Surface reflectances are metameric for a colour input device if they induce identical response under one light source and induce a set of distinct responses under a second light source. Depending on the device sensitivities, metamerism will be different in structure (which reflectances form metamer sets), cardinality (the number of reflectances in each set) and their perceptual magnitude (e.g. the colour mismatch region of a metamer set under a change of illuminant).In this paper we propose measures to quantify the differences in colour input devices from the point of view of metamerism. Specifically, three quantitative correlates are proposed: the proportion of potentially metameric reflectances (reflectances that give identical response under a canonical illuminant), the proportion of metameric reflectances (potentially metameric reflectances that result in a colour mismatch under any of the test illuminants), the magnitude of the colour mismatch (CIE ΔE's of metameric reflectances under all relevant illuminants). In addition we introduce frequency images that visualise the extent of metamerism for a particular set of spectral sensitivities and a multi–spectral image of interest.Our aim in this study is twofold: firstly, to provide a means for the study of colour input devices from the point of view of their degree of metamerism; secondly, to expose the relationship between the accuracy of reflectance estimation and the extent of metamerism of a particular device.To illustrate our approach we compare several devices of various spectral sensitivities (trichromatic and multispectral) as well as series of synthetic sensitivities designed to study two particular aspects: the number of sensors and their shape.