A real potential for calibration errors is the Test Patches used for display measurements, as it is possible that any patch generator may interpret the RGB triplet patch values sent by ColourSpace incorrectly.
Therefore the measurement values recorded by the probe would not match the correct calibrated values, and so provide inaccurate calibration, and inaccurate verification.
Test Patch Processing
Many display internal TPGs are not bit-accurate to the sent triplet values, which is also true of many software TPGs that run on devices such as Chromecast, Shield, FireStick, Apple TV, etc. The accuracy can depend on the hardware device, or the software TPG used. For example, LightSpace Connect has proven to be accurate in many applications, while the Calman VirtualForge has not.
The following shows the basic similarity with display internal TPG, and 3rd party external TPG applications.
Display Internal TPG
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Patch Generation
Patch colour defined by ColourSpace, and sent to display as RGB Triplet values via Network/USB connection
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Data Connection
USB/Network data connection to the display
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Patch Generator
Display Internal Test Patch Generator (TPG) located before the LUT
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Uploaded LUT
LUT uploaded into the display modifies the patch colour as defined by the RGB Triplet colour data
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Display Measurement
LUT corrected patch colour displayed and measured on monitor
3rd Party External TPG
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Patch Generation
Patch colour defined by ColourSpace, and sent to external TPG as RGB Triplet values via Network/USB connection
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Data Connection
USB/Network data connection to the external TPG
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3rd Party External Patch Generator
3rd Party External Test Patch Generator (TPG) located before the display
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Video Connection
Video connection from the TPG to the display
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Uploaded LUT
LUT uploaded into the display modifies the patch colour as defined by the TPG
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Display Measurement
LUT corrected patch colour displayed and measured on monitor
Depending on the location of the TPG, the potential for errors can stem from the display being YCbCr based, with errors in the conversion of the ColourSpace RGB Triplet patch values into YCbCr values for the TPG, and also within the re-conversion of the YCbCr values back into RGB before passing through the calibration LUT (as all 3D LUTs are RGB based) and then displayed on the display screen (again, as all display screens are RGB).
There can just as easily be issues with the TPG just not being accurate, and not displaying the ColourSpace defined Patch Triplet values correctly.
The above examples show an accurate External TPG vs. an inaccurate display Internal TPG, with both the low-light and high-light areas showing errors with the Internal TPG. The consequence is that any calibration LUT generated from the Internal TPG profile will not be accurate.
