This part of IEC 60904 gives guidance for the measurement of the relative spectral response of both linear and non-linear photovoltaic devices. This is only applicable to single-junction devices.
DS/EN 60904-8:1999 history
1999DS/EN 60904-8:1999 Photovoltaic devices - Part 8: Measurement of spectral response of a photovoltaic (PV) device