# Making Sense of Light: The Use of Optical Spectroscopy Techniques in Plant Sciences and Agriculture
## 5 Emerging Technologies for Portable Spectroscopy
In practice, this means that the DMD focuses successively on the different combinations of spectral bands emerging from the diffraction grating of the photodiode. Knowing the distribution of the bands in each combination (mask), it is possible to invert the data in order to resolve the spectrum. The DLP NIRscan (Texas Instruments, Sherman, TX, USA, [[https://www.ti.com/tool/DLPNIRSCANEVM](https://www.ti.com/tool/DLPNIRSCANEVM)]([https://www.ti.com/tool/DLPNIRSCANEVM](https://www.ti.com/tool/DLPNIRSCANEVM)); accessed on 10 October 2021) is based on this solution. It offers a resolution of 10 nm in a spectral range of 900-1700 nm. Replacing the array sensor with a single photodiode, the DMD-based spectrometer has a lower cost while yielding a good signal-to-noise ratio. In Figure 2e light selection is performed through a Fabry-Perot interferometer or etalon (9), which is an optical cavity made from two parallel reflecting surfaces. Light bounces back and forth, creating a superposition of waves that is destructive for all wavelengths, except for those resonant with the cavity, that is, when the etalon spacing is a half integer of the wavelength. The resonant wavelengths are transmitted, while the others are strongly attenuated. Tuning the filtered wavelength is just a matter of changing the etalon spacing. In this case, this is achieved by coupling one of the mirrors to a MEMS. A photodiode (8) aligned with the optical axis of the etalon performs the light detection in synchrony with the MEMS cycle, in a scan that originates the full spectrum. Etalon spectrometers such as the NIRONE (Spectral Engines, Helsinki, Finland, [[https://www.spectralengines.com/](https://www.spectralengines.com/)]([https://www.spectralengines.com/](https://www.spectralengines.com/)); accessed on 10 October 2021) or the MEMS-FPI (Hamamatsu, Japan, [[https://www.hamamatsu.com/eu/en/product/optical-sensors/](https://www.hamamatsu.com/eu/en/product/optical-sensors/)]([https://www.hamamatsu.com/eu/en/product/optical-sensors/](https://www.hamamatsu.com/eu/en/product/optical-sensors/))  
Figure 2: Fundamental schematics of major emerging technologies: (**a**) conventional, with slit (1), diffraction grating (2) and array sensor (3); (**b**) filter array (5) upon a CMOS photographic sensor (4); (**c**) linear variable filter (LVF) (6) and array sensor (3); (**d**) digital micromirror device (7), with slit (1), diffraction grating (2) and photodiode (8); (**e**) coupled micro-electro-mechanical systems (MEMS)-based Fabry–Perot etalon (9) and photodiode (8); (**f**) MEMS-based Michelson interferometer, constituted by a fixed mirror (10), beam-splitter (12), photodiode (8) and movable mirror (11) moved by a MEMS (13). spectrometers/mems-fpi-spectrum-sensor/index.html; accessed on 10 October 2021) are based on this principle. They feature a good throughput and signal-to-noise ratio at an affordable price. However, they have relatively narrow spectral ranges (typically 300 nm). In Figure 2f, the principle of measurement is that of Fourier Transform Infrared Spectroscopy (FTIR) [124]. It uses a MEMS-based Michelson interferometer, and it is constituted by a fixed mirror (10), beam-splitter (12), photodiode (8) and movable mirror (11). The novelty is that the movable mirror is acted on by a MEMS and all the optics are implemented in a small wafer. This technology allows bringing the power of FTIR to portable applications. In principle, they benefit also from the so-called Jacquinot's advantage. This concept is usually mentioned in the context of comparing FTIR spectroscopy and conventional dispersive spectroscopy. To understand it, it is necessary to remember that the spectral resolution in a dispersive spectrometer depends inversely on the width of the input slit. Thus, to obtain a high resolution, it is necessary to use a very narrow slit, which limits the amount of light that enters the device and, ultimately, the signal-to-noise ratio. There is, therefore, a tradeoff between the resolution and the signal-to-noise ratio in a dispersive spectrometer.