Electromagnetic Radiation (EMR) and its application in Remote

Sensing

Nevil Wyndham Quinn

Associate Professor in Applied Hydrology

• Gamma rays

IONISING RADIATION

• Visible light

• Infrared [heat]

• Microwaves

• Radio waves

NON-IONISING RADIATION

Low

er

EN

ERG

Y

Hig

he

r• X-rays

• High frequency ultraviolet

Nuclear reactions produce a full spectrum of

electromagnetic radiation, these waves travel through

space largely unchanged

• Cosmic raysExtra-solar

Lon

ger

WA

VEL

ENG

TH

S

ho

rte

r

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

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

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

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

• 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 colourappear white because

the pigment on the surface reflects all

incoming white light

Squares this colour appear greenbecause 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

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

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

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)

Digital camera How can spectral signatures be determined?

photosites

Image sensor

Mosaic filter

photons of light

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

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

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