Using a Digital Camera for Water Quality Measurements in Galway Bay

The potential of a digital camera, a Nikon Coolpix 885, as an optical instrument to estimate water quality parameters was explored. This study comprised technical tests as well as field surveys. To eliminate direct reflection from the water surface, a tube was fitted around the camera lens which breaks the surface such that only upwelling light from below was measured. The red, green and blue output of the digital camera, the RGB values of the resulting picture, was analysed. To obtain consistency between stations, it was determined that RGB values should be used where the total response (R+G+B) was a constant value.

The method was successfully used in Galway Bay, where the optical characteristics of the coastal waters are dominated by yellow substance (or dissolved organic material) and chlorophyll. A strong linear relationship was found between R/B and yellow substance. In addition, a log-log relationship between B/G and chlorophyll was found. Both relationships were consistent with previous measurements using more conventional narrow band irradiance sensors. The River Corrib plume in Galway Bay was mapped using the highly significant inverse proportionality between yellow substance, hence R/B, and salinity. Tests in the Celtic Sea and the North Atlantic indicated that the digital camera could also be used in clearer sea and oceanic waters. An optical reflectance model was developed to explain the optical signal of water-leaving light in terms of scattering and absorbing properties of pure water and its components. The model agreed with the observed relations, as well as with relations known from literature.

The use of a Nikon Coolpix 885 digital camera for simple water quality measurements was successfully demonstrated and indicated the possibility of using an ‘off the shelf’ digital camera as a low cost, easy to obtain and easy to operate alternative for optical water quality