# Spectral bands and the EM spectrum

Sensors receive energy derived from the sun that has been reflected off the earth surface. The energy is collected at the sensor is recorded in discreet sections of the electro-magnetic (EM) spectrum. These are known as spectral bands. 

In remote sensing and earth observation, these bands are measured as a range of wavelengths in nanometres or occasionally micrometres.  

For sensors collecting in the visible and near-infrared parts of the spectrum, these bands will related to the colours, red, green and blue, as well as the slightly longer wavelengths in near-infrared. It may also collect other sections of the spectrum.

Different sensors have different definitions for their spectral bands, and they do not overlap exactly. The image below illustrates how several sensor bands for specific parts of the spectrum do not cover exactly the same range of wavelength. 

![Credit: Susan Ustin, 2021 Current and near-term advances in Earth observation for ecological applications](/files/gNSU9bdh2JBQ7DITNGRa)

It is important to be aware of this when comparing results of spectral analysis from different sensors. 

It is generally possible to find easily find information online on the spectral band definition of a sensor. For example, [this page](https://www.usgs.gov/media/images/landsat-8-band-designations) details the band definition for Landsat 8.


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