  The Planck High Frequency Instrument (HFI) spectral response was determined
through a series of ground based tests conducted with the HFI focal plane in a
cryogenic environment prior to launch. The main goal of the spectral
transmission tests was to measure the relative spectral response (including
out-of-band signal rejection) of all HFI detectors. This was determined by
measuring the output of a continuously scanned Fourier transform spectrometer
coupled with all HFI detectors. As there is no on-board spectrometer within
HFI, the ground-based spectral response experiments provide the definitive data
set for the relative spectral calibration of the HFI. The spectral response of
the HFI is used in Planck data analysis and component separation, this includes
extraction of CO emission observed within Planck bands, dust emission,
Sunyaev-Zeldovich sources, and intensity to polarization leakage. The HFI
spectral response data have also been used to provide unit conversion and
colour correction analysis tools. Verifications of the HFI spectral response
data are provided through comparisons with photometric HFI flight data. This
validation includes use of HFI zodiacal emission observations to demonstrate
out-of-band spectral signal rejection better than 10^8. The accuracy of the HFI
relative spectral response data is verified through comparison with
complementary flight-data based unit conversion coefficients and colour
correction coefficients. These coefficients include those based upon HFI
observations of CO, dust, and Sunyaev-Zeldovich emission. General agreement is
observed between the ground-based spectral characterization of HFI and
corresponding in-flight observations, within the quoted uncertainty of each;
explanations are provided for any discrepancies.
