Limnology and Oceanography · 1968 · 313 citations · 0 references
EngineeringEarth SciencesPhysicsInstrumentationOpticsRadiation MeasurementCalibrationCurrent Measurement InstrumentsOptical InstrumentationOptical InstrumentsOptical MetrologyPhysical OpticsCalibration Of Optical InstrumentsDiffuse Attenuation CoefficientSecchi Disk Optical Measurements
The Secchi disc experiment is used to estimate the combined total and diffuse attenuation coefficients (α and K) of water. The study proposes calibrating the Secchi disc against modern measurements of α and K to determine a practical apparent contrast value. Independent α and K values require an additional measurement beyond the Secchi disc reading. Recent depth‑light measurements combined with Secchi disc readings suggest α and K can be independently determined, but if modern instruments exist, calibration of the disc is unnecessary.
The theory and practice of the Secchi disc experiment are discussed. It is shown, in theory, that the Secchi disc reading can be used to calculate the sum of the total and diffuse attenuation coefficients, α and K. To obtain independent values of α and K it is necessary to make some other measurement. Some recent measurements of the per cent of surface light reaching various depths, obtained in conjunction with Secchi disc readings, are discussed with respect to their usefulness in determining α and K independently. It is suggested that the Secchi disc could be calibrated against modern instrumental measurements of α and K and a practical value for apparent contrast thus determined for the Secchi disc experiment. However if modern instruments are available for measuring α and K, it would probably be better not to undertake such a calibration.