A method of categorizing the reliability of measurement data in a chromatic range sensor (e.g., an optical pen) which uses chromatically dispersed light to measure the distance to a surface. In one embodiment, the system performs a number of predetermined reliability checks which determine the reliability categories for the sets of measurement data. The reliability categories may be stored as metadata with the respective workpiece height measurements that are determined from the associated measurement data. The reliability categories may be reported to the user (e.g., as graphical reliability category indicators that accompany a graphical display of the measurement data). With these reliability categories, the user may make informed decisions regarding the measurement data (e.g., deciding to filter out data that is associated with certain reliability categories, making adjustments to the setup to achieve improved measurements, etc.).