BAG-18 Online Blue-green Algae Analyzer The blue-green algae sensor utilizes the characteristic that the cyanobacteria has an absorption peak and an emission peak in the spectrum, and emits a specific wavelength of monochromatic light to illuminate the water, and the cyanobacteria in the water absorbs the energy of the monochromatic light, releasing a single wavelength single. In shades of light, the light intensity emitted by cyanobacteria is directly proportional to the amount of cyanobacteria in the water.