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Color Temperature, Gray Levels

In this section we address the basis for the luminance and color fidelity of the display - generally know as the "gray scale".  Standard video signals consist of a "black and white" luminance component in addition to "color difference" components. The display starts with the luminance part of the video signal and then adds in the color difference parts to achieve the final colors for all parts of the display. The only way for the display to achieve accurate colors is to start with the correct "color temperature" for the display's white and gray levels. If this reference white "color" and gray scale are incorrect, virtually all the colors in the display will also be incorrect.  Are the "black and white" parts of your image truly colorless, are they at the correct level for the various input levels, and are the levels uniform over all screen areas?  In this section of Display Performance we'll help you find the answers.

Color Temperature:  When an object is referred to as white we think of it as colorless, while in fact there are a lot of "shades" to white. Characterizing an object's whiteness by "color temperature" is the most common way used to describe the shade of white. When a "black body" is heated to higher and higher temperature levels, it will start to glow dimly, the glow then becomes "red" hot, then it becomes "white" hot, and as it gets even hotter yet it becomes blue. The color temperature is simply the color of this black body at the specified temperature, which is in degrees Kelvin. Video standards have been chosen based on an ideal "white" color temperature of approximately 6500 degrees (which, not surprisingly, is fairly close to the Sun's actual surface temperature) - this is thus the reference white, with the D65 illuminant being selected as the precise standard. All the various colors in the image are referenced to this white by the "color difference" signals, which tell the display how much red and blue to add or subtract from the reference level.  Thus the starting color temperature of white must be correct in order for a display to produce not only the correct shades of gray and white, but also to produce the correct shades for all the colors in the image.

How is the color temperature created by a display?  In a full color display there are red, green and blue images that are combined in various amounts at the viewing screen.  The particular mixture at any one part of the screen determines the color we see in that area.  When there is no color in the image, thus making a "black and white" picture, the color temperature, or shade or white, is determined by the relative ratios of the red, green and blue images that are present.

The actual color temperature of a display can only be accurately checked and adjusted with appropriate, and relatively expensive, light meters.  These meters sense and measure the light either coming from the viewing screen, or alternatively, for front projectors, optionally measure the projector light that is illuminating the screen.  By using various level "window" test signals and evaluating the relative mix of the display's red, green and blue component colors, the meter is able to calculate the color temperature.  Ideally this will be at the D65 level, but if it is not, most displays can be adjusted by accessing the basic red, green and blue level settings in either in the user menu or the service menu.

Most home theater owners don't have access to specialized light meters, so what can he do to evaluate and set his display's color temperature?  For many displays the answer is to simply make the correct setting in a color temperature menu.  While the best color temperature selection will depend upon the particular display, usually it will be the 6500 setting, or a low setting.  Be aware, however, that frequently these settings aren't calibrated or exact, but are simply the manufacturer's standard settings that will be approximately correct.

How about using your eyes to check or adjust the color temperature?  After all, this is the point of the whole calibration exercise, isn't it, to make the display look natural to your vision?  The basic problem here lies in the great flexibility and adaptability of the human eye - in particular, how you see at any one instant depends upon the current environment and what you have recently viewed.  For a good demonstration, check the Animated Color Bar test pattern (WV-11). Simply keep your vision fixed at the screen center for a few cycles (placing the cursor at the center as a reference to lock onto may help), and note that after the color bars switch off, they magically reappear in your eye!  Local parts of the eye becomes desensitized to the color(s) that have been viewed, so that, for instance, where the green bar has been the eye has lower sensitivity to green, and that area then appears as magenta (red plus blue).  Thus setting color temperature by "eye" will yield inconsistent and usually incorrect results - the eye just isn't objective enough, like a meter is.  If necessary, however, you can use you eye with reasonable results if you can first neutralize any colorization set by the preceding viewing environment. This is best done by going outside where the sun is the illuminant, and produces a color temperature of about 5500 degrees.  When you go back inside to view your display, your eyes will be reasonably neutral, and black and white images should appear similar to the clouds you have just viewed outside.  Note that the page backgrounds here at WalVisions are all intentionally constructed as white or gray - this is to prevent the eye from becoming desensitized to any particular color so when the patterns are viewed the colors will appear natural.

Gray Scale:  The gray scale refers to the color temperatures for all the various levels of gray, ranging from the near black levels to the full white level. The most common check for grayscale is with a Stairstep test pattern (WV-32). When viewing such a pattern, all the steps should appear to be the same neutral, D65, color temperature. Check for any apparent shading of the different levels of gray. Note that the pattern has a mirror image on the bottom, so there are low and high levels at both the left and right sides - this is a check of left-right uniformity, as the equivalent level steps on both sides should appear the same neutral gray level.

Gray Uniformity:  Not only should the "color" of gray be correct at the screen center and at all levels, but it should be uniformly correct over the entire screen.  A lack of uniformity is known as shading, and it can be due to non-uniformity in any combination of the three display colors - red, green or blue.  The presence of shading can quickly be checked by looking at the Gray Field test patterns (
WV-61, Black; WV-62, 20%; WV-63, 40%; WV-64, 60%; WV-65, 80%; WV-66, White).  To check simply compare the gray levels and "color" over the entire screen - there should be no apparent change in either level or tint. Note that the background of this page and most other pages in WalVisions is gray, and should appear the same over all parts of the display.  The presence of shading will show as a variation in background color or intensity over the various areas of the display.

Gray Level Tracking - Gamma:  Most images are comprised of many luminance levels, ranging from relatively dark levels to relatively bright levels.  A good display will present these various levels the way they were intended to be seen, which is either how the scene was originally captured by the camera, or how the scene was intentionally altered by the director.  The way in which the video signal levels map to the display's luminance levels is known as the "gamma" characteristic.  The Gamma test pattern (WV-33) is a good check of the gamma characteristic for your display.  Each gray wedge is encoded to be 50% brighter than its dimmer neighbor, and on a standard CRT display the differences will range from 50% to 100% brighter.  Thus no matter which two adjacent wedges you compare, you should see approximately the same relative brightness difference, with the brighter wedge appearing almost twice as bright as its dimmer neighbor.  Many displays permit the user to change the gamma characteristic, and this will change the relative brightness of adjacent wedges.  The video standard was created for a gamma of 2.5 (matching a CRT display), which should be ideal in a somewhat darkened viewing environment.

Luminance Level Differences - Gradations:  In nature there are an infinite number of luminance levels - you can take any two closely spaced levels and always find a new level in between. The same is true for analog video signals, the signal can be at any level - you can take always any two levels and find a new level half way in between. With both digital signals and some digital displays such as DLP, however, we are constrained by the number of "bits" available (bit depth) to describe a video level.  At some point you can no longer find a new level between two closely spaced levels, so the level will have to be at either the upper or lower level. When you run out of finely spaced luminance levels the display will exhibit gradations, sometimes know as contouring or posterization.  This usually can be seen in larger image areas with subtle shading differences, such as either cloudy or blue skies, or in areas with dim gray levels.

Fortunately the human eye can only detect a change when there is about a 1% difference in adjacent luminance levels.  To see for yourself, check out the Level Differences test pattern (WV-35
).  This pattern has two almost white rectangles, both with a series of embedded rectangles that are different in level by 1%, 2%, 3%, 4% and 5%. The rectangles on the left are lower than the background level, and the rectangles on the right are brighter than the background level.  The series of rectangles in each row differ in either luminance, red, green or blue as noted by the letters above the larger rectangles.  You should be able to readily pick out the differences at the 5% level, but should only be barely able to see the 1% differences.  Note that this pattern will only be accurate with graphic cards with at least 24 bit color, and displays that have a gamma that is approximately correct (2.5).

To see how a display can handle all the pure (no color difference) luminance levels available with 256 levels (8 bit), see the 256 Gray Level Ramp test pattern (WV-54) and the 256 Level Vertical Gray Ramp
test pattern (WV-55). Both these patterns have every level from 0 to 255, and the small "dashes" mark the edges of each level bars.  Ideally the different level bars will smoothly transition to their neighbors, so you won't be able to see any bars that are distinctly different in level to the adjacent bars - it should look like a gray "ramp" of gradually increasing luminance.  You will be most likely to see differences at the lowest levels.  Another word of caution, however, these patterns will only display correctly with graphic cards with at least 24 bit color, and displays that have a gamma that is approximately correct (2.5), and even then, there may be issues with scaling or other processing that may create lines or bands.  Thus these patterns should be useful as a tool, but it there are any questions, any results should be checked by other means.


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