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Understanding Optical Windows – Material Properties
19/09/11

high transmittance UV and IR grade BaF2 optics

 

Material properties including the transmission, refractive index, and hardness of the window substrate can be critical for deciding which window is the best choice for your application. Several  key properties for selecting the appropriate window for your application include the index of refraction, Abbe number, density, and coefficient of thermal expansion. The selection guide below lists the optical, mechanical, and thermal properties of our available window substrates as well as their size and thickness ranges

 

Windows Selection Guide
Material Index of Refraction (nd) Abbe Number (vd) Density (g/cm3) Coefficient of Thermal Expansion (μm/m°C) Softening Temp (˚C) Knoop Hardness Size Range Thickness Range
B270 1.523 58.5 2.55 8.2 533 542 5 - 75 x 75mm 1.0 - 3.0mm
Barium Fluoride (BaF2) 1.48 81.61 4.89 18.1 800 82 5 - 50mm 1.0 - 3.0mm
BOROFLOAT® 1.472 65.7 2.20 3.25 820 480 5 - 200mm 1.75 - 6.5mm
Calcium Fluoride (CaF2) 1.434 95.1 3.18 18.85 800 158.3 5 - 50mm 1.0 - 3.0mm
Germanium (Ge) 4.003 N/A 5.33 6.1 936 780 10 - 75mm 1.0 - 5.0mm
Gorilla® Glass 1.509 N/A 2.44 9.1 843 5100 5 - 200 x 200mm 1.1mm
Magnesium Fluoride (MgF2) 1.413 106.2 3.18 13.7 1255 415 5 - 50mm 1.0 - 3.0mm
N-BK7 1.517 64.2 2.46 7.1 557 610 5 - 75 x 75mm 0.2 - 4.0mm
Potassium Bromide (KBr) 1.527 33.6 2.75 43 730 7 13 - 50mm 1.0 - 5.0mm
Sapphire 1.768 72.2 3.97 5.3 2000 2200 2.5 - 75mm 0.5 - 3.2mm
Silicon (Si) 3.422 N/A 2.33 2.55 1500 1150 10 - 50mm 1.0 - 3.0mm
Sodium Chloride (NaCl) 1.491 42.9 2.17 44 801 18.2 13 - 50mm 1.0 - 5.0mm
UV Fused Silica 1.458 67.7 2.20 0.55 1000 500 5 - 50 x 50mm 1.0 - 5.0mm
Zinc Selenide (ZnSe) 2.403 N/A 5.27 7.1 250 120 10 - 75mm 1.0 - 6.0mm
Zinc Sulfide (ZnS) 2.631 N/A 5.27 7.6 1525 120 12.5 - 50mm 2.0 - 3.0mm

 

Refractive Index
The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in an optical medium, which describes how light slows down as it passes through the material. The refractive index for optical glasses (nd) is specified at the Helium d-line wavelength of 587.6nm. Glasses with a low index of refraction are commonly referred to as "crowns" and glasses with a high index of refraction are referred to as "flints."

Abbe Number
The Abbe number (vd) describes the material’s dispersion, or variation of the refractive index with wavelength. It is 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), respectively. Low Abbe numbers indicates high dispersion. Crown glasses tend to have higher Abbe numbers than flints.

Density
The density of a glass is important to consider because it helps determine the weight of the optical assembly, which is critical for weight-sensitive applications. Generally, the refractive index of a glass increases as the density increases. However, the relationship between refractive index and density is not linear.

Coefficient of Thermal Expansion
The coefficient of thermal expansion describes how the size of the glass will change with changes in temperature. This property is a key factor in applications involving extreme temperatures and quick temperature differentials.

Koop Hardness
The Knoop hardness of a glass is a measure of its resistance to indentation. It is determined by using a fixed force with a given indenter and measuring the depth of the resulting indentation. The smaller the indentation, the higher the Knoop hardness. In general, materials with a high Knoop hardness are less brittle and can withstand greater pressure differentials than materials with a smaller Knoop hardness.