European Training Action
23 – 27 February 2009, Madrid
Functionality 2: Transparency
Marta C. de la Fuente
ElecroOptical Sensors
Sensors and Defence Electronics
INDRA, Spain
www.nanoker-society.org
Contents
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
1
Introduction
2
Transmittance
3
Absorption
4
Reflectivity
5
Scatter
6
Wavefront Quality
7
Conclusion
8
References
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Introduction
DEFINITION
TRANPARENCY is the material property of
allowing light to pass through.
TRANSMITTANCE of a medium is the ratio of
the transmitted to the incident flux
Materials MUST be clear for optical applications
that use refraction.
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
Marta C. de la Fuente
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Introduction
TYPE OF ELEMENTS – Windows and domes
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
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Introduction
TYPE OF ELEMENTS – Lenses & Prisms
European Training Action on
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23 – 27 February 2009, Madrid
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Introduction
Atmospheric transmission and useful materials
Optical glasses (BK7)
Fused silica
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
Silicon
MgF2, Ge, ZnSe, ZnS, Chalcogenides
(GASIR 1: Ge22As20Se58)
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Transmittance
There are three factors that affect the transmittance of a
material:
Absorption,
Reflection
Scatter
I 0  I A  I R  I S  IT
Second surface
First surface
I0
Optical
sample
IR
R2
R1
t
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
IT
IO = Incident flux density
IR = Reflected flux density
IA = Absorbed flux density
IS = Scattered flux density
IT = Transmitted flux density
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Transmittance
BK7
European Training Action on
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23 – 27 February 2009, Madrid
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Absorption
Internal Absorption
Absorption of electromagnetic radiation is the way by
which the energy of a photon is taken up by matter,
typically the electrons of an atom
Internal transmittance
t int  e
a t
a = absorption coefficient (cm-1) = f(l)
t = sample thickness (cm)
BK7
MgF2
Silicon
Germanium
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
@
@
@
@
1cm
1cm
1cm
1cm
&
&
&
&
400nm
2.8µm
4 µm
10 µm
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tint
tint
tint
tint
=
=
=
=
.997
.9945
.9929
.9787
Absorption
Dependency with temperature
Temperature range of operation:
-40ºC to +85ºC
Ge absorption coefficient for depends on:
• Temperature
• Resistivity
internal transmittance for t=3cm vs TRA & resistivity
Resistivity
(.cm)
2
4
10
40
TEMPERATURE (C)
30
40
50
91
89
87
93
92
89
94
94
87
91
87
80
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60
84
85
79
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Absorption
Infrared topic
Emissivity: fraction of the radiation of a blackbody that is
emitted.
  at
(for at<<1)
Besides, radiant emission is proportional to T4 (Plank law)
If T raises:
 a increases and emissivity increases
 radiant emission increases
For high temperatures, radiantion emitted from a window
can be so great as to obscure radiation from the scene
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
Marta C. de la Fuente
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Reflectivity
Refractive Index
Refractive index n is given as the ratio of the velocity of
light in vacuum c to that in the sample v
c
n
v
sin r 1

sin i n
Snell law for refraction depends on n:
BK7
MgF2
Silicon
Germanium
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
@
@
@
@
587.6nm
4µm
4µm
4µm
n
n
n
n
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=
=
=
=
1.5168
1.348829
3.425406
4.024610
Reflectivity
Fresnell Losses
Reflection of light at a smooth surface between two
transparent media:
1  sin 2 i  r  tan 2 i  r  
R  2

2  sin i  r  tan 2 i  r  
i is the angle of incidence,
r the angle of refraction
With normal incidence in air:
 n  1
R
 n  1
2R
Total reflectance of an element:  
1 R
European Training Action on
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2
Reflectivity
MgF2 vs angle of incidence
Reflectance at normal incidence:
BK7
MgF2
Silicon
Germanium
European Training Action on
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@ 1 element
0.08
0.04
0.46
0.53
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@ 5 elements
0.33
0.18
0.95
0.98
Reflectivity
Antirreflection coatings – High efficiency
The average transmission of the coated witness sample, Germanium 1mm
thickness and coated on both surfaces, between 7.45 and 10.5 microns shall
be at least 99% for incidence angles between 0º and 20º.
The average reflection of either coated surface between 7.45 and 10.5
microns shall not exceed 0.2% for incidence angles between 0º and 20º
The coating shall withstand all the following tests:
Humidity, per MIL-C-48497, Paragraph 4.5.3.2.
Moderate abrasion, per MIL-C-48497, Paragraph 4.5.3.3.
Adherence, per MIL-C-48497, Paragraph 4.5.3.1.
Temperature from -62ºC to +90ºC, per MIL-C-48497, Paragraph 4.5.4.1.
Solubility and cleanability, per MIL-C-48497, Paragraph 4.5.4.2.
Water solubility, per MIL-C-48497, Paragraph 4.5.5.3.
European Training Action on
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Marta C. de la Fuente
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Reflectivity
Antirreflection coatings – High durability
The average transmission of the coated witness sample, Germanium 1mm
thickness (second surface with high efficiency AR coating) between 7.45 and
10.5 microns shall be at least 90% for incidence angles between 0º and 20º.
The average reflection of either coated surface between 7.45 and 10.5
microns shall not exceed 2% for incidence angles between 0º and 20º.
The coating shall withstand all the following tests:
Adhesion, per MIL-M-1350C, Paragraph 4.4.6.
Humidity, per MIL-C-675C, Paragraph 4.5.8.
Severe abrasion, per MIL-C-675C, Paragraph 4.5.10.
Salt Solubility, per MIL-C-675C, Paragraph 4.5.7.
Salt Spray, per MIL-C-675C, Paragraph 4.5.9.
Temperature, per MIL-M-13508C, Paragraph 4.4.4.
Windscreen wiper action, no part of the coating shall show signs of
removal when exposed to 5,000 revolutions of a wiper blade under 40
grams load using a sand (to def. Stan. 07-55 type C)/ water mixture.
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
Marta C. de la Fuente
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Scatter
Light can be scattered away from incident direction when
it passes into inhomogeneous regions of the sample:
 grain boundaries
 voids
 impurities
Or when it passes into imperfections on the surfaces:
scratches
digs
dust
The fraction of incident light diverted from the incident
direction is called scatter
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
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Scatter
Internal Scatter
Effective absorption coefficient
a  a absorption  a scatter
European Training Action on
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23 – 27 February 2009, Madrid
Marta C. de la Fuente
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Scatter
External scatter
Total Integrated Scatter (TIS) vs. Surface Roughness
 4d 
TIS  
 l 
2
d is the RMS surface roughness
l is the wavelength
1E-001
VIS 540nm
LWIR 10000nm
TOTAL INTEGRATED SC ATTER
1E-002
l=540 nm
l=10 m
1E-003
1E-004
1E-005
1E-006
1E-007
1E-008
European Training Action on
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10
rms ROUGH NESS (Å)
100
Scatter
Effects of scatter
 degrades the sharpness of the image
 reduces the contrast of objects
 reduces resolution
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
Marta C. de la Fuente
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Wavefront Quality
Concept – Optical Path Difference OPD
Paraxial
sphere
Real aberrated
wavefront
Reference sphere
Paraxial focus
Optical path difference
Rayleigh Criteria: Diffraction limited system
OPD < l/4
European Training Action on
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Wavefront Quality
Inhomogeneity:
Gradual variation of refractive index within an optical
element
Caused by a variation of the chemical composition and
other defects within the bulk material
Effect on wavefront quality: OPD = (n-1)t
NON-COMPENSABLE
European Training Action on
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Wavefront Quality
Birrefringence
For anisotropic materials: light has different velocities
depending on the polarization vs the axis of the crystal.
Calcite crystal
NON-COMPENSABLE
European Training Action on
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23 – 27 February 2009, Madrid
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Wavefront Quality
Refractive index and dispersion - REPRODUCIBILITY
Tolerances:
n +/- 0.0002
v +/- 0.2%
European Training Action on
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23 – 27 February 2009, Madrid
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Wavefront Quality
dn/dT
BK7
Ge
KRS-5
~ 2x10-6/K
~ 400x10-6/K
~ -223x10-6/K
(Thallium bromo-iodide)
Thermal Range: -40ºC to +85ºC
Athermalization:
 optional in VIS
 required in IR
European Training Action on
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23 – 27 February 2009, Madrid
Marta C. de la Fuente
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Wavefront Quality
Manufacturing tolerances: Germanium singlet
European Training Action on
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23 – 27 February 2009, Madrid
Marta C. de la Fuente
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Conclusion
What is the ideal material?
Answer depends on the application, BUT
 low absorption coefficient
 low variation of absorption coeff. with temperature
 no birefringent ?
 low dn/dT
 good homogeneity
Other factors to be considered:
 manufacturability
 environmental conditions
 cost
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
Marta C. de la Fuente
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References
 Gerald C. Holst “Electro-Optical Imaging System Performace” SPIE &
JCD Publishing, 2000
 William Vaughan Ed. “Handbook of Optics” McGraw-Hill 1978
 Dwight E. Gray “American Institute of Physics Handbook” McGraw-Hill
1982
 Daniel C. Harris “Materials for Infrared Windows and Domes” SPIE
Press 1999
 Robert A. Clark “Design and Specification of Diamond Turned Optics”
Proc. SPIE CR38
 ISO 10110 “Optics and optical instruments – Preparation of drawings
for optical elements and systems”
 H.E. Bennet, D. W. Ricks “Effects of surface and bulk defects in
transmitting materials on optical resolution and scaterred light” Proc.
SPIE, 683, 153-159 (1986)
 MHO Technical note “Tight resistivity germanium for IR applications”
 SCHOTT Glass Catalog
 Solomon Musikant “Optical Materials: An introduction to selection and
apllication” Optical Engineering vol 6, Marcel Dekker (1985)
European Training Action on
Ceramic Nanocomposites
23 – 27 February 2009, Madrid
Marta C. de la Fuente
INDRA
www.nanoker-society.org