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4th SFERA Summer SchoolDLR
Hornberg, 16th May 2013
Aránzazu Fernández‐Garcí[email protected] Meyen (DR)Florian Sutter (DLR)
Specifications of reflectors based on guidelines and standards
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4th SFERA Summer SchoolHornberg, 16th May 2013
Contents
1. Introduction2. Reflectance definitions3. Reflectance models4. Requirements for instrumentation5. Commercial instruments6. Prototypes
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4th SFERA Summer SchoolHornberg, 16th May 2013
Introduction
3
• Goals of standardization of reflectance measurements
‒Relevant information in reported values‒Comparability of reported values, uniformity‒No market disturbance by misleading results‒No penalization of certain manufacturers‒Measurement with off the shelf instruments‒Applicability for anybody, not just well equipped labs‒High accuracy (aim 0.005)
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4th SFERA Summer SchoolHornberg, 16th May 2013
Introduction
4
• What do we need to get this goals
‒ A Guideline that everyone follows‒ Relevant parameters to be reported, which are
flexible to different collector design application‒ Adequate procedures and protocols‒ Adequate instruments to provide high accuracy‒ International agreement!‒ Translation into “official” standards
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4th SFERA Summer SchoolHornberg, 16th May 2013
Introduction
5
Topic of reflectance analysis for
different solar materials first
addressed by R.B. Pettit & others
1970s
A RRT was performed between NREL, CIEMAT & DLR.
SolarPACES paper
Spring 2010
Meeting at PSA to discuss methodology & content of reflectance measurement guideline. Participants: C. Kennedy, A. Fernandez, E.
Lüpfert & S.Meyen. Framework of document
June 2010
1st version interim guideline document
created by DLR, NREL & CIEMAT. Published at SolarPACES homepage
Spring 2011
Received comments & reviews presented at Task III meeting in Granada in by CK
Sep. 2011
• History of current guideline
Received comments & reviews presented at Task III meeting in Marrakech by SM
Sep. 2012
2nd version guideline document created.
Published at SolarPACES homepage
March 2013
2nd RRT organized by ENEA
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4th SFERA Summer SchoolHornberg, 16th May 2013
Introduction
6
• SolarPACES Task III: Solar Technology and AdvancedApplications
• Project: Development of guidelines for standards forconcentrating solar power (CSP) components
• Title: Parameters and method to evaluate the solar reflectance properties of reflector materials for concentrating solar power technology
• Authors (institutions): DLR (Germany), CIEMAT (Spain), ENEA (Italy), NREL (USA), Fraunhofer ISE (Germany)
• Authors (companies): Abengoa Solar (Spain‐USA), Flabeg(Germany), 3M (USA), Alanod (Germany)
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4th SFERA Summer SchoolHornberg, 16th May 2013
Introduction
7
Europe: CEN / TC 192 Glass in buildings /
WG5 Mirrors (Oct. 2012)
SolarPACESTask III
USA: ASTM E44.20 Solar Glass
International: IEC / TC 117 Solar Thermal Electric Plants / AG2
Systems and components(March 2012)
Spain: AENOR / CTN 206 Solar Thermal
Power Systems / GT2 Components
(Beginning 2010)
Germany: DKE / K374 Solar Thermal Facilities
for Electricity Generation(nov 2011)
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4th SFERA Summer SchoolHornberg, 16th May 2013
Contents
1. Introduction2. Reflectance definitions3. Reflectance models4. Requirements for instrumentation5. Commercial instruments6. Prototypes
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance definitions
• Interaction between light and matter
• Reflectance– Wavelength, λ– Incidence angle, θ– Light polarization unpolarized
9
ρ
τ
α
++ = 1
Law of conservation of energy
i
r
θ
Is a material property and its angular distribution depends on microscopic
surface flatness
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance definitions
• Laws of reflexion (followed by light)
• Hemispherical reflectance, ρh(λ,θ,h)
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θi First law of reflexion: same planeSecond law of reflexion: θi = θr
θr
Reflexion into the hemisphereFunction of λ and θ
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4th SFERA Summer SchoolHornberg, 16th May 2013
• Diffuse reflectance– Bundle of light rays impinges on an object with a rough or
microscopically structured surface (size≥λ), each individual ray encounters a different surface slope and therefore the law of reflection takes effect for a different θ to the surface normal at this point
– As a result the bundle of light is diffusely scattered in all directions in the plane of incidence
Reflectance definitions
11
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance definitions
• Specular reflectance, ρs(λ,θ,φ), and specularity
12
Reflexion close to the specularFunction of λ, θ and φ
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance definitions
• Specular reflectance function, ρs(λ,θ,φ)
– Hemispherical reflectance– Specular reflectance distribution (beam spread and scattering),
depending on surface roughness and microstructure– Convolution of Gaussians distributions, depending on λ and θKj: weighting factor for the several termsσ: standard deviation of the distribution of reflected light
13
M
j jjhs Kh
12
2
,2exp1,,,,
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance definitions
• Solar‐weighted reflectance
n
iiib
n
iiibi
h
E
ESW
1,
1,)(
)(
14
ASTM G173‐03λ=[280,2500] nmAir Mass AM 1.5
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance definitions
• The proper optical parameter to evaluate the quality of reflectors is the ρs(SW,θ,φ)– SW in the solar spectrum range– As a function of θ– As a function of φ for the angle of interested (application)
• No commercial instruments currently available• Some prototypes and models
15
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance definitions
• Required set of reflectance parameters
16
Spectral hemispherical reflectance, ρh(λ,θ,h)λ=[280,2500] nm, meas θ≤15ºIdeally: meas/model θ ≥15º
Specular reflectance, ρs(λ,θ,φ)At least 3 λ, 0 ≤ φ ≤20 mrad and θ≤15ºIdeally: meas/model as a function of λ, φ and θ
Solar‐weighted reflectanceρh(SW,θ,h) ASTM G173 λ=[280,2500] nm, AM 1.5, θ≤15ºIdeally: ρs(SW,θ,φ) as a function of θ and φ (at least 3)
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4th SFERA Summer SchoolHornberg, 16th May 2013
Contents
1. Introduction2. Reflectance definitions3. Reflectance models4. Requirements for instrumentation5. Commercial instruments6. Prototypes
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4th SFERA Summer SchoolHornberg, 16th May 2013
• ρs(SW,θ,φ) by Pettit approach (Pettit, 1982)
– Assumptions: ratio between specular and hemispherical is wavelength independent and small σ
– Only valid for reflectors with very good specularity– For example, it can be used in high quality silvered glass
reflectors– Check the valitity by comparing hemispherical and specular
reflectance results for several λ and φ
Reflectance models
),,(),,(),,(),,( hSW
hSW h
meash
measss
18
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4th SFERA Summer SchoolHornberg, 16th May 2013
• ρs(λ,θ,φ) by One Gaussians (Pettit, 1977)
Reflectance models
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4th SFERA Summer SchoolHornberg, 16th May 2013
• ρs(λ,θ,φ) by two Gaussians (Pettit, 1977)
Reflectance models
20
σ1 sharpσ2 broader
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4th SFERA Summer SchoolHornberg, 16th May 2013
• ρs(λ,θ,φ) by two Gassians (Gee, 2010)
Reflectance models
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance models
22
• ρs(λ,θ,φ) by Gaussians (Heimsath et al., 2010)
– Glass: b is the offset between reflection terms– Aluminum: τ is a parameter of the exponential decrease with
wide angle scattering– Coefficients are obtained with the Fraunhofer ISE experimental
set up
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance models
23
• Off‐normal solar reflectance (Montecchi, 2012)– Equivalent Model Algorithm (EMA)– Inittially proposed for architectural glazings – Based on refractive index calculation and reflector layers study
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4th SFERA Summer SchoolHornberg, 16th May 2013
Reflectance models
24
• Near‐specular reflectance with TIS (Montecchi, 2012)– Total Integrated Scatter (TIS) relashionship (Beckman and
Spizzichino, 1963)
– σφ is the equivalent roughness
– TIS is measured with the ENEA experimental set up– Coefficients A and σ are obtained
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4th SFERA Summer SchoolHornberg, 16th May 2013
Contents
1. Introduction2. Reflectance definitions3. Reflectance models4. Requirements for instrumentation5. Commercial instruments6. Prototypes
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4th SFERA Summer SchoolHornberg, 16th May 2013
Requirements for instrumentation
26
• Ideal instrument– λ=[280,2500] nm in 5 nm– Controllable that is variable within a wide range and over
small intervals– Measurement at various θ, from near normal to at least 50º or
as a function of θ– Adjustment options to account for different mirror thicknesses
and surface curvature– Absolute measurements, no reference mirror required– Measurement spot size as large as possible, with the option of
reducing size in case of interest– Measurements should be done at the same spot on the sample
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4th SFERA Summer SchoolHornberg, 16th May 2013
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• Ideal instrument (cont)– Non‐contac measurements to avoid damaging the surface – Portable for field measurements: high battery autonomy,
battery status display, light‐weight, small compact size, easy to handle, robust, and digital data storage
– A coordinate system incorporated in the instrument to identifythe position of defects points and study their evolution overtime
– Small concerning associated type‐Buncertainties– No influence by external stray light– No risk to human health or enviromental hazar involved
Requirements for instrumentation
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4th SFERA Summer SchoolHornberg, 16th May 2013
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• Minimun required instrument properties for hemisphericalreflectance, ρh(λ,θ,h)– λ=[280,2500] nm in 5 nm– Measurement at least at near normal incidence (θ <=15º)– Preferably absolute measurements, or with a calibrated reference– If measurements at θ > 15º are possible, the calibration of the
reference mirror must be available for the same θ– If integrating sphere is part of the setup: sphere size needs to be
adequate to minimize errors (diameter>150 mm)
Requirements for instrumentation
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4th SFERA Summer SchoolHornberg, 16th May 2013
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• Minimun required instrument properties for specularreflectance– Measurement at several (at least 3) narrow λ– Several and selectable acceptance apertures (0 ≤ ≤ 20 mrad). The
suitable depends on the collector design– Measurement at least at near normal incidence (θ <=15º)– Preferably absolute measurements, or with a calibrated reference– If measurements at θ > 15º are posible, the calibration of the
reference mirror must be available for the same θ– Adjustment options for correcting beam aligment (for different
mirror thicknesses and surface curvature)
Requirements for instrumentation
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4th SFERA Summer SchoolHornberg, 16th May 2013
Contents
1. Introduction2. Reflectance definitions3. Reflectance models4. Requirements for instrumentation5. Commercial instruments6. Prototypes
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Spectrophotometers
31
Lambda 1050 by Perkin Elmer Cary 5000 by Varian (Agilent)
‒λ = [185,3300] nm‒ Source: Deuterium (UV) + Halogen (VIS+NIR)‒Detectors: PM (UV+Vis)+ PbS (800‐2500) + InGaAs (2500‐3300)‒ Cost: ≈ 68000 €
Spectral Reflectance
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4th SFERA Summer SchoolHornberg, 16th May 2013
• Spectrophotometers Accesories
32
Spectral Hemispherical Reflectance:Integrating Sphere
Spectral Specular Reflectance:URA
Commercial instruments
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Spectrophotometers (Integrating Sphere)
33
‒ θ = 8 º‒ φ hemispherical‒ λ = [200,2500] nm‒ Detectors: InGaAs + PMT‒Cost: 2900 €
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Spectrophotometers (URA)
34
‒ θ = 8‐65 º‒ φ unknown‒ λ = [190,3100] nm‒ Detectors: PbS and Si‒Cost: 17000 €
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Spectrophotometers: comparison
35
0
10
20
30
40
50
60
70
80
90
100
250 500 750 1000 1250 1500 1750 2000 2250 2500
Ref
lect
ance
Wavelength range (nm)
4-mm silvered glass hem
URA 8º0
10
20
30
40
50
60
70
80
90
100
250 500 750 1000 1250 1500 1750 2000 2250 2500
Ref
lect
ance
Wavelength range (nm)
Aluminium1 hemURA 8º
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers
15R‐USB, D&S
36
‒ θ = 15º‒ φ = {3.5,7.5,12.5,23.0} mrad‒ λ = [635,685] nm; 660 nm‒ Source: LED‒ Resolution: 0.001‒ Spot size: 10 mm ‒ Optical aligment‒ Curvature and thinkness‒Time consuming‒ Cost: 16000 €
Monochromatic Specular Reflectance
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers
37
15R‐RGB (MWR), D&S
‒ θ = 15º‒ φ = {2.3,3.5,7.5,12.5,23.0} mrad‒ λ = {460,550,650,720} nm‒ Source: white light + filters‒ Resolution: 0.001‒ Spot size: 10 mm‒ Optical alignment‒ Curvature and thickness‒ Cost: 18000
Monochromatic Specular Reflectance
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers comparison: 15R and MWR
38
‐0,03
‐0,02
‐0,01
0,00
0,01
0,02
0,03
Δρs=ρs (D
&S, 660
nm) ‐
ρs (M
WR, 650
nm)
Sample
Difference between D&S and MWR specular reflecance
23 mrad
12.5 mrad
7.5 mrad
3.5 mrad
Glass: 0.001±0.001; Innovative: up to 0.016±0.009
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers comparison: MWR and spec
39
0,800
0,820
0,840
0,860
0,880
0,900
0,920
0,940
0,960
0,980
1,000
250 500 750 1000 1250 1500 1750 2000 2250 2500
Ref
lect
ance
Wavelength, nm
Comparison hemispherical reflectance and specular reflectance
Glass #1
3.5 mrad
7,5 mrad
12,5 mrad
23 mrad
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers comparison: MWR and spec
40
0,600
0,650
0,700
0,750
0,800
0,850
0,900
0,950
1,000
250 500 750 1000 1250 1500 1750 2000 2250 2500
Ref
lect
ance
Wavelength, nm
Comparison hemispherical reflectance and specular reflectance
3.5 mrad
7.5 mrad
12.5 mrad
23 mrad
Aluminium #2
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers
Condor, Abengoa
41
‒ θ = n/a‒ φ = 408 mrad‒ λ = {435,525,650,780,940,1050}nm‒ Source: LED‒ Resolution: 0.001‒ Spot size: 1 mm‒ No optical alignment‒ Easy to handle‒ Cost: ≈ 18000 €
Monochromatic Specular Reflectance
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers
42
SOC 410 Solar, Surface Optics
Monochromatic Hemispherical and Diffuse Reflectance
‒ θ = 20º‒ φ = 52.4 mrad‒ λ = 7 bands; 330‐2500 nm‒ Source: Tungsten‒ Resolution: n/a‒ Spot size: 6.35 mm‒ No optical alignment‒ Cost: 19000 €
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers
CM 700‐d, Konica Minolta
43
Spectral Hemispherical and Diffuse Reflectance
‒ θ = 8º‒ φ ≈ 50 mrad‒ λ = [400‐700] nm‒ Source: Xenon‒ Resolution: 0.0001‒ Spot size: 8 mm‒ No optical alignment‒ No curvature‒ Cost: 10000 €
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4th SFERA Summer SchoolHornberg, 16th May 2013
Commercial instruments
• Reflectometers
µScan, SMS
44
Monochromatic Specular Reflectance
‒ θ = 25º‒ φ = n/a‒ λ = 670 nm‒ Source: Laser‒ Resolution: 0.001‒ Spot size: 1 mm‒ No optical alignment‒ Small repeatability‒ Cost: ≈14000 €
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4th SFERA Summer SchoolHornberg, 16th May 2013
Contents
1. Introduction2. Reflectance definitions3. Reflectance models4. Requirements for instrumentation5. Commercial instruments6. Prototypes
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4th SFERA Summer SchoolHornberg, 16th May 2013
Prototypes
46
• Space Resolved Specular Reflectometer (SR)2: DLR
(Sutter et al., 2010)
Aperture wheel
Pinhole
Light source
Camera
Filter wheel / Density filter
Lens 0
Lens 1
Lens 2
Iris
2*Acceptance angle Mirror sample
Housing
‒ θ = 15º‒ φ = {3.5,6.0,12.5} mrad‒ λ = {410,500,656} nm‒ Resolution: 37 px/mm ‒ Spot size: 50 mm ‒ CMOS camera sensor
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4th SFERA Summer SchoolHornberg, 16th May 2013
• Space Resolved Specular Reflectometer (SR)2: DLR– Study of soiling and aging – Average specular reflectance– Detection of corroded area– Reflectance of corrosion spots– Reflectance of non corroded
parts of the surface
Prototypes
47
Exposure Time
0.10 M
0.25 M
0.49 M
0.94 M
(Sutter et al., 2010)
656nm 12.5mrad (D&S*) 12.5mrad(SR)²
Diff.
1.6‐mm silvered glass 96.3% 95.2% ‐1.1%
Aluminum 85.1% 89.2% +4.1%
0.95‐mm silvered glass 96.1% 97.4% +1.3%
4‐mm silvered glass 95.5% 96.2% +0.7%
Fist surface mirror 93.0% 94.6% +1.6%
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4th SFERA Summer SchoolHornberg, 16th May 2013
• Automatic device TraCS (DLR)•Automatic real time measurement of a test mirror with sun spectrum•High time resolution allows for identification of weather influences on soiling•Validated with hand measurement devices
0.5
0.6
0.7
0.8
0.9
1.0
0.5 0.6 0.7 0.8 0.9 1.0Reference cleanliness
TraC
S c
lean
lines
s[Wolfertstetter, 2012]
0.6
0.7
0.8
0.9
1.0
1.1
16-Jun-12 17-Jun-12 18-Jun-12 19-Jun-12 20-Jun-12 21-Ju
clea
nlin
ess
fact
or
Prototypes
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4th SFERA Summer SchoolHornberg, 16th May 2013
Prototypes
• Spectral Specular Reflectometer (SSR): NREL
– High‐speed measurements– Easy to use – removes operator error– Measure s(,) as a function of θ– Flat and curved samples
49
Fiber
Light SourceCollimating Lens
MirrorSample
Collection Optics
FiberVariableAperturePinhole
Spectrometer
(Kennedy, 2010)
‒ φ = [2‐50] mrad‒ λ = [300‐2500] nm
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4th SFERA Summer SchoolHornberg, 16th May 2013
Prototypes
50
• ENEA experimental setup
(Montecchi et al., 2012)
‒ θ = 2.9º‒ φ = {3.39,6.56,9.84, 13.97,16.83,19.95} mrad‒ Source: lasers 632.8, 543.5 and 405.5 nm‒ Detector: 150 mm IS
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4th SFERA Summer SchoolHornberg, 16th May 2013
Prototypes
51
• Fraunhofer ISE experimental setup
‒ θ = 8‐90º‒ φ = {3.5,6.0,12.5} mrad‒ Source: white light or laser‒ Aperture size: 0.2x20 mm ‒ Detector: photodiode
(Heimsath et al., 2010)
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4th SFERA Summer SchoolHornberg, 16th May 2013
Prototypes
52
• Fraunhofer ISE experimental setup
(Heimsath et al., 2010)
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4th SFERA Summer SchoolHornberg, 16th May 2013
Thank you for your attention!!!!!
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