The reflectance reference provides a method for calculating the reflectivity of a material from a measured spectrum of a sample. The experimental measurement is obtained in a particular wavenumber region for s-, p- or mixed polarization. The mid-infrared reflectance reference is suitable for use from 5000 cm⁻¹ to 400 cm⁻¹. The reflectance reference is perfectly suited to absolute reflectance measurements of materials. It is ideal for coatings on mirrors and reflective substrates.