The apparatus described in this paper consists of a minicomputer coupled to a rapid-scanning spectrometer that continuously measures the spectral profile during deposition of each layer for monitoring film thickness. The apparatus is demonetrated in the monitoring of a beam splitter made of three layers Performance of the apparatus: measuring band width: 333nm; spectral wavelength point: 50 point; transmittance accuracy: 1% spectral transmittance scan period: 12 sweep/sec.