Accurate photometric measurements depend on the linearity of the detection system, i.e., whether the output is strictly proportional to the incident light flux. The usual method for checking linearity is to introduce filters of known absorption into the optical path. Unfortunately, the many possible errors inherent in this method make it difficult to determine linearity in this way to better than 1%. By using three polarizers in series, keeping the axes of the outer two parallel and rotating the middle polarizer, it is possible to eliminate most of these sources of error. If polarizers of the highest quality are used, photometric linearity may be determined to better than 0.1%. Accurate values for the transmission of standard filters can also be determined with this instrument. The technique is particularly useful for calibrating filters having large optical densities. An error analysis and some experimental results obtained using a three-polarizer system are given.