An optical method for the quantitative study of the density distribution behind shock waves has been developed. The method, which uses a photoelectric recording, is based upon the integrated schlieren method originally devised by Resler and Scheibe. A detailed theoretical analysis is given. Excellent agreement with predicted performance was found in measurements of the density profiles behind shock waves in CO2. The method is very accurate and retains its high sensitivity for weak shocks. The pictures obtained show a nearly exponential approach to equilibrium of the density behind shock waves.