Optical Windows: the Ultimate Guide

VIS windows are a cost-effective option designed for the small range of visible light from 400-700nm. Although a UV window is suitable for similar applications, Optical Glass is a more specific and economical choice. N-BK7, the most commonly referred to as Optical Glass, has a transmission range of 350-2,000nm.

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A VIS window is frequently used in imaging and display systems. It also serves as a standard base substrate for mirrors and filter coatings. These windows possess a high index of refraction, excellent transmission, and superior material purity, making them crucial components in various optical systems. Additionally, N-BK7 exhibits a high degree of stain resistance.

IR Windows

The IR windows family includes the largest assortment of optical windows, frequently used in diverse applications. You can find the complete selection of optical windows on the Firebird Optics website.

Each specific window has unique properties and transmission profiles tailored for specialized applications. Below, we outline the benefits of each type of window, along with links for further information:

Barium Fluoride (BaF2)

BaF2 transmits from deep UV at 200nm to 12μm and is applicable in multiple setups within UV, VIS, and IR ranges. Key properties include resistance to high-energy radiation and a low index of refraction, often eliminating the need for AR coatings.

Calcium Fluoride (CaF2)

CaF2 shares similarities with BaF2, boasting a high damage threshold, low index of refraction, and a low absorption coefficient. Its transmission range of 130nm-9.5μm allows it to cover more extreme UV and IR ranges compared to UV Fused Silica. It is commonly used in laser and cryogenic applications.

Germanium (Ge)

Germanium's prominent attribute is its low dispersion, making it ideal for low-power CO2 laser applications, where a focused beam with minimal scattering is essential. With a range of 2-16μm, it ensures no unwanted radiation from UV, VIS, or most of the NIR interferes with measurements. Germanium is also chemically stable against air, water, and various acids.

Potassium Bromide (KBr)

KBr is a primary choice for FTIR spectroscopy, recognized for its extensive transmission range from 250nm to 26μm. It withstands high temperatures up to 300°C and mechanical shocks, although moisture should be avoided to prevent material degradation.

Potassium Chloride (KCl)

KCl is often interchangeable with KBr due to similar transmission properties (210-20μm). Still, it may be preferred for its high damage thresholds and low index of refraction. KCl is suitable for low-power CO2 laser applications and can be used across UV, VIS, and NIR ranges.

Sapphire (Al2O3)

Sapphire provides a large transmission range of 150nm-4.5μm and excels in robustness. Notably, it withstands extreme environments, displaying outstanding thermal conductivity, a high dielectric constant, and excellent chemical resistance. While less hard than diamond, sapphire can be manufactured to thin dimensions, enhancing transmission.

Sodium Chloride (NaCl)

NaCl is considered a disposable option within the IR window family. However, it is sensitive to moisture and thermal shocks, covering a wavelength range of 250-20μm. Its primary advantage lies in being a cost-effective, FTIR generalist.

Zinc Selenide (ZnSe)

ZnSe windows are primarily employed in high-power CO2 laser systems due to their excellent resistance to thermal shock, low absorption, and low dispersion properties, which enable concentration of high-energy radiation. Caution is advised, as ZnSe is softer and vulnerable to scratches, making it unsuitable for harsh environments. A sapphire window would be a better alternative.

Optical Glass

Optical glass is a particular type of glass designed for creating optical systems, such as lenses, prisms, or mirrors. Unlike regular window glass or crystal, which prioritize aesthetic appeal, optical glass incorporates additives that modify optical and mechanical qualities like refractive index, dispersion, transmittance, and thermal expansion.

Various elements, including silicon, boron, phosphorus, germanium, and arsenic (primarily in oxide form), are utilized to create glass. These elements confer a non-crystalline structure, and adding substances like alkali metals modifies the physico-chemical properties of the final glass to fit its intended function. Some optical glasses can have up to twenty different chemical components to achieve specific optical characteristics.

The purity and quality of optical glasses are critical in precision instruments. Defects in optical glass are measured and classified according to international standards, including bubbles, inclusions, scratches, and color variations.

History

The earliest optical lenses, dating back to before 700 BC, were crafted under the Assyrian Empire and made of polished crystals like quartz rather than glass.

With the rise of the Greeks and Romans, glass emerged as an optical material. It was occasionally fashioned into spheres filled with water for use as lenses in fire-starting and magnification applications.

The specific date of invention remains unclear, but accounts describe "glasses" made from materials like beryl or quartz, which assisted the elderly with impaired vision.

Things began evolving with the introduction of soda-lime glass, a common component of basic lenses, but improvements came slowly over centuries. Innovations, such as Angelo Barovier's "crystalline glass," significantly enhanced glass purity by removing impurities.

New optical instruments, like Galileo's telescope, originally used ordinary soda-lime glass—which wasn’t fit for precise optical applications. Inventions like lead crystal glass, developed by George Ravenscroft, aimed to improve resilience while enhancing optical performance.

Properties

The most critical physical properties of glass for optical applications include refractive index and constringency. These factors play a decisive role in designing optical systems, along with transmission, glass strength, and non-linear effects.

Index and Constringency

The refractive index indicates a glass's ability to bend light rays, dictating the extent of light deflection based on wavelength, which leads to component chromatic aberrations and distortions. This relationship can be graphically represented.

Optical glasses are categorized based on their refractive index, defined at specific wavelengths from the yellow helium line or green mercury line, depending on the application. The dispersion indicates how much two different wavelengths deviate, with glass classified as highly dispersive or not based on its Abbe number.

Transmission and Absorption

Glass’s transparency and absorption capability hold paramount importance. This attribute determines how effectively the lens transmits desired light spectra without interference from unwanted wavelengths.

Transmission measurements take into account material thickness, with variations present based on wavelengths and overall sample dimensions. Specific glasses, including those specializing in far-infrared or ultraviolet ranges, are selectively designed to optimize light transmission while minimizing absorption from impurities.

Ultraviolet Absorption

The drop in transmission in the UV spectrum is attributed to electronic transitions within glass materials. As shown by solid-state band theory, electron movements correspond to specific energy levels facilitated by light energy. Various types of glass demonstrate varied UV absorption characteristics based on their chemical structure, further influencing their efficiency in optical applications.

Infrared Absorption

Transmission drop in the infrared domain arises from molecular vibrations influenced by external energy absorption. Glass types exhibit different absorption properties based on their composition, including influences from humidity, which could cause further reductions in transmission. Chalcogenide glasses can improve IR transmission performance.

Emission and Non-Linear Phenomena

Under high illumination, lasers may yield properties where the refractive index results in non-linear behaviors influenced by light intensity. High fluences can result in complex optical phenomena, leading to potential damage to glass structures through molecular lattice disintegrations.

Fabrication

The production of optical lenses requires high-purity materials. Any contaminants can degrade performance, risking integrity through breakage or discoloration. Special production processes help ensure the required glass qualities are preserved throughout.

Types of Glass

Continuous advancements in optical glass technology have led to the development of various lens families characterized by unique mechanical and optical properties. Additional glass types exist, like halide and chalcogenide glasses.

Oxide Glass

Oxide glasses primarily consist of SiO2 or TiO2 compounds characterized by different transmission profiles tailored for specific applications.

Crown Glass Family

Borosilicate crowns, known for their excellent homogeneity, represent the most widely produced optical glass type. The barium and phosphate crown groups provide unique refractive characteristics beneficial for optical applications.

Flint Glass Family

Flint glasses achieve high refractive indices due to increased PbO concentrations and find applications in various optical contexts.

Classical Glass Designations

Common categories of optical lenses exist, accommodating a wide array of properties suitable for different applications.

Special Glasses

Specialized glasses designed for extreme conditions and purposes exist to fulfill specific optical functions beyond conventional applications.

Glass Quality

Optical components undergo rigorous testing based on established standards (MIL and ISO) to ensure consistency, quality, and performance across different applications.

Manufacturers

A variety of manufacturers produce specialized lenses, offering a wide selection of optical designs across diverse categories.

Applications

Optical glasses serve in numerous instruments like telescopes, microscopes, and lasers, as well as high-energy particle detectors and specialized optics for high radiation environments.

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