engineering

1. Electromagnetic Radiation (EMR)
and its application in Remote
Sensing
Francis Gitau
Taita Taveta University

2. • Gamma rays
IONISING
RADIATION
• Visible light
• Infrared [heat]
• Microwaves
• Radio waves
NON-IONISING
RADIATION
Lower
ENERGY
Higher
• X-rays
• High frequency
ultraviolet
Nuclear reactions produce a
full spectrum of
electromagnetic radiation,
these waves travel through
space largely unchanged
• Cosmic rays
Extra-solar
Longer
WAVELENGTH
Shorter

3. What is electromagnetic radiation (EMR)?
electromagnetic
an electric field a magnetic field
… is a wave that propagates (radiates) through a
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
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

4. What is electromagnetic radiation (EMR)?
1. Electromagnetic
energy is generated by
several mechanisms
• changes in the energy levels of electrons
• acceleration of electrical charges
• nuclear decay of radioactive substances
• thermal motion of atoms and molecules
• nuclear reactions (fission and fusion)
2. All objects above absolute
zero (-273.15 ℃) emit EMR
3. The amount of EMR and
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

5. Deep space as seen by NASA’s Hubble Telescope
• 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

6. 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…

7. Deep space as seen by NASA’s Hubble Telescope
• So what would you see if you lit it
by the white spotlight of your
passing spaceship?
You would see the colours as you see them
here…
But why do we see them anyway?
• What does this tell us about EMR?
Objects can show colour because:
• They are radiating EMR
• They are reflecting EMR
• They are absorbing EMR
Objects can also:
• Conduct (or transmit) EMR
Squares this colour
appear white because
the pigment on the
surface reflects all
incoming white light
Squares this colour appear green
because the pigment on the surface
absorbs most incoming white light,
but reflects green light
The underlying material
appears black because
the pigment on the
surface absorbs all
incoming white light

8. 99% of the energy of solar
radiation is contained in a
narrow band comprising:
• near ultraviolet
• visible
• near infrared
On a clear day on earth:
• 40% of solar radiation is visible light
• 51 % is infrared radiation (warmth)
The earth’s
atmosphere
modifies
incoming
solar
radiation

9. 100%
51%
4%
20%
6% Reflected by atmosphere
30%
16%
Absorbed by
Clouds
3%
19%
INCOMING SHORTWAVE RADIATION (light, warmth) OUTGOING LONGWAVE RADIATION (warmth, light)
Processes that
occur in the
atmosphere:
• Reflection
• Scattering
• Refraction
• Absorption

10. 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.

11. 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)

12. Digital camera
How can spectral signatures be determined?
photosites
Image sensor
Mosaic filter
photons of light

13. Multispectral imager
How can spectral signatures be determined?
SATELLITE: Sentinel-2 SENSOR: MultiSpectral Instrument (MSI)
• The MSI measures the Earth's
reflected radiance in 13 spectral
bands
• 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

14. The sun emits EMR, mostly in a
narrow band from near
ultraviolet through visible to
near infrared (shortwave
radiation)
Some is scattered and reflected by the
atmosphere, clouds and particles (26%); some
is reflected back from the surface (4%)
19% is absorbed
by clouds and
the atmosphere
Longwave radiation (heat)
in the infra-red is radiated
back out
51 % is absorbed by the earth
Multispectral sensors on
satellites measure reflected
shortwave (visible) and
radiated longwave (infrared),
providing samples of spectral
signatures for each pixel

15. 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