Four Color Matrix Meter Correction Table Method
The best choice to achieve the most color accurate results by measuring any display/projector technology will require to have both a spectrophotometer/spectroradiometer and a colorimeter.
Meter profiling is a process for improving the accuracy of a filter-based colorimeter, for a specific display. Profiling creates a calibration profile for the colorimeter, for the display currently being measured, based on display measurement data from a reference spectrophotometer/spectroradiometer.
When you profile a colorimeter, you measure the unique spectral characteristics of a display with a spectrophotometer/spectroradiometer and basically you transfer that accuracy to your colorimeter, for that display.
The process is known as the Four-Color Matrix Method (FCMM) for Correction of Tristimulus Colorimeter developed by Ohno and Hardis at NIST (National Institute of Standards and Technology).
The accuracy of this method has been studied further from NIST for spectral variations of displays including CRTs, LCDs, and OLED displays and it works well.
The procedure starts using a reference spectrophotometer/spectroradiometer which measures 4 color patches, the primary colors (red, green, blue) and white of a certain display technology and then using a colorimeter it will measure these 4 color patches (W,R,G,B) of the same display also.
From these 8 in total measurements (4 per each meter), a 3x3 XYZ matrix correction table will be created and it will transform the colorimeter results as close as possible into the results obtained by the spectrophotometer/spectroradiometer used. The correction matrix is not a simple x,y subtraction/offset, which would result in unpredictable and invalid results.
That generated matrix solution can then be used to transform any other color measured by the colorimeter on that specific display into accurate and corrected chromaticity coordinates and luminance values for any color within the gamut of the display primaries.
A matrix developed for one type of display will not work on another display of a different technology because the spectra are different, so the error correction will not be effective.
How often a new meter profiling is required?
Because the filter-photodiode combinations for each colorimeter may be a little different than another of the same brand and model, a set of matrices needs to be developed for each colorimeter and for each display technology. For that reason, sharing correction matrices between the same colorimeter users will not work.
It's recommended to create a new meter profiling on every new display you need to calibrate. Even if it's the same display model that you have calibrated before, manufacturers occasionally change the characteristics of a display screen or backlight in the middle of a manufacturing run, without changing the model number.
Color performance with aging
It is less widely known that the color performance of LED is actually more susceptible to temperature changes, driving current and aging than traditional displays.
While the CCFL and UHP, as well as the CRT phosphors mainly decrease in brightness and change chromaticity only slightly (UHP, CCFL) or not at all (CRT), with LED the colors both decrease in brightness and considerably change the chromaticity.
The CRT contains red, green and blue phosphors that emit light when a guided electron beam hits them. These phosphors do not change color with time, but do decrease in brightness along usage, which results in both a darker display.
Because different phosphors can decrease brightness with a different pace, the white balance will also be offset with time. So, the main challenge in CRTs was to maintain the color temperature (white balance).
Lamps (CCFL or UHP) with aging or increasing temperature, the different lamp spectral peaks decrease in intensity. Their peak wavelength and peak width also shift; however, the brightness decrease of the primaries is predominant so for practical purposes we can neglect the chromaticity change.
The LED spectrum has a different behavior than both phosphors and lamps. With aging and/or higher temperature, the LED spectral peaks: a) lower; b) widen; and c) shift to different wavelengths, all at the same time. The lowering is connected to decreasing brightness. But the shift and widening means that the color also changes in chromaticity.
Moreover, the only way to control brightness of an LED is with changing the current through it. But a different current means a different local temperature, so this again changes the spectrum.
The end result of LED aging is thus: lower brightness, offset white balance, and changed chromaticity of primary colors and all other colors in between.
Because of the color performance shifting/drifting from aging, the spectra of the RGB colors can change from display to display and technology to technology, a new meter correction table will be needed for the most accurate profiling results before each calibration run of the same display.
After a meter correction profile is created, the calibration process would then be performed using the colorimeter, which performs faster throughout and has higher sensitivity at low light levels.
The best choice to achieve the most color accurate results by measuring any display/projector technology will require to have both a spectrophotometer/spectroradiometer and a colorimeter.
Meter profiling is a process for improving the accuracy of a filter-based colorimeter, for a specific display. Profiling creates a calibration profile for the colorimeter, for the display currently being measured, based on display measurement data from a reference spectrophotometer/spectroradiometer.
When you profile a colorimeter, you measure the unique spectral characteristics of a display with a spectrophotometer/spectroradiometer and basically you transfer that accuracy to your colorimeter, for that display.
The process is known as the Four-Color Matrix Method (FCMM) for Correction of Tristimulus Colorimeter developed by Ohno and Hardis at NIST (National Institute of Standards and Technology).
The accuracy of this method has been studied further from NIST for spectral variations of displays including CRTs, LCDs, and OLED displays and it works well.
The procedure starts using a reference spectrophotometer/spectroradiometer which measures 4 color patches, the primary colors (red, green, blue) and white of a certain display technology and then using a colorimeter it will measure these 4 color patches (W,R,G,B) of the same display also.
From these 8 in total measurements (4 per each meter), a 3x3 XYZ matrix correction table will be created and it will transform the colorimeter results as close as possible into the results obtained by the spectrophotometer/spectroradiometer used. The correction matrix is not a simple x,y subtraction/offset, which would result in unpredictable and invalid results.
That generated matrix solution can then be used to transform any other color measured by the colorimeter on that specific display into accurate and corrected chromaticity coordinates and luminance values for any color within the gamut of the display primaries.
A matrix developed for one type of display will not work on another display of a different technology because the spectra are different, so the error correction will not be effective.
How often a new meter profiling is required?
Because the filter-photodiode combinations for each colorimeter may be a little different than another of the same brand and model, a set of matrices needs to be developed for each colorimeter and for each display technology. For that reason, sharing correction matrices between the same colorimeter users will not work.
It's recommended to create a new meter profiling on every new display you need to calibrate. Even if it's the same display model that you have calibrated before, manufacturers occasionally change the characteristics of a display screen or backlight in the middle of a manufacturing run, without changing the model number.
Color performance with aging
It is less widely known that the color performance of LED is actually more susceptible to temperature changes, driving current and aging than traditional displays.
While the CCFL and UHP, as well as the CRT phosphors mainly decrease in brightness and change chromaticity only slightly (UHP, CCFL) or not at all (CRT), with LED the colors both decrease in brightness and considerably change the chromaticity.
The CRT contains red, green and blue phosphors that emit light when a guided electron beam hits them. These phosphors do not change color with time, but do decrease in brightness along usage, which results in both a darker display.
Because different phosphors can decrease brightness with a different pace, the white balance will also be offset with time. So, the main challenge in CRTs was to maintain the color temperature (white balance).
Lamps (CCFL or UHP) with aging or increasing temperature, the different lamp spectral peaks decrease in intensity. Their peak wavelength and peak width also shift; however, the brightness decrease of the primaries is predominant so for practical purposes we can neglect the chromaticity change.
The LED spectrum has a different behavior than both phosphors and lamps. With aging and/or higher temperature, the LED spectral peaks: a) lower; b) widen; and c) shift to different wavelengths, all at the same time. The lowering is connected to decreasing brightness. But the shift and widening means that the color also changes in chromaticity.
Moreover, the only way to control brightness of an LED is with changing the current through it. But a different current means a different local temperature, so this again changes the spectrum.
The end result of LED aging is thus: lower brightness, offset white balance, and changed chromaticity of primary colors and all other colors in between.
Because of the color performance shifting/drifting from aging, the spectra of the RGB colors can change from display to display and technology to technology, a new meter correction table will be needed for the most accurate profiling results before each calibration run of the same display.
After a meter correction profile is created, the calibration process would then be performed using the colorimeter, which performs faster throughout and has higher sensitivity at low light levels.