An instrument used to perform in-situ measurements of the amount of chlorophyll in water. It does this by pulling water into two tubes, one measuring light absorption and the other attenuation. A beam of light with a wavelength rotating among three values is projected into each tube. The attenuation tube determines light absorption and scattering by measuring how much of the original light beam remains after it passes through the water inside the blackened tube. The absorption tube determines only how much light is absorbed by particles by measuring how much light is left of the original beam including that which has bounded off particles. This tube is lined with a quartz mirror which, in contrast to the absorbing black surface in the attenuation tube, reflects scattered light toward the detector. Chlorophyll causes a large change in the attenuation of light with a wavelength of about 676 nanometers, so a measurement of attenuation at the appropriate wavelength is a proxy measurement of chlorophyll concentration to first order. A fluorometer can also be used to measure chlorophyll.