The first thing to realise is that ColourSpace cannot introduce any colour or grey scale errors into any generated LUT by its own actions/processing.
The only way any grey scale issues can be generated is if the profile data is incorrect/invalid, or if the application of the LUT is in some way at fault.
LUT Accuracy & Grey Scale
Due to the advanced algorithms used with ColourSpace's colour engine the only way that any incorrect values can be added to any ColourSpace generated LUT is if the profile data is incorrect/invalid.
Such invalid/inaccurate measurements are obviously possible if the probe in use is not providing accurate readings throughout the volumetric display range, or the display itself is unstable, or if the TPG used is itself inaccurate.
Note: See Probe Accuracy for further information on potential probe induced profile errors.
Unfortunately, due to the understandable desire to assess grey scale as a separate entity to the full volumetric colour, such errors can be more easily visible, compared to the rest of the volumetric colours.
However, It must be remembered that there is no such thing as a grey scale as a separate entity compared to all other colours. All colour points are equally valid in all display calibration. Consequently, if there are invalid readings anywhere within the measured colour volume - not just the measured grey scale - the resulting calculated 3D LUT will contain inaccuracies.
Any LUT errors, regardless where they are located, will be due to original measurement/profiling errors, not generated by ColourSpace's colour engine.
This therefore means that when a calibration result appears to show any colour inaccuracies, in the grey scale, or anywhere within volumetric colour, the underlying profile data used to generate LUT will be the cause of such errors (assuming the application of the LUT is not at fault... which is also a possibility, as explained within the LUT Processing and Active LUT vs. LUT Upload guides.).
The various sections within this user guide explains the possible issues that can cause a less than perfect calibration result.
If required, any specific issues showing in the grey scale can be managed by the use of Augment Data when generating a LUT; by using an alternative LUT Processing option, such as Hybrid; or via the use of the LUT Adjustment Filters, such as Axis Blend.
Generating a LUT using Peak Chroma, and then a second LUT using Hybrid, is a good method to assess if the profile data has invalid/incorrect measurements, as a profile that has no invalid/incorrect measurements will produce an identical LUT, as is shown in the following graphs.
The above profile is from a display that cannot make Rec709 gamut; is extremely non-linear, but additionally has been profiled using a Tristimulus with no Probe Matching to a Spectro, meaning the probe measurements are inaccurate to the actual colours being displayed on the display's screen, as defined in the Probe Accuracy user guide.
(Click the following graphs to directly compare them)
As can be seen, the 3D Volumetric LUT is largely identical, with mainly the Grey Scale 1D LUT component showing significant variations. This shows the profile data is in some way inaccurate/invalid, specifically the points surrounding the grey scale measurements.
Focused Patch Set
To gain the best possible grey scale calibration, the use of a Focused Patch Set is highly recommended - especially when the native grey scale (RGB Balance) is not close to the target white point.
Note: Focused Patch Sets are not specific to grey scale improvements, and will improve the full volumetric calibration.
Reduced Gamut Patch Set
Another approach for difficult displays is the use of Reduced Gamut Profiling, as the Colour Engine within ColourSpace does not require a full gamut range of patches.
Using a Reduced Gamut patch set focused the profile measurements into the central gamut of the display, where critical accuracy is more important.
