Thermal Infra-red Remote Sensing

A few notes

  • All bodies with temperature above 0 K emit radiation.
  • 0 K = ‒273,15 °C
  • A blackbody is a theoretical object that completely absorbs all wavelengths of electromagnetic radiation incident on it.
  • When a blackbody is heated to a temperature above 0 K, it emits radiation.
  • Blackbody radiation at temperatures comparable to the temperature of the earth’s surface (~300 K) is the thermal infrared (TIR)
  • “Good absorbers are good emitters”, meaning that a blackbody emits 100% of the radiation it absorbs (so the absorbed radiation has no effect on the blackbody’s temperature)

Basic principles

It is necessary that the reader understands the following principles.

PRINCIPLEDESCRIPTION
Planck’s Law of blackbody radiationDescribes the electromagnetic radiation of a blackbody of a defined temperature
Stefan-Boltzmann LawCalculates the total electromagnetic radiation as function of a blackbody’s temperature
Wien’s Displacement LawCalculates the wavelength at which maximum spectral radiant exitance(emitted radiation) occurs
Kirchhoff’s Law➔Emittance at a given wavelength = absorbance at the same wavelength➔Blackbodies are theoretical; the behaviour of real objects can be described based on how close to being a blackbody they are.➔Emissivity (ε):◆ε = 1 ⇒ blackbody◆ε < 1 ⇒ real-life material

Planck’s Law of blackbody radiation

  • Created by Max Planck (1858–1947)
  • Blackbody radiation can be calculated by the object’s (absolute) temperature according to the formula:
M λ = 2 π h c 2 λ 5 e hc / λkT 1

where

  • λ: wavelength
  • Mλ: radiation at wavelength λ
  • Τ: temperature (absolute temperature in K).

Therefore, at any given wavelength we can have a spectral radiant exitance curve (“radiation”) according to the absolute temperature:

(to be continued)