Concentrations are estimated from ratios between spectral radiances.

A recently invented method of measuring concentrations of phycocynanin- pigmented algae and bacteria in water is based on measurement of the spectrum of reflected sunlight. When present in sufficiently high concentrations, phycocynanin- pigmented microorganisms can be hazardous to the health of humans who use, and of animals that depend on, an affected body of water. The present method is intended to satisfy a need for a rapid, convenient means of detecting hazardous concentrations of phycocynanin-pigmented microorganisms. Rapid detection will speed up the issuance of public health warnings and performance of corrective actions.