Methods and apparatus are described for assessing the reflective properties of mirrors at different angles of incidence without precise knowledge of the mirror's basic optical constants and/or without precise knowledge of the mirror's over-coating prescription. Reflectance values can be accurately calculated for multiple angles of incidence based upon measurement data collected for a single angle of incidence. The approach uses equations based on the Fresnel equations for reflectance in which reflectance is calculated as a function of the angle of incidence of incoming light to the scanned mirror used to collect the signal. The angle of incidence-based approach allows accurate reflectance values to be calculated over a broad range of wavelengths and angle of incidences without detailed knowledge of the optical properties of the coating material and the substrate underneath. The described methods and apparatus are particularly useful for calibrating measurements made with remote sensing instruments that use scanned mirrors.