This enumeration correctly describes the sense of the pixel whose numeric value is zero (minimum data value).

Values:

IG_TW_PF_CHOCOLATE	This pixel represents the darkest data value that can be generated by the device (the darkest available optical value may measure greater than 0).
IG_TW_PF_VANILLA	This pixel represents the lightest data value that can be generated by the device (the lightest available optical value may measure greater than 0).