Accurate thin-film thickness (TFT) monitoring is key technique of optical coating. Considering of the poor monitoring conditions of universal coating equipments with low stability

precision and automation level

a new photoelectricity-control-analysis system is developed. Based on double-frequency modulation

a photoelectric test system with four-light beams was put forward. Dual-lock-phase circuit system and comprehensive digital processing system with compound filter were used to process monitoring signals

and all automation-monitoring functions were achieved with control-analysis system. Experimental results show that standard deviation of TFT

which indicates duplication of TFT monitoring

is equal to or less than 0.55%

that signal-noise ratio approaches 1000. A very good linearity of numeral display is obtained and the display resolution limit on reflectivity is 0.02%. The linearity of static drift is very high

the drift ratio is less than 1.95%/h

and the static stabilities of the new system are double superior to the universal system. In short

the static and dynamic stabilities

the signal-noise ratio

and the control precision of TFT are extremely increased.