PRICE

Optical Materials 1OPTICAL GLASS

Optical Glass

There are a multitude of optical gradeglasses available from variousmanufacturers worldwide. They havea wide variety of optical character-istics which allows aberrations to becorrected to the level required forvery demanding imagingapplications. Optical glasses arepredominantly used from350nm−2500nm - in fact within thisspectral region there is usually noneed to consider any other material.Two of the most important opticalcharacteristics of an optical glass arethe refractive index and thedispersion. The dispersion is the indexdifference between the extreme endsof the spectral region of interest.Another figure sometimes moreconvenient in discussing theproperties of an optical material isthe Abbe number, which is related to,and calculated from the dispersion.Although these quantities can bedefined using any arbitrary wave-lengths, a particular choice which hasbecome the de facto standard in theindustry is the use of the threewavelengths d,C,F (587.6, 656.3,486.1nm respectively). In this casethe Abbe number V

dis defined as

For historical reasons the range ofglasses is divided into twosubgroups:− The crown glasses withn

d< 1.6 and V

d> 55 or n

d> 1.6 and

Vd

> 50; with the remaining glassesknown as the flint glasses.

The ideal glass prescription would bea glass with a high value of both n

d

and Vd, as this would enable

components of high refracting powerto be constructed from componentswith shallow curves. In addition thecolor aberrations and some of themonochromatic aberrations would begreatly reduced. Unfortunately, (inorder to increase the refractiveindex), oxides of heavier elementshave to be added to the basic SiO

2

matrix upon which most opticalglasses are made. This generallyresults in a loss of transmission at theblue end of the spectrum and anincrease in the optical dispersion.Glasses with high values of n

dtend to

be flints, and as a general rule anincrease in n

dand V

dboth lead to an

increase in material cost (in somecases by an order of magnitude).

By far the most popular optical glassis BK7(517642). It exhibits excellenttransmission from 350nm to2000nm. The transmission tails awayslowly in the infrared with a completeextinction beyond 2.7 microns. At theUV end of the spectrum the materialis not really usable at wavelengthslower than 300nm.

BK7 is a low index crown glass. Itexhibits a good resistance toatmospheric attack, acid and alkaliconditions and to staining - all of

these features being desirable both inthe final component and during themanufacturing processes that arerequired. The popularity of thematerial is such that all the majorglass manufacturers produce afunctionally identical equivalent,ensuring ready availability. Inaddition especially large pieces withimproved levels of homogeneity andfreedom from internal defects can beproduced, at additional cost.

The majority of the components forthe visible region in the EalingCatalog are manufactured from BK7,except where special optical orphysical properties are required.

Examples of this are in the use ofF2(620364) and SF10(728284) glassto give a high spectral separation insome of the dispersing prisms, andLaSFN9(850322) in the micro lenses.

Also a wide variety of crown and flintcombinations are used to correct theaberrations in compound lenses, suchas the cemented achromaticdoublets.

The nominal refractive indices of thefour glasses BK7, LaSFN9, F2 andSF10 at any wavelength in the region0.365 to 1.060 microns can becomputed using the followingexpression:

Sometimes the refractive index andAbbe number are combined into a sixfigure glass code, which allowssubstitute glasses from alternativesuppliers to be located with greaterease. This code is formed by using thefirst three significant figures after thedecimal point in the value of n

d

followed by the three mostsignificant digits in the value of V

d.

Irrespective of the glassmanufacturer, it is very common forthe Schott designation to be given inthe catalog listing of materialproperties. Most manufacturersproduce glasses which are, forpractical purposes, identical to manyof the more popular glasses in theSchott range, in addition to specialtyglasses of their own.

Vd

= nd−1

nF−n

c

Glass Type

Coefficient BK7 LaSFN9 F2 SF10

A0

2.2718929 3.2994326 2.5554063 2.8784725A

1−1.0108077x10−2 −1.1680436x10−2 −8.8746150x10−3 −1.0565453x10−2

A2

1.0592509x10−2 4.0133103x10−2 2.2494787x10−2 3.3279420x10−2

A3

2.0816965x10−4 1.3263988x10−3 8.6924972x10−4 2.0551378x10−3

A4

−7.6472538x10−6 4.7438783x10−6 −2.4011704x10−5 −1.1396226x10−4

A5

4.9240991x10−7 7.8507188x10−6 4.5365169x10−6 1.6340021x10−5

n2 = A0+A

1λ2+A

2λ−2+A

3λ−4+A

4λ−6+A

5λ−8

where λ is the wavelength in micronsand the coefficients for each glass areas shown below.

Further data and transmission curvesare shown on the next Page.

Ealing Catalog

2

3

4

5

Optical Materials 2OPTICAL GLASS

Material BK7 (517642) LaSFN9 (850322) F2(620364) SF10(728284)

Abbe Number Vd

64.17 32.17 36.37 28.41

Density (g/cm3) 2.51 4.44 3.61 4.28

Coefficient of 7.1 7.4 8.2 7.5Expansion (10−6/K)(Room Temperature)

Specific Heat (J/g/K) 0.858 − 0.557 0.465

Thermal Conductivity (W/m/K) 1.114 − 0.780 0.741

Youngs Modulus (103N/mm2) 81 109 58 64

Poissons Ratio 0.208 0.286 0.225 0.232

Knoop Hardness 520 510 370 370

Other important data and the transmission curves for these materials are given below.

Refractive index versus λ

λ Material

(nm) BK7 LaSFN9 F2 SF10

365.0 1.53626 − 1.66621 −404.7 1.53024 1.89844 1.65063 1.77578435.8 1.52669 1.88467 1.64202 1.76197486.1 1.52238 1.86899 1.63208 1.74648546.1 1.51872 1.85651 1.62408 1.73430587.6 1.51680 1.85026 1.62004 1.72825632.8 1.51509 1.84489 1.61656 1.72309656.3 1.51432 1.84526 1.61503 1.72085852.1 1.50981 1.82997 1.60672 1.708891060.0 1.50669 1.82293 1.60191 1.70229

100%

80%

60%

40%

20%

0%200 600 1000 1400 1800 2200

Wavelength in nanometers

Exte

rnal

tran

smis

sion

of 1

0mm

thic

k BK

7 w

indo

w (u

ncoa

ted)

Ealing Catalog

Refractive index versus λ

λ (nm) Index

214.4 1.53372248.3 1.50840265.2 1.50000302.2 1.48719334.1 1.47976365.0 1.47454404.7 1.47962435.8 1.46669486.1 1.46313546.1 1.46008587.6 1.45846656.3 1.45637852.1 1.45247

1014.0 1.450241530.0 1.444271970.0 1.438522325.4 1.43293

Optical Materials 3FUSED SILICA AND FUSED QUARTZ

Fused Silica and Fused Quartz

Both of these materials exhibittransmission characteristics whichbear some similarity to the opticalglasses with which they are closelyrelated. However, their simplerchemical composition, in particularthe absence of some of the heaviermetal oxides, leads them to haveimproved transmission in theUltraviolet region of the spectrum.Special grades of material may bespecified which are pre−treated toreduce the water content. Thiseliminates characteristic O−Habsorption lines in the near Infraredregion of the spectrum although atthe expense of transmission in theUltraviolet. The spectral transmissioncurves are given below.

The physical properties of FusedQuartz and Fused Silica are similar,and in applications where thematerials are chosen for theirstrength, chemical inertness or theirresistance to elevated temperature orshock, the less expensive FusedQuartz may be the preferred option.

The fundamental difference betweenFused Quartz and Fused Silica is inthe method of manufacture. MostFused Quartz is produced by themelting and re−fusing of silica sandand natural quartz. This means thattrace impurities in the raw materialsused can be carried across into thefinal material matrix. The presence ofheavy metallic ions is in partresponsible for absorption atUltraviolet wavelengths or forunacceptable levels of absorption inhigh power visible lasers.

By comparison Fused Silica isproduced by the flame hydrolysis of asilica halide. The resultant material ishence much purer, and free from thesites which could cause absorption inthe Ultraviolet or problems withinternal absorption in the use of highpower lasers.

Either of these materials is the mostsuitable choice for use in high powerlamp sources. As they are far moreresistant to thermal shock thanoptical glass, the final choice wouldbe dependent on the spectral rangerequired.

The refractive index of Fused Silicaand Fused Quartz at variouswavelengths may be calculated usingthe following Sellmeier type formulawith the coefficients due to Malitson.(J.Opt.Soc.Am. 55 p1205 (1965))

1

2

3

4

where A0=0.6961663

B0=0.004679148

A1=0.4079426

B1=0.01351206

A2=0.8974794

B2=97.934003

and λ is the wavelength in microns.

Abbe Number Vd: 67.8

Density: 2.203 g/cm3

Softening Point: 1730°CAnnealing Point: 1180°CSpecific Heat: 0.76 J/g/K at 20°CThermal Conductivity: 1.39 W/m/KThermal Expansion: 0.55x10−6 /°CYoungs Modulus: 72.8 x 103 N/mm2

Hardness (Mohs): 5−7

n2 = 1+A

0λ2

+A

1λ2

+A

2λ2

λ2−B0

λ2−B1

λ2−B2

100%

80%

60%

40%

20%

0%

150

200

300

400

500

600

700

800

900

1000

1250

1500

2000

2500

3000

4000

5000

Wavelength in nanometers

Exte

rnal

tran

smis

sion

of 1

0mm

thic

k sa

mpl

e

FusedSilica

FusedQuartz

Ealing Catalog

Optical Materials 4CALCIUM FLUORIDE

Calcium Fluoride

Calcium Fluoride (CaF2) has a useful

transmission over the spectral rangefrom 0.2−8.0 microns. The lowrefractive index (1.35−1.51) meansthat it may be used without recourseto an anti−reflection coating.

It may be used as an alternative toFused Silica in the Ultraviolet region,especially as a crown type material forassemblies which require reducedlevels of chromatic aberration. Inaddition, the high Abbe number inthe visible region, and the anomalousdispersion characteristics, render ituseful for correcting visible systemsfor secondary color aberration (forexample, apochromatic microscopeobjectives).

It is the least expensive material, withvisible transmission, that also coversthe complete 3−5 micron band.

Calcium Fluoride is slightly soluble inwater, although the surfaces shouldbe expected to withstand severalyears exposure to normalatmospheric conditions. CalciumFluoride is also susceptible to thermalshock, reducing its usefulnesssomewhat with high radiancesources.

The refractive index of CalciumFluoride at various wavelengths maybe calculated using the followingSellmeier type formula with thecoefficients due to Malitson.(Appl.Opt. 2,p1103(1963)).

where A0=0.5675888

B0=0.00252643

A1=0.4710914

B1=0.01007833

A2=3.8484723

B2=1200.5560

and λ is the wavelength in microns

Abbe Number Vd: 94.9

V3−5

: 21.7Density: 3.179 g/cm3

Melting Point: 1360°CSpecific Heat: 0.857 J/g/K at 0°CThermal Conductivity: 1.037W/m/K at 0°CThermal Expansion: 18.9x10−6/K at 20°CYoungs Modulus: 76x103N/mm2

Solubility: 0.00177 g/100 g H

2O (at 26°C)

3

4

5

n2 = 1+A

0λ2

+A

1λ2

+A

2λ2

λ2−B0

λ2−B1

λ2−B2

Refractive index versus λ

λ(nm) Index

248.3 1.46793265.2 1.46233334.1 1.44852365.0 1.44490404.7 1.44451435.8 1.43949486.1 1.43703546.1 1.43494587.6 1.43388656.3 1.43246852.1 1.43002

1014.0 1.428791530.0 1.426421970.0 1.424012325.4 1.422123507.0 1.413985018.8 1.398737464.4 1.36070

100

80

60

40

20

0.1 .2 .3 .4 .5 .6 .8.7 .9 1.0 2.0 4.0 5.0 103.0

Wavelength in microns

Perc

ent T

rans

mis

sion

Ealing Catalog

GUIDE

Optical Materials 5GERMANIUM

Germanium

Germanium is a semiconductormaterial, which has high internaltransmittance for radiation in thewavelength range from 1.8−12microns. It is particularly favored foruse in the 3−5 and 8−12 micronregions, for which the atmosphere istransparent. In addition the lowercost of the raw material compared tothe possible alternatives is a strongfactor in its favor.

Its high refractive index (approx 4.0)allows components of high refractivepower to be constructed withoutexcessive thickness or extremecurvatures. This keeps material usagedown and reduces the processingtime in quantity production.

The high index also ensures anexceptional single wavelengthperformance for a “best form” singletconstructed from Germanium. Theextremely high Abbe number in the8−12 micron band, and the relativelyhigh value in the 3−5 micron band,means that in most cases specialachromatizing measures are notrequired.A drawback of the high refractiveindex is the high uncoated surfacereflection losses of 36% per surface,giving an uncoated component anexternal transmittance of 40% . This

makes the application of an ARcoating essential in virtually allpractical situations. Fortunately thehigh refractive index can actuallysimplify the coating design in somecases, and a range of coatings isavailable to meet the majority ofrequirements.

The optimum form of material for usein most optical applications is n−typedoped material with a resistivity inthe range of 5−40Ωcm. This material,which is doped with a controlledamount of Antimony(Sb), exhibits alower absorption loss than otherforms of the material, provided thetemperature of the material is keptbelow 50°C.For high power applications,especially those which involve CWlasers or lasers with high energypulses and a fast repetition rate, it ispreferable to use Zinc Selenide. Theproblem with Germanium in theseapplications is its susceptibility tothermal runaway. If energy isabsorbed in the component the localtemperature near the beam mayincrease because the thermalconductivity of Germanium is too lowfor the heat to be dissipated. If thishappens then the local value of theabsorption coefficient will also rise.There is the possibility of a continualand uncontrolled rise in temperatureleading to component failure. Even

where water cooling is used, thethermal gradient required to allowthe heat away may only reach asteady state condition at a pointwhere thermal distortion of the beamhas upset the system performance.

The refractive index of Germanium atvarious wavelengths can becalculated using the followingformula due to Salzberg and Villa:−(J.Opt.Soc.Am. 43 p 579 (1953))

5

where A = 3.99931B = 0.391707C = 0.163492D =−0.0000060E = 0.000000053

Abbe Number V3−5

: 101.5V

8−12: 988

Density: 5.327 g/cm3

Melting Point: 936°CSpecific Heat: 0.32J/g/K at 0°CThermal Conductivity: 59.9W/m/K at 20°CThermal Expansion: 5.75x10−6/K at

20°CYoungs Modulus: 103x103Nmm−2

L =1

(λ2−0.028)

n = A+BL+CL2+Dλ2+Eλ4

Refractive index versus λ

λ(microns) Index

2.0 4.108272.5 4.066453.0 4.044953.5 4.032394.0 4.024394.5 4.018985.0 4.015145.5 4.012326.0 4.010186.5 4.008527.0 4.007217.5 4.006168.0 4.005318.5 4.004619.0 4.004039.5 4.00356

10.0 4.0031710.5 4.0028611.0 4.0026112.0 4.0022713.0 4.0021314.0 4.00217

100%

80%

60%

40%

20%

0%

Wavelength in microns

Exte

rnal

tran

smis

sion

of 1

0mm

thic

k G

e w

indo

w (u

ncoa

ted)

0 2 4 6 8 10 12 14 16

and λ is the wavelength in microns

Ealing Catalog

Optical Materials 6ZINC SELENIDE

Zinc Selenide

Zinc Selenide is a transparentpolycrystalline material with atransmission range from 0.5−15microns. The material is produced bya CVD (Chemical Vapor Deposition)process, which ensures that theextrinsic impurity levels are kept low.The purity of the material minimizesthe bulk absorption loss figures.

Although the material is moreexpensive than Germanium, theability to use visual alignment aids(such as a He−Ne laser), can reducethe costs incurred by the end user.This is especially so if many opticalcomponents are used in sequence.

The absorption losses are not onlylower than Germanium, they are alsofar more stable with temperature.This ensures that the risk of thermalrunaway leading to componentfailure is reduced, and this materialshould always be considered in anyhigh power laser application. Inaddition the possible dangers of

having an unpredictable paththrough misaligned optics is far lessbecause of the visual checks whichcan be carried out prior to firing upthe laser.

The average refractive index value ofapproximately 2.4 gives uncoatedreflection losses of 17% per surface,and an external transmittance of 69%in the principal beam. Usually themain issue is not the loss of radiationso much as the unwanted stray lightproblem. Some form of AR coating isrecommended on ZnSe componentsand coatings are available to meetboth narrow band and wide bandrequirements.

The refractive index of Zinc Selenideat various wavelengths may becalculated using the followingSellmeier type formula with thecoefficients due to Feldman,Horowitz, Waxler and Dodge."Optical Materials CharacterizationFinal Report February 1, 1978 −September 30, 1978", NBS TechnicalNote 993.

where A0=4.2980149

B0=0.036888196

A1=0.62776557

B1=0.14347626

A2=2.8955633

B2=2208.4920

3

4

5

Abbe Number V3−5

: 178V

8−12: 57.5

Density: 5.27g/cm3

Specific Heat: 0.34 J/g/K at 25°CThermal Conductivity: 18 W/m/K at 25°CThermal Expansion: 7.1x10−6/K at 0°CYoungs Modulus: 67x103N/Kmm−2

n2 = 1+A

0λ2

+A

1λ2

+A

2λ2

λ2−B0

λ2−B1

λ2−B2

and λ is the wavelength in microns.

Refractive index versus λ

λ(microns) Index

1.0 2.488821.5 2.457082.0 2.446202.5 2.440873.0 2.437583.5 2.435174.0 2.433164.5 2.431325.0 2.429535.5 2.427726.0 2.425846.5 2.423887.0 2.421817.5 2.419618.0 2.417288.5 2.414819.0 2.412189.5 2.40939

10.0 2.4064410.5 2.4033111.0 2.4000012.0 2.3928113.0 2.3848114.0 2.37593

100%

80%

60%

40%

20%

0%0 4 8 12 16 20

Wavelength in microns

Exte

rnal

tran

smis

sion

of 1

0mm

thic

k Zn

Se w

indo

w (u

ncoa

ted)

Ealing Catalog

PRICEGUIDE

Optical Materials 7SAPPHIRE

Sapphire

This material has excellent mechanicaland optical properties. It is extremelyhard (9 on the mohs scale) and has astrength which is retained even atelevated temperatures. In addition itis resistant to virtually all forms ofchemical attack. It has a transmissionband which extends from the UV ataround 0.18nm, throughout thevisible and near IR regions to 5.5microns.

The high cost of this versatile materialnaturally tends to limit its use to thefollowing areas:−

1) As a durable single lens in detectorapplications which do not requiremulti−component correction.

2) As a protective window to shieldmore delicate optics from harshenvironments such as debris fromlaser cutting processes. The windowcan frequently be kept extremelythin, saving on material cost andlowering any possible internalabsorption. Vigorous cleaningmethods can also be used with less

danger of scratching the window andcausing losses due to surface scatter.

3) It has also been used more recentlyin the form of spherical balls in fibercoupling applications.

Depending on the wavelength atwhich it is used the Fresnel reflectionlosses vary from 5% − 20% persurface. However at the wavelengthswhere these losses are highest therefractive index of 1.7−1.8 is almostideal for the application of a singlelayer coating of MgF

2.

The internal structure of the materialis anisotropic, which means thatcertain properties have a dependenceon the direction in which they aremeasured in the bulk material. Themost significant of these properties isthe birefringence introduced, whichcould be a problem in a particularlydemanding application. Fortunatelythe thinness of the substrates used isusually a sufficient protection againstthis source of image degradation.

The refractive index of Sapphire atvarious wavelengths may be

calculated using the followingSellmeier type formula with thecoefficients due to Malitson.(J.Opt.Soc.Am. 52 p 1377 (1962))

where A0=1.023798

B0=0.00377588

A1=1.058264

B1=0.0122544

A2=5.280792

B2=321.3616

and λ is the wavelength in microns

Abbe Number Vd: 72.2

Density: 3.98 g/cm3

Melting Point: 2055°CSpecific Heat: 756 J/kg/°C at 18°CThermal Conductivity: 24 W/m/K Thermal Expansion: 6.5x10−6/KYoungs Modulus: 345x103N/mm2

(Properties are dependent on orientation)

n2 = 1+A

0λ2

+A

1λ2

+A

2λ2

λ2−B0

λ2−B1

λ2−B2

λ(nm) Index

265.2 1.83365334.1 1.80181365.0 1.79358404.7 1.78582435.8 1.78120486.1 1.77558546.1 1.77077587.6 1.76823656.3 1.76494852.1 1.75887

1014.0 1.755461530.0 1.746591970.0 1.738352325.4 1.730553507.0 1.695015145.6 1.61510

100

80

60

40

20

0

Exte

rnal

Tra

nsm

issi

on o

f1m

m th

ick

win

dow

.1 .2 .3 .4 .5 .6 .8.7 .9 1.0 2.0 4.0 5.0 103.0

Wavelength in microns

Refractive index versus λ

Ealing Catalog