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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 is a fabulous tradeoff. 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.
Ambient Lighting 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 is utilizing, 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 the black-levels in displays. In particular, ambient light will quickly erode the CRT’s huge black-level advantage. So the more important that 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). Note that in brightly lit stores it’s virtually impossible to evaluate the relative black-levels between different models. Another reason is that they 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) due to several light adaptation mechanisms, so the threshold for the detection of 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 do video displays and projectors 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). This corresponds to Dynamic Range values of 10,000 and 100,000, although 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 are 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 that’s behind the display or surrounds 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 that’s 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.
Color TemperatureMost people are aware that white is not a single color – there is no such thing as “pure white.” Instead there is a whole range of colors that can be accurately referred to as white. However, if we are to have accurate color reproduction it is necessary to define one or more standard whites, which 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, called 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 that has 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, which is 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. On the other hand, 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 that is 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 that is 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.
Color Temperature Measurements
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 are result color temperature measurements and specifications can be quite misleading and should be used together with chromaticity coordinates. We’ll discuss these issues in detail in Part II of the article.
Peak BrightnessFor 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. Note that using a display or projector with more peak brightness than what you need often results in a higher black-level luminance, which is undesirable, however, phosphor and lamp aging will reduce brightness over time, so some reserve is necessary.
The above not withstanding, brightness is still the number that’s 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 manufacturer’s spec sheets don’t reference any standard so they are free to choose their own procedures. Frequently, what happens is 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 as much as 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, which is provided in all editions of DisplayMate for Windows. In many applications the 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 will 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 they interact.
Some technologies, particularly LCDs, also suffer from White Compression, where the gray-scale steps get closer and closer together near peak white (this will be discussed in Parts II and IV). Although this was not the case with the NEC LCD4000, it’s a severe problem on some LCDs. If you experience this problem then 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.
Peak Brightness Measurements 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, 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, like LCDs, this is often not possible, so 2% is a well-defined “red-line” for a precise specification. (Dr. Edward F. Kelley of the NIST, 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 less, and sometimes much less, 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:
Peak Brightness Measurements
The LCD has two entries, which depend on the backlight intensity setting. At its highest available color temperature setting of 9023 K the LCD produced 471 cd/m2, more than what NEC lists on their spec sheet, which is both unusual and commendable.
The values for the Plasma depend on the Average Picture Level, APL, which is 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 that are 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 spread sheets, for example, use a peak white background) but for most video applications it is 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 RangeDynamic 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:
Dynamic Range Measurements
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 is 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 because their Dynamic Range remains constant due to the fact that 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 ContrastDisplay 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, then 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.
Display Contrast Measurements
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 due to reflections within the rear projection optics. The LCD value decreased by only 2% on these scales because the 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 in spite of the fact that the blocks are a factor of five smaller in area than in the 4x4 checkerboard. The CRT value has fallen by an additional factor of 3 (a lot less 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.
Contrast Interpretation The term “Contrast” has been twisted in so many ways that its meaning is no longer clear. First of all, the ubiquitous “Contrast Control,” which is one of the most prominent controls found on virtually every display manufactured in the last 50 years actually controls Peak Brightness and does not affect contrast because 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). So 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. However, as we’ll see below, 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 that is designed to extract visual information together with the brain, which supplies the processing and interpretation.
In particular, the eye doesn’t really pick up on the large differences in Display Contrast that 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 roughly factor of 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 that there are 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 is 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. It’s definitely harder to read fine text on a CRT than on any of the flat panels. The eye takes clear notice of the differences in Display Contrast here because they affect perceptual content.
The optics in front and rear projectors also has a major impact on Display Contrast because each element in the light path scatters a small fraction of the light that reflects off or 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 will 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 its interpretation is often ambiguous and misleading, so 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: it’s called image contrast, and as we’ll see it depends on the shape of the gray-scale, and particularly on the widely misunderstood parameter of Gamma.
What’s Coming NextIn Part II we’ll first examine the 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 they 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.
AcknowledgementsSpecial 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 Theoretical 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. If you have any comments or questions about the article, you can contact him at dtso@displaymate.com.
Article Links Part II: Gray-Scale and Color Accuracy Part III: Display Artifacts and Image Quality Part IV: Display Technology Assessments
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