# Making Sense of Light: The Use of Optical Spectroscopy Techniques in Plant Sciences and Agriculture
## 5 Emerging Technologies for Portable Spectroscopy  
The massive adoption of portable spectrometers, observed in the last twenty years, was achieved thanks to small spectrometers with USB connection, which allowed the instruments to be reduced to the size of a cards deck. These spectrometers reproduce, on a small scale, the optics of bench instruments and generally include mirrors for collimation and focusing, a diffraction grating and an array sensor, usually a charge couple device (CCD) or a complementary metal oxide semiconductor (CMOS). This geometry, however, needs some spacing to allow the light to separate properly, which places limits on miniaturization. On the other hand, the quantity of optical components required also limits the price, and a high-quality array sensor weighs heavily on the final cost of the spectrometer. In recent years, the miniaturization of spectrometers has continued to advance towards reducing size and weight. New technological approaches played a crucial role, which allowed to overcome the limitations mentioned above. Thus, there are currently available alternatives for light separation that eliminate the need for a diffraction grating, saving space. On the other hand, there are also alternative schemes that use just a simple photodiode instead of a CMOS array, lowering the price of the spectrometer. There are still alternatives that combine the two advantages. Obviously, this simplification does not come without a compromise in terms of quality, signal-to-noise ratio and sensitivity, which will be explored further below. Figure 2 shows the fundamental concepts associated with the main emerging technologies. It is important to note that these are simplified schemes. In all cases, components are omitted, so that the figure retains only the fundamental process associated with each technique. In Figure 2a the conventional setup is shown. At its core, there is a diffraction grating (2) and an array sensor (3). Light is dispersed by the grating and focused on the sensor array (focusing optics not represented). Each sensor pixel receives light in a narrow band determined by the entrance slit (1), the grating characteristics and geometrical factors through appropriate algorithms. However, this approach does not offer the best spectral resolution. The SCo spectrometer (Israel, www.consumerphysics.com/scio-for-consumers/; accessed on 10 October 2021) uses this type of technology, working in the range 740-1070 nm. In Figure 2b, a filter array (5) upon a CMOS photographic sensor (4) is one of the conceptually simpler approaches to obtain a spectrum from non-dispersive elements. The filter array may be composed of Fabry-Perot elements with tailored characteristics. Knowing the transmission curve of each filter allows retrieving more bands than the number of filters. In Figure 2c, a linear variable filter (LFV) (6), which is a wedged filter, and whose spectral properties vary linearly, allows to separate light as a function of the position. An array sensor (3) placed immediately after the filter acquires the spectral information. This technique offers the advantage of a higher light throughput, while offering at the same time a compact arrangement. The MicroNIR spectrometer (Viavi, San Jose, CA, USA, www.viavisolutions.com/en-us/osp/products/micronir-spectrometers;accessed on 10 October 2021) is based in this solution, with a resolution of 6 nm in the range 900-1700 nm. In Figure 2d, a digital micromirror device (DMD) (7) is at the core of the system. It receives light already sent through a slit (1) and separated in a diffraction grating (2) and redirects it into a single photodiode (8). The DMD consists of hundreds or thousands of micromirrors acted on by coupled micro-electro-mechanical systems (MEMS). The purpose of the DMD is to implement an Hadamard spectrometer, which is a multiplex device that observes more than one wavelength at a time using (Hadamard) masks.