1. Lecture 2 Remote Sensing: Radiation Theory and Solar Radiation
Professor Menglin S. Jin
Department of Meteorology and Climate Science
San Jose State University
Electromagnetic radiation (Wallace and Hobbs, chapter 4)
This lecture provides a brief review of the E&M theory that are needed to
understand the physical principles of remote sensing. Most of this material should have been covered in previous meteorology/physics
course. This review will take at least two lectures. If you have not seen this
material before, you should read the chapter several times to pick up the terminology and important
results. No matter if you have covered this material before, you may need to go back to your old text
books and review some basic physics.
The topics that you will need to understand for later in the course include: refractive index - n, flux
density - F, the electromagnetic spectrum, absorption and absorption length, reflection, polarization and Doppler shift.
A second set of topics treated later include: diffraction and resolution, Snell's law, thermal radiation,
radiance, and irradiance.

2. This is to review How much energy is emitted by some medium?
What “kind” of energy (what frequency/wavelength) is emitted by some medium?
What happens to radiation (energy) as it travels from the “target” (e.g., ground, cloud...) to the satellite’s sensor?

3. Brief history
Since the 1960s, most remote sensing has been conducted from satellites
Prior to that remote sensing is associated mainly with aerial photography, using cameras mounted in aircraft that fly at various altitudes (with scale emcompassed)
Aircraft remote sensing continues through today but is usually directed towards specific tasks and missions.

4. "Remote" and "Proximal" Sensing
“Remote” sensing involves making measurements and collecting data for (and from) objects, classes, and materials that are not in contact with the sensor (sensing device) whereas the “Proximal” sensing includes making direct contact with these targets

5. if the objective is to measure a person's bodily temperature
the proximate approach would be
the remote approach would be
to place a thermometer in or on the body
to hold a radiometer sensitive to thermal energy
at some distance from the body
Today, some of the most challenging problems in climate remote sensing are related to
developing stable calibration for thermal infrared sensors
Both need “Calibrated”
its response as a sensor must be transformable into a good
approximation of the actual temperature by determining the response
using a target whose temperature range is specifically known.

6. DEM: Digital Mapping System

7. Passive and Active Remote Sensors
Remote sensing systems which measure energy that is naturally available are called Passive Sensors. (Sun, surface emission, etc)
Active sensors, on the other hand, transmit short bursts or 'pulses' of electromagnetic energy in the direction of interest and record the origin and strength of the backscatter received from objects within the system's field of view. Passive systems sense low level microwave radiation given off by all objects in the natural environment.

8. Example of passive and active remote sensing
In this figure, find out passive and active remote sensing environment

9. diagram for remote sensing for surafce (1) –solar radiation

10. diagram for remote sensing for (2) – longwave emission

11. Electromagnetic Spectrum
Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each traveling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons.

12. A photon has no mass, but its energy is E = hν
E = hν = hc/λ (since c = λν)
h = “Planck’s constant,” which is a VERY small number(6.63 x in our units).
Frequency (ν)
Energy
Put these
lights into the
right place!! 12

13. Electromagnetic Spectrum
Remote sensing relies on measurements in the
electromagnetic spectrum (except sonar)
Remote sensing of the ground from space
• Need to see through the atmosphere
• The ground must have some feature of interest in that spectral region
• Studying reflected light requires a spectral region where solar energy dominates
Radar approaches mean we need frequencies that we can generate
• Also need to ensure that we are not affected by other radio sources
• Atmosphere should be transparent at the selected frequency
SONAR: SOund Navigation And Ranging
Sound is a vibration of some medium that it travels through, like water or air
Light is a vibration of magnetic and electric fields--a vibration of pure energy

14. Print this out for the students (teacher’s note)

15. The wavelengths we are most interested in for climatology and meteorology are between 0.01 and 100 μm

16. The Wavelength of Visible Light
The typical unit of measurement for λ is the
Ångstrom (Å) = m= 0.1 nanometer
• Red = 6500 Å • Yellow = 5800 Å
• Green = 5300 Å • Blue = 4800 Å

17. Units (in case you need)
The hertz (symbol Hz) is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon
One hertz simply means "one cycle per second"
1 micrometer = nanometers 1 μm = nm
1 μm = 10-6 meter
kHz (kilohertz, 103 Hz)
MHz (megahertz, 106 Hz)
GHz (gigahertz, 109 Hz)
THz (terahertz, 1012 Hz)
SI is the International System of Units

18. Class activity
PM frequency is 860x106/s what is its wavelength in light? (light speed is 3x108m/s, nm=nanometer=10-9m)
Sound wave is v = 343 m/s, what is its wavelength in sound wave?
frequency = light velocity / wavelength
PM = 860/s ->wavelength=3.49 x 10^8 nm
v = 343 m/s λ = (343 m / s) / (860 x 10^6 s^-1) = 3.99 x 10^-7m x (10^9 nm / 1 m) = 3.99 x 10² nm = 399 nm

19. Need to know Solar constant solar radiaiton at TOA
TOA radiation budget
Basic definition

20. Measuring energy: (Important!)
Radiant energy: Total energy emitted in all directions (J)
Radiant flux: Total energy radiated in all directions per unit time (W = J/s)
Irradiance (radiant flux density): Total energy radiated onto (or from) a unit area in a unit time (W m-2)
Radiance: Irradiance within a given angle of observation (W m-2 sr-1)
Spectral radiance: Radiance for range in 

21. Radiance Toward satellite Normal to surface
Solid angle, measured in steradians
(1 sphere = 4 sr = sr)
Radiance is what satellite sensor can measure, but in specific wavelength

22. Blackbody radiation
Examine relationships between temperature, wavelength and energy emitted
Blackbody: A “perfect” emitter and absorber of radiation... does not exist

23. (sometimes indicated as E*)
Stefan-Boltzmann Law
M BB = T 4
Total irradiance
emitted by a blackbody
(sometimes indicated as E*)
Stefan-Boltzmann constant
The amount of radiation emitted by a blackbody is
proportional to the fourth power of its temperature
Sun is 16 times hotter than Earth but gives off 160,000 times
as much radiation

24. Planck’s Function
Blackbody doesn't emit equal amounts of radiation at all wavelengths
Most of the energy is radiated within a relatively narrow band of wavelengths.
The exact amount of energy emitted at a particular wavelength lambda is given by the Planck function:

25. Planck’s function c1-5 B  (T) = exp (c2 / T ) -1
First radiation constant
Wavelength of radiation
c1-5
B  (T) =
exp (c2 / T ) -1
Absolute temperature
Second radiation constant
Irridance:
Blackbody radiative flux
for a single wavelength at temperature T (W m-2 m-1)
Total amount of radiation emitted by a blackbody is a function of
its temperature
c1 = 1.19x10-16 W m-2 sr-1
c2 = 1.44x10-2 m K

26. Planck curve

27. Wein’s Displacement Law
mT = m K
Gives the wavelength of the maximum emission of a
blackbody, which is inversely proportional to its temperature
300K: ~
6000K: ~
10 μm
0.5 μm

28. Rayleigh-Jeans Approximation
B (T) = (c1 / c2) -4 T
When is this valid:
1. For temperatures encountered on Earth
2. For millimeter and centimeter wavelengths
At microwave wavelengths, the amount of radiation emitted
is directly proportional to T... not T4
B (T)
TB =
(c1 / c2) -4
Brightness temperature (TB) is often used for microwave and
infrared satellite data, where it is called equivalent blackbody
temperature. The brightness temperature is equal to
the actual temperature times the emissivity.

29. Emissivity and Kirchhoff’s Law

Actual irradiance by
a non-blackbody
at wavelength 
Emittance: Often referred to as emissivity
Emissivity is a function of the wavelength of radiation and the viewing angle and) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperature
absorbed/ incident
Absorptivity (r , reflectivity; t , transmissivity)

31. Solar spectrum composition
The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K.
The Sun does, however, emit X-rays, ultraviolet, visible light, infrared, and even Radio waves
UV – μm
visible – μm (namely so called light)
infrared – 0.7 μm – 1mm

32. Intensity and Wavelength of Emitted Radiation : Earth and Sun

33. window
Atmosphere
Window

34. Solar constant (see Wallace ch. 4)
Question: How to calculate solar radiation?
Assuming sun’s surface temperature is 5780K,
Average distance between Sun-Earth is 1.5x108 km
mean Sun radius is 7x105 km.
The solar constant is defined as the quantity of solar energy (W/m²) at normal incidence outside the atmosphere (extraterrestrial) at the mean sun-earth distance. Its mean value is W/m². The spectral distribution is given in the figure.
Energy from the Sun (E)
Using the Stefan-Boltzmann law, calculate t
the average irradiance of the sun.
b. Reverse law
The inverse square law is used to
calculate this constant: So = E(sun) x (R(sun)/r)2
The solar constant includes all
wavelengths of solar electromagnetic
radiation, not just the visible light

35. How to calculate solar radiaiton at TOA?
The Earth receives a total amount of radiation determined by its
cross section (π·RE²), but as it rotates this energy is distributed
across the entire surface area (4·π·RE²).
Hence the average incoming solar radiation is one-fourth
the solar constant (approximately 342 W/m²)

36. Is solar radiation at TOA a real constant?
Does this value vary with latitude and season, and local hour?
Answer: At any given moment, the amount of solar radiation received
at a location on the Earth's surface depends on the state of the
atmosphere and the location's latitude.

37. solar energy incident to earth
Sun Spot numbers
solar energy incident to earth 37

38. Solar Zenith Angle (important)
The angle between the
local zenith and
the line of sight to the sun.

40. In general, the (solar) azimuth angle varies with the latitude and time of year and
the full equations to calculate the sun's position throughout the day.
There is equation for calculation this angle (not discussed in this class)

41. the "solar constant" images above, from a variety of calibrated satellite instruments aboard SOHO.
SOHO was launched in December 1995 by an Atlas Centaur rocket and became operational in March SOHO weighs about two tons and with its solar panels extended stands about 25 feet across.  SOHO will continue operating well past the next solar maximum in (Image credit: Alex Lutkus)

42. HW2 – solar radiation and Black Body
IDL tutorial for HW2