Lecture 2 Remote Sensing:
Radiation Theory and Solar
Radiation
Professor Menglin S. Jin
Department of Meteorology
San Jose State University
• 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?
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.
"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
if the objective is to measure a
person's bodily temperature
• the proximate approach would be
to place a thermometer in or on the body
• the remote approach would be
to hold a radiometer sensitive to thermal energy
at some distance from the body
•
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.
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.
Example of passive and active
remote sensing
In this figure, find out passive and active remote sensing environment
diagram for remote sensing for
surafce (1) –solar radiation
diagram for remote sensing for (2)
– longwave emission
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.
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
• The wavelengths we are most interested
in for climatology and meteorology are
between 0.01 and 100 μm
Need to know
•
•
•
•
Solar constant
solar radiaiton at TOA
TOA radiation budget
Basic definition
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 
Radiance
Normal
to surface
Toward satellite
Solid angle, measured in steradians
(1 sphere = 4 sr = 12.57 sr)
Radiance is what satellite sensor can measure, but in wavelength
Blackbody radiation
• Examine relationships between
temperature, wavelength and energy
emitted
• Blackbody: A “perfect” emitter and
absorber of radiation... does not exist
Stefan-Boltzmann Law
M BB = T 4
Total irradiance
Stefan-Boltzmann constant
emitted by a blackbody
(sometimes indicated as E*)
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
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:
Planck’s function
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
Planck curve
Wein’s Displacement Law
mT = 2897.9 m K
Gives the wavelength of the maximum emission of a
blackbody, which is inversely proportional to its temperature
Earth @ 300K: ~10 m
Sun @ 6000K: ~0.5 m
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.
Emissivity and Kirchoff’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)
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 – 0.1-0.4 μm
visible – 0.4-0.7 μm (namely so called light)
infrared – 0.7 μm – 1mm
Intensity and Wavelength of Emitted
Radiation : Earth and Sun
window
Atmosphere
Window
Solar constant
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
a. Energy from the Sun (E)
(extraterrestrial) at the mean
Using the Stefan-Boltzmann law, calculate t
the average irradiance of the sun.
sun-earth distance. Its mean
value is 1367.7 W/m². The
spectral distribution is given in
b. Reverse law
the figure.
The inverse square law is used to
calculate this constant:
The solar constant includes all
wavelengths of solar electromagnetic
radiation, not just the visible light
So = E(sun) x (R(sun)/r)2
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)
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.
Solar Zenith Angle (important)
The angle between the
local zenith and
the line of sight to the sun.
correction for the elliptical orbit - declination
the extraterrestrial solar illuminance (Eext), corrected for the elliptical orbit by using
the day number of the year (dn), is given by
See handout
Declination is analogous to latitude on
Earth's surface, and measures an angular
displacement north or south from the
projection of Earth's equator on the
celestial sphere to the location of a
celestial body. See Celestial Sphere
Figure.
http://www.srrb.noaa.gov/highlights/sunrise
/glossary.html
Sun Spot numbers
solar energy incident to earth
• 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 1996. 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 2001.
(Image credit: Alex Lutkus)
HW1 – solar radiaiton
• IDL tutorial for HW1