*2.4. Close-Range (Ground- or Lab-Based) Hyperspectral Imaging*

Close-range hyperspectral imaging, including ground (Figure 4a–c) or lab based (Figure 4d,e), is an emerging technology in recent years, and it is capable of acquiring super-high-spatial-resolution (e.g., cm or sub-cm level) hyperspectral imagery [137–139]. Therefore, this imaging technology can be used for investigating fine-scale (e.g., leaf and canopy level) vegetation features and thus greatly support the investigation of crop growing status and detection of early signs of crop stress (e.g., disease, weeds, or nutrition deficiency). Sensors are mounted on moving or static platforms (e.g., linear stages, scaffolds, or trucks) that can be deployed indoors or outdoors for collecting images. Lamps (e.g., halogen lamp) or the sun are used as light sources in these platforms, respectively.

Researchers have utilized different types of platforms and hyperspectral sensors for collecting super-high-spatial-resolution hyperspectral imagery to study different agricultural features, as shown in Table 4.




**Table 4.** *Cont.*

Overall, the close-range hyperspectral imaging platform is capable of acquiring super-highspatial-resolution hyperspectral imagery that is critical for investigating fine-scale crop or soil features. These features provide detailed information about the plant's biophysical and biochemical processes and how plants respond to environmental stresses and diseases. However, the image collection and processing also suffer from different issues, such as uninformative variability caused by the interaction of light with the plant structure (i.e., illumination effects), influences of shadows, and expanding applications of the platform to a large scale [141,146]. Further research in these areas is warranted.

**Figure 4.** Close-range imaging platforms used in previous studies. Figures were reproduced with permission from corresponding publishers: (**a**) American Society for Photogrammetry and Remote Sensing (ASPRS), Bethesda, Maryland, asprs.org [139]; (**b**) SPIE [148]; (**c**) Elsevier [138]; (**d**) Springer Nature [144]; (**e**) Elsevier [149].

In summary, different hyperspectral imaging platforms, including satellites, airplanes, helicopters, UAVs, and close-range, have different advantages and disadvantages for applications in precision agriculture. Detailed comparisons of these platforms for agricultural applications are shown in Table 5. In brief, satellite-based systems provide images covering large areas but suffer from medium spatial resolution and limited data availability (e.g., a limited number of operating sensors and long

revisit time). Airplane- and helicopter-based imaging platforms acquire data with suitable spatial coverage and resolution for most of the agricultural applications. However, they are limited by a high mission cost and scheduling challenges and thus are not suitable for repeated monitoring. UAV-based systems are capable of acquiring high-spatial resolution images repeatedly and have high flexibility. However, they can only cover a small area due to the limited battery endurance and aviation regulations. The close-range imaging systems are capable of obtaining super-high-spatial-resolution images, but they can only be used at leaf or canopy levels. Therefore, the following factors should be taken into consideration when selecting a platform for a specific research project: spatial resolution needed for the study, flight area and flight endurance, weight of the imaging system, platform payload capacity, flight safety and regulations, operation flexibility, and cost.


**Table 5.** Comparison of hyperspectral imaging platforms.

\* The number of publications was counted based on which specific platform was used in each of the literature reviewed.
