The various forms of the turning-point method of controlling the optical thickness of dielectric layers during deposition are described briefly with particular emphasis on their shortcomings. It is pointed out that the human operator has attributes which cannot be reproduced with simple electromechanical devices without memory. However, the availability of microprocessors enables this restriction to be overcome effectively. A new method of terminating the deposition of dielectric layers very precisely when their optical thicknesses are an integral number of quarter-waves is described. It involves the use of a microprocessor for control and data processing based on a ‘least-squares’ fit of the reciprocal transmittance of the coating approximated to a quadratic function of time in the vicinity of the turning-points. The qualities of control of a human operator and of the automatic microprocessor system are compared in terms of the reproducibilities of the wavelength of peak transmittance of narrow-band spectral filters prepared by the two methods.