SPECTROSCOPY 1. Spectrophotometer - an instrument which can measure the optical density of a sample at any wavelength. Light Lens Slit Monochromator Sample Detector Quantitative Analysis SlitsSPECTROSCOPY 1. Spectrophotometer - an instrument which can measure the optical density of a sample at any wavelength. Light Lens Slit Monochromator Sample Detector Quantitative Analysis Slits