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:
    
      
    
    
    
    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.)