Abstract: A device for converting a position of a float member in a rotameter in to an electrical output. Light from a light source is collimated into parallel light rays, at either or both opposing sides of a rotameter tube to greatly enhance accuracy of readings. The lightrays passing through the tube are detected by a linear optical array. A range of movement capable of being read by the linear optical array is extended by use of either a fiber optical array or a movable illumination device and linear optic array. The collimator(s) may be a linear array of one or two rows of cylindrical lenses sandwiched between a pair of plates. The linear optic array maybe arranged on opposite sides of the rotameter tube or may be on the same side there of. A light guide is used in the latter arrangement to guide light from the light source to a side of the rotameter tube opposite the linear optic array which may be a charge-coupled device(CCD).