What is the optical window?

Optical Windows are flat, optically transparent plates that are typically designed to maximize transmission in a specified wavelength range, while minimizing reflection and absorption. They are often used to protect optical systems and electronic sensors from an outside environment. Because windows introduce no optical power into a system, Silicon windows should be selected based on the material transmission properties, optical surface specifications, and the mechanical properties that match your application.

Material Properties

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. The figure below highlights the transmission regions of the different materials offers as windows.

Several other 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.

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)(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)(vd) describes the material’s dispersion, or variation of the refractive index with wavelength. It is defined as (nd−1)(nF−nC)(nd−1)(nF−nC) where nFnF and nCnC 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.

Optical Surface Specifications

The surface specifications of optical windows affect the optical performance and must be considered when selecting or specifying a window. It is important to make sure your optical window has the appropriate specifications with respect to tightness to meet your application requirements, but over-tolerancing the window will unnecessarily increase the cost.

Surface Quality

The surface quality of an optical window is an evaluation of surface imperfections that may be caused during manufacturing or handling. These defects typically cause small reductions in throughput and small increases in scattered light, which have little to no adverse effect on the overall system performance in most imaging or light gathering applications. However, some surfaces are more sensitive to these defects, such as surfaces at image planes, because surface defects are in focus. Windows with high power levels are also sensitive to surface defects because they can cause increased absorption of energy and damage the window.

Surface quality is often described by the scratch-dig specification in the U.S. Standard MIL-PRF-13830B. The scratch designation is determined by comparing the scratches on a surface to a set of standard scratches under controlled lighting conditions. This is not a direct measurement of the scratch dimensions themselves. On the other hand, the dig designation directly relates to the size of the dig. The dig designation is calculated by taking the diameter of the dig in microns and dividing it by 10.