Electromagnetic Spectrum

Electromagnetic spectrum (EMS) represents the continuum of electromagnetic radiation (EMR) arranged on the basis of wavelengths or frequency. Electromagnetic spectrum ranges from shorter wavelengths (gamma rays to x rays) to the longer wavelengths (microwave and radio waves). Most common remote sensing systems operate in one or several of the visible, infrared and microwave portions of the electromagnetic spectrum. Within the infrared portion of the spectrum it should be noted that only thermal infrared energy is directly related to the sensation of heat; not the near and mid infrared ones. Before discussing about EMS with reference to remote sensing it is important to understand it fully. 

Different radiations which constitute the EMS are as follows: 

Radio waves 

These are the longest wavelength (lowest frequency) radiations of the EMS. The wavelength of radio waves is more than 100 cm. These passes through Earth's atmosphere easily. Radio signals are used in radios, televisions, aircrafts, ship etc. These are also emitted by stars. 

Electromagnetic Spectrum 

Microwaves Their wavelength ranges between 1mm & 1m. RADAR (Radio Detection And Ranging) is the most common device used in Microwave Remote Sensing. Other applications are in cooking food (microwave oven), in broadcasting transmissions etc. 

Infrared Radiations 

When we feel hot it is because of infrared (IR) radiations. For common understanding we can call them as 'Heat'. The wavelength of IR is longer than visible light and shorter than microwaves approximately ranges between 1micron to 100 microns. These are very useful radiations for remote sensing. Thermal Imaging Systems detect objects by recording their temperature (infrared emissions). 

Visible Radiations 

As their name itself suggests these are the EMRs which are visible to our eyes in different colours. These ranges between 700nm to 400nm. Most of the remote sensing systems and camera records images in this range. 

Ultraviolet (UV) Radiations 

These radiations have wavelength shorter violet colour of visible light and longer than X-rays. UV radiations can be divided into near UV (400–200nm), far UV (200–10) and extreme UV 1–31 nm. 

X-rays 

These are very short length electromagnetic radiations wavelength in the approximate range from 0.01 to 10 nanometers. In EMS these falls between UV radiations and Gamma-rays. Mostly used in medical sciences. 

Gamma-rays Gamma-rays are the electromagnetic radiations with shortest wavelength in the range of the range of 10^-11 m to 10^-14 m. Their very high energy can cause serious damage to living cells.

Types of Remote Sensing

Based on Source of energy

The sun provides a very convenient source of energy for remote sensing. The sun’s energy is either reflected, as it is for visible wavelength or absorbed and then re-emitted (for thermal infrared wavelength).

Remote sensing systems, which measure this naturally available energy, are called passive sensors. This can only take place when the sun is illuminating the earth. There is no reflected energy available from the sun at night. Energy that is naturally emitted can be detected day and night provided that the amount of energy is large enough to be recorded.

Remote sensing systems, which provide their own source of energy for illumination, are known as active sensors. These sensors have the advantage of obtaining data any time of day or season. Solar energy and radiant heat are examples of passive energy sources. Synthetic Aperture Radar (SAR) is an example of active sensor.

Based on Range of Electromagnetic Spectrum

Optical Remote Sensing

The optical remote sensing devices operate in the visible, near infrared, middle infrared and short wave infrared portion of the electromagnetic spectrum. These devices are sensitive to the wavelengths ranging from 300 nm to 3000 nm. Most of the remote sensors record the EMR in this range, e.g., bands of IRS P6 LISS IV sensor are in optical range of EMR.

Thermal Remote Sensing

The sensors, which operate in thermal range of electromagnetic spectrum record, the energy emitted from the earth features in the wavelength range of 3000 nm to 5000 nm and 8000 nm to 14000 nm. The previous range is related to high temperature phenomenon like forest fire, and later with the general earth features having lower temperatures. Hence thermal remote sensing is very useful for fire detection and thermal pollution. e.g., the last five bands of ASTER and band 6 of Landsat ETM+ operates in thermal range.

Microwave Remote Sensing

A microwave remote sensor records the backscattered microwaves in the wavelength range of 1 mm to 1 m of electromagnetic spectrum. Most of the microwave sensors are active sensors, having there own sources of energy, e.g, RADARSAT.

These sensors have edge over other type of sensors, as these are independent of weather and solar radiations.