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Home , Radiation

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 full spectrum of electromagnetic radiation, these travel through largely unchanged • Visible

• [] NON-IONISING RADIATION

•

Lower Higher ENERGY Lower Longer Shorter WAVELENGTH Longer • Radio waves What is electromagnetic radiation (EMR)? … is a that propagates (radiates) through a electromagnetic vacuum at the (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 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 ‘’ of light called What is electromagnetic radiation (EMR)? • changes in the energy levels of 1. Electromagnetic • acceleration of electrical charges energy is generated by • nuclear decay of radioactive substances several mechanisms • nuclear reactions (fission and fusion) • thermal of atoms and molecules

2. All objects above absolute 3. The amount of EMR and zero (-273.15 ℃) emit EMR the 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 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 background which is 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 ’s 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 • 16% • Absorption Absorbed by 3% 19%

51% 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 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- 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 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: 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