How to select an optical parallel window suitable for a specific application?

Spectral range

First, it is necessary to clarify the spectral range required by the application. For example, if it is used for visible - light imaging, such as a protective window for an ordinary camera lens, then an optical parallel window with high transmittance in the wavelength range of 400 - 700nm can be selected. BK7 optical glass has good transmittance performance in this band and can meet basic requirements. However, if it is used for an infrared thermal imaging device, materials with high transmittance in the infrared band (such as 3 - 5μm or 8 - 14μm) need to be selected, such as parallel optical windows made of materials like zinc sulfide (ZnS) or zinc selenide (ZnSe).

                                                          ZnSe  parallel Optical Window

Parallelism requirements

For high - precision optical systems, such as astronomical telescopes or high - precision microscopes, extremely high parallelism is required. The parallelism error usually needs to be controlled within a few seconds (e.g., ±5 seconds) to reduce aberration. If it is a general industrial inspection machine vision system, the requirement for parallelism is relatively low. A parallelism error within a few minutes (e.g., ±10 minutes) may be sufficient to meet the needs, which can reduce costs to a certain extent.

Transmittance and reflectance

Select a window with appropriate transmittance according to the requirements of the luminous flux in the application scenario. In occasions where high light transmission is required, such as optical windows in lighting systems, products with high transmittance should be selected, and windows with antireflective coatings can be considered to further improve light transmittance. For some applications that need to reduce reflection interference, such as windows inside optical instruments, in addition to choosing materials with low reflectance itself, the reflectance can also be reduced by coating.

Refractive index uniformity

In applications such as imaging and high - precision optical signal transmission, the uniformity of the refractive index is very important. For example, the parallel optical window used in lithography technology requires a highly uniform refractive index inside the material to ensure accurate light propagation and thus achieve high - precision pattern transfer. If the refractive index non - uniformity exceeds a certain limit, it will lead to blurred imaging or optical signal distortion. Generally, high - quality optical quartz materials perform well in terms of refractive index uniformity. 

Consider physical properties

Size and shape

The appropriate size and shape of the parallel optical window should be selected according to the actual installation space and the layout of the Optical System. For example, in small handheld optical devices, windows with a small size and regular shape (such as circular or square) need to be selected to adapt to the compact design of the device. In large optical experimental devices or astronomical telescopes, larger - sized windows may be required, and the shape may be designed as circular, rectangular, or other special shapes according to the optical path.

Mechanical strength and stability

If the optical parallel window is applied in harsh environmental conditions, such as outdoor optical monitoring equipment or industrial environments with mechanical vibrations, the mechanical strength and stability of the window need to be considered. For example, in optical equipment in the aerospace field, the optical window needs to be able to withstand factors such as vibration, acceleration during the launch process, and temperature changes in the space environment. Selecting materials with high hardness and good impact - resistance performance (such as sapphire materials) can improve the mechanical stability of the window.

Thermal performance

In some applications that are sensitive to temperature changes, such as high - precision optical measuring instruments or laser devices, the thermal performance of the optical parallel window is crucial. Different materials have different thermal expansion coefficients. Selecting materials with a small thermal expansion coefficient can reduce the impact of temperature changes on optical performance. For example, quartz materials have a low thermal expansion coefficient and can still maintain good optical performance in an environment with large temperature changes, making them suitable for high - precision optical systems.

Consider cost and availability 

Material cost

The costs of parallel optical windows made of different materials vary greatly. BK7 optical glass is relatively inexpensive and suitable for general optical applications in large - scale production, such as protective windows for ordinary camera lenses or some industrial inspection equipment. High - end materials such as sapphire are expensive, but they have superior performance and are suitable for occasions with extremely high requirements for optical performance and stability, such as high - end military optical equipment or aerospace optical instruments. Under the premise of meeting application requirements, the material cost needs to be weighed.

Manufacturing process cost

Complex manufacturing processes will increase costs. For example, optical windows with extremely high requirements for parallelism and surface finish require high - precision grinding, polishing, and coating processes, which will increase production costs. If the application does not have particularly high requirements for optical performance, windows with relatively simple manufacturing processes can be selected to reduce costs.

                                                         2m ring polishing machine

Availability

Ensure that the selected parallel optical window materials and specifications are easily available. Some special materials or windows with customized sizes and shapes may have limited supply, which will affect the project progress and cost. For example, some new optical materials may have limited supply channels due to reasons such as small production scale. When selecting, the stability and timeliness of their supply need to be considered.

MG-Optics offers ultra-Large High-Precision Windows,including fused silica, SiC, and Zerodur. The size can reach 1000mm diameter and RMS achieves λ/80@1064nm, Just send your specifications to info@mirrorganize.com, we will provide you evaluation and quotation.