An optical measurement instrument includes a measurement head and mechanical support elements arranged to support a sample well plate. The measurement head is moved towards the sample well plate and, after the measurement head has touched the sample well plate, the measurement head is moved backwards away from the sample well plate so as to provide a desired distance between the measurement head and the sample well plate. A sensor device is attached to the mechanical support elements and arranged detect a mechanical effect occurring in the mechanical support elements due to force directed by the measurement head to the sample well plate. Hence, the situation in which the measurement head touches the sample well plate can be detected without a need to provide the measurement head with a force sensor. This is advantageous because the measurement head can be a changeable module of the optical measurement instrument.