Some Problems in the Measurement of Radiation Under Water: A Review

A brief review is given of generally agreed current practice in the terminology and methodology of radiation measurement underwater. The use of energy units is recommended for the presentation of ecological irradiance data, together with details of the spectral composition. Absolute values may be determined above the water but relative values, determined, with selenium photocells, are more usual underwater. Some controversial or historically confused topics are discussed in more detail. The uses of ‘absorption’ and ‘extinction‘ are critically examined. Current proposals to replace extinction by ‘attenuation‘ are endorsed with the qualifications that absorption is only used when losses of radiant energy by transformation are related to path length and Bouguer‐Beer constants, and that attenuation is used when losses of radiant energy are related to depth and empirical coefficients. The advantages of optical density for calculations combining and analysing the transmissions of different components of equipment or the environment are presented. Calculations for designing equipment for the analysis of spectral energy distributions and the interpretation of the results are described in detail. Use of photometers with opal discs can introduce errors when control readings on a surface cell are compared with submerged readings. The sources of these errors are described and procedures suggested to overcome them.