Digitally Fabricated Mobile Spectrometer for Multipoint Continuous Spectroscopic Analysis of Light Environment in Greenhouse Tomato Canopies

Accurate measurements and control of light environment can significantly affect plant growth; thus, light measurement and control techniques are important for increasing plant yield. The lowest-cost option amounts to measuring the illuminance, for which instruments are readily available; more advanced techniques address measurements of radiation and photon fluxes. For the evaluation of photosynthesis under various light sources, the most informative quantity is the flux of photosynthetic photons in the 400―700 nm range of wavelengths (Tibbitts et al., 1983). To comprehensively evaluate the light environment including photomorphogenesis, spectroscopic measurements and analysis are becoming indispensable. Many spectrometers have been introduced into plant research laboratories, leading to some significant advances (Kendrick and Kronenberg, 1986). However, because spectrometers are much more expensive than illuminance meters, this approach is not practical for use in small-scale fields, such as the ones that are cultivated by many farmers in Japan. Consequently, these relatively expensive devices remain unpopular. The development of the micro electro mechanical systems (MEMS) technology has contributed to miniaturization and cost reduction of sensors (Kaajakari, 2009). Some micro-spectrometer MEMS devices that combine reflective concave brazed gratings and complementary metaloxide-semiconductor (CMOS) linear image sensors have been commercialized (Hamamatsu Photonics, 2017). To fabricate a measuring instrument that uses the MEMS technology, a computer system for controlling and a case for housing are required. Up to date, it has been difficult to solve these problems in research laboratories, and the existing prototypes have been bulky and costly. Recently, digital fabrication technology has been proposed for overcoming these technical problems in a small lot production (Walter-Herrmann and Büching, 2014). Based on the increasing use of open source hardware (computers) and three-dimensional (3D) printers, a framework for developing and manufacturing equipment for digital gadgets, called FabLab, was established (Pearce, 2012). Owing to this technology, fabrication of prototypes has become much less expensive. A low-cost spectrometer, featuring an MEMS device and open source hardware, was designed and manufactured using a 3D printer. Four manufactured spectrometers were installed inside and outside of a tomato canopy in an experimental greenhouse, and were tested to determine their usability for continuous measurements and spectroscopic