Electromagnetic Radiation (EMR) and its application in Remote Sensing Nevil Wyndham Quinn Associate Professor in Applied Hydrology Extra-solar • Cosmic rays • Gamma rays • X-rays IONISING RADIATION Nuclear reactions produce a • High frequency full spectrum of ultraviolet electromagnetic radiation, these waves travel through space largely unchanged • Visible light • Infrared [heat] NON-IONISING RADIATION • Microwaves Lower ENERGY Higher ENERGY Lower Longer WAVELENGTH Shorter WAVELENGTH Longer • Radio waves What is electromagnetic radiation (EMR)? … is a wave that propagates (radiates) through a electromagnetic vacuum at the speed of light (just under 300 000 m/s) and transfers energy from one place to another … these waves carry energy as synchronized oscillations of electric and magnetic fields an electric field a magnetic field that are perpendicular to each other and perpendicular to the direction of travel … although it is a wave, it also can be detected as discrete ‘particles’ of light called photons What is electromagnetic radiation (EMR)? • changes in the energy levels of electrons 1. Electromagnetic • acceleration of electrical charges energy is generated by • nuclear decay of radioactive substances several mechanisms • nuclear reactions (fission and fusion) • thermal motion of atoms and molecules 2. All objects above absolute 3. The amount of EMR and zero (-273.15 ℃) emit EMR the wavelengths emitted depend on the temperature of the object 4. As the temperature of an object increases, the total EMR increases while the wavelength of the peak shortens • What are these? galaxies • Why are they different colours? they represent the combined temperature of all the stars in each galaxy • How cold is space? -270.45 °C or 2.7 K Deep space as seen by NASA’s Hubble Telescope • What would happen to a Rubik Cube, if you could throw it into space? If it was far from any stars or planets it will eventually come into thermal equilibrium with the cosmic microwave background which is thermal radiation of 2.7 K It would not glow… Deep space as seen by NASA’s Hubble Telescope Squares this colour appear green • So what would you see if you lit it because the pigment on the surface by the white spotlight of your absorbs most incoming white light, but reflects green light passing spaceship? You would see the colours as you see them Squares this colour here… appear white because the pigment on the But why do we see them anyway? surface reflects all incoming white light • What does this tell us about EMR? The underlying material Objects can show colour because: appears black because • They are radiating EMR the pigment on the • They are reflecting EMR surface absorbs all • They are absorbing EMR incoming white light Objects can also: • Conduct (or transmit) EMR Deep space as seen by NASA’s Hubble Telescope The earth’s atmosphere modifies incoming solar radiation On a clear day on earth: • 99% of the energy of solar 40% of solar radiation is visible light • radiation is contained in a 51 % is infrared radiation (warmth) narrow band comprising: • near ultraviolet • visible • near infrared INCOMING SHORTWAVE RADIATION (light, warmth) OUTGOING LONGWAVE RADIATION (warmth, light) 30% Processes that occur in the 6% Reflected by atmosphere 100% atmosphere: 20% • Reflection 4% • Scattering • Refraction 16% • Absorption Absorbed by 3% Clouds 19% 51% Albedo is a measure of how much light that hits a surface is reflected without being absorbed. Something that appears white reflects most of the light that hits it and has a high albedo, while something that looks dark absorbs most of the light that hits it, indicating a low albedo. Spectral signatures are the characteristic curves associated with different surfaces (including types of vegetation) that show the extent to which EMR of a particular wavelength is reflected. Whereas the albedo tells you the proportion of total radiation that is reflected, the spectral signature tells us which wavelengths are either absorbed (a trough in the spectral signature) or reflected (a peak in the spectral signature) How can spectral signatures be determined? Digital camera photons of light photosites Image sensor Mosaic filter How can spectral signatures be determined? Multispectral imager • The MSI measures the Earth's reflected radiance in 13 spectral bands SATELLITE: Sentinel-2 SENSOR: MultiSpectral Instrument (MSI) • 290km swath width • Light collected by a 3-mirror telescope • Focussed by a dichroic beam splitter which separates into into 2 channels: • Visible + Near Infrared [VNIR] • Short Wave Infrared [SWIR] • 2 separate arrays of 12 staggered detectors to cover the full field of view Multispectral sensors on satellites measure reflected The sun emits EMR, mostly in a shortwave (visible) and narrow band from near radiated longwave (infrared), ultraviolet through visible to providing samples of spectral near infrared (shortwave signatures for each pixel radiation) Some is scattered and reflected by the Longwave radiation (heat) atmosphere, clouds and particles (26%); some in the infra-red is radiated is reflected back from the surface (4%) back out 19% is absorbed by clouds and the atmosphere 51 % is absorbed by the earth Image credits: Slide 1: The sun https://solarsystem.nasa.gov/solar-system/sun/overview/ Slide 3: Electromagnentic waves https://www.researchgate.net/publication/320616988_The_reflection_of_two_fields_-_Electromagnetic_radiation_and_its_role_in_aerial_imaging/figures?lo=1 Slide 5: Deep space from Hubble: https://www.nasa.gov/image-feature/goddard/2016/hubble-sees-a-legion-of-galaxies Star colour and temperature https://www.smore.com/esdkf-twinkle-twinkle-little-star Slide 8: Emission and wavelength http://www.seos-project.eu/modules/remotesensing/remotesensing-c01-p02a.html Slide 9: Earths atmosphere https://eapsweb.mit.edu/news/2016/study-pinpoints-timing-of-oxygens-first-appearance-in-earths-atmosphere Energy at the the top/bottom of the atmosphere https://www.visionlearning.com/img/library/large_images/image_9438.png Radiation budgets https://science.nasa.gov/ems/13_radiationbudget Slide 10: Reflection albedo http://www.reportingclimatescience.com/2016/05/19/albedo/ Albedo graphic https://climate.ncsu.edu/edu/Albedo Slide 11: Spectral signatures http://www.seos-project.eu/modules/classification/classification-c01-p05.html Slide 12: How a digital camera works https://www.cambridgeincolour.com/tutorials/camera-sensors.htm Spectral signature http://www.seos-project.eu/modules/classification/classification-c01-p05.html Slide 13: Comparison of Sentinel-2 and others https://twitter.com/usgslandsat/status/773939936755982336 Sentinel-2 MSI https://www.researchgate.net/publication/318093467_Band_Selection_in_Sentinel-2_Satellite_for_Agriculture_Applications/figures?lo=1 Sentinel-2 image https://www.esa.int/Our_Activities/Operations/Sentinel_mission_control_taking_shape.