A colorizer for use in the provision of an ophthalmic tint is described. The design differs from that of an earlier model in that the spectral power distribution of the light in the instrument is very similar to the spectral power distribution obtained when tinted glasses are worn under typical lighting. The similarity in spectral power is obtained notwithstanding the fact that the instrument uses an additive mixture of filtered light, whereas tinted lenses use a subtractive mixture of dyes. The precision of color selection is high, and this precision is shown to be necessary to optimize reading fluency. Because of the similarity in spectral power distribution, it is possible to examine the effect of ophthalmic tints not only on reading fluency but also on the perception and naming of colored surfaces, and the examination can be undertaken by patients who have color vision anomalies. Compared with the earlier design, the use of a diffuse source and seven colored filters reduces the require ment for precision in alignment of components; the variation in color from one instrument to another is, therefore, small, allowing a single calibration for all instruments. This calibration permits the matching of colored ophthalmic lenses to a given colorizer setting, using a computer algorithm which enables practitioners with color vision deficiencies to undertake ophthalmic colorimetry.