||This APG contains parameters pertaining to the optical properties of seawater. Assigning parameters to this group encounters two areas of difficulty. Optical techniques, such as transmissometers and nephelometers, are used to quantify SPM concentration. The 'usual' parameter for transmissometer measurements is optical attenuance (or its natural log), which is clearly an optical property. Consequently, attenuance, percentage transmission and their precursor voltages have been included in this APG. However, the usual calibrated parameter from a nephelometer is turbidity, which is clearly more closely related to the suspended load of the water column. Consequently, nephelometer parameters, including raw voltages, have been included in the Suspended Particulate Matter APG. Light intensity, such as downwelling irradiance, may be measured by sensors placed either above or below the surface of the water. It might be argued that the latter belonged in the Meteorology APG. However, some irradiance records include measurements both above and below the sea surface to quantify light loss across the air-sea interface. Consequently, both surface and subsurface irradiance have been placed in the Optical Properties APG.