Basic Concepts of Video Calibration

Essential Equipment

• Color analyzer. This is a USB device that you point at the display so it can read the color and light output of the display and then connect to a laptop computer. The most accurate device under $1000 is the X-Rite i1Pro, though it can be difficult to use. A close second is the X-Rite Chroma 5. A much less expensive option that is somewhat less accurate is the X-Rite Display 2, also sold as the Pantone Eye-One Display LT for less than $150. I would avoid any less expensive colorimeters, as they will not offer what I consider to be a reasonable degree of accuracy.
• Calibration Software. You need this to interpret the data that the meter receives, analyze the data, and plot the results on easy-to-read charts/graphs.
• Video Test Patterns. Finally, you'll need some way to get a test pattern on the screen. The easiest way to do this is with a DVD. Avia Guide to Home Theater works well and sells for about $30. An even better choice is the GetGray disk, which can be downloaded and burned to DVD for about $25. AVS members have created a very nice set of HD patterns for Blu-ray and HD-DVD players. Finally, I have created a very simply calibration disc that provides all the patterns necessary for the steps in this guide.

Once you have the items in this list, you are ready to calibrate your display.

Basic principles

Display performance is measured in several ways:

• White and black levels. The most basic aspect of video calibration is getting the correct white and black levels. White level—the intensity of light the display produces when the video signal is at 100% intensity—is adjusted by the contrast control. Black level—the intensity of light the display produces when the video signal is a 0% intensity is adjusted by the brightness control.
• Sharpness. This adjustment is a holdover from the days of analog video and generally should be turned way down or completely off.
• White Balance . This is the aspect of color performance that gets the most attention. It concerns the display's ability to provide a neutral shade of white. The shade of white all the way from darkest black to the brightest white is called the grayscale. If the display can't reproduce a neutral white, then it will add a color cast to all images that looks very unnatural.
• Color Decoding. This term refers to a process that is used to lower bandwidth requirements by encoding the native RGB signal into YPbBr (analog) or YCbCr (digital) and then decoding back to RGB prior to display. There are different encoding/decoding standards, so sometimes a poor design may lead to color decoding errors. These errors are primarily seen as primary colors with incorrect brightness and/or secondary colors with incorrect hues. All commercial displays include a Color and Tint control. These are basically color decoding controls, though their effectiveness is extremely limited because Color adjusts the brightness of ALL of the colors and Tint effects hue of ALL of the secondaries. The problem is that to often displays have color decoding errors that effect the colors differently. For example, you could adjust Color/Tint to get the correct brightness of blue and the correct hue of cyan, but the brightness of green and red may still be inaccurate. You could adjust the color control to get red right, but then blue and green would be inaccurate. See the problem? A full set of color decoding controls addresses this problem by offering color/tint controls that operate on red/magenta and green/yellow independently. Then you can use the main Color/Tint controls to adjust blue/cyan.
• Color Gamut. This is the range of colors that the display is capable of rendering. The gamut is appears as a triangular pattern plotted on a standard tongue-shaped chart. The chart is defined by the xy points of the primary colors (red, green, and blue). The secondary colors (cyan, magenta, and yellow) are derived from the primaries. These color points have specific definitions for both standard and high definition signals. All commercially available video material is mastered according to these standards. If the display cannot reproduce the gamut accurately, then the image will suffer. In recent years it seems that digital displays have gotten increasingly inaccurate in this respect. The only way to fix it is with a Color Management System (CMS). A CMS can make a profound difference to the performance of the display, but few offer one and of those that do not all work properly.
• Gamma. This performance parameter describes how the the display responds to increasingly more intense signals. As a signal gets more intense, if the display rises out of black very fast, then it has a low gamma. If it rises out of black slowly, then it has a low gamma. The optimal gamma is expressed numerically. Aim for a gamma in the 2.2-2.35 range.

It is important to understand that these aspects of display performance can be interactive, but are for the most part independent. You can have good gray scale tracking and poor color decoding. You can have good color decoding and a very inaccurate color gamut. The bottom line is that each needs to be adjusted separately.

Essential Terminology

• xyY - A common method for precisely measuring color performance. x and y are coordinates that plot out a color on the triangular CIE chromaticity chart mentioned above. This graphically represents the established definitions of the color spectrum. Y is the brightness of the color. This is not plotted by these xy coordinates, but instead constitutes the brightness of the color and must be represented separately.
• Saturation - the colorfulness of the color relative to its own brightness. A color's saturation is measured on the CIE chart as the distance from the white point. Add saturation to a color and it will begin to appear excessively deep and rich. Under saturate a color and it will begin to appear as a neutral gray of a similar brightness.
• Hue - the primary characteristic of color that allows us to distinguish one color from another. A color's hue is measured by its angle to the white point. When a color's hue is off, its appearance will seem contaminated by other colors. For example, red that is too yellowish will begin to seem orange. Blue that is too reddish will begin to appear purplish.
• Color Brightness - the luminance of color. The brightness of any color (or even white) can be measured by a simple light meter.

Each color can be expressed by xy coordinates on a chromaticity chart that establish its saturation and hue. The Y value defines its brightness. The correct xy coordinate for all primary and secondary colors is defined by reference points on the triangular CIE chromaticity chart shown below.

If the color deviates from the reference point by appearing shifted towards other colors on the chart, then its hue is wrong and needs correcting. If a color is shifted closer to or father from the white point on the chart relative to the reference, then its saturation is wrong and needs correcting. Finally, if the color is too bright or too dim relative to the establish standard (not shown on the chart, but determined mathematically), then its brightness is wrong and needs correcting.

Calibration Order

Although there are no hard and fast rules about this, I would make adjustments in the following order:

• Black and White levels
• Sharpness
• White Balance
• Color/Tint (Color decoding)
• Color gamut
• Gamma

When finished, go back and remeasure each parameter, because changes in one may have affected the readings for another. Video calibration is a reiterative process.

Setting White Level (Contrast)

The Contrast control determines the peak output your display will provide. Set too low you lose image punch and lower contrast ratios. Set this too high and you lose color accuracy and detail in bright scenes. You may even suffer from eye strain.

The standard method for setting Contrast requires that you look at a test pattern that has a just-below-white stripe against a white background. You are supposed to set Contrast as high as you can without losing the ability to distinguish the just-below-white stripe from full white. I included such a pattern on the calibration DVD.

However, there are a couple of problems with this method.

• Many modern digital displays will never suffer from loss of high level detail even with Contrast set to 100%. This method will recommend a setting that is much too high.
• This method does not take into consideration color performance. Many displays will lose their ability to track a neutral grayscale at high output levels with Contrast set as high as this method recommends.

Thus, I think that a better method for setting Contrast is to just set it at a level consistent with good color performance and reasonable light output for a given display device. What's a reasonable level?

• CRT tubes: 30-40 fL
• Plasma: 30-40 fL
• LCD flat panel: 30-50 fL
• Digital rear projection: 30-40 fL
• Digital front projection: 12-16 fL

Setting Black Level (Brightness)

The typical method for setting black level is to use a pluge pattern that displays just above and just below black information against a black background. You set brightness so that the just-above-black is barely visible and the just-below-black is invisible.

However, if you have calibration equipment there is a less subjective method for setting brightness.

1. Set the contrast as described above and then measure and record the Y (luminance) of a 100% white test pattern.
2. Display a standard brightness test pattern and set brightness by eye as best you can. You may find that it is hard to distinguish between one or two ticks on the brightness control by eye alone. If so, continue to the next step.
3. Display a 10% test pattern.
4. Adjust the brightness setting so that this test pattern measures as close as possible to 0.6% of the Y (luminance) of the 100% white window. This sets your gamma at 2.22 for the the 10% stimulus level. In the great majority of cases this will be the correct setting for Brightness.
5. IF this adjustment falls within the one or two tick range you arrived at by eye alone, then this is the correct setting. If not, then leave brightness where it was using the purely subjective method. In short, use the objective method to refine the subjective method, but the subjective method defines the range of possible adjustments and that range should be VERY small.

There is one problem with the method just described. How do we set black level for broadcast sources where no test pattern is available? Fortunately, there is one approach that will get a correct black level even without a test pattern, but you must have a recorded source of broadcast material, either from a DVR or DVD.

1. Record a television source that includes a "fade to black" sequence that typically occurs in between commercials or between commercials and network programming.
2. Play back the sequence and pause at the "fade to black" section.
3. Using a colorimeter or a light meter, measure the light output of the black screen.
4. Adjust the black level up and down. You will find a place where additional adjustments of the Brightness setting will not affect the light output of the panel. That point just where the panel's light output becomes responsive to increases or decreases in the Brightness setting is the correct setting.

Setting Sharpness

This one is simple. Just use the sharpness pattern to look for ringing or faint outlines along the edges of the horizontal and vertical lines in the test pattern. Set the Sharpness control to the highest point you can that minimizes ringing (you may not be able to eliminate it entirely). On some sets, the sharpness should be set to zero. But for most it is usually at about the 1/3 point. I include a test pattern for setting sharpness on the calibration DVD.

Adjusting the White Balance

Briefly, white balance adjustment simply involves adjusting specialized controls that allow a display to track a neutral white throughout its entire range from the blackest black to the whitest white.

Unlike a good CMS, which is rare, virtually all displays have white balance controls. Sometimes they are in the user menu, but more often they are buried in a service menu that can only be accessed by a specific key sequence on the remote. The goal is to get an xy measurement as close as possible of x0.3127, y0.329. The calibration software will provide these raw numbers and a graphical representation of RGB relative to the target white point.

To calibrate the white balance:

1. Aim the meter at the display.
2. Select a 80% stimulus window on the calibration DVD.
3. Adjust the RGB Contrast controls until RGB is balanced at 80% stimulus or until you read x0.3127, y0.329.
4. Select a 20% stimulus window from the calibration DVD and use the RGB Brightness controls to balance RGB or achieve x0.3127, y0.329.
5. Repeat the last two steps as many times as necessary until both the 80% stimulus window and the 20% stimulus window measure neutral gray. This may take several sets of measurements.
6. Finally, take an entire series of grayscale measurements at 10% intervals from black to 100% to ensure that the display tracks white accurately throughout the entire range.

Sometimes you may find that even though 80 and 20% stimulus are neutral white, the mid range 40-60% stimulus is not. This means that your display won't track a good grayscale and you have to make some compromises. The general rule of thumb is to focus on getting the mid range to track neutral white. Then get the low end right. Sacrifice accuracy at the top end if you have to.

Note: There is no industry-wide accepted terminology for white balance controls. You may see RGB Contrast/Brightness, RGB Gain/Bias, RGB Gain/Offset, RGB Drives/Cuts. They all mean the same thing. Contrast, Gains, and Drives are for adjusting the bright end of the grayscale and Brightness, Biases, Offsets, and Cuts are for adjusting the dark end of the grayscale.

Setting Color/Tint

(Note: If your display has a full-featured CMS, then this step is not necessary.)

The standard method for doing this involves looking at a SMPTE color bar through a blue filter. This method has 2 drawbacks. First, at best it is an approximation of the correct setting. Second, and more importantly, for some displays it simply does NOT work. On some plasmas in particular I have noticed that this method will recommend a grossly inaccurate setting. Here's a foolproof method for setting Color/Tint that does not use filters.

Color

1. Point the colorimeter or light meter towards the screen and display a 75% or 100% white test pattern.
2. Measure the Y value (luminance) of white.
3. Display a 75% or 100% Red test pattern, and measure the Y value here as well. You will notice that as you move the Color control up and down, the Y value of Red increases and decreases, but white stays the same.
4. Set the color control at the point where Red Y measures closest to 21% of the white reading.

Note: It is not really important whether you use 75% or 100% patterns in this test, so long as you use the SAME level of intensity for both.

Tint

1. If you have not already done so, adjust the gray scale and get it as close to D65 (x=0.3127, y=0.329) across the entire range as possible.
2. Point the colorimeter towards the screen and display a cyan test pattern.
3. Put the Tint control at its neutral mid setting.
4. Use the software controls to plot the hue of cyan.
5. Adjust Tint up or down until the reading places the hue of cyan as close to the target (0% error) as possible
6. If you had to substantially adjust Tint from the neutral point to get an accurate hue of cyan, then check the other secondaries. You may have to select another setting that gets all 3 secondaries as close to correct hues as possible.

Adjusting Color using a Color Management System (CMS)

1. Point your colorimeter towards the screen, display a white window, and then take a xyY measurement.
2. Repeat the step above for all of the primary and secondary colors (red, green, blue, yellow, cyan, magenta).
3. Use the controls on your calibration software to plot the amount of error in hue, saturation, and brightness each color shows relative to the chosen standard. I would select the Rec. 709 (High-Definition) standard. Your software should allow you to set that as the desired gamut.
4. Use the CMS on the display to adjust the colors so that they show the lowest error in each of the 3 dimensions as possible. dE is a good single numerical metric for judging the amount of color error. The lower the dE value, the better.
5. You probably won't be able to get all of the colors lined up perfectly, but get them as close as you can.
6. If your software only shows xy errors on a CIE chart, then it is important to understand that some poorly designed CMSs are such that as you change the xy values to get correct hue/saturation and a good looking CIE chart, the Y value will change as well. It is very important that you check the color brightness—which does not show on the chart—after making these changes. Otherwise, you could have made the color worse without knowing it. If your software doesn't support this, you'll have to do it manually.

Note: The human eye is not equally sensitive to all colors and all color differences. For example, it is more important to get red and green right than blue. It is also more important to get correct hues than correct saturation.

Adjusting Gamma

You want to ensure that your display has a gamma response that is both within the accepted range and that it is reasonably linear. I suggest 2.22 as a good gamma value to aim for, but you can experiment with higher gammas if you like. Above 2.35 you will likely find loses shadow detail and makes the image appear somewhat contrasty.

1. Display a 100% white test pattern and record the Y (brightness) value.
2. Display a 90% white test pattern and record the Y (brightness) value.
3. Repeat until you have recorded the intensity of white all the way down to 10%.
4. Use your display's controls to make necessary adjustments to achieve a gamma value as close to 2.22 as possible at every point throughout the grayscale.

That's it. Now you should go back and remeasure black/white levels, grayscale, color decoding, saturation/tint, and gamma because there may have been interaction between these adjustments. You may have to go through two or three rounds of measurements until all are correct.