Development of PAR Sensor Applicable to Greenhouses and Smart Farms Using a Ripple-Free Red/Blue TiO₂/SiO₂ Dual-Band Bandpass Filter

The sunlight that promotes photosynthesis is called photosynthetically active radiation (PAR). Most plants use a large amount of red (R) and blue (B) wavelength light for photosynthesis, but also to a limited extent use green (G) wavelength light. The quantitative light value required for photosynthesis in farming is referred to as the photosynthetic photon flux density (PPFD). Most commercial PAR sensors are well calibrated and precisely developed to measure PPFD, but have complex structures, optical systems, and a multiple number of bandpass filters and/or channels. To address these complex problems, we propose using a dual-band bandpass filter (DBPF) that can transmit light in two bands at the same time. Here, we simulated and developed a ripple-free DBPF that transmits R and B wavelength regions and simultaneously reflects G and infrared (IR) wavelength regions. Our ripple-free DBPF was manufactured by nonperiodically stacking TiO2 and SiO2 films using e-beam evaporation with the help of an optical simulation. In addition, an IR-cutting filter was deposited on the back of the substrate to block IR light that is absorbed by the Si photodiode (PD) used as the light receiver of the PAR sensor. Finally, our dual-color detectable PAR sensor was developed by combining the general Si PD and ripple-free DBPF. The measured PPFD value was calibrated in a wide PPFD range of 0.05 to 3048 μmol/m2/s. Reliability was confirmed from an average measurement error of 3.7% in five light-emitting diode (LED) lighting environments, including white light and monochromatic light. Due to the easy modification of both the R and B transmission bands of the developed DBPF, it can be easily adapted to slightly different PAR spectra of various plants or crops.