In remote sensing process, electromagnetic energy is subjected to various interactions with earth as well as atmospheric features. In this article, we will learn about how electromagnetic energy (EMR) interacts with earth surface features. Understanding the EMR interaction with earth surface features is very important in various remote sensing studies.
Types of Interaction Between Earth Surface Features and EMR:
The type of interaction happening between earth surface objects and EMR varies depending on nature of surface of the objects, wavelength of energy and object characteristics. Following are Different types of interactions that primarily take place between earth surface objects and EMR.
As per the principle of conservation of energy, E (i) = E (r) + E (a) + E (t), Where E (i) = incident energy, E (r) = reflected energy, E (a) = absorbed energy and E (t) = Transmitted energy.
Two important things associated with these interactions are as follows. Firstly, the amount energy reflected, absorbed and transmitted is different for different features. Again, for a same object the proportions of these energies may change based on object condition. For example, water absorbs most of the energy, while same water in solid form such as ice reflects most of the energy.
Secondly, the interactions vary with wavelength of incident energy for a given feature. This means, an object may be a good reflector in the green (0.6-0.7 Micrometers) region, but the same object can be a poor reflector in the infrared region.
These 2 characteristics play very important role in studying the properties of objects using remote sensing.
Reflection can be defined as a process in which incident energy is reverted back into the atmosphere in such a way, where angle of incidence is angle of reflection.
Scattering is a special case of reflection, where energy is diffused into multiple directions after hitting the object.
The electromagnetic energy incident on the surface can be reflected or scattered depending on the roughness of the surface. If roughness of the surface is less than wavelength of the radiation, the radiation gets reflected otherwise it gets scattered.
Reflection is very important phenomena in remote sensing, as this is the portion of energy which is sensed by the sensors for imaging the objects. Each object reflects energy uniquely over the electromagnetic spectrum. A graph which shows the reflectance of an object over various wavelengths of electromagnetic spectrum is called spectral reflectance curve. These curves are used for distinguishing between different objects.
Reflectance can be defined as the percentage of the ratio of energy reflected to energy incident on given surface. Geometry of reflection changes with wavelength of radiation, angle of incidence, composition and roughness nature of object surface. Based on the geometry, reflection is classified as follows.
This is also called as mirror type reflection. This type of reflection takes place on smooth and flat surfaces only. The angle of reflection is equal to angle of incidence. Maximum amount of energy is reflected only in one direction. Specular reflection doesn’t have much significance in remote sensing as detection of energy from objects becomes very difficult for the sensor.
This is also called as lambertian reflection. Energy reflected uniformly into all directions. Diffused reflection is very important in remote sensing, as the energy propagating in multiple directions contain information regarding the objects. Further, diffused reflection aids in placing the sensor at any location as energy is moving in all directions.
If wavelength of the energy is smaller than the surface roughness or particle size of surface diffused reflection will take place. Otherwise, specular reflection dominates the reflection phenomena. In reality, neither perfect specular reflection or perfect diffused reflection happens as most of the objects in the nature deviate from the ideal conditions required for such reflections.
In real world the reflections that happen can be considered as near specular and near diffused reflections only.
Absorption is the process where a portion of incident energy is retained (absorbed) by the object. It can be defined as the percentage of ratio between absorbed to incident energies over an object surface at a given wavelength region. This absorbed energy is stored by the earth surface objects and released later in the form infrared radiation at longer wavelengths.
This is the process where a portion of incident energy is transferred into other objects. This transferred energy may be stored by the other objects or it may be converted into other forms of energy. Transmittance can be defined as percentage of ratio of the transmitted energy to incident energy.
You may read the following resources also for understanding this concept: