Download Overview

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Topics:
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Statistics
The Human Visual System
Color Science
Radiometry/Photometry
Geometric Optics
Tone-transfer Function
Image Sensors
Image Processing
Displays & Output
Colorimetry & Color Measurement
Image Evaluation
Psychophysics
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Outline
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Definition of Terms and Units
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Radiometry
Solid Angle, Point Sources, Projected Area
Sources of Radiation
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Gas Discharge
Fluorescent
Blackbody Radiation
Incandescent
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The Sun and daylight
SOURCES LABORATORY
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Outline
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Radiance
 Radiance invariance
 Lamberts Cosine Law and Lambertian Surfaces
 Relationship between radiance and exitance
 Relationship between flux and intensity
Photometry and Color Temperature
 HVS short overview
 Photometric terminology
 Planckian locus
 Color temperature
 Distribution temperature
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Outline
 Spectral radiometry
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Diffraction gratings
Spectrometers
Wavelength selectors
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Absorption and interference filters
IR rejection
ND filters
Monochromators
CALIBRATION LABORATORY
Irradiance Variation with Distance and Angle
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Point source (inverse square law)
Line source
Plane source
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Outline
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Irradiance Variation with Distance and Angle
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Point source (inverse square law)
Line source
Plane source
INV SQ LAW LABORATORY
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Outline
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Radiometry and Imaging Systems
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Non-Point Sources
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Lens falloff and exposure
Irradiance onto the focal plane
How to handle non-point source calculations
Some governing formula’s
Integrating Spheres (if time permits)
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Governing sphere equation
Applications of integrating spheres
Spectroradiometry
Emmett Ientilucci, Ph.D.
Digital Imaging and Remote Sensing Laboratory
Chester F. Carlson Center for Imaging Science
Spectroradiometry- Overview
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Diffraction Gratings
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Line spacing, lpm
Grating types
Order convention
Spectrometers
Monochromators
Spectral Considerations
n(l) = speed in vacuum c
speed in medium v(l)
-n, will always be greater than one
-So, the index is a function of wavelength
-Therefore, the amount of refraction is different
-This effect is called chromatic dispersion
n(l)
d
Normal Dispersion Region
1.5
1.0
0.4
0.7
1.0
l
Dispersing Prism
d is the deviation angle
What is a Diffraction Grating?
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Simply another device to disperse polychromatic light
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Extension of Young’s double slit experiment
 Is based on both diffraction and interference
2nd-order maximum
1st-order maximum
0th-order maximum
D
Relative Brightness
Slits widths are not much greater in width than the wavelength of light
So, what is a Diffraction Grating?
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Extension of Young’s double slit
 Instead of two slits, we not have N small transmitting slits that are
comparable to the wavelength of light.
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Move from 2-slit to N-slit interference problem
Reflection Grating
+
1
-
+ 2
2
Monochromatic
Beam
1
-qr
qi
d
- It is clear that ray 2 travels a greater distance than ray 1
which is the general grating equation using the pos/neg angle conventions above.
Properties
Location of the m = 0 order:
When the incident angle, qi = 0:
Lines per Millimeter
The distance, d between grooves would be
Distance can also be expressed as
L - is the total length of the grating
N - is the total number of grooves
How lpm does a DVD have?
Grating Equation Observations
As the number of slits increases the
PM narrows and gets sharper.
The principal maxima (PM) are called
spectrum lines
2 Slits
4 Slits
See grating applets:
http://library.thinkquest.org/26938/diff.html
http://www.physics.uq.edu.au/people/mcintyre/applets/grating/grating.html
20 Slits
What is the Geometry of This Pattern?
Spectrum
Red
Using a two wavelength source
l1 = 400 nm Blue
l2 = 600 nm Red
Blue
Order Convention
Pos Orders
+ -
Neg Orders
m=0
qi
+ -
a+1
qi
a+1
-qr
a-1
a-1
-qr
m=0
+ -
Spectrometer
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A spectroscopic instrument that may scan
wavelengths individually or the entire spectra
simultaneously.
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It may employ a prism or grating for means of
dispersing incident light.
Grating Spectrometer
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Comes in many configurations
Grating Spectrometer
Ocean Optics USB 650
25 um Slit
200-1100 nm range
Spectral Calculation
Spectral Calculation
Neutral Density Filters
- Absorbs light uniformly in VIS
- Can vary exposure while
maintaining a particular focus
Transmission = 10-ND
ND = -log(transmission)
http://dvdreamtime.com.au/images/tiffen/ndcompare580433.jpg
IR Cut Filters
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Simply blocks NIR radiation and passes VIS radiation
Why you would want to do this?
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Can be done via absorption filters
 Optical glass that is absorptive in the NIR
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Can be done via reflection filters
 Short-pass interference filters that reflect NIR
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Typically block 640 – 1100 nm (i.e., NIR)
IR Cut Filters
Reflective
Filters
Absorptive
Filters
http://www.optics-online.com/OOL/pics/IRCcurve40.gif
Reflective
Filters
(For Security Cameras)
CALIBRATION LABORATORY
Irradiance Variation with
Distance and Angle
Emmett Ientilucci, Ph.D.
Digital Imaging and Remote Sensing Laboratory
Chester F. Carlson Center for Imaging Science
Irradiance Falloff From a Point Source
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Lets use some radiometric terms to defines useful
expressions
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How does the irradiance vary with distance from a point
source?
 E2 = (E1 r12) / r22
[W / m2]
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This formula only works with the following assumptions
 You have a point source
 Non intervening medium (transmission = 1)
 Photons travel in straight lines
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The flux from the first distance is the same flux at the second
distance.
Irradiance Falloff From a Point Source
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What if I knew the flux per solid angle?
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How is irradiance related to this?
Intensity, I [W /sr]
Irradiance, E [W / m2]
Solid Angle, [sr]
Distance, r
? ? ?
E = I / r2
[W / m2]
Example
Using Sun
And Earth
Irradiance Falloff from a Line Source
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How does irradiance vary from a line source?
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What is a line source?
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Infinite number of point sources next to each other
So, we just need to be far enough away to consider it a line (use
our factor of ten rule)
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E2 = (E1 r1) / r2
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i.e., 1 inch tube, place yourself 10 inches away
 (don’t want to see extents of the tube)
[W / m2]
Irradiance varies inversely with distance
Assumptions
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It is infinite in length
Transmission is one
Plot of 1/xn
Irradiance Falloff From a Plane Source
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What is a flat, plane, broad source?
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Infinite number of point sources on (x,y) plane
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E2 = E1
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Irradiance does not vary from an infinite plane source
INV SQ LAW LABORATORY
Radiometry and Imaging Systems
Emmett Ientilucci, Ph.D.
Digital Imaging and Remote Sensing Laboratory
Chester F. Carlson Center for Imaging Science
Lens Falloff
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To this point
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We have done radiometry in the absence of imaging
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Lets incorporate what we know about radiometry with the
simplest of imaging systems
 Pinhole camera
http://www.dkimages.com/discover/previews/756/196161.JPG
Spam Pinhole Camera
Pinhole
http://zedomax.com/blog/tag/pinhole_camera/
Lens Falloff – Ideal Camera
For a simple system like this, what is the “uniformity” like on the image plane?
Uniform
Radiance
(e.g., 18%
Gray Card)
Lens Falloff
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This is for a lens-less pinhole system
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The lens helps to reduce optical vignetting
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The cos q due to the effective smaller aperture
Therefore, a lens system typically has n < 4.
Lens Falloff
http://en.wikipedia.org/wiki/Image:RioparaguayJune2005.jpg
To Compensate, use Anti-vignetting filter
Exposure would be too long, if one used a “perfect” Anti-vignetting filter
Trade off between level of “uniformity” and exposure
Vignetting
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A derived term from vignette
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From the same root as vine (French)
Originally referred to a decorative border in a book
Later, used to describe a photographic portrait which is
clear in the center, and fades off at the edges.
Vignetting
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A decrease in geometrical transmission
A reduction in the illumination at some image point (usually
off-axis)
Vignetting
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3 types of vignetting
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Optical
Natural
 Both of these are inherent to each lens design
 Collectively, these are referred to as “lens falloff”
Mechanical
 Due to the use of improper attachments to the lens
 Does not produce “gradual” falloff
 Abrupt transition with entirely black image corners
http://www.vanwalree.com/optics/vignetting.html
Vignetting
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Optical Vignetting
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Strongest when the lens is used wide open
Stopping down can reduce
optical vignetting
But
You collect less light
for off-axis points
hence image corners
will be darker.
http://www.vanwalree.com/optics/vignetting.html
f/1.4
f/5.6
Vignetting
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Natural Vignetting
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Natural light falloff, cos4 q
 Our “Lens falloff” derivation
 Remember, not all four cosines may be present
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Inherent to each lens design and becomes more
troublesome for wide angle lenses (i.e., short f)
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Is not cured by stopping down
 Stopping down helps “optical vignetting”
 Can help fix “natural vignetting” with anti-vignetting filter
http://www.vanwalree.com/optics/vignetting.html
Vignetting
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Mechanical Vignetting
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Mechanical extensions (e.g., lens hood) to a lens protrude
into its FOV
Lens hood prevents flare
http://www.vanwalree.com/optics/vignetting.html
Camera Equation
We
Camera Equation
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A way to relate the radiance at the front of a camera
system to the irradiance at the focal plane
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Metric of optical throughput
G# = L/E [sr-1]
Lens
System
Aperture
Camera System
Focal Plane
Camera Equation
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Factoring in system element variables we have:
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Valid for:
 On optical axis
 For small apertures (i.e., large f/#)
 Focused at infinity
 Transmission is uniform
Camera Equation
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Camera equation of ANY f-number:
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Valid for:
 On optical axis
 Focused at infinity
 Transmission is uniform
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For off-axis conditions,
 Reduce irradiance by cos3q
Area / Non-Point Sources
Emmett Ientilucci, Ph.D.
Digital Imaging and Remote Sensing Laboratory
Chester F. Carlson Center for Imaging Science
Area Sources
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What is an area source?
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How do we perform radiometry when point source
approximations produce large errors?
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source-radius to source-distance is small?
Overview
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Energy Exchange Between Parallel Disc’s
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Small disc source and small receiver (pt source solution)
Large disc source and small receiver (non-pt source solution)
Comparison of point source and non-point source solutions
 Examples
Energy Exchange Between Paralled Plates
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Large plane source and small receiver
Large plane source and large receiver
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Energy Exchange Between Arbitrary Surfaces
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Derivation of G# for Small f-number’s
Energy Exchange Between Parallel Disc’s
What is the flux from a Lambertain source of radiance L,
onto a receive at distance r?
We will first use point source approximations…..that is, we are at least
10 x the radius of the source (i.e., 1% error).
Energy Exchange Between Parallel Disc’s
From the viewpoint of the “source”
L
F0
Energy Exchange Between Parallel Disc’s
Now calculate when the distance is small compared to the source radius
Now, what is the flux from a small segment annulus?
Assume “it” is a point source.
Then integrate over all small segments
Energy Exchange Between Parallel Disc’s
If source is Lambertian, (i.e., no L(x,y)), the Non-Point Source case yields:
Where q1/2 will be the “half-angle” or ½ the FOV
(same as q in all our figures)
Non-Point Source Case
(rectilinear coordinates)
Point Source Case:
q
Recall, from point source solution,
Ratio the fluxes…..
If distance “r” is big, they are the same
Using our point source rule, r = 10R
We get a 1% error.
Example 1
What is flux on receiver ?
Example, with a Lambertian blackbody, (D) diameter = 1.2m, T=320 K.
Receiver is a 0.05 m diameter at a distance, r = 3m. Assume t=1
R
r
L
Note: 0.6m x 10 = 6m
We are at 3m.
-Can’t use pt source approx.
Integrating Spheres
Emmett Ientilucci, Ph.D.
Digital Imaging and Remote Sensing Laboratory
Chester F. Carlson Center for Imaging Science
Integrating Spheres
Integrating Sphere
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Used to spatially integrate radiant flux
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Internal or external source or radiation
Integrating Sphere
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Applications Include:
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Light collection from internal or external source (called
lamp measurement photometry)
Uniform light sources
Laser power measurement
LED spectral and flux measurement
Reflectance of either specular or scattering samples
Total or diffuse-only transmittance measurement
Cosine receptor
Irradiance From an Ideal Integrating Sphere
Want to relate flux from source to
total irradiance exiting sphere
Assume:
- Entrance/exit port are small
- Baffles/wires, etc are also small
- Inside surface is Lambertian and uniform
r
F
Irradiance From an Ideal Integrating Sphere
5
5
4
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3
1- 
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2
1
1
0
0
0
0
0.1
0.2
0.3
0.4
0.5
0.6
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Reflectance
0.7
0.8
0.9
1
1
Ideal Integrating Sphere, Example
Find efficiency of a source
-Measure irradiance
-Measure electrical power
(16)
Real Integrating Spheres
Baffle
Baffle
Baffle
Detector
Not perfectly Lambertian  Specular Reflection  Baffles
Integrating Sphere Coating
Coating types offered by Labsphere
Total Flux from a Source
Can be configured for Photo- or Radiometric measurements (integrated or spectral)
Diffuser
- Originated at turn of 20th century
- Fast, simple, method for comparing
output of different lamps.
Calibrated Photopic
Detector
2/3 r
Sphere Photometer
- What is flux from source?
LASER and LED Power Measurements
- Detectors can’t handle laser power density
- Integrating sphere prevents damage
- LED (divergent or non-symmetric beam)
- Overfill active detector area
- Integrating sphere spatially integrates
Laser
Or LED
Detector
- What is the LASER power?
Cosine Receptor - Ideal Diffuser
- Due to its insensitivity to angular alignment,
integrating spheres make excellent ‘collectors’
Protective Weather Dome
Monochromator
Detector
Global Irradiance Monitor
-What is the irradiance / illuminance
of a horizontal surface?
Total Transmittance of Sample
Optional
Monochromator
Irradiation Beam
Sample
Detector
Baffle
t=
SignalSAMPLE
SignalOPEN PORT (No sample)
Diffuse Transmittance of Sample
Optional
Monochromator
Irradiation Beam
Sample
Detector
t=
SignalSAMPLE
SignalOPEN PORT (No sample)
Baffle
Specular
Trap
Same as before,
with added specular trap
Total Reflectance or Sample
Optional
Monochromator
q = Up to 10o
Irradiation Beam
=
q
SSAMPLE
Baffle
SREF (Halon)
Detector
Sample
Collects specular and
diffuse component
Diffuse Reflectance of Sample
q = Up to 10o
Optional
Monochromator
Optional
Monochromator
q = 0o
Irradiation Beam
Specular
Trap
Irradiation
Beam
q
Baffle
Detector
Sample
Collects diffuse component
Detector
Sample
Collects diffuse component
Uniform Source
Radiance Standard
- Many tungsten halogen lamps placed near port
- Must have a regulated current supply