Masthead
Header

Atmospheric Absorption & Transmission

Introduction

Sun

The Earth is constantly bombarded with electromagnetic radiation, but the atmosphere protects us from the exposure to higher energy waves that can be harmful to life – i.e. X-Ray and Gamma Rays. The Earth's atmosphere reflects, absorbs and scatters electromagnetic radiation. This has significant implications to remote sensing, as most radiation detected by passive remote sensors passes through the atmosphere where it interacts with the molecules and particles in the atmosphere. In portions of the electromagnetic spectrum, significant amounts of energy are absorbed by the Earth's atmosphere, with very little of the energy reaching the Earth's surface. All radiation detected by remote sensors passes through the atmosphere for some distance, this distance is known as path length. The path length can vary, it could be a short path (i.e. hand-held camera) or a very long path (space based sensors).

Absorption

A portion of the incoming solar radiation is absorbed by gases in the Earth's atmosphere. These gases absorb electromagnetic energy at certain wavelengths, therefore in certain portions of the electromagnetic spectrum very little energy is absorbed (for example the visible) while in other portions like the Ultraviolet, nearly all incoming energy is absorbed. The portions of the spectrum that are absorbed by atmospheric gases are known as absorption bands.

Atmospheric Absorption

The primary gases that are responsible for the majority of the atmospheric absorption of energy are water vapor, carbon dioxide, and ozone.

Transmission

In contrast to the absorption, transmission is when electromagnetic energy is able to pass through the atmosphere and reach the Earth's surface. Visible light, largely passes (is transmitted) through the atmosphere.

Transmission The above graph shows the transmittance of electromagnetic radiation across the spectrum. Values close to 1, represent 100% transmittance, indicating the all radiation is able to pass through the atmosphere at the given wavelength. Conversely, values close to 0 indicate that all radiation is blocked and no radiation is able to pass through the atmosphere at the given wavelength.

Some types of electromagnetic radiation easily pass through the atmosphere, while other types do not. The ability of the atmosphere to allow radiation to pass through it is referred to as its transmissivity, and varies with the wavelength of the radiation. The gases that comprise our atmosphere absorb radiation in certain wavelengths while allowing radiation with differing wavelengths to pass through. In contrast to the absorption bands, there are areas of the electromagnetic spectrum where the atmosphere is essentially transparent (with minimal to no absorption of radiation) to specific wavelengths. These regions or wavelengths are known as "atmospheric windows" since they allow the radiation to pass through the atmosphere to Earth's surface. For example visible light and radio waves can pass relatively freely through the atmosphere, while X-Rays can not.

It is always important to keep atmospheric windows in mind when designing and selecting senors for remote sensing applications. Most remote sensing instruments on aircraft or satellite platforms operate in one or more of these windows.

← Back

Next →

Module Home