# Making Sense of Light: The Use of Optical Spectroscopy Techniques in Plant Sciences and Agriculture
## 3 Main Techniques
### Elastic Spectroscopy  
Elastic scattering represents the simplest and most efficient interaction between photons and matter. Thus, elastic spectroscopy has found numerous applications in various domains of biology [24]. Essentially, this methodology examines the spectrum of photons scattered elastically, that is, without changing the energy and wavelength, reflected from the sample under study (Figure 1). The incident and reflected spectra differ because part of the photons is extinguished in the path between source and detector. Photon extinction is due to wavelength-dependent absorption and scattering. The light input is obtained from an illuminator that emits a continuous or line spectrum in the range of interest. The active setups use broadband light sources based on flames, high-temperature filaments (e.g., tungsten lightbulbs), discharges (arc lamps) or tunable (dye, semiconductor, free-electron, etc.) lasers. The passive setups use external broadband light sources, primarily solar radiation. Usually, active setups are chosen for the in situ and/or \"point\" spectral measurements on both plants and fruit, where a specific site is targeted, producing an average spectrum for that site only. Otherwise, multi- and hyperspectral (MHS) imaging, applied to plant or fruit sections, may use both active (indoors) and passive (outdoors) setups. MHS produces a \"cube\" of data, the three dimensions corresponding to the 2D pixel matrix times the 1D spectral component. This cube may be sectioned along a plane of constant wavelength to create an image of the measured areas/sections at that wavelength band or along constant x or y to create a spatially resolved reflectance map [26]. The main difference between multi- and hyperspectral modes is the number of wavebands used and how narrow the bands are [27]. Multispectral imaging uses a set of filters and a common digital camera to deliver typically no more than ten bands, while hyperspectral cameras merge imaging and spectral separation in the optical hardware to produce hundreds of contiguous wavebands [26]. The light-source-free spectral imagery equipment is efficient and lightweight; therefore, it has found wide application in the air- and spaceborne diagnostics of individual plants and vegetation landscapes [28; 29]. Among the various elastic spectroscopy techniques used in plant sciences and in precision agriculture, for \"point\" spectral measurements and MHS imaging, either in situ or remotely, visible-near infrared reflectance spectroscopy (Vis-NIRS) has undoubtedly a prominent and versatile role in the assessment of whole plants, fruit, field crops, orchards and forestry [1; 26; 30]. Vis-NIRS covers the electromagnetic spectrum wavelength range of 400-2500 nm, although the upper limit of 2500 nm is not consensual among authors. The choice made here is the most common, and one of the reasons to adopt it is that it coincides with the detection limit of the InGaAs detectors equipping the standard NIR spectrometers. The NIR radiation was discovered by [PERSON] in 1800 and was first used in agricultural applications to measure the moisture in grain in the late 1960s [31]. The first Vis-NIRS application was commercialized in Japan in 1989 to sort peaches based on their soluble solids content (SSC) in an automated grading line. However, the research on its principles, applications and on the development of new customized systems have only followed some decades later, being a flourishing area nowadays [10; 11; 32]. Overall, devices and systems have been developed and used to produce a large bulk of significant and cutting-edge knowledge, which has been published throughout several decades [32]. Furthermore, there are more and more commercial applications available by industrial vendors, which are used on a daily basis, particularly in agriculture and the agro-food sector [10; 11; 33; 34]. Figure 1: The basics and principles of different spectroscopic methods. The red circles represent a molecule generically.