Argyll XRGA conversion

## Conversion to/from XRGA

After purchasing Gretag-Macbeth, X-Rite harmonized the subtly different calibration standards the two companies were using for their reflective measurement instruments, in order to minimize changes with the introduction of new instruments. The result was the XRGA standard, introduced in September 2010.
Current X-Rite instruments are natively calibrated to XRGA, while older instruments from X-Rite are calibrated to XRDI, and Gretag-Macbeth instruments to GMDI standards. To smooth the interchange of old and new instruments, a spectral conversion is possible between these different standards. While such a conversion is not perfect, it reduces the discontinuities between old instrument families and current X-Rite instruments.
By default, Argyll will use the native calibration of Gretag-Macbeth and X-Rite instruments. These are summarized here:

DTP20 "Pulse" | XRDI
DTP22 Digital Swatchbook | XRDI
DTP41 | XRDI
DTP51 | XRDI
Spectrolino/ SpectroScan | GMDI
Eye-One Pro Rev A-D | GMDI
ColorMunki Design or Photo | XRGA
Eye-One Pro2 (Eye-One Pro Rev E) | XRGA

You can override this and force reflective data to be a particular calibration standard in two ways:
Using the -A command line option in spotread and chartread:
-A N|A|X|G
The N argument sets the calibration to Native.
The A argument sets the calibration to XRGA.
The X argument sets the calibration to XRDI.
The G argument sets the calibration to GMDI.
Or by setting an environment variable:
(Note that command line options will override this.)
ARGYLL_XRGA
If the ARGYLL_XRGA environment variable is set to:
XRGA, then all Getag-MacBeth/X-Rite reflective measurements will be converted to XRGA.
XRDI, then all Getag-MacBeth/X-Rite reflective measurements will be converted to XRDI.
GMDI, then all Getag-MacBeth/X-Rite reflective measurements will be converted to GMDI.
If not set (or set to some other string), then the native calibration will be used. Note that the values must be upper case.
All of the above except the DTP51 can have a conversion applied.
(The DTP51 returns only colorimetric data, while the conversion requires spectral data.)