Abstract – A fast and reliable surface monitoring and diagnostic technique is essential to develop a real-time automatic control system for laser cleaning of artworks. In this paper, an in-line surface monitoring and diagnostic system based on chromatic modulation using tristimulus detectors and fibre optics is presented. The system produces measurements that are dependent on the spectral signature of the incident light but are independent of intensity. In order to demonstrate its usefulness and versatility, the technique was applied to surface monitoring in the laser cleaning of metal and stone. Results show that the spectral parameters derived from chromatic detection not only provide a clear indication of the surface cleanliness and surface damage but also much surface chromatic information from its versatility. It is also shown in this paper how a chromatic modulation technique may be utilised as a robust method for monitoring and diagnosing the surface during laser cleaning. © 2000 E ´ ditions scientifiques et me´ dicales Elsevier SAS