To measure the true shape, particularly the true maximum of an absorption band, the spectral bandwidth Δ λ of the instrument must be much less than the width of the absorption band. A spectral bandwidth error results from using too large a bandwidth relative to the absorption band being measured.
Source:
PAC, 1988, 60, 1449. 'Nomenclature, symbols, units and their usage in spectrochemical analysis - VII. Molecular absorption spectroscopy, ultraviolet and visible (UV/VIS) (Recommendations 1988)' on page 1456 (https://doi.org/10.1351/pac198860091449)