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|
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."
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.
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.
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.