Introduction to Remote Sensing The Electromagnetic (EM) Spectrum.

Introduction to Remote Sensing

The Electromagnetic (EM) Spectrum

Electro-magnetic radiation is a wave

Radiation

• All objects emit electro-magnetic radiation in some form

• This radiation moves through space until it hits something

• The thing it hits may then absorb the radiation and get its energy

• Alternatively it may deflect, scatter or reflect the radiation

• It may even simply allow the radiation to pass through - transmission

Wavelength• We can describe the radiation by:

– Wavelength• The actual length (metres) between wave

peaks.• Wavelengths for radiation vary greatly

– radio waves (100 cm to 160 metres)–Light (10-9 metres).– I will often talk in cm or mm for

microwaves–μm for IR–nm or Ǻ for visible

Frequency

• Frequency

– The number of wave crests that pass by a point per second (Hz). Often we will talk in terms of GHz (109)

• We also sometimes refer to a wavenumber; this is 1/λ, or the number of wavelengths in a unit length (normally in cm-1)

λ & These two quantities are inversely proportional

because

cWhere c is the speed of light (which varies with the medium, but not very much for the media we will consider)

Radiation

The distance between wave crests is the wavelength.

Shorter waves: x-rays, UV, visible light

Longer waves: infrared, microwave, radar, TV, radio

One Wavelength

Solar Spectrum

max = 0.55 m

© 1998 Wadsorth Publishing -- From Ahrens Essentials of Meteorology

Passive instruments

• These are instruments that simply receive radiation.– Eg radiometer, camera, retina– Most satellite instruments are passive

Active instruments

• These send out a signal and measure what is returned to them– Eg radars, lidars– We will pretty much ignore these– We will occasionally present examples of both

active and passive images for discussion

Other classifications

• We can also classify instruments by their position:– Ground based

• Fixed• Mobile

– Airborne– Satellite borne

• Geostationary• Other orbits

Ground based

• Most common is radar, but also profilers, ceilometers etc

• Advantages?

• Disadvantages?

Satellite borne• Mostly passive visible and radiometers

• Advantages?

• Disadvantages?

More spectrum

Wavebands

• We often, in this field, talk of wavebands. These can be broad, e.g. the visible band (400nm - 700nm) or narrow, e.g. the radar S-band (around 10cm)

• These are related to the sensitivities of particular receivers

Today’s quiz

• What is the speed of light?

• What is your favourite colour?

• What is its wavelength?

• What is its frequency?

Units

• You will become familiar with many concepts, including things we measure. These will have units.

• Temperature: in Kelvin (K)

• Radiance in Wm-2

• Albedo (unitless)

• And many more

Why do things have colour?

• Emission

• Scattering (reflection) and absorption

Interactions with matter

• When radiation hits matter 3 things can happen– Scattering (radiation bounces off – includes

reflection which is a special kind of scattering)– Transmission (radiation passes through)– Absorption (radiation is… erm… absorbed)

Can define more thingsAbsorptance

aλ = radiation absorbed / incident

Reflectance

ρλ = radiation scattered / incident

Transmittance

Tλ = radiation transmitted / incident

Aλ + ρλ + Tλ = 1

What we need to do

• How do we get values of temperature and albedo (reflectance) using the instruments on the satellites?

Albedo

• Albedo varies with wavelength

• Many substances have high albedo (reflectance) in the visible (e.g. snow), but low albedo in the microwave (e.g. snow)

• Can also have different albedo for different colours and therefore appear coloured (e.g. leaves)

Specular and Lambertian reflection

Reflection/scattering

• The type of reflection is determined by the smoothness of the surface– This is really the ratio of the wavelength of the

radiation to the size of uneven features

• Very few natural surfaces produce true specular reflection

• Give me some examples!!

Solid angles

• The solid angle is the proportion of the surface area of a sphere subtended by the 2 dimensional angle. (See picture drawn on board)

• It is measured in steradians – sr.

Measuring Reflectance (albedo)

• Measure the energy impacting the sensor in the visible waveband channel

• In Wm-2sr-1μm-1

• Energy reflected per unit time per unit area: Normalised for width of waveband and solid angle view.