Abstract: A dual beam spectophotometer is described in which a beamsplitter is at least partially surrounded by a chopper, the combination of which provide sample and reference beams. An oscillating grating produces a monochromatic light beam scans a selected spectral range of wavelengths. The movement of a second chopper and of the oscillating grating is controlled to alternately pass sample and reference spectrums for detection by a detector and to block all radiation. Detected signals are processed for storage by suitable electronics and a computer. The electronics also control the motion of the oscillating grating and chopper. A set of beamsplitters with distinctly different sample-to-reference beam ratios are provided to add versatility to the spectrophotometer.