Optical Windows are flat optical components that are designed to transmit light according to the wavelength range required by a specific application. They protect sensitive equipment and can produce a range of optical effects with additional optical coatings.
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In this first of our two articles on these widely used optical components, we look at the key properties of Optical Windows as well as their use in a huge range of scientific, medical, defence and industrial sectors and applications.
We also run through some of our most popular optical windows including Germanium Windows and Sapphire Windows and their respective options, applications and properties.
Optical Window specifications may be based on a range of material properties and criteria including:
Their use should not impact on magnification of a system.
Optical Windows can be optically polished. Surfaces can be coated with AR coatings for maximum transmission performance across a particular wavelength from the UV to IR.
Optical Windows are used by scientific and industrial companies across the world of medical, defence, instrumentation, laser, research and imaging.
Depending on the requirements specified, optical windows can be made in a range of materials:
UQG Optical Windows products include wedges, substrates, discs, flats, plates, rods, protection windows, laser windows, camera windows, light guides and domes.
All our Optical Windows are available from a standard range stock, while custom sizes and specifications can be custom-made to your specific design.
We stock a wide range of optical windows. Here we look at some of the most popular:
AR coating of windows, glass plates and substrates reduces glare and reflection, thereby increasing transmission and brightness. AR coatings are used widely used in LCD modules, display panels and medical devices and can be applied in a range of custom thicknesses to 0.10mm thick. Coating layers can be applied to both sides of the optical window substrate for the visible wavelength or to a single side for the near IR.
BK7 windows are made from high quality borosilicate-crown glass material for applications and instrumentation in the medical, scientific and industrial sectors. BK7 windows are supplied as diameters, squares and rectangles from standard stock range or to custom specifications and provide excellent transmission in the visible and near IR areas of the spectrum. AR coatings can be applied for increased transmission.
Fused silica is an extremely pure material with a very fine, low scatter surface quality which is ideal for optical windows. Fused silica windows are widely used in high-energy applications such as imaging and laser systems where microscopic defects could potentially lead to damage due to distortion, autofluorescence or scattering. AR coatings can be applied for UV, visible or IR applications.
Germanium windows and lenses are used widely in thermal imaging systems, defence and aerospace industries, life and medical sciences, optics for industrial OEMs and many other infrared applications. They have excellent transmission across 2-14 µm waveband and can be used in both mid-wave infrared (IR) and long wave IR systems. Transmission reduces as the temperature of the window increases. Germanium windows can be supplied uncoated, AR both sides coated and AR/DLC coated in a range of sizes.
Fused quartz windows are used in lamps, instruments, viewports and other sensitive equipment where protection from harsh environmental extremes is required. This is because fused quartz is very hard, has high temperature stability to °C and is resistant to thermal shock. Fused quartz windows offer transmission ranges from 260nm to over 2 microns, multiple grade options from commercial to precision and custom options including surface flatness, coatings, shapes and dimensions make them a cost-effective choice for industrial and optical applications.
Optical diffusers are used to alter the divergence of incident light in imaging techniques and for illumination control in optical sensing and military instrumentation. They can be supplied in several options: unmounted, mounted and AR coated on N-BK7 substrates and unmounted on a UV fused silica substrate.
Sapphire windows and plates are extremely hard and scratch resistant. Sapphire as a material has excellent thermal conduction and heat radiation properties and is resistant to common chemical acids. Manufactured in a range of sizes and thicknesses, sapphire windows can withstand high pressures and are suited for applications in harsh environments for (UV) ultraviolet, (VIS) visible and (IR) infrared transmission.
Silicon windows from UQG Optics are made from 99.9% pure silicon. Silicon has a transmission range of 1.2-8 microns (uncoated) but for 9 microns it has a strong absorption band good for use at 3-5 microns. Silicon windows have low density making them ideal for use in delicate and weight-sensitive optical instruments requiring hard surfaces in the military, imaging and scientific sectors. AR coatings can be applied to silicon windows.
Definition: flat transparent plates with optical quality, used for protection against the environment
More specific term: Brewster windows
Category: general optics
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For purchasing optical windows, use the RP Photonics Buyer's Guide – an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.Most optical windows are made in the form of flat plates of a transparent medium (e.g. glass, crystal or polymer). They are often used for isolating optical systems or components against detrimental influences from the environment. For example, most photodiodes and other kinds of photodetectors often contain an optical window above their light-sensitive area to protect it against dirt, corrosive influences and mechanical damage. Similarly, housings of lasers are often protected with optical windows to keep the housing free of any dust.
In some cases, for example for the active tubes of gas lasers like helium–neon lasers, there are optical windows separating the inside low-pressure gas volume from the outside atmosphere. Similarly, windows are needed for multipass gas cells as used in spectroscopy. If such windows are not rigidly connected, one may require some suitable type of seals to get a housing reliably air-tight. There are special vacuum windows built into vacuum viewports, coming together with suitable seals and mounting parts.
Some laser viewing windows are made to transmit visible light for inspection purposes, while blocking laser light (e.g. in the infrared) for laser safety reasons.
There are also strongly curved optical windows, which are called optical domes.
Common optical materials used for optical windows are glasses like fused silica and BK7 for visible or near-infrared light. For infrared optics at longer wavelengths, one also uses various types of crystalline materials such as calcium fluoride, also semiconductors like zinc selenide, silicon and germanium. Particularly for low-cost mass applications, some polymer materials are also often used, e.g. PMMA acrylic. They may be equipped with anti-scratch coatings for making them more resistant.
In some cases, an optical element such as a lens or a mirror can at the same time fulfill the function of an optical window, so that no separate part is required for that. Note, however, that a separate optical window may be advantageous in rough environments, since it is both easier and cheaper to exchange it, compared with exchanging a high quality optical element.
Usually, it is important to avoid significant losses of optical radiation going through an optical window. Such losses can occur due to different effects:
Note that the power losses at an optical window can be polarization-dependent, if the incidence of light is far from normal incidence. An extreme case is that of a Brewster plate, where the angle of incident needs to be at Brewster's angle, so that the reflection losses are very small for p polarization (without using a coating), but rather high for s polarization. Such windows are called Brewster windows. They are often used for the tubes of gas lasers, for example.
It has already been mentioned above that beam distortions (wavefront errors) may be caused by optical windows of insufficient quality. That may also lead to a loss of beam quality of laser beams, or image distortions in viewing devices and cameras.
The surface quality of optical windows is often quantified with scratch–dig values according to the U.S. standard MIL-PRF-B, or alternatively in a more rigorous fashion based on ISO -7. In addition, there are certain tolerances for surface flatness and irregularity. The article on laser mirrors, where surface quality is of particularly vital importance, contains some more details on such issues.
Optical window of particularly high quality are also sold as interferometer flats; this marks them as being suitable for use in interferometers, where low beam distortions are often of particular importance.
Besides deficiencies of the material itself, there can be inhomogeneities induced by mechanical stress. Therefore, optical windows should be mounted such that stress effects are avoided.
A good surface quality should not only be achieved in production, but also be maintained by carefully transporting and mounting optical windows, and by avoiding adverse effects during operation. For example, the performance can be degraded by fingerprints (when touching surfaces), deposited dust and dirt, or by scratching the surface when touching them with hard parts. In applications involving intense laser pulses, e.g. from Q-switched lasers, dust and other dirt may be burned into a surface, making it difficult afterwards to remove it.
Some kinds of dirt can be removed from optical windows with appropriate cleaning procedures. For example, one may use a soft cleaning tissue and a few droplets of a suitable solvent (e.g. cleaning alcohol or acetone) to wipe the surface if it is well accessible. One should avoid wiping back and forth, only distributing dirt; instead, one should systematically wipe in one direction, getting any dirt outside the sensitive area. At the same time, care must be taken not to damage optical surfaces e.g. by touching them with any hard tools.
For applications in rough environments, one may use special holders which facilitate the quick exchange of damaged optical windows. For example, some windows are used as debris shields in laser material processing, and may have to be exchanged regularly. They are also called sacrificial windows. One may also try to protect windows to some extent e.g. with hard tube structures.
Most optical windows are parallel windows, having quite precisely parallel surfaces. The parallelism is often quantified, e.g. as <1 arcsec. With parallel faces, there is no change in beam direction, but only a slight beam position offset, dependent on the angle of incidence and the thickness; there is then often no need for precise alignment of the window.
The beam offset also has a slight dependence on the optical wavelength, since the wavelength-dependent refractive index leads to a wavelength-dependent beam direction within the plate. It is rare, however, that such effects cause problems.
There are also wedged windows, having a well defined angle between their surfaces. This is sometimes required for avoiding interference effects between the parasitic reflections from the two surfaces. Note that in this case there is some level of beam deflection, dependent on the orientation of the window. See also the article on wedge prisms.
For applications with very high optical powers, e.g. in laser material processing, even some small residual absorption in an optical window may cause some level of thermal lensing. (A small amount of dirt on a surface can of course strongly increase the strength of heating and its consequences.) Such thermal effects can depend on multiple properties of the material, in particular on the absorption coefficient, the thermal conductivity, the temperature dependence of the refractive index and photoelastic coefficients. Special high-quality materials may have to be chosen for such applications.
Various other detailed properties of optical windows may be relevant for applications. Some examples of such properties:
Some suppliers offer custom windows with special specifications, often for special application areas such as aerospace and military.
Contact us to discuss your requirements of Custom Reticles. Our experienced sales team can help you identify the options that best suit your needs.