Definition: flat transparent plates with optical quality, used for protection against the environment
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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.
If you are looking for more details, kindly visit Sapphire Optical Window.
Blue sapphire has long symbolized honesty, sincerity, and faithfulness. When a king wore a sapphire ring, it meant he was pledging to be a just ruler. During the Middle Ages, Cardinals and other high church officials wore blue sapphires to symbolize their vows to their faith. Blue sapphire also symbolized heaven and the reward of faith and its celestial association continues still today.
The ancient Persians believed the earth rests on a giant sapphire which colors the sky blue.
Sapphire’s association with royalty goes back at least one thousand years. The historic St Edward’s sapphire, which can be traced to Edward the Confessor in , is set in the cross of the Imperial State Crown of the British Crown Jewels, along with the 104-carat royal blue Stuart Sapphire. The story goes that St Edward, King of England, met a beggar and, having no coins, gave him his sapphire ring. The beggar was St Peter in disguise, who returned the ring to the charitable king. The sapphire was later set in the crown as a symbol of the divine right of royal rule.
Sapphires are valuable so they have long been coveted by jewel thieves too. In New York, the 563-carat Star of India star sapphire, in the collection of the American Museum of Natural History was stolen in an infamous burglary carried out by Jack Murphy, AKA Murph the Surf, and two other men. The huge sapphire was recovered several months later.
Sapphires have been mined since 500 BC on the beautiful island once known as Ceylon, now called Sri Lanka. The island was the origin for the valuable gems given by King Solomon to the Queen of Sheba in the Old Testament. Sri Lankan sapphires are ancient Roman rings dating from the third century BC. Marco Polo visited Sri Lanka in , writing about its streams filled with sapphires and other gemstones.
In , blue sapphires were discovered in Kashmir at an elevation of 13,000 feet near the village Sumjam in the Himalayan mountains. Kashmir sapphires have a saturated velvety blue color with a silky clarity that seems to intensify the color. Because Kashmir sapphires from this famous deposit were only produced for a short period, they are rare. Sapphires with an independent laboratory report confirming Kashmir origin from Gubelin, SSEF, or the GIA command a premium in the market.
Another famous source for blue sapphire is Burma, also known as Myanmar. Burmese sapphire is generally cleaner and darker in tone. Burma is famous for producing the best rubies and its sapphires too command a premium when origin is confirmed by an independent laboratory report.
In recent years, most fine color natural sapphires have been produced from mines in Madagascar. Blue sapphire is also produced in Montana in the United States as well as Australia, Cambodia, Tanzania, and Thailand.
The color of sapphire is very restful. Like gazing out at the ocean, it makes you feel more peaceful and serene. Is blue your favorite color? You aren’t alone: blue is the most popular color worldwide. That’s why so many people love sapphire.
Sapphire is also one of the most durable gemstones: it’s corundum, with a hardness of 9 on the Mohs scale, second only to diamond’s 10. Sapphire is also tough. In fact it’s even tougher than diamond, resisting chips and damage. Sapphire is so tough that lab-grown sapphire is used for supermarket scanners, spacecraft windows, and watch crystals.
We also love how natural sapphires aren’t as common as diamonds. Choosing a blue sapphire engagement ring instead of a diamond engagement ring sets you apart from the crowd. Every sapphire has a unique personality.
In fact, if you aren’t in love with faceted blue sapphire, you have other natural sapphire options available. All the rainbow colors of corundum are sapphires except red corundum, which is ruby. Sapphires in colors other than blue are known as fancy sapphires. Sweet pastel or vibrant pink sapphire is almost as popular as blue. These feminine rosy sapphires are a good alternative to pink diamonds at a more affordable price. The rarest color of sapphire is pinkish-orange padparadscha sapphire, with the color of the Sri Lankan sacred lotus blossoms that gave padparascha its unusual name. Yellow sapphire has a golden hue that’s like sunshine. You can also choose white sapphire as a beautiful natural diamond alternative.
And blue sapphire is also lovely cut in a smooth domed cabochon shape instead of being faceted. Some cabochon sapphires, known as star sapphires, will display a phenomenon known as asterism, a six-legged star that moves across the dome of the cabochon with the light.
The most important thing to consider when buying a sapphire is its color. Sapphire with more vivid blue is more expensive than sapphires that are a watery or blackish blue. The most valuable blue sapphire hue is a rich and saturated medium dark blue. This is sometimes called “cornflower blue,” after the bright blue flower.
Some people assume that the darker a sapphire’s color, the better, but that’s actually not the case. Blue sapphire can be too dark, with a midnight blue that’s almost black. You’ll see these midnight blue sapphires that are almost opaque in chain jewelers. These gemstones are less valuable than a rich medium royal blue color.
At RockHer, we’ve matched our preferred sapphire colors to Pantone’s system as a buying guide to communicate the rich blue of their hue. Which blue is your favorite?
Because the blue hue of sapphire is so strong, inclusions and haziness don’t distract the eye as much as they would in a colorless gem and so don’t affect the value as much. Avoid any inclusions that distract your eye but don't worry about examining a sapphire’s clarity under magnification like you would a diamond.
Look for a lively sparkling cut that sparkles evenly across the entire sapphire with no dull, washed out, or lifeless areas. Your sapphire should be cut for brilliance not to maximize carat weight.
You’re more likely to see oval shape sapphires and cushion cut sapphires. These two shapes are the most suited to the shape of the rough. Of course small sapphires will be cut in the round brilliant shape and are beautiful in eternity bands and also as accent gems. Other fancy shapes will be a bit more difficult to find in larger carat weights.
Most blue sapphires have been heat treated after they are mined in order to bring out the maximum blue color. You should assume that your blue sapphire has been heat treated unless you have a lab report that says there is no evidence of heat treatment.
Some sapphires on the market are treated in a process called diffusion. Diffusion treated sapphires have titanium and other coloring elements added in a surface layer when the sapphire is heated to create a rind of color around the outside of the gem. This layer can be very shallow like a coating so these stones can’t be repolished or recut. As a result, they are much less expensive. If you see sapphires with vivid color that are surprisingly affordable they are probably diffusion treated. Jewelers should disclose this treatment to you in writing on your invoice. At RockHer, we don't sell diffusion treated sapphire because they don't hold their value.
Blue Sapphire Buying Guide Quality Checklist
Sapphire has a hardness of 9 on the Mohs scale and is also extremely tough. Sapphire dust is even used to cut and polish other gemstones. That shows that sapphires are the most durable of gems.
Sapphire is excellent for rings you wear every day. It’s more durable than its precious metal setting, which can be damaged by chemicals or contact with other metal, like weights or machines at the gym. Avoid wearing any rings at the beach or in the pool where they can slip off and be lost.
Clean sapphire jewelry with mild dish soap: use a soft brush behind the stone where dust can collect. Using ultrasonic cleaners is also safe for sapphire jewelry.
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