EM spectrum

We call the total range of wavelengths of Electro-magnetic (EM) radiation the Electro-magnetic spectrum. Figure 1 illustrates the wide range of EM spectrum. We refer to the different portions of the spectrum by name: gamma rays, X-rays, UV radiation, visible radiation (light), infrared radiation, microwaves, and radio waves. Each of these named portions represents a range of wavelengths, not one specific wavelength. The EM spectrum is continuous and does not have any clear-cut class boundaries.

Different portions of the spectrum have differing relevance for Earth Observation, both in the type of information that we can gather and the volume of geospatial data acquisition (GDA). The majority of GDA is accomplished by sensing in the visible and infrared range. The ultra-violet (UV) portion covers the shortest wavelengths that are of practical use for Earth Observation. UV radiation can reveal some properties of minerals and the atmosphere. Microwaves are at the other end of the useful range for Earth Observation; they can, among other things, provide information about surface roughness and the moisture content of soils.

The “visible portion” of the spectrum, with wavelengths producing colour, is only a very small fraction of the entire EM wavelength range. We call objects “green” when they reflect predominately EM radiation of wavelengths around 0.54 μm. The intensity of solar radiation has its maximum around this wavelength and the sensitivity of our eyes is peaked at green-yellow. We know that colour effects our emotions and we usually experience green sceneries as pleasant. We use colour to distinguish between objects and we can use it to estimate temperature. We also use colour to visualize EM radiation we cannot see directly. Section RGB elaborates on how we can “produce colour” by adequately “mixing” the three primary colours red, green and blue.

Radiation beyond red light, with larger wavelengths in the spectrum, is referred to as infrared (IR). We can distinguish vegetation types and the stress state of plants by analysing near-infrared (and mid-infrared) radiation—this works much better than trying to do so by colour. For example, deciduous trees reflect more near-infrared (NIR) radiation than conifers do, so they show up brighter on photographic film that is sensitive to infrared. Dense green vegetation has a high reflectance in the NIR range, which decreases with increasing damage caused by plant disease (see also Spectral signature). Mid-IR is also referred to as short-wave infrared (SWIR). SWIR sensors are used to monitor surface features at night.

Infrared radiation with a wavelength longer than 3 m is termed thermal infrared (TIR) because it produces the sensation of “heat”. Near-IR and mid-IR do not produce a sensation of something being hot. Thermal emissions of the Earth’s surface (288 K) have a peak wavelength of 10 m. A human body also emits “heat” radiation, with a maximum at λ ≈ 10 m. Thermal detectors for humans are, therefore, designed such that they are sensitive to radiation in the wavelength range 7–14 m. NOAA’s thermal scanner, with its interest in heat issuing from the Earth’s surface, detects thermal IR radiation in the range 3.5– 12.5 m. Object temperature is a kind of quantity often needed for studying a variety of environmental problems, as well as being useful for analysing the mineral composition of rocks and the evapotranspiration of vegetation.