Optical windows are flat plates made of optically transparent material, designed to allow light into an optical instrument. They can also be used to protect a light source from an outside environment.
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These windows are designed to minimize both reflection and absorption while maximizing transmission over a target wavelength range. When choosing an optical window, you will have to keep in mind optical surface specifications, material transmission properties, and the mechanical properties required by your application.
Our state of the art factory is able to manufacture both large optical windows and micro windows for nano-sized optical assemblies. A wide variety of optical substrates are available to choose from, and since production is carried on in-house our design team will be able to match your exact specifications when carrying out your order.
Substrate properties and optical surface specifications are two attributes you will want to keep in mind as you select custom optics for your application.
The material properties of the substrate used will determine transmission, refractive index, and hardness. Our Potassium Bromide substrates, for instance, will transmit UV visible and infrared light. They have a density of 2.75 g/cm3 and an index of refraction of 1.527. Zinc Selenide, on the other hand, blocks UV as well as some visible light and transmits higher wavelength visible light and infrared. It has an index of refraction of 2.631. Fused silica has a density of 2.202g/cm and a index of refraction of which varies from 1.55 to 1.40.
The index of refraction quantifies how much the light is slowed down as it is transmitted through the substrate. It is calculated as the ratio of the speed of light in a vacuum to the speed of light through the substrate. For instance, the index of refraction of Zinc Selenide, 2.631, means that light travels through a vacuum 2.631 times faster than it does through ZnSe.
The refractive index of our sapphire windows is 1.76-1.77. For optical windows, the refractive index is typically specified at 587.6 nm, the Helium d-line wavelength. Optical glasses with a high index of refraction are sometimes called ‘flints’, while windows with low index of refraction are called “crowns”.
Another important specification, the Abbe Number, describes how the refractive index varies with wavelength. The lower the Abbe number, the higher the dispersion. The Abbe number of sapphire is 72.24, and for BK7, 64.17.
MgF2 windows are hard and durable, with a refractive number of 1.378 and an Abbe number of 106.22. They have a high laser damage threshold, and very good broadband transmission from 120 nm to 8 μm. Our MgF2 windows are often used with UV radiation sources and receivers.
If your specifications application is weight-sensitive, you may need to pay attention to substrate density. Although the refractive index of optical materials tends to increase as density increases, this is not always the case, and the relationship is not always linear.
Surface flatness is described in terms of the deviation from a completely flat surface, and is often measured with a precise reference piece called an optical flat. Deviations from perfect flatness can be quantified and are given in what is called waves, abbreviated λ. Lower λ implies higher flatness. While flatness of 1λ is sufficient for most applications, precision optics such as high power lasers may require surface values of as low as λ/20.
Surface quality refers to the presence or absence of surface imperfections: scratches and digs. It is quantified with a two-part scratch dig number, as specified by the United States Military Performance Specification MIL-PRF-B. The lower the number the better the surface quality.
A scratch-dig number of 40-40 or even 80-50 is appropriate for most optical windows, especially those used for imaging systems. Some precision applications may require 40-20. High power laser systems require high surface quality, perhaps 20-10 or 10-5. The lower the scratch-dig number, the higher the manufacturing cost, and there is a high cost premium associated with 20-10 or 10-5 windows.
Most of our windows are coated with an anti-reflective coating that increases durability and efficiency. Since these AR coatings allow the window to maximize transmission of the desired wavelength of light, it is wavelength specific. The full spectral range of your system must be considered before a selection is made.
Shanghai Optics offers wedged windows in addition to our range of parallel window offerings. These optical components, which feature a controlled wedge in the optical path, are ideal for laser systems because they prevent common problems that a parallel window setup might entail. For instance, power spikes through unwanted reflections, interference effects, and mode-hopping can all be eliminated or control through the use of wedged windows.
We can also customize optical filters for your device, as requested. These are optical windows with dyes injected into the substrate in manufacturing, or with special coatings, and they transmit selective bands or colors of light.
Our high quality optical windows are suitable for a wide range of applications, from military defense to scientific experimentation, manufacturing, lasers, and high precision photography.
Alongside our regular range of high quality optical windows, Shanghai Optics offers optical flats for measurement purposes or for use in applications where precision is at a premium.
These flat discs feature highly polished surfaces with flatness options of λ/4, λ/10, and λ/20. When these optical flats are placed in contact with a test surface, the presence of light and dark bands provide a straightforward visual determination of the surface quality of the test piece.
Contact us to discuss your requirements of Custom Optical Windows. Our experienced sales team can help you identify the options that best suit your needs.
Our optical flats are carefully tested with precision metrology equipment to ensure they fully meet the flatness level specified. Do be aware that optical flats do deteriorate over time and with use, and should be periodically recalibrated.
Optical windows are utilized in a variety of industrial and research applications, including equipment inspection, process monitoring in manufacturing, medical and mechanical experiments, and spectroscopy. When designing an optical window, it is crucial to account for both the application environment and the spectral range to ensure optimal performance. These factors are key in selecting the appropriate manufacturing material. In this article, we will explore the three primary material categories and the key criteria for choosing components that ensure the best results for an optical window.
An optical window is a transparent interface that separates two environments while allowing specific wavelengths of light to pass through. Designed to maximize light transmission within a particular spectrum (ultraviolet, visible, or infrared) and minimize reflection and absorption, optical windows are made from materials with specific mechanical and optical properties.
They can protect equipment by reducing the effect on incoming or outgoing optical signals, and they can serve as viewing or measurement lenses. Additionally, optical windows prevent external elements like water, dust, or air from contacting the interior of the equipment.
- Plane parallel windows: These are used to minimize distortion in light beams transmitted at specific wavelengths.
- Prismatic windows: These windows direct transmitted light at a specific angle, helping to reduce rear reflection.
Optical windows come in various shapes (round, rectangular, oval, or freeform) based on their application and environment. They are widely used in industries such as aeronautics, defense, security, transportation, and petrochemicals, as well as in research institutes and laboratories (both public and private) for tests, measurements, inspections, and studies in fields like UV spectroscopy and IR thermography.
Choosing the right material for an optical window is crucial to ensuring it meets the intended application and spectral range. Materials generally fall into three main categories: glass, polymer, or crystal. Let's explore the key characteristics of these material families.
There are three primary categories of materials used in the creation of optical windows, each offering distinct properties:
1. Glass: Optical glass is transparent, hard, and easy to polish, making it a traditional choice for lenses and mirrors. It has a low refractive index, meaning it doesn't significantly deflect visible light. Different types of glass, such as silica or borosilicates, offer various mechanical, thermal, and optical properties. The choice of glass depends on the specific spectral range (ultraviolet, visible, or infrared) required for the application. The main drawback of glass is its brittleness, which makes it prone to cracking under certain conditions.
2. Polymer: Materials like cell acetate, polycarbonate, polyurethane, or acrylic (PMMA) are lightweight, impact-resistant, and inexpensive to manufacture. These polymers have a lower refractive index than glass, which can be beneficial for some applications. However, polymers are less transparent and durable than glass under certain conditions and have lower heat resistance.
3. Crystal: Crystals, such as quartz or sapphire, offer excellent optical properties and are commonly used in precision optics. They are highly transparent with low dispersion, meaning they do not significantly deflect light across different wavelengths. Each crystal type alters the light passing through it in unique ways. While crystals are more expensive to produce than glass or polymers, they provide superior optical performance in specialized applications.
The choice of optical window material depends on the intended application. Optical glass is typically used for visible light applications, while crystals such as sapphire, CaF2, ZnSe, ZnS, and germanium are preferred for UV or IR applications. These two material categories are the most widely used. To select the most suitable material for a given application, key properties such as transmission, refractive index, and hardness must be carefully considered.
When choosing a material for an optical window, several key criteria must be taken into account, including transmission, refractive index, and Knoop hardness.
Transmission refers to a material's ability to allow light to pass through at a specific wavelength. The percentage of transmission is determined by how much light is reflected or absorbed by the material. For applications such as imaging or spectroscopy, high transmission is critical for obtaining high-quality images or data.
The refractive index defines how much light bends as it passes through the material. This factor is vital when designing optics like lenses or prisms. The refractive index varies based on the material and the wavelength of light, making it an essential consideration for precise optical applications.
Knoop hardness measures the material's resistance to indentation and is important for optical windows used in demanding environments, such as those involving high pressure, temperature, or abrasive conditions. Materials with higher Knoop hardness offer better mechanical strength and scratch resistance.
In designing an optical window, it is essential to establish clear specifications regarding the intended use and application conditions. This ensures that the most suitable material is selected based on its transmission, refractive index, and hardness to meet performance requirements.
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