A protection member for an optical measurement device, such as a break-beam tool setting device for a machine tool. The protection member includes a conduit through which light and air can pass. The conduit is configured such that, in use, a beam of light is passed through the conduit along an optical axis and a stream of air is guided out of the conduit along an airflow axis. The optical axis is non-parallel to the airflow axis and the conduit has a varying cross-sectional profile along the airflow axis. Improved measurement repeatability is provided.