Understanding Optical Windows
Optical Windows are flat, plane-parallel plates that are traditionally used to provide a barrier between electronic sensors or detectors from an outside environment. Because windows introduce no optical power into a system, windows should be selected based on the material transmission properties as well as the mechanical properties that match your application. The figure below highlights the transmission regions of different window materials available.
Figure 1: Transmission regions for Edmund Optics® window substrates. Wavelength range for N-BK7 is representative for the majority of substrates used for visible wavelengths such as B270, N-SF11, and BOROFLOAT®.
Several other key properties are important for selecting the appropriate window for your application including 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||450||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). 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. A glass’s density also denotes how difficult it is to work with the glass and is somewhat proportional to the material cost.
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.
Equivalent Glass Types
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). Edmund Optics uses ECO-friendly glasses in many of our TECHSPEC® products, but this designation may not be reflected in the product descriptions. For products not manufactured by Edmund Optics, the inclusion of ECO glasses differs by manufacturer. Once an item has been switched to ECO glass, Non-ECO-friendly glasses will not be utilized again. Based on availability, we reserve the right to substitute an equivalent ECO glass in our production runs. The table below shows the glass equivalents for common optical glasses.
|Glass Material Equivalents|
|Listed Glass Name||Glass Number||Schott Equivalent||Ohara Equivalent||CDGM Equivalent|
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- Sizes Ranging from 3 - 500mm with Thickness Starting at 0.2mm
- Dozens of Materials from Standard Glass Catalogs of Schott, Hoya, Ohara, and CDGM
- Infrared Materials Including Fluorides, Ge, Si, ZnS, and ZnSe
- Wide Variety of Anti-Reflection Coatings Including Broadband, Laser-Line, and ITO Conductive
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