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For an accurate assessment of the f 1 value of a photometer head it is important to determine the maximum step size for sampling and an appropriate bandwidth of the monochromator for an acceptable uncertainty contribution. A simulation is carried out to evaluate these uncertainty contributions for fine photometer heads, with a mismatch characteristic f 1 3% and uncertainty contributions f 1 0.1% taken as the limits for acceptance. Assuming a stable measurement set-up, the results show that a 5 nm step size and a 5 nm bandwidth or less are needed for this assessment. The uncertainty of the f 1 value caused by the wavelength uncertainty of the monochromator is analysed additionally. For extended bandwidths a method is proposed to correct the spectral responsivity data of the photometer head and reduce the bandpass uncertainty contribution effectively.