Electromagnetic radiation and the atmosphere
Chapter 1 of Remote sensing.
Electromagnetic waves
Remote sensing needs energy that the target reflects or emits: electromagnetic radiation. It consists of an electrical field, which varies in strength perpendicular to the direction the radiation travels, and a magnetic field at right angles to the electrical field. Both travel at the speed of light, c = 2.998 × 10⁸ m/s.
Two properties matter most for remote sensing: the wavelength λ (in metres) and the frequency f (cycles per second). They are related by
c = f × λ
so a shorter wavelength means a higher frequency. The frequency depends on the source, from the low frequencies of power lines to the very high frequencies of gamma rays from atomic nuclei. The whole range is the electromagnetic spectrum.
The electromagnetic spectrum
The spectrum is divided into regions by wavelength. The boundaries are approximate, and neighbouring regions overlap. Units: 1 µm = 0.001 mm; 1 nm = 0.001 µm.
Visible light, about 0.4–0.7 µm, is the narrow band the human eye sees:
| Colour | Wavelength (µm) |
|---|---|
| Violet | 0.40–0.43 |
| Indigo | 0.43–0.45 |
| Blue | 0.45–0.50 |
| Green | 0.50–0.57 |
| Yellow | 0.57–0.59 |
| Orange | 0.59–0.61 |
| Red | 0.61–0.70 |
Infrared, 0.7–300 µm, is divided further:
| Region | Wavelength (µm) | |
|---|---|---|
| Near infrared (NIR) | 0.7–1.5 | reflected infrared: the infrared part of sunlight reflected by the surface |
| Short-wave infrared (SWIR) | 1.5–3 | reflected infrared |
| Mid-wave infrared (MWIR) | 3–8 | thermal infrared: radiation emitted by the surface, from which surface temperature is calculated |
| Long-wave infrared (LWIR) | 8–15 | thermal infrared |
| Far infrared (FIR) | > 15 |
Microwaves, from about 1 mm to 1 m, are used by radar (see radar sensors).
Solar radiation
Optical remote sensing uses the sun as its only source of light and measures the part of it that the surface reflects. Sunlight reaching the ground has passed through the atmosphere, which has absorbed and scattered part of it. What remains at the surface lies mostly between 0.25 and 3 µm: visible light and reflected infrared. Some wavelengths are absorbed so strongly that they tell more about the atmosphere than about the surface.
Spectral reflectance
Radiation that reaches a surface is transmitted, absorbed or reflected. Remote sensing is concerned with the reflected part, because that is what the sensor measures. How much is reflected depends on the material and on the wavelength. The reflectance of a material is the fraction of the incoming radiation that it reflects, from 0 (nothing) to 1 (everything). Reflectance across many wavelengths is the material’s spectral signature (see Spectral signatures and vegetation indices).
Effects of the atmosphere
On its way through the atmosphere radiation is absorbed or scattered by gases and particles. Absorption turns the radiation’s energy into energy of the molecules; scattering sends it off in all directions. Both take energy out of the beam.
Scattering
- Rayleigh scattering is caused by gas molecules and particles much smaller than the wavelength. It depends strongly on wavelength: the shorter the wavelength, the more scattering. It is why the sky is blue, and why sunsets are red: with the sun low, light travels a longer path through the atmosphere, the blue is scattered away, and more of the red reaches the ground.
- Mie scattering is caused by pollen, dust, smoke, water droplets and other particles in the lower atmosphere that are about as large as the wavelength or larger. It makes clouds white.
- Non-selective scattering is caused by particles much larger than the wavelength, in the lower atmosphere. It affects all wavelengths alike and is the main cause of haze.
Atmospheric windows
Each gas absorbs radiation in its own wavelength bands. Only the regions between the main absorption bands can be used to observe the Earth’s surface. These atmospheric transmission windows lie in the visible, the near infrared, parts of the short-wave and thermal infrared, and the microwave region. Sensors built to observe the land place their bands inside them.
Effects on images
- Absorption mainly affects the visible and infrared bands. It weakens both the incoming sunlight and the reflected radiation, by an amount that varies with wavelength, and so alters the measured spectral signature of the target.
- Scattering matters most in the visible and near infrared. Sunlight scattered towards the sensor without reaching the ground gives images a hazy look. The effect is strongest in blue light, because Rayleigh scattering is strongest at short wavelengths.
Correcting for both is the step from top-of-atmosphere to surface reflectance (see top-of-atmosphere and surface reflectance).