Data type: char
Must appear in a looped list
May match a value of '_diffrn_refln_wavelength_id'
An arbitrary code identifying each value of _diffrn_radiation_wavelength. Items in the DIFFRN_RADIATION category are looped when multiple wavelengths are used. This code is used to link with the _diffrn_refln_ list. It must match with one of the _diffrn_refln_wavelength_id codes.