Electromagnetic radiation

Geospatial Data Acquisition (GDA) challenges us to make choices: on which one of the many sensors available should the agronomist rely for accurate yield predictions? If he or she chooses a sensor producing several images, such as a multispectral scanner, which image or which combination of images to use? How to properly process sensor recordings to increase the chances of a correct interpretation? When interpreting a colour image, what causes the sensation red? Instead of writing a thick book of recipes to answer such questions for every application, we can better review the physics of RS. Understanding the basics of electromagnetic (EM) radiation will help you in making more profound choices and enable you to deal with sensors of the future.

A standard photograph is an image of an object or scene that very closely resembles direct sensing with our eyes. The sensation of colour is caused by EM radiation. Red, green and blue relate to forms of radiation that we commonly refer to as light. Light is EM radiation that is visible to the human eye. As we are interested in Earth Observation, our light source is the Sun. The Sun emits light, the Earth’s surface features reflect light, and the photosensitive cells (cones and rods) in our eyes detect light. When we look at a photograph, it is the light reflected from the photograph that allows us to interpret the photograph. Light is not the only form of radiation from the Sun and other bodies. The sensation warm, for example, is is the result of thermal emissions. Another type of emissions, ultraviolet (UV) radiation, triggers our body to generate vitamin D and also produces a suntan.

It is important to understand the basic characteristics of EM radiation, its sources and what we call the EM spectrum, the influence of the atmosphere on EM radiation, interactions of EM radiation with the Earth’s surface, and the basic principles of sensing EM radiation and generic properties of sensors.