Srsc 04 lecture 2 source based radiometry

2004 - Lecture 2: Page 1
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Spectroradiometry Short Course
Lecture 2: Source-based Radiometry

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Outline
1.Source Metrology
2.Blackbodies
3.Spectral Radiance Sources
a) Tungsten Strip Lamps
b) Integrating Spheres
c) Diffusely Reflecting Surfaces
4.Spectral Irradiance Sources
a) Lamps
5.Scale Realizations

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Traceability and measurement equation
A1 A2
d
Source Radiometer



 d
s
C
v )
(
)
(


)
(
  

s
Source flux, radiance, or irradiance is known, radiometer response is not known.
The source may be broadband or monochromatic (single wavelength). It may be
a primary standard or a transfer standard.

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NIST spectral Irradiance calibrations
Typical Irradiance Values
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600
Wavelength [nm]
Irradiance
[W
cm
-3
]
FEL
XEXON
ISS w/fiber output
D2
FOS-FEL
FOS-D2
LE7

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NIST spectral radiance calibrations
Typical Radiance Values
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600
Wavelength [nm]
Radiance
[W
cm
-3
sr
-1
]
RFL
L460
L450-12U
L450
L420
V40
LE7
2800 K BB
1100 K BB

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Radiance and Irradiance
1.Radiance Sources
a) Overfill the field-of-view of the radiometer
b) Extended source that is spatially uniform
c) Radiance is independent of view angle
d) Radiance is independent of distance to radiometer
2.Irradiance Sources
a) Underfill the field-of-view of the radiometer
b) Approximate a point source (follows 1/d2 law)
c) Uniform irradiance at the entrance pupil of the radiometer

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The Right Source for You
1.Describe the Source you want to Measure
a) spectral shape, size and angular extent, magnitude,
polarization, stability
2.Choose the Standard Source
a) as similar to the unknown source as possible
b) practical considerations apply
3.If Mismatches occur
a) thorough instrument characterization is required
4.Define the Acceptable Uncertainty!

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Blackbody Sources
Cavity temperature, T [K]
In an enclosed cavity, where the walls are in thermal equilibrium
with the radiation field inside the cavity, the radiation
• depends ONLY on the cavity temperature (T), and
• not on the shape, surface structure, material, etc. of the cavity.
• The radiant flux is a function of the wavelength [].
All materials above absolute temperature (zero kelvin)
emit thermal radiation.

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“Ideal” Blackbody Sources
Enclosed isothermal cavity with a small hole for observation of the radiant flux
The effect of the hole is truly negligible so that the radiation field and the cavity
walls remain in complete thermal equilibrium (the cavity remains isothermal).
The cavity absorbs all incident radiation, and the emitted spectral radiance is
independent of direction. No surface can emit more thermal radiation than a
blackbody at that temperature.
Lb() = spectral radiance of ideal blackbody
Ideal blackbody is difficult to engineer, but we can
still calculate this ideal situation: Lb() is known as
a function of blackbody temperature—this is
Planck’s Law.
Natural Examples:
A large underground cavern
Deep inside a stellar atmosphere

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“Practical” Blackbody Sources
An enclosed cavity with a small hole for observation of the radiant flux
The effect of the hole is small so that the radiation field and the cavity walls
remain in approximate thermal equilibrium.
L () = spectral radiance of real blackbody
)
(
)
(
)
(
b 



L
L

Concept of Emittance (emissivity) ():
Ratio of what you have to the ideal situation.
Depends on cavity isothermality, cavity design,
surface emittance (which may depend on
temperature), temperature of surrounds,
wavelength, direction, and angular acceptance
Graybody: () = 

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Planck’s Law
 
  1
/
exp
1
)
(
2
5
2
1L
b


T
n
c
n
c
L



radiation
of
wavelength
constant
radiation
second
air)
for
1
(
medium
of
refraction
of
index
)
(
radiance
spectral
for
constant
radiation
first
2
1L







c
n
c
Ideal Blackbody
Non ideal Blackbody: L() = Lb() ()
Note nonlinear relationship between Spectral Radiance
and Blackbody Temperature
c1L = 1.19 x 108 [W m4 m-2 sr-1]
c2 = 14 388 [m K]

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Spectral Distribution, Lb()
Wavelength [m]
0.1 1 10
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
2800
o
C
1064.18
o
C
232
o
C
Spectral
radiance
[W
m
-2
sr
-1
m
-1
]
 
  1
/
exp
1
)
(
2
5
1L
b


T
c
c
L



T
c3
max 

 
 
T
c
c
L 

 /
exp
)
( 2
5
1L
b 

Wien Approximation:
c3 = 2898 [m K]
(1337 K)
(505 K)
(3073 K)

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Stefan-Boltzmann Law
• Total exitance M: sum L() over all directions
(into the hemisphere above the opening) and sum
L() over all the electromagnetic spectrum (all
wavelengths)
• For an ideal blackbody, the spectral radiance
lambertian
• With ()   and n()  n, the sums
yield
•  = Stefan-Boltzmann constant
 = 5.670 400 x 10-8 [W m-2 K-4]
)
1
with
(
4
4
2


 n
T
T
n
M 




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Values of Constants
• c1L = 1.191 042 722(93) x 108 [W m4 m-2 sr-1]
• c2 = 14 387.752(25) [m K]
• c3 = 2897.768 6(51) [m K]
•  = 5.670 400(40) x 10-8 [W m-2 K-4] Gold-point Blackbody
CODATA Internationally recognized values of constants (http://physics.nist.gov)
300 400 500
1.26
1.28
1.30
1.32
1.34
1.36
1.38
1.40
1.42
1.44
Signal
[V]
Time [min]

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Example—peak emission
1. What is the wavelength for which the spectral
radiance of an ideal blackbody at 22 C is a
maximum?
a. Convert to kelvin: 22 + 273.15 = 295.15 K
b. Rule: max = c3/T, so max =2898/295 = 9.82 m
c. Application: The room is radiating in the 8 m to
14 m region

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Problems with Blackbodies
1.Temperatures above 3000 K are very difficult to
achieve
2.Expensive to produce accurate systems (testing and
modeling)
3.Not very transportable
4.Slow time constants

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Blackbody Alternatives
1.Lamps, arc sources (many types), heated refractories,
light emitting diodes, lasers, synchrotron radiation
2.Examples:
a) tungsten filament strip lamps
b) tungsten quartz-halogen lamps
c) deuterium (D2) gas discharge lamps
d) xenon arc lamps
e) Nernst glower and Globar

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Tungsten strip lamp features
18
• Spectral Radiance or Radiance
Temperature standards
• Vacuum or Gas-filled
• Quartz or glass windows
• Good stability (especially for the
vacuum type)
• Small target area (0.6 mm wide
by 0.8 mm tall)
• Careful alignment procedures
required
• Calibrated by comparison to a
blackbody or another strip lamp at
0.654 m
• Suited for Devices Under Test
with small field-of-views

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Emittance of Tungsten
0.4
0.41
0.42
0.43
0.44
0.45
0.46
0.47
0.48
0.49
250 350 450 550 650 750 850
Wavelength [nm]
Emissivity
1600 K
2400 K
)
ln(
1
1
2



c
T
T

 =1510 K at 1600 K and 660 nm
Spectral and
temperature
dependence of
tungsten.
Radiance
Temperature

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Tungsten strip lamp output
Gas-filled Lamps (to
suppress tungsten
evaporation)
0
5000
10000
15000
20000
25000
200 600 1000 1400 1800 2200
Wavelength [nm]
Spec.
Rad.
[uW/cm2/sr/nm]
40.4 A
For Spectral Radiance
10
15
20
25
30
35
40
800 1200 1600 2000 2400
Radiance Temp. [deg C]
Lamp
Current
[A]
655.3 nm
For Radiance Temperature

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Comparison of blackbodies and tungsten strip lamps
0 500 1000 1500 2000 2500
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Radiance
(
W
/
cm
3
/
sr
)
Wavelength ( nm )

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Integrating Spheres
1. Features:
a) Spherical geometry
b) Low absorbance
c) Diffuse reflectance
2. Result
a) Flux “averager”
3. Applications
a) Radiance source (add lamp,
laser, LED, etc)
b) Irradiance collector
c) Internal or external sources and
detectors

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Sphere Performance
1.Flux transfer equations yield
2.Baffles to shield direct view of lamps
3.Integrated monitor detectors to record performance
4.Stable power supplies and reflectance of interior wall
 
 
f
A
L




1
)
(
1
)
(
)
(
)
(







A
f


areas
port

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Reflectance and Throughput
0 500 1000 1500 2000 2500
0.75
0.80
0.85
0.90
0.95
1.00
0
5
10
15
20
25
30
Reflectance
Wavelength [nm]
() (Barium Sulfate)
Throughput
()/(1-0.98())

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Sphere Source Protocols
1.Geometry for uniform illumination
a) Lamps baffle
2.Document operation
a) Lamp current, lamp voltage drop, monitor detector signals,
Lamp operating hours
3.Keep coating clean
4.Recalibrate
5.Map spatial uniformity and dependence on view angle

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Radiance of Integrating Spheres
0 500 1000 1500 2000 2500
0
10
20
30
40
50
Spectralon (TM)
Barium Sulfate
Earth Systems
Spectral
Radiance
[

W
/
(cm
2
sr
nm)]
Wavelength [nm]

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Temporal changes in the sphere output
400 500 600 700 800 900
-1.0
-0.5
0.0
0.5
1.0
NIST
UA
3 Lamp Config. 18 June 1997
Percent
Difference
(
Begin
/
End
Runs
)
Wavelength ( nm )
14:22:07 15:22:07 16:22:07 17:22:07 18:22:07
4.60E-006
4.62E-006
4.64E-006
4.66E-006
4.68E-006
4.70E-006
4.72E-006
4.74E-006
4.76E-006
4.78E-006
4.80E-006
Photodiode
Voltage
Time ( hr: min: sec )
Photometer measurements
Changes at 400 nm
are more pronounced

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“Lamp-Plaque” Method
1. Radiance standard
a) substitute for strip lamp,
blackbody, or integrating
sphere
b) Combines irradiance standard
and reflectance standard
2. Measurement Equation
a)  = offset from front post and
radiometric center
b) R =  BRDF
)
50
,
(
)
(
)
50
(
)
,
45
,
0
(
)
,
( 2
2





 E
d
R
d
L






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Halogen Filament Lamps
•Illumination, heating, &
irradiance standards
•Wide commercial selection
•Select on features:
•lifetime
•color temperature
•lumen efficacy
•current or voltage
•built in lens
•base configuration
•Maximum wavelength
range: 250 nm to 2.6 m

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FEL Lamp Irradiance Standards
• 1000 W output
• Coiled-coil structure to increase
emittance
• FEL type (a model number)
• Modified by addition of bipost
base
• Calibrated by comparison to a
high temperature blackbody
• 50 cm from front of post
• 1 cm2 collecting area
• Selected and screened for
undesirable features

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FEL alignment system

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FEL Lamp Screening
1. Inspect, test, anneal, age, pot into base
2. Spectral line screening (currently 0 % pass rate)
a) 250 nm to 400 nm in 0.1 nm steps with 0.04 nm bandpass (emission
and absorption lines)
3. Temporal stability (90 % pass rate)
a) <0.5 % before and after 24 h continuous operation at four
wavelengths in UV to near infrared
4. Geometric (95% pass rate)
a) < 1% in  1 at 655 nm

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FEL Output
0
5
10
15
20
25
30
200 600 1000 1400 1800 2200 2600
Wavelength [nm]
Spectral
Irrad.
[uW/cm2/nm]
50 cm
Calibration Data, FEL at 8.2 A
240 260 280 300 320 340 360 380 400 420
0
1
2
3
4
5
Absorption Lines
Emission Lines
Signal
[V]
Wavelength [nm]
Undesirable Lines
a. 256.97 nm (256.80 nm)
b. 257.67 nm (257.51 nm)
c. 308.48 nm (308.22 nm)
d. 309.47 nm (309.27 nm)
e. 394.57 nm (394.40 nm)
f. 396.27 nm (396.15 nm)

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Dependence on horizontal and vertical angles
-8 -6 -4 -2 0 2 4 6 8
6
4
2
0
-2
-4
-6
Percent different from center
Horizontal Angle [  ]
Vertical
Angle
[

]
-7.5
-6.5
-5.5
-4.5
-3.5
-2.5
-1.5
-0.5
0.5
1.0


50 cm

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Protocols for FEL Standard Lamps
1. Orientation
a) 50 cm from front of posts, entrance pupil diameter
of 1 cm2, use special alignment jig for FELs
2. Electrical
a) maintain polarity, constant current, log voltage drop
and burning hours
b) Similar sensitivity to error in current as strip lamps
3. Operational
a) 30 min warm-up; recalibrate every 50 h
b) transfer to user working standards
c) don’t touch the envelope; don’t enclose the lamp
during operation; baffle properly

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Power Supply Feedback Loop
16 bit DA
Converter
Lamp
0.01  Shunt
Resistor
Computer
Power Supply
Digital
Voltmeter
Voltage to
current
conversion in the
power supply
8.2 A  1 mA
stabilization
~ 5 s

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Vertical Side Horizontal
Optic
Axis Radiometer
Aperture
Lamp Orientations

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Frame, Lamp, and Radiometer
Lamp
Frame
Radiometer
(Shutter)
At NIST
Orientation dependence of the FEL
300 400 500 600 700 800 900 1000
0.92
0.93
0.94
0.95
0.96
0.97
0.98
0.99
1.00
Ratio
to
Initial
Vertical
Signals
Wavelength [nm]
Side
Horizontal
Vertical

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D2 Irradiance Standards
•30 W output
• Stable relative spectral irradiance
distribution
• 200 nm to 350 nm
• Modified by addition of bipost base
(same as FEL)
• Calibrated by a) relative distribution
from wall stabilized hydrogen arc and
b) FEL at 250 nm
• 50 cm from front of post
• 1 cm2 collecting area
• Selected and screened for undesirable
features

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Deuterium, Xe and FEL
200 300 400 500 600 700 800 9001000
1000
0.01
0.1
1
10
100
1000
Spectral
Irradiance
[
W
/
cm
3
]
Wavelength [ nm ]
FEL 1000 W
Xe
Deuterium lamp

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Synchrotron Ultraviolet Radiation Facility (SURF) SURF III
BL 2 Beamline for instrument
calibrations
BL 3 “Whitelight” beamline for deuterium lamp calibrations
BL 4 “UV” beamline for detector calibrations
BL 3
BL 4
BL 2
Source spectral radiance calculated
according to the Schwinger equation

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Spectral Output of Synchrotron Radiation
10 MeV < E0 < 380 MeV
1 10 100 1000 10000 100000
Wavelength  (nm)
0
1014
2*1014
3*1014
4*1014
5*1014
6*1014
Flux
(Photons
/
s)
at

=
4°,
100
mA,
1%
b.w.
380 MeV, c = 8.5 nm
331 MeV, c = 13 nm
284 MeV, c = 20 nm
234 MeV, c = 36 nm
183 MeV, c = 76 nm
131 MeV, c = 210 nm
78 MeV, c = 976 nm
37 MeV, c = 9533 nm
Blackbody 3000 K
DUV
Visible
EUV

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NIST Radiance Scale Realization
1. Gold point blackbody, T = 1337.33 K
2. Lamp 1 at 1337.33 K
a) transfer at 655 nm
3. Lamp 2 at 1530 K
a) transfer at 655 nm, “whiter”
4. Variable temperature blackbody
1610.7 K to 2654.2 K
a) transfer at 655 nm, use from ~ 300 nm to 2400 nm
b) small aperture, high emittance
Expanded uncertainties 0.5% to 1%, depending on wavelength

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View of the VTBB
VTBB = 0.9987
2 mm
diameter
opening of
the
blackbody

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Irradiance Scale Flow Diagram
High Accuracy Cryogenic
Radiometer & Lasers
Aperture Area
Measurement Facility
Detector Transfer Standard
(Trap Radiometer)
Irradiance Filter
Radiometers
High Temperature
Blackbody
Working Standard Spectral
Irradiance Lamps

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NIST Irradiance Scale Realization
Detector-
based
Radiometry

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Irradiance Realization
1000 W
Irradiance
Standard Lamp
Precision
Aperture
High
Temperature
Blackbody
Filter
Radiometer
Spectro-
radiometer

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Reduction in E() Uncertainties
0 500 1000 1500 2000 2500
0
1
2
3
4
5
1990 Scale
2000 Scale
2000 Scale (issued lamps)
Expanded
Uncertainties
(
k
=
2
)
[
%
]
Wavelength [ nm ]

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Summary
1.Sources are required to calibrate radiometers
2.Sources must be
a) selected for the application
b) characterized for performance
c) calibrated – made traceable to SI units
3.For best performance
a) incorporate monitor detectors
b) observe all protocols
c) intercompare with others

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References
1. C.E. Gibson, B.K. Tsai, and A.C. Parr, Radiance Temperature Calibrations, NIST Special
Publication 250-43, Washington, DC, (1998) 56 pp plus appendices.
2. D.G. Goebel, Generalized integrating-sphere theory, Appl. Optics, 6, (1967) 125-128.
3. J.H. Walker, R.D. Saunders, A.T. Hattenburg, Spectral Radiance Calibrations, NBS Special
Publication 250-1, Washington, DC, (1987) 26 pp plus appendices.
4. J.H. Walker, R.D. Saunders, J.K. Jackson, D.A. McSparron, Spectral Irradiance Calibrations,
NBS Special Publication 250-20, Washington, DC, (1987) 37 pp plus appendices.
5. G.J. Zissis, Ed., Sources of Radiation, The Infrared and Electro-Optical Systems Handbook,
Vol. 1, ERIM (Ann Arbor, MI) and SPIE(Bellingham, WA) co-publishers, (1993) 373 pp.

Srsc 04 lecture 2 source based radiometry

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