A method of determining an absorption or turbidity coefficient of a liquid involves storing a set of data describing a plurality of droplets or other discrete bodies of liquid of different shapes, sizes and absorption or turbidity coefficients. Each body is captured as a combination of a measurable transmission parameter obtained by modelling the interaction of light with a drop, and of one or more dimensional measurements selected from lengths, areas and volumes. The absorption or turbidity coefficient is indicated also. By measuring the transmission of light through a real body of liquid, and making measurements allowing the droplet to be specified, the absorption or turbidity coefficient associated with a droplet giving rise to the same behaviour in transmitting light can be identified from the data set.