Miscellaneous / Blog / 20060914
Since some people are still looking for this article from our old website in 2005, I have reposted it here...
This article looks at some of the issues involved in choosing an LCD for a color calibrated work flow. Integrated Color Corporation's ColorEyes Display was used for the calibration and verification results, and a Samsung SyncMaster 910T was used as an example of a quality (with some uniformity problems) inexpensive 16.7M color 19" LCD display. Tom's Hardware and BE Hardware are two good sources for LCD reviews which give calibration results.
Delta-E
This term describes the perceptual difference between two colors in the LAB color space. A Delta-E value greater than 1 represents the beginning of a visible color difference, and a Delta-E of 100 represents the difference between the display's black and white. Sometimes Delta-E is abbreviated as DE.
Uniformity
Intensity and color output are not uniform across the full area of any LCD screen. This can be measured by using the calibration software to verify the corners of the screen after verifing the center. The SyncMaster 910T had the following maximum Delta-E measured in the four corners: 2.29, 2.73, 3.15, 4.43, while having a maximum Delta-E of 0.44 in the center. The first image below is the validation results from the center, the second is from the worst corner.
Tom's Hardware gives uniformity results as an intensity percentage of the brightest white. So 100 represents perfect uniformity. In order to convert this percentage into a Delta-E value use the following equation, Delta-E = 100 - (116 * (Intensity_Percentage/100)^(1/3) - 16). This equation basically translates from CIEXYZ Y (which is intensity) to CIELAB L (which is perceptual brightness). Here are a few translations of the ranges used in Tom's Hardware reviews,
95 -100% = 1.97 - 0.00
90 - 95% = 4.00 - 1.97
85 - 90% = 6.12 - 4.00
80 - 85% = 8.32 - 6.12
75 - 80% = 10.6 - 8.32
70 - 75% = 13.0 - 10.6
Even Pro LCDs costing over $1500 have corners in the 85-90% range which equates to a 6.12-4.00 Delta-E difference. Given that uniformity problems have such a drastic effect on display calibration, good uniformity should be a very high priority.
16.7 or 16.2 Million Color Display
The 16.7M color display has 256 output shades for each of the three primaries (RGB). This gives 256*256*256 or 16,777,216 possible colors. A 16.2M color display has only 64 output shades. This gives 64*64*64 or 262,144 possible colors (which is only 1.6% of the number of colors a 16.7M display can produce).
So how does a display that only has 0.2M colors produce 16.2M colors? The 16.2M displays use a dithering pattern that continously changes while the screen refreshes. This dithering can simulate 253 shades for each of the three primaries. This gives 253*253*253 or 16,194,277 simulated shades. I have not used a 16.2M color display, but I might be tempted to try one if it had better than 95% uniformity over the entire screen.
VGA or DVI Input
Color calibration works by adjusting LUT (Look Up Tables) which translate an 8-bit intensity to a higher 10-bit to 16-bit intensity. For quality LCD displays that have a built-in LUT, use the DVI-D input which sends the 8-bit intensity data digitaly to the display. In this case the color calibration software will calibrate using the LUT in the display, and the result will be higher quality output than using the VGA cable. For displays without a built-in LUT, use the VGA cable so that the LUT on the video card will be used to calibrate the display.
Native White Point
LCD native white points tend to be in the range of 5500K to 6500K. Calibration performance and maximum brightness drop off as the calibrated white point deviates from the native white point, partly because the display runs out of adjustability. For example, on the 910T (which has a 6150K native white point) at full contrast and brightness the R/G/B ratio for a 5000K white point is 50/18/3. In this case the blue channel is only 3 points higher than its lowest setting.
ColorEyes Display does not run the user through the step of calibrating the LCD's white point, as it does for calibrating a CRT. In order to get the best possible calibration, white point adjustment should be done manually, if possible. At the Choose Color Temp option, first choose the color temp. Then choose the Chromaticity Coordinates option. Write down the values for x and y, then click on the Measure button. This will bring up the Measurement Patch which meters the white point. Make sure to click the Show Uncalibrated button before measuring. Below is a screen shot taken during the process.
Now while metering the white point, adjust the monitor's R/G/B ratios until the x and y settings in the dialog are as close as possible to what was written down earlier. The display's brightness and contrast should be adjusted at this point also. The (upper case) Y is a measure of intensity. To get the best results, first get the R/G/B ratios close, then adjust the brightness and contrast, and finally go back and fine tune the R/G/B ratios. Increasing green will mostly decrease y, increasing red will mostly increase x, and increasing blue will mostly decrease x. All settings are interdependant so one will need to experiment with this. When the adjustments are complete, click the Accept button, re-enter the target white point and Continue to the next step.
On the 910T good results for a 180cd/m2 5000K white point were obtained using the following settings. Gamma mode 3 was used because it produced the darkest uncalibrated image (meaning highest gamma) which allocates more of the 256 output brightness levels to the darker intensities where they are needed the most.
Picture:Brightness = 80
Picture:Contrast = 80
Color:Color Tone = Custom
Color:Color Control:Red = 50
Color:Color Control:Green = 34
Color:Color Control:Blue = 17
Color:Gamma = Mode3
Black Level and Contrast Ratio
Contrast ratio is the ratio of the darkest and lightest intensities a monitor can produce. Generally, a higher contrast ratio is preferable because if the contrast ratio is too low, the black level can be too bright causing a dull display. However, some high contrast LCD displays are very bright, and in the process of calibrating these displays, lowering the brightness to an acceptable editing level can yield a low contrast ratio and thus a dull display.
On a Mac, find the contrast ratio of a display by double clicking the ICC profile (which will probably be found in your user directory under Library/ColorSync/Profiles) and use the ColorSync Utility to find the Media black-point tristimulus value. The Y (or intensity) of white is always 1 so the contrast ratio is 1/Black_Y. So for the 910T profile the contrast ratio is 1/0.0027 or 370:1. The 910T's white intensity is 180cd/m2 so its black level is 180/370 or 0.49cd/m2.
Viewing Angle
LCD display output is effected by viewing angle. On laptops this can be a significant problem. When editing using a LCD display with distortion specific to viewing angle, one way to minimize the problem is to always edit using the same viewing angle.
The following PNG images are designed to be tiled on the desktop background. They exhibit a very visable color shift in the midst of distortion caused by viewing angle. They can be used both to check for distortion and also to come back to a consistant viewing angle. Below are three images intended for use with the following gamma settings: 1.8, 2.2, and 2.5 respectively. Feel free to use these images.
Note that gamma images like these should NOT be used to calibrate a display or to test if a display is calibrated. They will always exhibit some color even though mathematically they are designed to show as gray. It is the color shift from how the image appears in the center to across various areas of the screen that is actually useful.
Calibration Settings
Using ColorEyes, the 16-bit Lookup Tables option gives the best ICC profiles. Since monitors have a limited number of shades (256 per channel) it would be best if these shades were spaced out perceptually uniformly across the full range. A Gamma curve (such as 1.8 or 2.2) cannot do this, but this is exactly what ColorEyes Display's L* option does. When editing 16-bit images in a color calibrated environment such as Adobe Photoshop, the L* option provides the best results regardless of the Gamma of the RGB working space. However, when working in a non-color managed environment, such as viewing images from the web, the L* option lightens the darks considerably. So having a second monitor calibration with a 6500K white point and Gamma of 2.2 (similar to the sRGB standard) could be used when not editing images.
Last Words
For photographers, finding a good LCD can be a tough process. There is little in the realm of technical comparison between different displays on the internet, and the technology is moving so fast that it is hard to keep up. LCD display price is not a good indicator of quality, as some high end displays are lower performing then some economy models. Perhaps the best option for those comfortable with monitor calibration is to keep purchasing and returning LCD displays until you are satisfied with one.