The in situ absorption meter, based on the reflective tube absorption meter principle in which both scattered and directly transmitted light are measured by a single receiver, was originally proposed as a alternative means to measuring in situ concentrations of chlorophyll a and phaeophytin. By measuring differential absorption between two wavelengths, 676 nm and 712 nm, a scattering correction mechanism was provided which provides accurate absorption measurements in natural waters. As the instrument design evolved six wavelengths were eventually installed to measure absorption throughout the visible and near IR spectrum. An operational overview of the instrument describes the primary optical and electrical components of the instrument and provides a basic understanding of how the absorption measurement is performed. After initial field tests, laboratory tests were performed to quantify the instrument's operational characteristics. Precision, linearity, and performance in the presence of a scattering medium were tested to determine the instrument's utility in performing in situ quantitative analysis of chlorophyll. The instrument demonstrated precision approaching 0.02 (mu) g/1 at a 7 Hz acquisition rate, excellent linearity over a 40 (mu) g/1 range, and less than two percent error in measurement accuracy under scatterer to absorber concentration ratios in excess of 1000:1.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.