—more than what NEC lists on the spec sheet, which is both unusual and commendable.

The values for the plasma depend on the average picture level APL, or the average intensity level for each of the red, green, and blue sub-pixels over the entire screen. For example, a full screen of peak intensity white has an APL of 100%, but it's only 33% for pure green (because red and blue are off). In our case, APL refers to the percentage of pixels set to peak white. When 5% of the pixels are at peak white, the brightness is 212 cd/m2.

As the APL increases, power and heat dissipation restrictions reduce the maximum brightness that can be safely produced, so the display automatically reduces the peak brightness. When 100% of the pixels are at peak white, the brightness is only 53 cd/m2, which requires subdued ambient lighting for good viewing. For most computer applications the APL is rather high (because word processors and spreadsheets, for example, use a peak white background) but for most video applications it's relatively low (because the images are generally dimmer and are colored, not gray or white). As a result, plasma displays are generally used for video.

Dynamic Range
Dynamic range is simply the ratio of peak white luminance to black-level luminance that a display can produce. The values are measured separately—one screen for peak white and the other for the black level. This is frequently referred to as contrast, full field contrast, or full on/off contrast, but the term contrast should be reserved for measurements on a single image, not on different screens. The ratio of the peak white to black-level luminance values tells us the maximum range of brightness that the display can produce. So dynamic range is especially important in imaging and home-theater applications, where, for example, bright day scenes and dark night scenes both need to be rendered accurately. The higher the dynamic range, the better the display will be able to reproduce wide differences in scene brightness. Note that a high dynamic range will also yield a dark black level unless the peak brightness is very high. Here are the ratios calculated from the peak white and black-level values measured above:

Product: Sony PVM-20L5 NEC LCD4000 NEC 61XM2 Optoma RD-50
Measurement: 17,600 595 505, 5% APL
317, 25% APL
193, 50% APL
126, 100% APL
1,381


The CRT wins by a huge factor. (We've measured dynamic range values as high as 36,500 for a CRT using a sensitive photometer.) The CRT's enormous lead in dynamic range is another major reason why it remains the technology of choice for home theater perfectionists. There are four values for the plasma, depending on the Average Picture Level of the peak white field. Note that there's only a single value listed for the LCD, because the peak white and black-level values track exactly with the backlight intensity. For the flat panels, the DLP wins by more than a factor of 2, and the plasma trails the LCD by 15% for low APL and by much larger factors for high APL. Remember that these values were measured in a completely dark lab. Ambient room lighting will decrease the above values because the black-levels will be higher.

Note that if you lower peak brightness with the contrast control, you will also be reducing the dynamic range (and the contrast, discussed below) at the same time because the black-level luminance generally doesn't change. This turns into a major advantage for the backlight control found on many LCDs and the iris aperture control on many projectors: Their dynamic range remains constant, because the black-level luminance decreases together with the peak luminance. (In many cases, reducing an iris aperture will actually increase the dynamic range because spurious light paths within the projector optics are attenuated, so the black-level luminance actually decreases faster than the peak luminance.)

Display contrast is another highly advertised specification, but this number flaps in the wind more than any other spec. It's supposed to tell you the ratio of the brightest white to the darkest black that a display can produce within an image. Internal reflections within a display or display optics cause light from the bright areas of the image to bleed and contaminate the dark areas so they can't get as dark as the black-levels listed above. This means that display contrast is always less than dynamic range. If the display's contrast falls too low, images will appear washed out (see below). Remember, unless you see a standard like ANSI next to the contrast specification, it's most likely some form of dynamic range.

Contrast Measurements
A standard way to measure display contrast is to use a black-and-white checkerboard test pattern and measure the luminance at the center of the white blocks and then the black blocks. The smaller the blocks, the greater the bleed, resulting in lower contrast values. We've done this for a 4x4 checkerboard, which is a standard pattern, and then for a much finer 9x9 checkerboard to see how much more the contrast falls when the blocks are reduced by an additional factor of 5 in area. Note that this measurement is tricky because a similar contamination effect (called veiling glare) also affects the measuring instrument. We used heavy black felt masks to eliminate this common source of error in contrast measurements. All of the displays had their controls carefully adjusted as described previously. The measurements were made in a completely dark lab, so there was no contamination from ambient room lighting.

Product: Sony PVM-20L5 NEC LCD4000 NEC 61XM2 Optoma RD-50
4x4 checkerboard contrast: 219 586 475, 5% APL
305, 25% APL
188, 50% APL
124, High APL
332
9x9 checkerboard contrast: 75 577 449, 5% APL
294, 25% APL
184, 50% APL
122, High APL
274
Plasma Note: the checkerboard pattern has a 50% APL. Values for the other APLs were calculated by applying the same form factors for the light bleed to the Peak White luminance values. The High APL entry uses the values for 100% APL.

Comparing the 4x4 checkerboard values with dynamic range above, we see that the CRT value has fallen the most, by a factor of 80, because of heavy reflections within its thick glass faceplate. (For a larger screen size the effect would have been somewhat smaller.) The DLP value has fallen by a factor of 4, primarily because of reflections within the rear-projection optics. The LCD value decreased by only 2% on these scales, because its glass is thin, and multiple reflections are heavily absorbed. For similar reasons, the plasma value also shows a relatively small 6% decrease from the dynamic range values.

Continuing on to the much finer 9x9 checkerboard, we see a comparatively smaller decrease, despite the blocks' being five times smaller in area than in the 4x4 checkerboard. The CRT value has fallen by an additional factor of 3 (a lot lower than the previous factor of 80), the LCD by only 2%, the plasma by 5%, and the DLP by 17% (again because of the rear projection optics). It would be tempting to go an additional factor of five smaller in area, to a 20x20 checkerboard, but the effects of veiling glare make it much harder to perform accurate measurements at smaller scales.
The term contrast has been twisted in so many ways that its meaning is no longer clear. First of all, the ubiquitous "contrast control," one of the most prominent controls found on virtually every display manufactured in the last 50 years, actually controls peak brightness and doesn't affect contrast. Rather, it proportionally increases or decreases the entire gray-scale (by controlling the video gain), so none of the brightness ratios change (unless the display's gamma is not constant, see Part II). When people adjust this control, they mistakenly believe that the changes they see on-screen are due to a change in contrast. Another twist is that almost all manufacturer's "contrast" specifications actually refer to the display's dynamic range rather than anything indicative of the brightness ratios that will be generated for an image by the display. Checkerboard Display Contrast certainly falls within the definition of contrast that we have been discussing, but it generally doesn't correspond well with the eye's own sense of visual contrast.

It's not surprising to see the checkerboard display contrast continuing to decrease as we move to smaller scales, and taken at face value, you would think that images on a CRT would appear washed out compared with the flat panels. For the most part, they do not. Of course, what really matters is the eye's perception of contrast, and that seems to differ noticeably from the checkerboard luminance measurements. The eye is, after all, not a camera or an instrument, but rather an image processing system designed to extract visual information for the brain, which processes and interprets the information.

The eye doesn't actually pick up on the large differences in display contrast we've measured. Side by side, the checkerboard patterns on all of the displays appear to have roughly the same visual contrast, even though the instrumentation tells us otherwise. The eye can detect that there are differences, but they appear to be small differences instead of the factor of roughly 3 in the 4x4 checkerboard and factor of 8 in the 9x9 checkerboard. This has much more to do with human visual perception than optics. It seems that on these scales, the brain interprets large brightness differences between the adjacent bright and dark checkerboard blocks but is less concerned with their precise ratio, because there is no perceptual content involved. There's no question that if the checkerboard contrast falls too low, the eye will at some point take full notice of the effect—it just didn't happen with these displays.

It's an entirely different story for the smaller scales used in fine text and graphics. For black text on a white background, the eye immediately notices that characters on the CRT show up as light-gray on white instead very dark-gray on white for the flat panels. So the differences in display contrast are clearly significant in this case. Reading fine text on a CRT is definitely harder than on any of the flat panels. The eye takes clear notice of the differences in display contrast here, because they affect perceptual content.

Optics in front and rear projectors also have a major impact on display contrast, because each element in the light path scatters a small fraction of the light that reflects off it, passes through it, or both. That's why the rear projection DLP experienced a significant decrease from the dynamic range value. CRT, LCD and LCoS projectors experience similar declines. (Plasma displays are not suited for projection.) Front projectors generally perform better than rear-projection units in this regard because they don't need mirrors to fold the light path into a compact enclosure, and they use a front surface reflecting screen rather than a thick transmissive screen that scatters image light from both its front and rear surfaces. Projection CRTs also perform better than direct view CRTs, because reflections within the faceplate are better controlled.

From this discussion, we see that measuring checkerboard display contrast is tricky, and interpreting it is often ambiguous and misleading, meaning its usefulness is limited. We need another parameter that corresponds well with the eye's own sense of visual contrast. Next we'll consider a better and more important measure of contrast—image contrast. As we'll see this depends on the shape of the gray scale, and particularly on the widely misunderstood parameter, gamma.

In Part II, we'll first examine gray scale and gamma in detail and see how they affect image contrast and contribute to color hue and saturation errors. Then we'll measure the primary chromaticities and color gamut for each display and discuss how these affect color accuracy. In Part III, we'll examine the complex world of display artifacts for each of the display technologies, and in Part IV we'll analyze and assess each of the display technologies in detail and tie together all of the results from Parts I to IV.

Acknowledgements
Special thanks to Dr. Edward F. Kelley of the NIST, National Institute of Standards and Technology, for many interesting discussions and for generously sharing his expertise, and to John P. Pytlak of Eastman Kodak for supplying data on film density, dynamic range and black-levels. Special thanks to the Konica Minolta Instrument Systems Division for providing editorial loaner instruments whenever and wherever they have been needed and for providing the CS-1000 Spectroradiometer on a long-term loan for this project.

About the Author
Dr. Raymond Soneira is President of DisplayMate Technologies Corporation of Amherst, New Hampshire. He is a research scientist with a career that spans physics, computer science, and television system design. Dr. Soneira obtained his Ph.D. in Physics from Princeton University, spent 5 years as a Long-Term Member of the world famous Institute for Advanced Study in Princeton, another 5 years as a Principal Investigator in the Computer Systems Research Laboratory at AT&T Bell Laboratories, and has also designed, tested, and installed color television broadcast equipment for the CBS Television Network Engineering and Development Department. He has authored over 35 research articles in scientific journals in physics and computer science, including Scientific American.

Copyright ©2004 DisplayMate Technologies Corp.

 
 
 
 
 
 
 
 
 
























 
 
 
 
 
 

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