In a polychromatic photometer a sample wheel containing angularly spaced sample compartments and a filter wheel containing angularly spaced filters are rotated by a common drive arrangement so that the sample compartments and filters are in turn rotated through and aligned with the optical path of a photometric measuring device. The number of sample compartments is different from the number of filters. The number of sample compartments and the number of filters are not evenly divisible by a common integer. The ratio of the angular velocity of the sample wheel to the angular velocity of the filter wheel is substantially equal to the ratio of the number of filters to the number of sample compartments.