Reflection and Absorption
Paths of light
Reflection means that something is thrown back. If, for example, a ball is thrown against a wall and it comes back, one could say that it has been reflected. The same happens with sunlight meeting the surface of the earth: It is being reflected and can thus be collected by a remote sensing sensor. If the light makes contact with a smooth surface, then the angle of incidence equals the angle of reflection.
If light makes contact with a body, it doesn’t mean it is being reflected in any case. It can pass through the body (transmission) or can be refracted (refraction). In case of refraction, the light passes through the body but changes its propagation direction. However, the most important interactions regarding the principles of remote sensing are reflection and absorption.
Types of reflection
There are three types of light reflections. Note that the reflection depends on the surface roughness of an object.
- Specular (mirror-like) reflection: The light ray meets a smooth surface and the angle of incidence is identical with the angle of reflection.
- Diffuse reflection: The light ray meets a rough surface and is equally reflected in all directions.
- Mixed reflection: The light ray meets a very rough surface and is unequally reflected in all directions. This type of reflection is the most common one in nature!
Absorption and Albedo
Surfaces do not only reflect light; they also (partially) absorb it. In the process of absorption, energy is absorbed by the molecules of a given body and is then transformed into kinetic energy. The rise in movement of the molecules produces heat which is radiated to the surroundings.
The principles of absorption can be transferred to daily life: A black t-shirt absorbs more sunlight than a white one. This is the reason why we are sweating more wearing a black shirt in summer. The albedo of a body is crucial for the percentage of absorbed sunlight. It measures the degree of reflection of materials in different spectral ranges. An albedo of 100% indicates than no absorption takes place; accordingly, an albedo of 0% stands for no reflection. The table below lists values of albedo for different materials in the visible range of light.
As visualized in the animation from ESA below, you can see the varying surface reflectance, which is mainly caused by seasonal effects of disappearing snow cover.
|Water (low angle)||22%|
|Water (high angle)||5%|
Behaviour of real life ‘targets’
In previous topic we also about how the Earth’s atmosphere impacts the radiation and your also learned which kind of paths the light can take on its course. Now, we will take a look at how the radiation interacts with objects, or, as we say in remote sensing perspective, ‘targets’, to gain better understanding of what is actually being interpreted in the imagery. Please note that the interactions described below are mostly referring to the scenarios where a passive sensor is used, driven by sunlight.
Paved and smooth surfaces
Built-up areas or paved roads are a good example for specular reflection behaviour. Another example could be a still water surface or glass/metal objects. Seen as a smooth surface, this area will reflect most of the energy away from its illumination source with a reflection angle that is equal to the incidence angle.
With a “rougher” surface comes a more diverse reflection scheme. As a rule of thumb diffuse reflection will dominate, if the wavelength is significantly smaller than the object it hits. As given on the right, a tree is subject to different reflection processes. If microwaves of certain wavelengths (> 10 cm) they will mainly interact with the tree trunk, as the waves are to big to even recognize the leaves and smaller branches. However, the visible and near-infrared portion of the light heavily interact with the leaves as well.
Taking a closer look we can also explain, why leaves appear green to the human eye. Chlorophyll strongly absorbs EM radiation in the red and blue, but reflects the green wavelength. In remote sensing near-infrared channels are well known for the potential to monitor plant vitality, because the cell structure of healthy vegetation are well functioning diffuse reflectors.
Water absorbs most longer wavelengths and reflects the shorter ones. The appearance of water surfaces/body strongly varies with the amount of particles that are present. A very sad but famous case to observe changes in the reflection can be seen in the shrinking Aral sea. When water levels were high, most of the short radiation was absorbed but as the levels decreased and the water got more and more shallow, the reflection increased again. The greenish tone of the water originates from algae and the chlorophyll it contains.