## 2 Main Factors Affecting Spectral Reflection Characteristics of Leaves  
Plant leaves are the places where visible light is absorbed and photosynthesis occurs. When radiation is projected onto the blade surface, a part of it is diffusely reflected by the interface, while the remaining part enters the blade. The interior of leaves is a cell stack structure, and radiation entering leaves will be reflected and refracted within this structure [22]. The optical properties of plants mainly depend on the pigments, dry matter, and water in leaves, which will absorb incident radiation [23]. When radiation is transmitted to the abaxial surface, a part of it will be transmitted out of the blade, and the remaining part will be reflected into the blade again, where refraction and absorption occur. According to the law of energy conservation, the sum of reflected, absorbed, and transmitted energy is equal to the incident radiation energy. A spectrophotometer can be used to test the solar spectral reflectance and transmittance of leaves. Figure 2 shows the solar reflectance and transmittance of leaves [24]. Clearly, the transmittance and reflection curves of leaves both are characteristic of green peaks (550 nm), red edges (680-720 nm), near-infrared plateaus (800-1300 nm), and water absorption valleys (1450-1940 nm). The green reflection near 550 nm is associated with chlorophyll. Chlorophyll can be divided into chlorophyll a, which mainly absorbs red-orange light, and chlorophyll b, which mainly absorbs blue-violet light. Chlorophyll absorbs less green light, so the solar spectrum reflection curve of leaves is characteristic of a green reflection peak [25]. The red edge is located near 700 nm. In this band, the absorption of incident radiation by chlorophyll gradually weakens, while the scattering characteristics of the cell stack structure slowly increase, so the reflectivity of leaves rises significantly [26]. The near-infrared plateau is also associated with the stacking structure of cells in leaves. The collimated incident radiation changes into almost isotropically uniform diffuse light through scattering in leaves, forming a highly reflective area. The water absorption valley located near 1450 and 1940 nm is related to the characteristic absorption of incident radiation by water in leaves [27]. The reflection peak of the green band near 550 nm, the red edge between 680 and 750 nm, the near-infrared plateau between 800 and 1300 nm, and the moisture absorption band after 1300 nm are common on the reflection spectra of all healthy green vegetation. As the reflectance spectral curves of different angiosperm leaves exhibit consistent shapes, peak positions, and key turning points, the vegetation reflectance spectra formed by combining the reflectance spectra of various plants are also very similar. However, because the spectral reflectance values of various plant leaves vary, there are differences in spectral reflectance among different environments, regions, seasons, and types of vegetation. Though the growth and development of vegetation, type of vegetation, irrigation, fertilization, climate, soil, topography, and other factors affect the spectral characteristics of vegetation, they are also expressed through the influence of three internal factors: pigments, structure, and moisture.