# Application of Reflectance Indices for Remote Sensing of Plants and Revealing Actions of Stressors
## 2 Optical Methods and Platforms of Remote Sensing of Plants
### Multi- and Hyperspectral Imaging  
The light interacts with all the environmental objects; the interactions include light absorption, reflectance, and scattering, which are dependent on the optical characteristics of the material of these objects. Measurements of the intensities of reflected light at narrow spectral bands are an effective approach of plant remote sensing that can be used for the estimation of the growth and development of plants [10], investigation of their physiological processes and biochemical compositions [12; 13], analysis of water exchange [45], and others. Changes in the reflectance spectra of plants can be detected by hyper- and multispectral cameras, which are tools for hyperspectral and multispectral imaging, respectively. In accordance with [PERSON] [44] and [PERSON] et al. [22], a camera measuring more than 20 wavelengths can be classified as a hyperspectral camera; in contrast, a camera measuring less than 20 wavelengths is a multispectral camera. Hyperspectral data can be presented as a set of frames (\"hypercube\"), where each frame shows the spatial reflectance distribution at a specific wavelength; as a result, the whole spectrum of the reflected light in each point of the investigated object can be analyzed. In contrast, when using a multispectral camera, the information is strongly minimized because images are measured at only several specific wavelengths. The hyperspectral cameras seem to be more interesting for application by researchers because they can be used for the solution of a broad range of tasks in the fields of the plant sciences and plant remote sensing. A simple variant in the architecture of the hyperspectral camera can include the camera lens, prism (or diffraction grating) for the division of light with different wavelengths, and matrix for the registration of the light signal (Figure 3a). This architecture is used in the \"push broom\" camera (Figure 3b). These sensors scan the surface line by line and simultaneously measure the light spectra in each point of the current line; an image is formed from these lines [22; 44; 46]. This type of sensor is widely used in plant remote sensing; particularly, flying mobile platforms of remote sensing, including copters, drones, airplanes, and satellites, are mainly equipped with these sensors. Figure 3: Some variants of hyper- and multispectral cameras. **(a)** The simple schema of the hyperspectral camera with line-by-line scanning. **(b)** The schema of the line-by-line scanning (push broom). **(c)** The schema of the point-by-point scanning (whisk broom). **(d)** The schema of scanning of the whole hypercube (snapshot). **(e)** Multispectral camera with several elementary cameras equipped by narrow band spectral filters. **(f)** Multispectral camera with single camera and wheel of narrow band spectral filters. Another variant of sensor, namely \"whisk broom\", scans point by point [22; 44]. The rotating mirror scans the surface; further, light is transmitted through the prism, and the spectrum of each point is registered [46]. The result of the hyperspectral measurement is the hypercube, which includes a set of images with reflectance at specific spectral bands. It should be noted that the possibility of using this method on the basis of the mobile platforms is rather limited. The more complicated variant of sensors is the \"snapshot\" technique, including a group of alternative methods of remote sensing that can scan the whole hypercube in a small time (Figure 3d) and, therefore, have an increased speed of measurement [47; 48]. The snapshot imaging is based on different technical solutions, which can be complicated (e.g., see review [47] for details). Potentially, snapshot imaging seems to be very promising for plant remote sensing and can compete with push and whisk broom cameras in the future. The detectors of hyperspectral cameras with a spectral range from 300 nm to 1000 nm are two-dimensional matrices of photosensors performed on the basis of silicon.