Electromagnetic Spectrum

Electromagnetic Spectrum

EMR is a form of energy exhibiting wave-like behaviour as it travels through space. EMR ranges from very high energy radiation such as gamma rays and X rays through ultraviolet light, visible light, infrared radiation and microwaves to radio waves. 

The range of frequencies of EMR is known as the electromagnetic spectrum.

The Sun produces a continuous spectrum of energy from gamma rays to radio waves that continually bathe the Earth in energy.

The visible portion of the spectrum may be measured using wavelength (measured in mm or nm) or electron volts (eV) - All units are interchangeable.

Classification of Electromagnetic Radiation

Infrared radiation (750 nm - 1 mm)
The infrared region can be divided into two categories based on their radiation properties - the reflected IR, and the emitted or thermal IR. The reflected IR covers wavelengths from approximately 0.7 micrometres to 3.0 micrometres. The thermal IR covers wavelengths from approximately 3.0 micrometres to 100 micrometres.

 Far-infrared (1 mm – 10 μm):

The lower part of this range may also be called microwaves or terahertz waves. The water in Earth's atmosphere absorbs so strongly in this range that it renders the atmosphere in effect opaque. However, there are certain wavelength ranges (windows) within the opaque range that allow partial transmission and can be used for astronomy.

 Mid-infrared (10 –2.5 μm):

Hot objects (black-body radiators) can radiate strongly in this range, and human skin at normal body temperature radiates strongly at the lower end of this region.

 Near-infrared (2,500–750 nm):

Physical processes that are relevant for this range are similar to those for visible light. The highest frequencies in this region can be detected directly by some types of photographic film, and by many types of solid-state image sensors for infrared photography and videography.

Thermal IR Remote Sensing:

Thermal infrared radiation refers to electromagnetic waves with a wavelength of between 3 and 20 micrometres. Most remote sensing applications make use of the 3 - 5 and 8 – 14 nm range (due to absorption bands). The main difference between thermal infrared and near infrared is that thermal infrared is emitted energy, whereas the near infrared is reflected energy, similar to visible light.

Microwave region about (1 mm to 1 m):

Longer wavelength microwave radiation can penetrate through the cloud, fog, haze etc as the longer wavelengths are not susceptible to atmospheric scattering which affects shorter optical wavelengths Active Microwave has Night Vision Capability.

Visible Light: 

It is the part of the EM spectrum the human eye is the most sensitive to. This narrow band of electromagnetic radiation extends from about 400 nm (violet) to about 700 nm (red) above infrared. 

Red: 610 - 700 nm

Orange: 590 - 610 nm

Yellow: 570 - 590 nm

Green: 500 - 570 nm

Blue: 450 - 500 nm
Indigo: 430 - 450 nm
Violet: 400 - 430 nm
Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another. This action allows the chemical mechanisms that underlie human vision and plant photosynthesis.

Ultraviolet radiation

Next in frequency comes ultraviolet (UV) rays, which are shorter than the violet end of the visible spectrum but longer than the X-ray.

Visible Blue Band (0.45-0.52 Micrometers)

Greatest water penetration

Greatest atmospheric scattering

Greatest absorption

Used for: water depth water characteristics detection of subsurface features soil and vegetation discrimination.

Visible Green Band (0.52-0.60 Micrometers)

Vegetation discrimination

Urban Infrastructure

Less affected by atmospheric scattering

Chlorophyll Concentration 

Visible Red Band (0.63-0.69 Micrometers)
Chlorophyll absorption band of healthy green vegetation.
Vegetation type
Plant condition
Least affected by atmospheric scattering
Less water penetration but good near surface information i.e. Water quality, sediment, and chlorophyll.