In a method and apparatus for measuring the optical absorption of samples having a light source (1), a photoelectric converter (8), a measurement beam path extending between the light source (1) and the converter (8), in which path the sample to be examined is arranged, a reference beam path extending between the light source (1) and the converter (8), in which path a reference sample is arranged, and a motor-driven chopper disc (10), the chopper disc (10) is configured with a first number of first openings (15) unblocking only the measurement beam path and a second number of second openings (16) unblocking only the path for the reference beam. A lock-in amplifier (21) and a device (17) for synchronising the lock-in amplifier (21) with the chopper disc (10) is connected to the converter (8) and an evaluation circuit (26) establishes the quotient of the transmitted intensity of the reference beam path detected by the converter (8) and the transmitted intensity of the measurement beam path detected by the converter (8) as a measurement for the concentration of the sample in such a way that the decrease in the intensity of the beam by absorption leads to an increase in sensitivity.