31/07/2021
How does remote sensing work?
Our eyes see one part of sunlight (visible light), and we distinguish objects by the visible light they reflect. There are other parts of sunlight that we cannot detect with our eyes, for example infrared radiation. In this sense, and this is important for diagnostic purposes, the amount of reflected light is different for different surfaces, and although plants of different species and growing in different conditions reflect light differently, there are some characteristics of sunlight reflection that are common to all plants. For example, vegetation reflects about 5-15% of the visible light it receives and 40-80% of the infrared radiation, while bare soil devoid of vegetation reflects about 15-25% of the visible light it receives and 30-35% of the infrared radiation (Figure 1). This ratio of reflected light to the amount of incident light is known as reflectance.
In more detail, within visible light, vegetation shows a maximum reflectance of green light, i.e. vegetation reflects green light in greater proportion than the portion of light of other colors, and that is why the human eye sees vegetation as green. Conversely, the reflectance of red and blue light is minimal, because the plant (chlorophyll, more specifically) absorbs blue and red light to carry out photosynthesis. Finally, vegetation reflects a large portion of infrared light. The reflectance of infrared light increases with the amount of vegetation, and the plant architecture and water status of the vegetation also influence the reflectance of a crop.
The light reflected by an object can be measured and analyzed by a sensor, such as a conventional camera (for visible light) or an infrared camera. Earth observation satellites are equipped with visible light and infrared light sensors, among others. The reflectance measurements are processed for representation as an image of the surface.