Water bodies can be characterised optically by the euphotic depth, zeu(1% of surface light), the diffuse attenuation coefficients,Kdfor down‐welling light andKufor upwelling light, and the reflectance coefficient,R(ratio of upwelling to downwelling light), all in terms of photosyntheti‐cally active radiation (PAR). The estimation of all of these quantities from underwater PAR measurements is simple in principle but very difficult in practice under changing ambient light conditions caused by drifting cloud patches. A method of PAR measurement using a two‐channel instrument which records the ratio of the response of 2 PAR sensors and thus overcomes the problems of changeable ambient light is described. Relative PARQ(z)/Qr,whereQ(z)is PAR (either upwelling or downwelling) at depth z, andQTis reference (downwelling) PAR, is much less dependent on ambient light when the reference sensor is located in water than when located in air (deck sensor). The profiles of relative PAR can be used to calculate zeu,Kd,andKu. Rcan be measured directly as the ratio of upwelling to downwelling PAR.