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Resources / Application Notes / Optics / Optical Glass
Optical Glass
Edmund Optics Inc.

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. When dealing with applications involving extreme temperatures and quick temperature differentials, a glass' coefficient of expansion also 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).

Table 1: Glass Material Equivalents
Listed Glass NameGlass NumberSchott EquivalentOhara EquivalentCDGM Equivalent
N-BK7 517/642 N-BK7 S-BSL7 H-K9L
N-K5 522/595 N-K5 S-NSL5 H-K50
N-PK51 529/770 N-PK51
N-SK11 564/608 N-SK11 S-BAL41 H-BaK6
N-BAK4 569/561 N-BAK4 S-BAL14 H-BaK7
N-BAK1 573/576 N-BAK1 S-BAL11 H-BaK8
N-SSK8 618/498 N-SSK8 S-BSM 28
N-PSK53A 618/634 N-PSK53A S-PHM52
N-F2 620/364 N-F2 S-TIM 2 H-F4
S-BSM18 639/554 S-BSM18 H-ZK11
N-SF2 648/338 N-SF2 S-TIM 22 H-ZF1
N-LAK22 651/559 N-LAK22 S-LAL54 H-LaK10
S-BAH11 667/483 S-BAH 11 H-ZBaF16
N-BAF10 670/472 N-BAF10 S-BAH 10  H-ZBaF52
N-SF5 673/322 N-SF5 S-TIM 25 H-ZF2
N-SF8 689/312 N-SF8 S-TIM 28 H-ZF10
N-LAK14 697/554 N-LAK14 S-LAL14 H-LAK51
N-SF15 699/301 N-SF15 S-TIM35 H-ZF11
N-BASF64 704/394 N-BASF64
N-LAK8 713/538 N-LAK8 S-LAL8 H-LAK7
S-TIH18 722/293 S-TIH18
N-SF10 728/284 N-SF10 S-TIH10 H-ZF4
N-SF4 755/276 N-SF4 S-TIH4 H-ZF6
N-SF14 762/265 N-SF14 S-TIH14
N-SF11 785/258 N-SF11 S-TIH11 H-ZF13
SF65A 785/261 SF65A S-TIH23
N-LASF45 800/350 N-LASF45 S-LAM66 H-ZLaF66
N-LASF44 803/464 N-LASF44 S-LAH65 H-ZLaF50B
N-SF6 805/254 N-SF6 S-TIH 6 H-ZF7LA
N-SF57 847/238 N-SF57 S-TIH53 H-ZF52
N-LASF9 850/322 N-LASF9 S-LAH71
S-NPH2 923/189 S-NPH2
N-SF66 923/209 N-SF66

Optical Glass Properties

Today, the quality and integrity of optical glass is a fundamental assumption made by optical designers. Nearly 125 years ago, Otto Schott began a revolution by systematically researching 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, dispersion, and transmission.

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."

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.

Transmission

Standard optical glasses offer high transmission throughout the entire visible spectrum and beyond in the near ultraviolet and near infrared ranges (Figure 1). 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 (AR) coating.

Sample Optical Glass Transmittance Curve
Sample Optical Glass Transmittance Curve
Figure 1: Sample Optical Glass Transmittance Curves

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.

Table 2: Essential Values for All Types of Glass
Glass NameIndex of Refraction (nd)Abbe Number (vd)Density (g/cm3)Coefficient of Thermal 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
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
S-BaH11 1.667 48.30 3.76 6.80 575
N-BAF10 1.670 47.20 3.76 6.80 580
N-SF5 1.673 32.30 4.07 8.20 425
N-SF8 1.689 31.20 4.22 8.20 422
N-LAK14 1.697 55.41 3.63 5.50 661
N-SF15 1.699 30.20 2.92 8.04 580
N-BASF64 1.704 39.38 3.20 9.28 582
N-LAK8 1.713 53.83 3.75 5.60 643
N-SF18 1.722 29.30 4.49 8.10 422
N-SF10 1.728 28.40 3.05 7.50 454
S-TIH13 1.741 27.80 3.10 8.30 573
N-SF14 1.762 26.50 4.54 6.60 478
Sapphire** 1.768 72.20 3.97 5.30 2000
N-SF11 1.785 25.80 5.41 6.20 503
N-SF56 1.785 26.10 3.28 8.70 592
N-LASF44 1.803 46.40 4.46 6.20 666
N-SF6 1.805 25.39 3.37 9.00 605
N-SF57 1.847 23.80 5.51 8.30 414
N-LASF9 1.850 32.20 4.44 7.40 698
N-SF66 1.923 20.88 4.00 5.90 710
S-LAH79 2.003 28.30 5.23 6.00 699
ZnSe 2.403 N/A 5.27 7.10 250
Silicon 3.422 N/A 2.33 2.55 1500
Germanium 4.003 N/A 5.33 6.10 100

*10-6/°C
**Sapphire is a birefringent material. All specifications correspond to parallel to C-Axis.

Optical Glass Selection

EO Abbe diagram with Coefficient of Thermal Expansion and Relative Partial Dispersion
Figure 2: EO Abbe diagram with Coefficient of Thermal Expansion and Relative Partial Dispersion

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 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 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 (Figure 2).

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.

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.

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