cee 6100 / css 6600: remote sensing fundamentalsw.d. philpot, cornell univ. fall 2010radiation,...

CEE 6100 / CSS 6600: Remote Sensing Fundamentals W.D. Philpot, Cornell Univ. Fall 2010 Radiation, Radiometry and Blackbodies 1

The EM spectrum and associated detectors

0.1 µ

Ultraviolet

Multispectral scanners

Infrared film &silicon detectors

Ordinaryfilm

Infrared instruments THz

X-rays andgamma rays

VisibleHuman eye

Film & silicon

NearIR

(NIR)IR film

&silicon

Short-waveIR

(SWIR)HgCdTeInGaAs

Mid-wave(MIR)HgCdTeInAsSb

ThermalIR

(TIR)HgCdTe

PbS

0.4 µ 1 µ0.7 µ 3 µ 10 µ 30 µ

Infrared

0.4 µ 0.5 µ 0.6 µ 0.7 µ

IRUV Blue Green Red


EM radiation in the microwave

1mm 10cm

passive microwave systems

active systems (radar)

1cm

Ka X C S LK KuUHF

PBand designations:

1m


Radiative Transfer Concepts (Phenomenological Optics)

• Angular measures• plane angles• solid angles

• Radiative terms• Power (Flux)• Radiance• Irradiance

• Blackbody radiation• Reflectance• Attenuation (absorption & scattering)• Transmission


Plane angles: examples

Plane angleh = altitudes = perpendicular distance (at range h) = the angle subtended by s at range h

altitude (h) IFOV () length (s) 100 m 1 mrad 10 cm

0.1 mrad 1 cm0.01 mrad 1 mm

1 km 1 mrad 1 m10 km 1 mrad 10 m

800 km 1 mrad 800 m (AVHRR)0.1 mrad 80 m (MSS; 1972)

0.01 mrad 8 m (SPOT; 1986)0.001 mrad (1 µrad) 0.8 m (Quickbird; 2001)

If< 5° small angle approximationsin θ ~ tan θ ~ θ in radians

s ~ h

s = 2h tan (

h

s

100 m 0.3 mrad 3 cm (visual resolution)


Solid Angles

A solid angle is defined as an area on the surface of a sphere, divided by the square of the radius of the sphere,

= A/r2 ~ A'/r2

• for small angles, the area on the surface of the sphere is approximated by the area of the intersecting planar surface.

rA

A' ~ A for small angles

Solid Angle = A/r2 r


Summary of Radiative Transfer Quantities1. Radiation quantities Symbols Units

(new) (old)

a. Radiant flux or radiant power PW=J s-1

The time rate of flow of radiant energy. We may imagine the flow to be in the form of swarms of tiny colored particles -- photons. Each photon contains a well-defined amount (a quantum, hf) of radiant energy.

b. Irradiance (also exitance, emittance) E, M H, M W m-2

Power per unit area. The radiant flux incident on (irradiance) or emitted by (exitance, emittance) a surface divided by the area of that surface. (See Planck's equation.)

c. (Radiant) Intensity I J W sr-1

The radiant flux emitted by a source into a cone centered on the given direction divided by the solid angle of that cone. Particularly useful in describing radiation from a source that is perceived as a point source.

d. Radiance L N W m-2 sr-1

Radiant flux per unit solid angle per unit projected area. If a source is perceived/measured as an extended source, radiated power is described by radiance, L.


reflectivity or reflectance, , defines the proportion of incident radiation (e.g., from the sun) that will be reflected (returned from the object).

absorptivity or absorbance, , defines the proportion that will be absorbed (removed by the object).

transmissivity or transmittance, , defines the proportion that will be transmitted (passed through the object.)

NOTE: all quantities are a function of wavelength (spectral)

Reflectance, absorption and transmittance

Solar Irradiance inside the atmosphereAbsorption. A photon interacts with a particle by imparting its energy to an electron which moves to a higher orbit. Absorption is very wavelength dependent and can often provide a characteristic signature for a particular substance.

Absorption in the atmosphere is primarily due to oxygen (O2), carbon dioxide (CO2), ozone (O3), and water (H20).

Atmosphere © William Philpot, Cornell University, Fa10

8

L* = Path radiance Lsat = v Lt + L*

Lsat = v [(sunEsuncos sun Esky)] Rrs(v) + L*

RT Model: Path Radiance

vvrs v

d

L( )R ( )

E

earth's surfaceEsky

L*

M

A

Atmosphere © William Philpot, Cornell University, Fa10

9

Esun

sunEsuncos sun

sun

10 CEE 610: RS Fundamentals – Frame Cameras

H

As

FOV

θFOV

FOV @ aperture = 2 H tan(θFOV)

The frame camera as radiometer

source element

[w m-2 sr -1]

Radiance at the aperture

as a a s

P PL

A A

s

s = IFOV = As / H2 sec3 = angle subtended by a source element at a distance R seca

a =Aa / H2 sec3 = angle subtended by a camera aperture at a distance R sec


emulsion

anti-halationcoating

base

film,glass, or

paper

Photographic materials Solid State Detectors

• emulsion: most commonly a silver halide in a gelatin base

• base: paper, film, or glass

• anti-halation coating: an absorbing layer to suppress "halos" -- the spreading of a photographic image beyond its proper boundaries. Halation is caused primarily

• Each photodiode collects light independently

• The active area is smaller than the photodiode area

http://home.sandiego.edu/~ekim/photodiode/pdtech.html




Characteristic (D log E) curve Response function

• Each film has a unique D log E curve

• The shape of the curve varies with conditions of development

• A film's sensitivity is determined from the film's characteristic curve.

• "Gross fog" = film base density + net fog (dark current for film)

Solid state analogs: characteristic curve response functionDmin dark currentDmax saturation

The gain of a photodiode under reverse bias is extremely linear with irradiance (illuminance) over a very wide dynamic range.

Photographic materials Solid State Detectors

gross fog

log E (Exposure)

Den

sity

(D

)

linearresponse

range

Dmax

Dmin

(dar

ken

ing

on t

he

neg

ativ

e)

101

logdensitytransmission

13

Photographic materials Solid State Silicon Detectors

• Both are frame cameras, i.e., they image the entire 2-D array at one time.

• The fundamental optics are identical, both in terms of advantages and limitations.

• The spectral range of sensitivity is from the near UV through the NIR

• Film is a random medium – there is no orientation associated with the response

• Digital arrays are arranged in a rectangular grid.

• There are no gaps in film between detection elements.

• The detection surface is discontinuous.

• For a given speed and aperture setting, film products are limited to roughly 8-bits of information (distinct gray levels).

• Digital arrays (professional cameras) commonly capture up to12-bits of (reproducible) information

• The spectral range and specificity of film can range from near UV through the NIR

• The spectral range of solid state detectors (other than silicon based) can be extended into the SWIR, MIR and TIR

• Color film provides full resolution for all three channels

• Single array cameras generally use separate color filters over individual detector elements (e.g,. the Bayer Pattern) to produce color imagery.

© W. Philpot, Cornell University, Fall-09 CEE 6100: Remote Sensing Fundamentals 14

Digital Cameras have only one 2-D array which is essentially a grayscale device.

A Color Filter Array (CFA) is placed between the lens and the sensors. A CFA typically has one color filter element for each sensor.

Bayer Pattern

The Bayer pattern uses the three additive primary colors, red, green and blue (RGB), for the filter elements. Since there are two Green elements in every set of 4 elements, this is sometimes referred to as an RGBG mask. The extra Green is used to create contrast in the image.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

400 700 1000 1300 1600 1900 2200 2500

Refle

ctan

ce

wavelength (nm)

Laboratory ReflectancePaving Asphalt

Const. Asphalt

Galv. Steel 1

Galv. Steel 2

Bare Soil

vegetation

W. Philpot, Fa 10 CEE 6100 / CSS 6600: Photogrammetry 16

Photogrammetry

Photogrammetry:

Making precise measurements from images

• Close range photogrammetry: with camera focus set to a finite value.

• Far range photogrammetry: with camera focus setting to indefinite (infinity)


Depth of field

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html

• A lens focused on the yellow dot generates a yellow dot on the image plane. The yellow dot and all objects having the same subject-lens distance will appear sharp.

• The white dot, with larger subject-lens distance, will be out of focus. Its image is actually formed somewhere in front of the image plane and the image of this white dot on the image plane is a circle (the circle of confusion). As the subject-lens distance increases, the size of this circle increases.

• The same holds true for a subject in front of the yellow dot (e.g., the green dot).

• The size of a circle of confusion is proportional to the amount of light that can pass through the lens tube. Thus, smaller circles of confusion will be formed if less light can pass through. Therefore, a smaller aperture means smaller circles of confusion and a sharper image.

aperture


Basic Optics: thin lens equation

o i

imag

e

f

obje

ct

1 1 1

o i f

i h ' image sizeM

o h object size

thin lens equation:

magnification:

h

h'

depth of field

optic axis


Long range photogrammetry: focus at infinity

1 1 1

o if

f

h 'M Scale

o h

f

thin lens equation:

magnification:

i ≈ fo


Horizontal resolution

w dScale S

H FOV D

f

H = altitude

opti

c ax

is

FOV

w

d

D

f = focal length

Resolution will depend on:

• inherent resolution of the film or spacing of detectors in a digital array

• depth of field (circle of confusion)

• characteristics of the optics– lens quality – focal length– imaging geometry


fa

a'bt

rb'rb

ra'ra

da

RARB

A

A'

H

hT

hB hA

• •

• •

•• • •

datum ground

photo

nadir point

•

Relief displacement

a Aa

r hd

H•b'

rt

W. Philpot, Fa 10CEE 6100 / CSS 6600: Photogrammetry 22

f

H-h

dp1

o2o1

x1

h

H

dp2

o2o1

x2

imageplane

rearnode(lens)

O1 O2

dP

P = absolute parallax

dP1dP2

parallax difference: dP = dP1 – dP2

h dP

H h P

H dP

hP dP

Change in height

1

1

dpS

dP


perspective center

t

ff

d'

d d''

n'

n

Pa a''

a'equivalent vertical photo

•

•

•

••• •

i

tilted photo•

AB

• Tilt displacement is always relative to the isocenter.• Scale change is in the direction of tilt.• The nadir point is always on the down side of the axis of tilt and

opposite the principal point from the isocenter

Tilt displacement

A point that would have been imaged at a' on a vertical photo is actually imaged at a on the "up side" of the tilted photo.

The tilt displacement of points on the "up side" of the tilted photo is then toward the isocenter while points on the "down side" are displaced away from it.

•


Flight Planning

Factors to consider:

1. General issues• focal length• film size (format)• photo scale / FOV• overlap / sidelap (continuous coverage, stereo, …)

2. Issues specific to the application• spectral considerations (film / filter) • time of day (illumination, sun orientation, tidal stage, …)• season (crop calendar, leaf on, leaf off, …)• sun orientation, sun angle


Sidelap

flightpath

• Plan for 30% overlap (sidelap) in order to insure complete coverage (no gaps).

cee 6100 / css 6600: remote sensing fundamentalsw.d. philpot, cornell univ. fall 2010radiation,...

Documents

em radiation

remote sensing fundamentalsw

small angles solid angle

unit solid angle

h tan h s

radiant flux incident

altitude s

radiant intensity