Abstract: This paper presents an optical film design method used in plant factory spectrum monitoring, and the optical film is synthesized using different kinds of cut-off filters. Three optical film filters are disigned—red light filter, blue light filter, and red and blue light filter, with central wavelengths at 460 nm for blue light and at 660 nm for red light, and the bandwidth is about 50 nm. The central wavelengths of red light and blue light are set according to the demand of plant. The central wavelengths and bandwidths can be changed when used in different kinds of applications. The filters are used to make spectrum monitoring cells used in facility agriculture and cultural research, and the spectrum monitoring cell can be made by covering an optical filter onto a solar cell panel, then the light intensity with specified wave-range can be monitored by measuring the output of the solar cell panel.