To calibrate its grayscale tracking, a display may provide only two-point white balance controls, only multipoint white balance controls, or it may provide both. If both types of controls are provided, it is generally best to adjust the two-point controls first, and then adjust the multipoint controls, if necessary…
Multipoint Grayscale Calibration – Multipoint grayscale calibration is performed with typically either ten or twenty sets of white balance controls. Red, green, and blue balance controls are provided for each of the ten or twenty grayscale levels. Multipoint grayscale is adjusted either in 10% increments (10/11-point) or in 5% increments (20/21-point). See the Multipoint Grayscale Adjust Help topic for information on performing multipoint grayscale calibration.
Two-Point Grayscale Calibration – Two-point grayscale calibration is performed with two sets of white balance controls. There are high brightness Red Gain, Green Gain and Blue Gain controls that are actually individual Contrast controls for each primary color. There are also low brightness Red Offset, Green Offset and Blue Offset controls that are actually individual Brightness controls for each primary color. These white balance controls may be given different names in different displays:
|White Balance Controls
|Other Names Used
|Used to Calibrate
|Drive, High, Contrast
|High brightness white balance
|Cutoff, Bias, Low, Brightness
|Low brightness white balance
Two-point grayscale calibration is performed at two grayscale levels (points), typically at a high brightness level of either 70% or 80% (with the Gain controls) and at a low brightness level of either 20% or 30% (with the Offset controls). High brightness and low brightness test patterns are measured, and adjustments are made to change the display’s balance of red, green, and blue light, to produce as close as possible to the target white/gray color as possible, at these two grayscale levels. The display’s natural linearity then determines the white balance accuracy across the rest of the grayscale, with the tracking error often being higher at the dark and bright ends of the grayscale.
NOTE: To perform two-point calibration, both RGB Balance charts need to be in Relative mode, with their center target lines labeled as “100.” If the charts are in Absolute mode, with their center target lines labeled as “0,” they need to be switched to Relative mode, or the display’s gamma will be calibrated improperly. Right click on each RGB Balance chart, select “Properties,” then select “Relative” in the Reference section.
Two-Point Grayscale Calibration Procedure:
1. Configure your meter and test pattern source correctly to Calman and to the display, if you haven’t already done so.
2. Under Calman Settings / Workflow Basic Options / Calibration Points, select the desired number of calibration points for the display being calibrated.
3. On the Calman Grayscale Calibration page, press the Read Series meter button to measure the display’s grayscale tracking performance.
High Brightness Gain Controls
To minimize interaction between the Gain and Offset controls, adjust the high brightness gain controls first. Adjust the Gain controls to optimize the balance of red, green, and blue at a high brightness level. The Gain controls are typically less sensitive, requiring significant change to achieve a desired effect.
4. Select the desired high brightness adjustment level (typically 70% to 80%), on the bottom Calman Level Select bar.
5. Press the Read Continuous meter button to start continuous measurements.
6. Adjust the display’s red and blue Gain controls to balance them each to the green level, as indicated on the single level RGB Balance chart. (If the center target line on the RGB Balance chart is labeled “0,” see the Note above.)
Low Brightness Offset Controls
When the white balance is optimal at high brightness, switch over to adjusting the low brightness Offset controls. Adjust the Offset controls to optimize the balance of red, green, and blue at a low brightness level. The Offset controls are typically more sensitive, requiring less change than the Gain controls to achieve a desired effect.
7. Select the desired low brightness adjustment level (typically 20% to 30%) on the bottom Calman Level Select bar.
8. Adjust the display’s red and blue Offset controls to balance them each to the green level, as indicated on the single level RGB Balance chart.
When the white balance is optimal at low brightness, switch back to check the high brightness white balance. You may find that the original color balance at high brightness has drifted, due to interaction with the low brightness adjustment. Repeat the high brightness gain adjustment and the low brightness offset adjustment until the control interaction is minimized. The amount of control interaction varies greatly from one display to another.
9. Repeat the high brightness gain adjustment and the low brightness offset adjustment until both adjustments are optimized.
When both Gain and Offset adjustments are optimized, perform a Read Series measurement, to view the display’s grayscale tracking performance on the full scale RGB Balance chart and to evaluate the error on the DeltaE chart. Mouse over the error bars on the DeltaE chart to view a visual representation of each grayscale color error. Color errors of 1 dE or less are typically not visible. For an in-depth discussion of the color comparator, see the paper on Visual Color Comparison. Also, view familiar program content to confirm the picture improvement.
10. Press the Read Series meter button to measure the display’s grayscale tracking performance.
11. View familiar program content to confirm the improved picture quality.