Temperature-drift of spectrum, means the spectrum of a optical filter will drift to the left, or said to the shortwave side, while the work temperature increase to a certain level, such as from 25 C-degree to 70 C-degree, the spectrum will drift -3nm or -5nm to the left, which will cause the centre wavelength to leave the point expected. It is not a problem for most filters, such as wide bandpass filters or longpasss filters, but really do harm to narrow bandpass filters, especially the interference filters with very narrow band required, such as FWHM=30nm or 20nm etc.