A method for determining the polarization of a light source is to observe the transmitted light intensity through a polarizer set at two azimuthal positions with respect to any arbitrary axis. If the polarizer is mechanically rotated at a constant frequency, there are just four angular positions per revolution at which cross extinction occurs and onlyI∥or I⊥are transmitted. We have constructed a rotating polarizer in which eitherI∥or I⊥is transmitted independent of the azimuthal angle for nearly one‐half a cycle. Effectively, the transmitted light is square wave modulated with respect to two azimuthal positions of a stationary polarizer which differ by 90°. This rotating polarizer has been exceedingly valuable for precision and efficiency of polarization measurements where on‐line computer sampling of experimental data is used.