When setting a new value for this attribute, the new value can be supplied either
directly as a frequency, or indirectly as a wavelength or energy, in which case the
supplied value is converted to a frequency before being stored. The nature of the
supplied value is indicated by appending text to the end of the numerical value
indicating the units in which the value is supplied. If the units are not specified,
then the supplied value is assumed to be a frequency in units of GHz. If the supplied
unit is a unit of frequency, the supplied value is assumed to be a frequency in the
given units. If the supplied unit is a unit of length, the supplied value is assumed to
be a (vacuum) wavelength. If the supplied unit is a unit of energy, the supplied value
is assumed to be an energy. For instance, the following strings all result in a rest
frequency of around 1.4E14 Hz being used: "
1.4E5"
, "
1.4E14 Hz"
, "
1.4E14
s-1"
, "
1.4E5 GHz"
, "
2.14E-6 m"
, "
21400 Angstrom"
, "
9.28E-20 J"
, "
9.28E-13 erg"
, "
0.58 eV"
, etc.
When getting the value of this attribute, the returned value is always a frequency in units of GHz.