Process monitoring (optical engineering) is the systematic observation, measurement, and analysis of critical parameters and characteristics during the fabrication, assembly, or testing processes of optical components and systems. This activity involves employing various techniques, frequently optical in nature, to acquire data indicative of the process state, performance, and output in real-time or near real-time. Positioned as a fundamental element within 'manufacturing planning' and 'manufacturing strategy', it serves as a prerequisite for effective 'process control' by providing the necessary feedback to detect deviations, assess quality, and optimize efficiency. Its primary significance lies in ensuring the repeatability, reliability, and adherence to specifications in the production of 'optical systems', thereby enabling predictable manufacturing outcomes and high product quality.