Semidirect level monitoring has error compensation capabilities of potential benefit to optical coatings of certain types. A review of the principles of level monitoring using circle diagrams shows how to design a level monitoring scheme for various cases, including the use of precoated monitoring chips. The film thickness sensitivities for various optical monitoring strategies differ considerably. Optimum level trigger point monitoring offers improved sensitivity of change in reflectance vs change in optical thickness. This procedure is also expected to give small thickness errors when optically monitored. Optimum level trigger point monitoring, and its application, is fully described.