The Spectral Irradiance Monitor (SIM) is a spectrometer aboard the NASA Solar Radiation and Climate Experiment (SORCE) that provides long-duration solar spectral irradiance measurements in the visible and near infrared (Vis/NIR) electromagnetic spectrums. SIM measures how sunlight is distributed among the ultraviolet, visible, and infrared wavelengths. The wavelength coverage is primarily from 300 to 2400 nm, with an additional channel to cover the 200 to 300 nm ultraviolet spectral region. SIM contains two completely independent and identical (mirror-image) spectrometers to provide redundancy and self-calibration capability.