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BEST OF
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Featur
e
ALL NE s
MATERIAW
L!
TECHSPEC®
Diode and
Broadband
Laser Mirror
TECHSPEC® Fused
Silica Ball Lenses
TECHSPEC®
Fused Silica
Cylinder
Lenses
O
ssue
I
1
0
1
ptics
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Table of Contents
Anti-Reflection (AR ) Coatings …………………………………………………………….2
Cleaning Optics ……………………………………………………………………………………..5
Metallic Mirror Coatings ……………………………………………………………………….7
Optical Glass ………………………………………………………………………………….………10
Understanding Ball Lenses ……………………………………………………………………14
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Table of Contents
Written by EO engineers for you, our customers, we hope you find these application notes informative and fun. Let us
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Thasha Ramdas
Technical Content Editor
page 1
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Anti-Reflection (AR) Coatings
Edmund Optics® offers all TECHSPEC® transmissive optics with a variety of anti-reflection (AR) coating options that vastly improve the
efficiency of the optic by increasing transmission, enhancing contrast and eliminating ghost images. Most AR coatings are also very
durable, with resistance to both physical and environmental damage. For these reasons, the vast majority of transmissive optics
include some form of anti-reflection coating. When specifying an AR coating to suit your specific application, you must first be fully
aware of the full spectral range of your system. While an AR coating can significantly improve the performance of an optical system,
using the coating at wavelengths outside the design wavelength range could potentially decrease the performance of the system.
COATING THEORY
Figure 1: MgF2 Anti-Reflection Coating Performance
Antireflective MgF2 Coating Properties
6.0
Figure 2: Illustration of Light Interacting with Thin Film
Substrate: BK7
Air
no
Uncoated at 45°
Reflectance %
5.0
Thickness, t
Optical Thickness = n f * t
Uncoated at 0° (Avg.=4.25%)
4.0
3.0
1.0
Thin Film
Coated at 0° (Avg.=1.5%)
400
450
500
550
600
650
700
=
+
nf
Coated at 45°
2.0
Substrate
ns
750
Wavelength (nm)
Why Choose an Anti-Reflection Coating?
As light passes through an uncoated glass substrate, approximately 4% will be reflected at each interface. This results in a total
transmission of only 92% of the incident light. Applying an AR coating on each surface will increase the throughput of the system and
important if the system contains many transmitting optical elements. Also, many low-light systems incorporate AR coated optics to
allow for efficient use of light. Figure 1 demonstrates the difference between an uncoated and coated single surface BK7 substrate.
The coating used is a ¼ wave of MgF2 centered at 550nm.
How Does an Anti-Reflection Coating Work?
The transmission properties of a coating are dependent upon the wavelength of light being used, the substrate’s index of refraction,
the index of refraction of the coating, the thickness of the coating, and the angle of the incident light.
The coating is designed so that the relative phase shift between the beam reflected at the upper and lower boundary of the thin film
is 180°. Destructive interference between the two reflected beams occurs, cancelling both beams before they exit the surface. The
optical thickness of the coating must be an odd number of quarter wavelengths (l/4, where l is the design wavelength or wavelength
being optimized for peak performance), in order to achieve the desired path difference of one half wavelength between the reflected
beams, which leads to their cancellation.
The equation for determining the index of refraction of the thin film needed for complete cancellation of the two beams is:
nf is the index of refraction of the thin film
n0 is the index of refraction of air (or the incident material)
nf = (n0 + ns)½
ns is the index of refraction of the substrate
Figure 3: UV and Visible Anti-Reflection
Coating Performance
Figure 4: NIR Anti-Reflection
Coating Performance
UV-VIS (250-700)
¼λ MgF2 @ 550nm
300
400
500
600 700 800
Wavelength (nm)
900
1000 1100
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
550
¼ Wave MgF2
NIR I (600-1050nm)
NIR II (750-1550nm)
Telecom-NIR (1200-1600nm)
AR Coating
VIS-NIR (400-1000nm)
Reflectance (%)
Reflectance (%)
VIS 0° (425-675nm)
UV-AR (250-450nm)
VIS 0, 45
NIR Anti-Reflection Coatings
UV and Visible Anti-Reflection Coatings
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
200
Figure 5: Wavelength Selection Chart
Anti-Reflection (AR) Coatings
reduce hazards caused by reflections traveling backwards through the system (ghost images). Anti-reflection coatings are especially
VIS-NIR
UV-VIS
Telecom-NIR
UV-AR
NIR I
NIR II
200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600
750
950
1150
Wavelength (nm)
1350
1550
1750
UV
Visible
Near-IR
Wavelength (nm)
page 2
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Anti-Reflection (AR) Coatings
COATING SPECIFICATIONS
Anti-Reflection Coating Options
Edmund Optics® offers all TECHSPEC® lenses with an optional single layer dielectric, anti-reflection (AR) coating to reduce surface
reflections. In addition, custom single-layer, multi-layer, V, and 2V coatings are available for both our off-the-shelf and large volume
custom orders.
¼λ MgF2: The simplest AR coating used is ¼λ MgF2 centered at 550nm (with an index of refraction of 1.38 at 550nm). MgF2 coating
is ideal for broadband use though it gives varied results depending upon the glass type involved.
VIS 0° and VIS 45°: VIS 0° (for 0° angle of incidence) and VIS 45° (for 45° angle of incidence) provide optimized transmission for 425 –
675nm, reducing average reflection to 0.4% and 0.75% respectively. VIS 0° AR coating is preferred over MgF2 for visible applications.
VIS-NIR: Our visible/near-infrared broadband anti-reflection coating is specially optimized to yield maximum transmission (>99%) in
the near infrared.
Telecom-NIR: Our Telecom/near-infrared is a specialized broadband AR coating for popular telecommunications wavelengths from
1200 – 1600nm.
UV-AR and UV-VIS: Ultraviolet coatings are applied to our UV fused silica lenses and UV fused silica windows to increase their
coating performance in the Ultraviolet region.
wavelengths of common fiber optics, laser diode modules and LED lights.
Anti-Reflection Coating Information
Name
Wavelength Range
Reflection Specifications
MgF2
¼ Wave @ 550nm
Rave ≤ 1.75% 400-700nm (N-BK7)
VIS 0°
425-675nm
Rave ≤ 0.4% 425-675nm
VIS 45°
425-675nm
Rave ≤ 0.75% 425-675nm
UV-AR
250-450nm
Rabs ≤ 1.0% 250-425nm
Rave ≤ 0.75% 250-425nm
Rave ≤ 0.5% 370-420nm
UV-VIS
250-700nm
Rabs ≤ 1.0% 350-450nm
Rave ≤ 1.5% 250-700nm
VIS-NIR
400-1000nm
Rabs ≤ 0.25% @ 880nm
Rave ≤ 1.25% 400-870nm
Rave ≤ 1.25% 890-1000nm
NIR I
600-1050nm
Rave ≤ 0.5% 600-1050nm
NIR II
750-1550nm
Rabs ≤ 1.5% 750-800nm
Anti-Reflection (AR) Coatings
NIR I and NIR II: Our near-infrared I and near-infrared II broadband AR coatings offer exceptional performance in near-infrared
Rabs ≤ 1.0% 800-1550nm
Rave ≤ 0.7% 750-1550nm
Telecom-NIR
1200-1600nm
Rabs ≤ 0.25% 1295-1325nm
Rabs ≤ 0.25% 1535-1565nm
Rave ≤ 0.25% 1200-1600nm
page 3
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Anti-Reflection (AR) Coatings
TECHSPEC® Anti-reflection coating Products
Lenses
o
Plano-Convex PCX Lenses
o
Achromatic Lenses
o
Double Convex DCX Lenses
o
Cylinder Lenses
o
Plano-Concave PCV Lenses
o
Drum Lenses
o
Double Concave DCV Lenses
o
Dispersion Prisms
o
Retroreflection Prisms
o
Right Angle Prisms
o
Specialty Prisms
o
Image Rotation Prisms
o
Penta Prisms
o
Visible Windows
o
UV and IR Windows
Prisms
More Information Online: Product Downloads
o
Spec Sheets
o
2D Drawings &
o
Filter & Lens Curves
3D Models
o
Zemax Files & More!
Anti-Reflection (AR) Coatings
Windows
page 4
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Cleaning optics
After purchasing an optical component, exercising proper care can maintain its quality and extend its usable lifetime. Choosing the
proper cleaning products and using the proper methods are as important as cleaning the component itself. Improper cleaning practices can damage polished surfaces or specialized coatings that have been used on optics such as lenses, mirrors, filters, or gratings,
degrading the performance in almost any application. Also, be aware of your clothing and your environment while cleaning optics;
shirts with zippers and buttons can scratch your optics, likewise dirty or dusty environments are not well suited for optical applications.
CLEANING PRODUCTS
There are a variety of cleaning products and cleaning methods to use depending upon the type of optic to be cleaned and the nature of the care needed, ranging from removing dust to smudges on the surface. Products such as Pick-Up Tools, Tweezers, Gloves,
Compressed Air, Cotton-Tipped Swabs, Lens Tissue, Lens Cleaners, Reagent-Grade Isopropyl Alcohol, Reagent-Grade Acetone, and
De-Ionized Water can be used to ensure a long product lifetime. Each type of cleaning product has its own unique benefit: Pick-Up
tools and Tweezers are useful for holding optics in place while cleaning, Gloves provide a protective barrier to optics from any moisture
or oils on your hands, Compressed Air effectively removes surface dust without directly contacting any coating an optic may have,
Cotton-Tipped Swabs and Lens Tissue offer an effective means to wipe away any dirt without scratching an optic, and Lens Cleaners,
Reagent-Grade Isopropyl Alcohol and Acetone, and De-Ionized Water each safely clean an optic.
An important point to stress is that you should NEVER clean plastic optics or optics in plastic housings with Acetone because it
will damage the plastic. Therefore, if you have a plastic optic, then you should use Compressed Air, Reagent-Grade Alcohol, or
De-ionized Water. If you are unsure about the type of optic that you have or the reactivity of your optical substrate or coating, then using
De-Ionized Water and a little bit of dish soap is the safest way to make sure the optic is not damaged by harsh chemicals.
Want to see one of our Applications Engineers demonstrate how to clean optics?
View our Cleaning Optics video: www.edmundoptics.com/videos
Cleaning Methods - Lenses
Dust is the most common contaminant and can usually be removed using Compressed
Air. If more cleaning is necessary, hold the lens in Lens Tissue and apply a few drops of
Reagent-Grade Isopropyl Alcohol, Reagent-Grade Acetone, or Lens Cleaning Solution.
Slowly turn the lens while applying pressure in the center and working outward, to pull
should be cleaned as soon as possible to avoid staining or damaging the optic. Larger
dirt particles, however, should be removed with a Dust-Free Blower before attempting
to clean the optic with lens tissue. Larger particles trapped under the cloth will scratch
the surface you are attempting to clean. If the lens is still dirty - for instance, if the oil
was just redistributed and not cleaned off the optic - then a mild soap solution can be
used to gently wash the lens. Repeat the procedure with Reagent-Grade Isopropyl
Alcohol or Reagent-Grade Acetone to eliminate streaks and soap residue.
Cleaning Methods - Mirrors
After blowing off dirt and dust with Compressed Air, the Drag Method of cleaning can
be used to remove fingerprints or other contaminants on mirrors. In the Drag Method,
cleaning optics
dirt off the lens instead of redistributing it on the surface. Fingerprints on a coated lens
lens tissue saturated with Reagent-Grade Isopropyl Alcohol or Reagent-Grade Acetone
is slowly dragged across the surface. If done correctly, the solvent will evaporate uniformly without leaving streaks or spots. Bare metallic coatings are delicate and cannot be cleaned in this manner. Dirt and fingerprints will permanently damage a bare
metal-coated mirror, so preventive measures should be taken to prolong the lifetime
of the coating.
page 5
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Cleaning optics
Cleaning Methods - Filters
Filters can be cleaned using the same methods as lenses or mirrors. The preferred
method is to use Compressed Air or an Air Blower to remove dust. If additional cleaning is required, then using a Cotton-Tipped Swab or Lens Tissue saturated with Reagent-Grade Isopropyl Alcohol, Reagent-Grade Acetone, or Lens Cleaning Solution will
work too.
Cleaning Methods - Gratings
Due to the construction of diffraction gratings, the only recommended cleaning method is to use Compressed Air or an Air Blower to remove surface dust. Avoid methods
that require any direct contacting of the grating surface. Ultrasonic cleaning should not
be used as it may separate the grating surface from the glass substrate.
Note: The same precautions taken with Gratings should be applied to Wire Grid Polarizers in order to ensure a long product lifetime.
Cleaning Methods - Micro Optics
Micro Optics may also be cleaned using Reagent-Grade Isopropyl Alcohol or ReagentGrade Acetone but, due to their extremely small size, they require special handling and
care. For example, micro lenses typically refer to lenses smaller than 3mm in diameter.
Delicate tweezers, such as Non-Marring, Bamboo, and Plastic Tweezers, may be used
to securely hold a micro optic by its edge, or a Vacuum Pick-Up Tool may be used.
1. Always handle optics by the edges, never touch the optical surface with your fingertips. The moisture on your fingertips can
sometimes damage the coating on optics, and if a fingerprint is left on an optical surface for a long time, it can become a
permanent stain. Even if you are wearing gloves, avoid touching the optical surface.
2. Never handle optics with metal implements. Reduce the chance of damage by using wooden, bamboo, or plastic implements
to handle optics. Vacuum pens are handy for small optics.
3. Always place an optic on a soft surface, especially if the optical surface is convex. Resting on a hard tabletop can cause
scratches in the surface.
4. For lens systems or assemblies, always replace the lens cap when not in use to protect the optical surface from damage.
cleaning optics
5 Tips for Handling ANY Optics to Keep Them in Good Condition
5. To store optics, wrap them individually in clean, lint free Lens Tissue and put in a safe place. Never store unwrapped optics
together in a box or bag, as they will damage each other if they touch. Never store optics with heavier items on top of them.
Want to View our Cleaning Optics Whitepaper?
Download a PDF Today: www.edmundoptics.com/downloads
page 6
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Metallic Mirror Coatings
Just as with anti-reflection (AR) coatings, metallic mirror coatings are also designed for different regions of the spectrum.
Edmund Optics® offers a series of protected metallic coatings capable of providing high reflectance values for applications using wavelengths ranging from 250nm to beyond 10μm. Our standard metallic mirror coatings include Protected Aluminum, Enhanced Aluminum, UV Enhanced Aluminum, Protected Gold and Protected Silver. Protected Aluminum and Enhanced Aluminum are typically used
for visible applications. UV Enhanced Aluminum can be used for UV and visible applications. Protected Gold offers high reflectance for
Infrared or near-Infrared wavelengths. Protected Silver provides the highest reflectance between 500-800nm but is best suited as a
rear surface reflector due to its sensitivity to tarnishing.
INTRO TO FIRST SURFACE AND SECOND SURFACE MIRRORS
Figure 1
Figure 2
ΘI
Incident
Light
Reflected
Light
Θr
Reflected
Light
Incident
Light
ΘI = Θr
First Surface is Coated
First Surface is Uncoated
Substrate
Substrate
Second Surface is Uncoated
Second Surface is Coated
Edmund Optics® offers a variety of mirror substrates and geometries coated with our standard metallic mirror coatings, laser-line coatings, as well as, custom broadband, narrowband, single laser line, dual laser line and laser-line beamsplitter coatings. All of our mirror
products are first surface mirrors, Figure 1, featuring a high reflectivity coating, such as metallic or multi-layer dielectric, deposited on
one surface of the glass substrate (we also offer a few families of metal substrate mirrors). By contrast, second surface mirrors, Figure
2, are manufactured in the same manner as first surface mirrors but feature an additional cover glass on the coated surface, usually
silver-plated, for protection. This cover glass protects the coating layer from scratching and oxidization. Edmund Optics recommends
1. Light incident on a second surface mirror is subject to dispersion from reflection off the cover glass. Glass tends to disperse
light, causing different wavelengths to refract at different angles.
2. Due to Fresnel reflections at the cover glass, reflected light exhibits ghost images, indicated by the dashed red line.
3. Due to Fresnel reflections, responsible for approximately 4% reflection loss at each interface, reflection efficiency is reduced
by using a second surface mirror.
Reflectance Curves for Metallic (Mirror) Coatings
Ultraviolet
Visible
Infrared
STANDARD METALLIC MIRROR COATING
SPECIFICATIONS
100
Reflectance Curves for Metallic (Mirror) Coatings
Visible
Reflectance %
Ultraviolet
90
Infrared
100
Reflectance %
90
80
70
60
50
80
Figure 3: Metallic Mirror Coating Reflectance Curves rises 0.7
gradually
10μm
0.2 Theoretical
0.3
0.4reflectance
0.5
0.6
0.8 through
0.9
1.0
1.1
70
Wavelength in Microns (µm)
60
UV Enhanced Aluminum (R >85% 0.25-0.7 Microns)
Protected Aluminum (R>85% 0.4-0.7 Microns)
Enhanced Aluminum (R >95% 0.45-0.65 Microns)
Protected Gold (R>97% 0.8-2 Microns, R>94% 0.7-0.8 Microns)
Protected Silver (R >98% 0.5-0.8 microns, R >98% 2-10 microns)
50
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Wavelength in Microns (µm)
0.9
1.0
1.1
UV Enhanced Aluminum (R >85% 0.25-0.7 Microns)
Protected Aluminum (R>85% 0.4-0.7 Microns)
Enhanced Aluminum (R >95% 0.45-0.65 Microns)
Protected Gold (R>97% 0.8-2 Microns, R>94% 0.7-0.8 Microns)
Protected Silver (R >98% 0.5-0.8 microns, R >98% 2-10 microns)
Metallic Mirror Coatings
first surface mirrors to second surface mirrors for the following three reasons (Figure 2):
Protected Aluminum
Protected Aluminum is our most popular mirror coating for applications in the visible and near-Infrared. A half wavelength of Silicon
Monoxide (SiO) is used as an overcoat to protect the delicate aluminum. This treatment provides an abrasion-resistant surface while
maintaining the performance of aluminum.
page 7
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Metallic Mirror Coatings
Enhanced Aluminum and UV Enhanced Aluminum
A multi-layer film of dielectrics on top of aluminum is used to enhance the reflectance in the visible or ultraviolet regions. Enhanced
Aluminum is ideal for applications requiring increased reflectance from 400nm-650nm while UV Enhanced Aluminum yields increased
reflectance from 250nm-400nm. The multi-layer film also provides the handling characteristics of the protected aluminum coating.
Protected Gold
Protected Gold is effective for applications requiring high reflectance in the near-Infrared and Infrared regions. Since a durable coating
is necessary for handling purposes, we offer gold with a Silicon Monoxide overcoat. The performance of gold (96% reflectivity from
750nm-1500nm) is maintained, while providing a more durable finish.
Protected Silver
Protected Silver offers excellent reflectivity in the visible and infrared regions, making it an excellent choice for broadband applications
that span multiple spectra. Silver offers a reflectivity >98% from 500-800nm. A protective coating reduces silver’s tendency to tarnish
but the coating is ideally suited to low humidity environments.
Metallic Mirror Coating Information
Name
Reflection Specifications
400-700nm
R(ave) >85%
Enhanced Aluminum
450-650nm
R(ave) >95%
UV Enhanced Aluminum
250-700nm
R(ave) >85%
Protected Gold
700-800nm
R(ave) >94%
800-10000nm
R(ave) >97%
500-800nm
R(ave) >98%
2000-10000nm
R(ave) >98%
Protected Silver
CUSTOM MIRROR COATINGS AVAILABLE
Edmund Optics® is a leading designer and manufacturer of laser coatings for mirrors, beamsplitters, windows, and other optical
components. We work with a number of testing laboratories to independently qualify and certify our coatings for resistance to laser
damage. All specified damage threshold values have been obtained by testing multiple coating runs to ensure an accurate result. The
values specified are conservative, with typical data showing that our mirrors are able to withstand 2-3 times the specified threshold.
We are able to design and manufacture coatings for single or multiple laser lines, as well as for broadband tunable laser sources. Coatings designs are also available for partial reflectors, output couplers, and etalons, and for any angle of incidence or polarization state.
Our world class manufacturing facilities are able to produce substrates with surface accuracies of < 1/20λ, surface qualities of 10-5,
parallelism of <0.5 arcseconds, and surface roughness of <5 Angstroms in any glass and most crystalline substrates. We will also coat
customer-supplied substrates for prototype or production volumes.
For high-power applications, coating designers choose materials with intrinsically low absorption at the relevant wavelengths. But the
customer also needs to be aware that the choice of coating materials for high-power applications is limited. The system-designer does
well to design for optics with the appropriate damage thresholds from the beginning of the optical design process.
Coatings for use with high-power ultraviolet (UV) lasers are made of different materials from those for use in the visible and near-
Metallic Mirror Coatings
Wavelength Range
Protected Aluminum
infrared (IR). The core structure of high-reflection coatings is typically a repeating stack of high- and low-index layers, each a quarterwavelength thick. The design of the coating can significantly alter the damage threshold. Simply adding a half-wave of low-index material (normally silicon dioxide) as the final layer can result in measurably higher damage thresholds. Silicon dioxide (SiO2) is the generally
accepted and ubiquitous choice for low-index layers, and dielectric metal oxides in general are preferred materials for UV, visible, and
near-IR laser applications. Choosing a material for high-index layers is not as straightforward: oxides of titanium, tantalum, zirconium,
hafnium, scandium, and niobium are all popular high-index materials.
page 8
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Metallic Mirror Coatings
BBHR and NBHR (Notch Filter)
oVisible Broadband: Rave >98%, o
Custom Coatings Available
Broadband and Narrow Band High Reflectors
for Wavelengths between
Visible
Broadband
Reflector
oNotch Filter: R >90% at Center 300-1800nm (BBHR) and
Wavelength (CWL), FWHM
350-850nm (Notch)
< 0.12 CWL, Tave >90% for
Out of Band Wavelengths
o
Max Reflectance at 1 or 2
o
Single Line: R >99.5%, at
Customer Selected
Design Wavelength
Wavelengths from
o
Dual Line: R >98.5%, at
190-3000nm (Single Line)
Both Wavelengths of Interest
or 350-1700nm (Dual Line)
o
Partial Reflectance from 5%R
o
45°, Random and
to 95%R, per Customer
Non-polarizing Versions
o
Design Wavelengths from
Figure 4
100
90
80
Reflection (%)
70
Single Stack
High Reflector
Narrow Band
Reflector
(Notch Filter)
60
50
40
425-675nm, 0-45° AOI
30
20
10
0
400
450
500
550
600
Wavelength (nm)
650
700
Single and Dual Laser Line Reflector
Figure 5
Reflection (%)
Dual Laser Line Reflector
100
90
80
70
60
50
40
30
20
10
0
400 500 600 700 800 900 1000 1100 1200 1300 1400
Wavelength (nm)
Laser Line Beamsplitter
Laser Line Beamsplitter Coatings
Requirement
Reflection (%)
100
90
80
70
60
50
40
30
20
10
0
1000
1200
1400
1600
1800
Wavelength (nm)
2000
2200
2400
o
Reflectance Specification:
R% = ±2%; Typical: R% = ±1%
250-3000nm
Standard metallic mirror coatings Products
Prisms
Mirrors
o
Dispersion Prisms
o
Diverging Mirrors
o
Right Angle Prisms
o
Flat Mirrors
o
Image Rotation o
Focusing Mirrors
o
Specialty Mirrors
Prisms
o
Metallic Mirror Coatings
Figure 6
Retroreflection Prisms
o
Specialty Prisms
page 9
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Optical Glass
Optical Glass Specifications
Selecting a glass material is important since different glass types have different characteristics. Edmund Optics® offers a wide variety
of glass types which can be selected on the basis of the following characteristics.
The index of refraction and Abbe number of a glass are typically used by designers as degrees of freedom when designing systems.
The index of refraction refers to the ratio of the speed of light in a vacuum to the speed of light through a given material at a given
wavelength, while the Abbe number of a material quantifies the amount of dispersion (variations in index) for a specific spectral range.
For instance, a higher index of refraction generally bends light more efficiently so there is less of a need of curvature in the lens. Spherical aberration is less present in lenses with higher indices of refraction, while light travels faster through materials with lower indices
of refraction. A high Abbe number generally gives less color dispersion and reduces color aberration. Also, certain glass types have
different transmission wavelength regions.
The density of a glass helps determine the weight of the optical assembly and, along with lens diameter, becomes critical for weight
sensitive applications. Density also generally denotes the ability to work with the glass and is somewhat proportional to the cost of
the material. When dealing with applications involving extreme temperatures and quick temperature differentials, a glass’ coefficient of
expansion becomes a key factor. Opto-mechanical designers need to keep this in mind when designing optical assemblies.
Many glass manufacturers offer the same material characteristics under different trade names and most have modified their products
and processes to be ECO-friendly (free of lead and arsenic).
Essential values for all types of glasses
Abbe Number (vd)
Density (g/cm3)
Coefficient of Linear
Expansion*
Max Operating
Temp (°C)
CaF2
1.434
95.10
3.18
18.85
800
Fused Silica
1.458
67.70
2.20
0.55
1000
Schott BOROFLOAT™
1.472
65.70
2.20
3.25
450
Corning Pyrex 7740®
1.474†
65.40†
2.23
3.20
490
S-FSL5
1.487
70.20
2.46
9.00
457
N-BK7
1.517
64.20
2.46
7.10
557
N-K5
1.522
59.50
2.59
8.20
546
B270/S1
1.523
58.50
2.55
8.20
533
Schott Zerodur®
1.542
56.20
2.53
0.05
600
N-SK11
1.564
60.80
3.08
6.50
604
N-BaK4
1.569
56.10
3.10
7.00
555
N-BaK1
1.573
57.55
3.19
7.60
592
L-BAL35
1.589
61.15
2.82
6.60
489
N-SK14
1.603
60.60
3.44
7.30
649
N-SSK8
1.618
49.80
3.33
7.10
598
N-F2
1.620
36.40
3.61
8.20
432
BaSF1
1.626
38.96
3.66
8.50
493
N-SF2
1.648
33.90
3.86
8.40
441
N-LaK22
1.651
55.89
3.73
6.60
689
page 10
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Optical Glass
Index of Refraction (nd)
Glass Name
Optical Glass
Essential values for all types of glasses
Glass Name
Abbe Number (vd)
Density (g/cm3)
Coefficient of Linear Expansion*
S-BaH11
1.667
48.30
3.76
6.80
N-BaF10
1.670
47.20
3.76
6.80
N-SF5
1.673
32.30
4.07
8.20
N-SF8
1.689
31.20
4.22
8.20
N-LaK14
1.697
55.41
3.63
5.50
N-SF15
1.699
30.20
2.92
8.04
N-BaSF64
1.704
39.38
3.20
9.28
N-LaK8
1.713
53.83
3.75
5.60
N-SF18
1.722
29.30
4.49
8.10
N-SF10
1.728
28.40
4.28
7.50
S-TIH13
1.741
27.80
3.10
8.30
N-SF14
1.762
26.50
4.54
6.60
Sapphire**
1.768
72.20
3.97
5.30
N-SF11
1.785
25.80
5.41
6.20
N-SF56
1.785
26.10
3.28
8.70
N-LaSF44
1.803
46.40
4.46
6.20
N-SF6
1.805
25.39
3.37
9.00
N-SF57
1.847
23.80
5.51
8.30
N-LaSF9
1.850
32.20
4.44
7.40
N-SF66
1.923
20.88
4.00
5.90
S-LAH79
2.003
28.30
5.23
6.00
ZnSe
2.403
N/A
5.27
7.10
Silicon
3.422
N/A
2.33
2.55
Germanium
4.003
N/A
5.33
6.10
*microns/m°C (-30 to 70°C) **Sapphire is a birefringent material. All specifications correspond to parallel to C-Axis.
Pyrex 7740® nd and vd specified at 589.3nm
†
More Information Online: Application notes
o
Application Examples
o
Graphical Illustrations o
Equations
& More!
Optical Glass
Index of Refraction (nd)
www.edmundoptics.com/appnotes
page 11
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Optical Glass
Optical Glass Properties
Today, the quality and integrity of optical glass is a fundamental assumption made by optical designers. Until recently, however, that
was not the case. Nearly 125 years ago, Otto Schott began a revolution by systematicallyresearching and developing glass compositions. His development work on composition and the production process took glass manufacturing from the realm of trial and error to
its state today as a truly technical material. Now optical glass properties are predictable, reproducible and homogeneous - the essential
prerequisites of a technical material. The fundamental properties that characterize optical glass are:
Refractive Index
Refractive Index is the ratio of the speed of light in a vacuum to the speed of light in the specified material - a description of how light
slows down as it passes through an optical material. The refractive index for optical glasses, nd, is specified at a wavelength of 587.6nm
(Helium d-line). Materials with a low index of refraction are commonly referred to as “crowns” whereas materials with a high index of
refraction are referred to as “flints.” The typical index of refraction tolerance for components in our catalog is ±0.0005.
Dispersion
Dispersion is a description of the variation of the refractive index with wavelength. It is specified using the Abbe number, vd, defined
as (nd - 1) / (nF - nC) where nF and nC are the refractive indices at 486.1nm (Hydrogen F-line) and 656.3nm (Hydrogen C-line). A low Abbe
number indicates high dispersion. Crown glasses tend to have lower dispersion than flints. The typical Abbe tolerance for components
in our catalog is ±0.8%.
Transmission
Standard optical glasses offer high transmission throughout the entire visible spectrum and beyond in the near ultraviolet and near
infrared ranges. Crown glasses tend to have better transmission in the NUV than do flint glasses. Flint glasses, because of their high
index, feature higher Fresnel reflection loss and thus should always be specified with an Anti-Reflection coating.
Figure 1: Sample Optical Glass Transmittance Curves
1
1
0.9
0.99
0.98
Pyrex 7740®
0.97
Pyrex 7740®
0.96
N-BK7
N-BK7
0.6
N-K5
0.5
B270
0.4
N-BaF10
N-SF5
0.3
N-SF10
0.2
N-SF11
0.1
N-LaSFN9
0
Transmittance (5 mm)
Transmittance (5 mm)
0.7
BoroFloat™
300
400
500
Wavelength (nm)
600
BoroFloat™
N-K5
0.95
B270
0.94
N-BaF10
0.93
N-SF5
N-SF10
0.92
N-SF11
0.91
700
0.9
1400
N-LaSFN9
1500
1600
1700
1800
1900
2000
2100
2200
Wavelength (nm)
Optical Glass
0.8
Additional Properties
When designing an optic that will be used in an extreme environment it is important to realize that each optical glass will have slightly
different chemical, thermal and mechanical properties. These properties can be found on the glass datasheet.
page 12
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Optical Glass
Optical Glass Selection
Figure 2: Schott Abbe Diagram
νd
2.05
95
n d 2.00
d
90
85
80
Abbe Diagram
75
70
65
60
55
50
40
1.90
1.85
35
33A
34
LAK
1.70
14
12
7
1.65
1.60
PSK
PK
FK
90
νd
85
80
75
2
5
7 7
10 BK ZK7
5
CaF2
nd
47 43
10
BAK
K
2
4
5
SSK
5
8
BAF
52
KZFS4
LF
LLF 1
KF
5
4
BALF
KZFS2
KZFS5
KZFS11
2
1.80
14
4
7
BASF
51
6
56A 11
KZFS8
64
9
1.85
SF
45
LAF
2
35
1.90
57
40
10
8
22
21
16 4
2
53A 14
SK
5
11 57 14
3
1.55
52A
2.05
LASF
9
1.75
51
66
67
46A
31A
44
21
33
34
95
20
1.95
1.80
51A
25
2.00
41
1.50
30
nd – νd
Description of Symbols
N- or P-glass
Lead containing glass
N-glass or lead containing glass
Glass suitable for Precision Molding
CaF2 or Fused Silica
1.95
1.45
45
5
8
15
1
1.75
10
1.70
1.65
2
2
1.60
F
5
1.55
9
10
1.50
FS
70
1.45
65
60
55
50
45
40
35
30
25
20
Optical systems have to be optimized for a total set of functional characteristics. Geometrical and color induced aberrations can be compensated only by the use of more than one glass type. In most cases three or more glass types are used. The requirements on optical
of glass types has been developed. Traditionally they are shown in the refractive index versus dispersion diagram - the Abbe diagram.
The Abbe diagram, first introduced by SCHOTT in 1923, is a long established survey of the optical glass program. Glass types are
given in a two-dimensional coordinate system with the Abbe number (vd) as x-axis and the refractive index (nd) as y-axis. The x-axis is in
reversed direction with numbers increasing to the left side.
In the Abbe diagram, glass materials are divided into type denominations like BK, SK, F, SF, etc. These “glass families” correspond to
the regions in the Abbe diagram defined by the blue lines. There is a major line that separates crown glass types (last letter “K” from
German “Kron” for crown) from flint glass types (last letter “F”). This line starts upwards from the bottom at Abbe number 55, steps
aside at refractive index 1.60 to Abbe number 50, and continues upwards to the top.
Optical Glass
systems for different applications cover a range so wide that they cannot be met with just a small set of glass types. So a wide range
The leading letters in the glass name characterize an important chemical element used in the glass type: F - Fluorine, P – Phosphorus,
B – Boron, BA – Barium, LA – Lanthanum. Deviating from this rule are the glass types from the crown - flint series, which progresses
from K (“Kron”) to KF (“Kronflint” – crownflint) to flints of increasing lead content and hence density: LLF (“Very light flint”), LF (“Light
flint”), F (“flint”), and SF (“Schwerflint” – heavy flint). Another deviation is the SK and SSK glass types: SK (“heavy crown”) and SSK
(“heaviest crown”). LAK, LAF and LASF mean Lanthanum crown, flint and dense flint glass types, respectively.
page 13
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Understanding ball lenses
Ball lenses are great optical components for improving signal coupling between fibers, emitters and detectors. They are also used in
endoscopy, bar code scanning, ball pre-forms for aspheric lenses and sensor applications. Ball lenses are manufactured from a single
substrate of glass and can focus or collimate light, depending upon the geometry of the input source. Half-ball lenses are also common and can be interchanged with (full) ball lenses if the physical constraints of an application require a more compact design.
Essential Equations for Using Ball Lenses
D d
Figure 1: Key Parameters
BFL
EFL
P
There are five key parameters needed to understand and use ball lenses (Figure 1): Diameter of Input Source (d), Diameter of Ball Lens
(D), Effective Focal Length of Ball Lens (EFL), Back Focal Length of Ball Lens (BFL) and Index of Refraction of Ball Lens (n).
EFL is very simple to calculate (Equation 1) since there are only two variables involved: Diameter of Ball Lens (D) and Index of Refraction (n). EFL is measured from the center of the ball lens, indicated by R in Figure 1. BFL (Equation 2) is easily calculated once EFL
and D are known. Numerical Aperture NA (Equation 3) is dependent on EFL and d. It is a commonly referenced term and often used
in lieu of d/D.
EFL =
nD
D
BFL = EFL –
4(n-1)
Equation 1
NA =
2d(n-1)
nD
2
Equation 2
Equation 3
1.600
Fused Silica
N-BK7
N-SF8
Sapphire
LaSFN9
S-LAH79
Numerical Aperture (NA)
1.400
1.200
1.000
0.800
Since NA is often used, Figure 2 illustrates
how it increases as the Diameter of the Input
Source (d) also increases.
Figure 2: Numerical Aperture vs. Diameter for Ball
Lens Glass Types offered by Edmund Optics®.
0.600
0.400
0.200
0.000
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
Understanding ball lenses
Equations
1.00
d/D
page 14
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Understanding ball lenses
Application Examples
Figure 3: Laser to Fiber Coupling
D d
Example 1: Laser to Fiber Coupling
When coupling light from a laser into a fiber optic, the choice of ball lens is dependent on the NA of the fiber and the diameter of
the laser beam, or the input source. The diameter of the laser beam is used to determine the NA of the ball lens. The NA of the ball
lens must be less than or equal to the NA of the fiber optic in order to couple all of the light. The ball lens in contact with the fiber as
shown in Figure 3.
Initial Parameters
Calculated Parameter
Diameter of Input Laser Beam = 2mm
Diameter of Ball Lens
Index of Refraction of Ball Lens = 1.517
Numerical Aperture of Fiber Optic = 0.22
D=
2d(n-1)
2 x 2mm(1.517 -1 )
= 6.2mm
1.517 x 0.22
A N-BK7 ball lens (index of refraction of 1.517) of 6-8mm in diameter would be ideal for coupling a 2mm laser source into a 0.22NA
fiber optic. One can easily try different indices of refraction in order to find the best ball lens for a laser to fiber coupling application.
Example 2: Fiber to Fiber Coupling
To couple light from one fiber optic to another fiber optic of similar NA, two identical ball lenses are used. Place the two ball lenses in
contact with the fibers as shown in Figure 4. If the fiber optics have the same NA, then the same logic as in Example 1 can be applied.
Fiber Coupler
Figure 4: Fiber to Fiber Coupling
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Understanding ball lenses
nNA
D=