IR Windows Innovations: Empower 2024 with Infrared Windows

Author: Pradyumn – R&D Intern

Editor: Qu Yingli – R&D Director

Editor: Bryan Ng – Marketing Manager

Published on:

Last edited:

Optical windows are selectively transparent components designed to allow specific wavelengths of light to pass through, based on their material. These windows are carefully designed to maintain optical clarity, withstand environmental conditions, and minimize any distortion or alteration of the light passing through them. They are primarily used to protect delicate optical components, facilitate measurements, and enable observation or imaging in different applications.

1. Introduction to Optical Windows and Materials

IR Windows Innovations: Empower 2024 with Infrared Windows 1
Wavelength Opto-Electronic Infrared Windows

Optical windows allow specific wavelengths to pass through while reflecting, absorbing, or blocking others. Optical windows can be made from an extended range of materials, such as fused silica, Silicon, Calcium Fluoride (CaF2), Germanium (Ge), Potassium Chloride (KCl), Potassium Bromide (KBr), Sapphire (Al2O3), N-BK7, and Zinc Selenide (ZnSe).

Each of these materials has its own transmission profile, which is chosen based on the application. Material properties including the transmission, refractive index, and hardness of the window substrate can be critical for deciding which window is the best choice for the application. Before we delve into infrared windows (IR windows), let’s glimpse at other types of optical windows to gain a better understanding of a whole category.

1.1 A Brief Walkthrough of Other Windows

For transmission in the visible spectrum (350nm – 750nm), N-BK7 and fused silica are commonly used as substrates for optical windows. With fused silica having a high thermal shock resistance and high LIDT (laser-induced damage threshold), it is ideal for applications such as those involving high-power laser optics, visible range imaging systems, and spectroscopy.

Part NumberWavelength (nm)MaterialDiameter (mm)Thickness (mm)Application
WFS-170-31030-1090Fused Silica170.03.0Protective
WFS-150-31030-1090Fused Silica150.03.0Protective
WFS-140-41030-1090Fused Silica140.04.0Protective
WFS-110-31030-1090Fused Silica110.03.0Protective
WFS-110-2.51030-1090Fused Silica110.02.5Protective
WFS-104-3U343-355Fused Silica104.03.0Protective
WFS-104-31030-1090Fused Silica104.03.0Protective
WFS-90-3U343-355Fused Silica90.03.0Protective
WFS-70-9.51030-1090Fused Silica70.09.5Protective
WFS-55-1.51030-1090Fused Silica55.01.5Protective
WFS-50-1.51030-1090Fused Silica50.01.5Protective
WFS-43-2G515-545Fused Silica43.02.0Protective
WFS-37-71030-1090Fused Silica37.07.0Protective
WFS-36-21030-1090Fused Silica36.02.0Protective
WFS-30-51030-1090Fused Silica30.05.0Protective
WFS-28-41030-1090Fused Silica28.04.0Protective
WFS-25-31030-1090Fused Silica25.03.0Protective
WFS-22-31030-1090Fused Silica22.03.0Protective
WFS-20-2-YG515-545 / 1030-1090Fused Silica20.02.0Protective
WFS-18-31030-1090Fused Silica18.03.0Protective
WFS-16-1.55-YG515-545 / 1030-1090Fused Silica16.01.6Protective
WFS-15-21030-1090Fused Silica15.02.0Protective
WFS-3.5-1E2940Fused Silica3.51.0Medical Laser Er:YAG
WFS-1.5-51030-1090Fused Silica38.15.0Protective
WFS-1-3UG1030-1090/515-545/343-355Fused Silica25.43.0Protective
WBK-108-2.5GR532 / 650N-BK7108.02.5Protective
WBK-84-2YG532 / 1064N-BK784.02.0Protective
WBK-24-1.4-YG532 / 1064N-BK724.01.4Protective
WBK-16-1A755/633N-BK716.01.0Medical Laser Alex
WBK-1.5-4R694/633N-BK738.14.0Medical Laser Ruby
WBK-1.5-4N1064/532N-BK738.14.0Medical Laser Nd:YAG
WBK-1-3R694/633N-BK725.43.0Medical Laser Ruby
WBK-1-3N1064/532N-BK725.43.0Medical Laser Nd:YAG
WBK-0.75-2.5R694/633N-BK719.12.5Medical Laser Ruby
WBK-0.75-2.5N1064/532N-BK719.12.5Medical Laser Nd:YAG
WBK-0.6-2R694/633N-BK715.22.0Medical Laser Ruby
WBK-0.6-2N1064/532N-BK715.22.0Medical Laser Nd:YAG
WBK-0.5-2R694/633N-BK712.72.0Medical Laser Ruby
WBK-0.5-2N1064/532N-BK712.72.0Medical Laser Nd:YAG

Table 1: Wavelength Opto-Electronic Fused Silica (WFS series) and N-BK7 (WBK series) Windows

N-BK7, on the other hand, can be used for camera lenses, optical filters, and general-purpose visible light optics, due to its excellent optical transmission in the visible spectrum (approximately 350 – 2200nm). BK7 is also relatively hard and shows good scratch resistance. However, it is not recommended for temperature-sensitive applications such as precision mirrors.

UV-grade fused silica has additional properties such as high optical transmission in the UV range (approximately 175 – 400nm), low absorption and fluorescence in the UV region. These make it suitable for applications within the UV spectrum such as UV spectroscopy, excimer laser optics, and semiconductor lithography.

Calcium fluoride can also be used for UV optics as it has a wide spectral range and can be used for deep UV to infrared applications because of its non-birefringent properties. It can also be used without an AR (anti-reflection) coating due to its low index of refraction. It has a transmission above 90% between 0.25 and 7µm and is commonly used for excimer laser optics due to its low absorption and high damage threshold. CaF2 has a high coefficient of thermal expansion, which makes it unsuited for applications with a high operating temperature environment.

Diameter Tolerance: +0/-0.25mm
Thickness Tolerance: ±0.25mm
Parallelism: ≤ 10arcsec or 30 ± 5arcmin
Clear Aperture: ≥ 85% of central diameter
Surface Quality: 10–5 Scratch & Dig
AR Coating: R≤0.25% per surface @ 1064nm (Single Wavelength) | R≤0.3% @ 1064nm (Dual Wavelength)
Damage Threshold: 10J/cm2, 10ns, 20Hz @ 1064nm (Single Wavelength) | 3.5J/cm2, 10ns, 20Hz @ 532nm (Dual Wavelength) | 7J/cm2, 10ns, 20Hz @ 1064nm (Dual Wavelength)

Specifications 1: Wavelength Opto-Electronic Optical Glass Windows

2. IR Windows

IR Windows Diagram
Figure 1: IR Window Diagram

For applications within the IR spectrum, materials such as Zinc Selenide, Sapphire, Silicon, and Germanium are used. An optimal IR window should allow all infrared radiation to pass through it with zero losses. Such windows are typically used for separating environments of varied pressures or temperatures while allowing light energy at a specified electromagnetic wavelength to pass between the two environments.

Part NumberWavelength (nm)MaterialDiameter (mm)Thickness (mm)Application
WSP-1-31064/750Sapphire25.43.0Medical Laser
WSP-15.7-1.11064/750Sapphire15.71.1Medical Laser
WZ-15x18-110600/9400ZnSe15.0 x 18.01.0Protective
WZ-31.75x31.75-410600/9400ZnSe31.7 x 31.74.0Protective
WZ-50x80-310600/9400ZnSe50.0 x 80.03.0Protective
WZ-65x85-310600/9400ZnSe65.0 x 85.03.0Protective
WZ-90x60-310600/9400ZnSe90.0 x 60.03.0Protective
WZ-92x68-310600/9400ZnSe92.0 x 68.03.0Protective
WZ-95x95-310600/9400ZnSe95.0 x 95.03.0Protective
WZ-150x105-310600/9400ZnSe150.0 x 105.03.0Protective
WZ-185x125-610600/9400ZnSe185.0 x 125.06.0Protective
WZB-0.5x1.3-210600/9400ZnSe12.7 x 33.02.0Protective
WZB-0.5x1.3-2C(Corner cut)10600/9400ZnSe12.7 x 33.02.0Protective
WZB-0.6x1.5-210600/9400ZnSe15.2 x 38.12.0Protective
WZB-0.7x1.8-210600/9400ZnSe17.7 x 45.72.0Protective
WZB-0.75x1.5-310600/9400ZnSe19.0 x 38.13.0Protective
WZB-1.0x2.6-310600/9400ZnSe25.4 x 66.03.0Protective
WZB-1.5x3.9-410600/9400ZnSe38.1 x 99.14.0Protective
WZB-2.0x5.2-510600/9400ZnSe50.8 x 132.15.0Protective
WZB-20.3x52.8-310600/9400ZnSe20.3 x 52.83.0Protective
WZB-25x50-310600/9400ZnSe25.0 x 50.03.0Protective
WZB-25x66-310600/9400ZnSe25.0 x 66.03.0Protective
WZB-26.42x10.16-210600/9400ZnSe26.42 x 10.162.0Protective
WZB-30x75-510600/9400ZnSe30.0 x 75.05.0Protective
WZB-53x20-310600/9400ZnSe53.0 x 20.03.0Protective

Table 2: Wavelength Opto-Electronic Sapphire (WSP series) and ZnSe (WZ series) Windows

These windows are made up of special panes of transparent and infrared material set in a frame. Such windows are often used in FTIR (Fourier transform infrared) spectroscopy, FLIR (forward-looking infrared), medical systems, thermal imaging, and a range of other applications within the IR spectrum.

In thermography and infrared imaging applications, IR windows are highly utilized for identifying hot spots resulting from electrical malfunctions, faults, or thermal leaks in various electrical distribution equipment such as circuit breakers, switches, switchboards, switchgear, and transformers. These windows are also used to ensure both personnel safety and equipment protection.

Dimension Tolerance: +0/-0.13mm
Thickness Tolerance: ±0.25mm
Parallelism: ≤3 arc min.
Clear Aperture: >90%
Surface Flatness: λ/4 per 1″Dia@632.8nm
Surface Quality: 60-40 S-DAR
Coating: R<0.2% per surface @10.6μm
Angle of Incidence: Brewster Angle @ 10.6μm

Specifications 2: Wavelength Opto-Electronic ZnSe Windows

What Is Infrared Optics? Thermal Imaging 2
Figure 2: Thermal Imaging Monitoring

Additionally, they empower inspections of live, energized components and connections within electrical cabinets without requiring the removal of their covers. When using these windows for industrial purposes, it’s crucial to ensure they meet the requisite strength and environmental standards specific to the equipment they are installed in. These windows come in a variety of sizes and thicknesses to allow for proper installation.

2.1 Difference Between IR Windows and Other Windows

Infrared light can consist of near IR (NIR), short-wavelength (SWIR), mid-wavelength (MWIR), long-wavelength (LWIR), and far-infrared (FIR). For applications within the infrared region, germanium is often used as a substrate material for optical windows. Unlike other materials like fused silica and N-BK7, which allow the transmission of wavelengths of light from the visible and UV regions of the electromagnetic spectrum, germanium, and silicon are opaque to UV and visible light, but have a wide transmission range in the infrared region.

Materials such as sapphire, Zinc Selenide, Zinc Sulfide, and calcium fluoride have a wide transmission band that ranges from UV to MWIR for Calcium Fluoride and Sapphire and from the visible spectrum to LWIR for Zinc Selenide and Zinc Sulfide. Hence, applications requiring the transmission of solely IR waves should use Germanium or Silicon windows.

2.2 Germanium IR Windows and Applications

Germanium IR Windows Transmission
Figure 3: Germanium Transmission Profile

As seen from the transmission profile, Germanium serves as a long-pass filter for wavelengths greater than 2µm. Due to its high index of refraction (4.0 from 2µm to 14µm), it has minimal chromatic aberration and anti-reflection coating is used in it. In addition, it demonstrates scratch resistance, and inertness to air, water, alkalis, and a variety of acids. Its relatively high density (5.323 g/cm3), should be considered in applications where weight is a restriction.

Part NumberWavelength (nm)MaterialDiameter (mm)Thickness (mm)Application
WGE-152X120X6.54-BB8000-12000Ge152.0 x 120.06.5Protective

Table 3: Wavelength Opto-Electronic Germanium IR (WGE series) Windows

In addition, the transmission characteristics of Germanium are significantly affected by temperature. As the temperature reaches 100°C, absorption increases to the extent that germanium becomes nearly opaque, and at 200°C, it loses all transmissive properties. Germanium optical windows are extensively utilized in the defense and aerospace industries, life and medical sciences, industrial OEM, and a variety of other infrared applications. Refraction makes it suitable for wide-angle lenses and microscopes. In thermal imaging systems, Germanium is commonly used for IR windows and lenses.

Laser Engraving Application
Laser Engraving

One of the more common applications for Germanium windows is in low-power CO2 laser systems. With a LIDT (Laser-Induced Damage Threshold) of 10 J/cm2, Germanium windows are not suited for high-power or continuous wave (CW) lasers. Part of the reason for this is higher powered lasers cause temperature increases, dramatically dropping transmission properties over 100ºC and eventually damaging the substrate itself once temperatures near 600ºC are reached. On the other hand, an AR-coated germanium is well suited in a low-power pulsed laser setup. One particularly noteworthy application is in quantum cascade lasers (QC), which are used in high-end materials science. 

2.3 Silicon IR Windows and Applications

In addition to Germanium, Silicon (Si) is also widely used for IR windows. Silicon is one of the hardest minerals and optical materials available for use in the NIR (1µm) to about 6µm. Optical quality Silicon is usually doped (5 to 40 ohm-cm) to prevent absorption bands within the transmission waveband. Silicon has a lower refractive index than germanium and has a lower density that makes for less weighty optical designs.

Silicon Infrared Windows Transmission
Figure 4: Silicon Transmission Profile

Silicon is ideal for use as windows in the 3 to 5µm (MWIR) waveband and as a substrate for optical filters and silicon’s low density (half that of germanium or Zinc selenide) makes it ideal for weight-sensitive applications, especially those applications between the 3 – 5µm range. It has a density of 2.329 g/cm3 and a Knoop hardness of 1150, so it is harder and less brittle than germanium.

With its high thermal conductivity, Silicon is better suited for high-power lasers compared to Germanium. This is particularly important in fields such as industrial inspections and surveillance. However, as seen from its transmission profile, it has a strong absorption band at 9µm, which does not make it suitable for CO2 laser applications.

Surveillance Application

Silicon windows are used in various applications. It is an integral component in thermal imaging devices, enabling the detection of temperature variations in objects and environments. It is also widely used in IR spectroscopy equipment, to analyze the composition of different materials, as well as in the defense and security industry for target detection and night vision goggles.

2.4 Difference Between Germanium and Silicon IR Windows

Silicon and germanium are both semiconductor materials used in various applications, including window technologies. The main difference lies in their physical properties and optical characteristics. Silicon windows offer better transparency in the SWIR and MWIR but are less efficient in LWIR. On the other hand, germanium windows have superior infrared transparency for LWIR, making them ideal for thermal imaging and infrared spectroscopy applications.

However, germanium is generally more expensive and more fragile than silicon. Silicon is a commonly found compound on the earth’s surface. On the other hand, Germanium is a rare material that is commonly found in lead, silver, and copper deposits. Additionally, the processing costs of Germanium are also higher than those of Silicon, which makes Germanium a more expensive compound. The choice between silicon and germanium windows depends on the specific requirements of the application, such as wavelength range, cost, and mechanical durability. 

3. Windows Coating

IR Windows Innovations: Empower 2024 with Infrared Windows 3
Wavelength Opto-Electronic Coating Machine

Anti-reflection (AR) coatings are often put on optical windows to maximize transmission in the desired wavelength range. Most AR coatings are also very durable, providing resistance to both physical and environmental damage. For these reasons, the vast majority of transmissive optics include some form of anti-reflection coating.

When choosing an AR coating for a window, the full operating spectral range of the specific application must be thoroughly considered. While an AR coating can significantly improve the performance of an optical system, using the coating at wavelengths outside the design wavelength range could potentially decrease the performance of the system. It is recommended that an AR coating is used for germanium windows. 

4. Conclusion

ToleranceStandardPrecisionHigh Precision
MaterialsGlass: Borosilicate Glass (BK7), Optical Glass, Fused Silica, Fluoride
Crystal: ZnSe, ZnS, Ge, GaAs, CaF2, BaF2, MgF2, Si, Fluoride, Sapphire, Chalcogenide
Plastic: PMMA, Acrylic
DimensionMinimum: 4 mm, Maximum: 200 mm
Clear Aperture80%90%95%
Irregularity (P-V)λ/4λ/10
Wavelength Range200nm-14μm200nm-14μm190nm-14μm
Surface Quality80-5040-2010-5
CoatingBroadband Anti-Reflection, Narrowband Anti-Reflection
Table 4: Wavelength Opto-Electronic Optical Windows Manufacturing Capabilities

Optical windows are crucial in the optics industry and are used for a range of applications with varying purposes. Various materials can be used as optical windows to filter out specific wavelengths of light, based on their transmission profiles. Zinc Selenide, Zinc Sulfide, Sapphire, and Calcium Fluoride are a few compounds used for windows that allow for the transmission of light in the visible and IR spectrum. On the other hand, Germanium and Silicon are useful in applications requiring only wavelengths from the IR spectrum to pass through.

Various factors can affect which material should be chosen for a specific application. In addition to transmission range, this includes factors such as density, hardness, operating temperature, nature of operation, and cost. Other considerations include the addition of an anti-reflective coating, which can alter the transmission profile to be within the desired range of application. In the ever-evolving field of optics, the significance of optical windows remains crucial, serving as a gateway to unlock the potential of optical technologies and applications.

Wavelength Opto-Electronic design and manufacture optical windows of different materials from standard to high precision specifications. Our engineers are equipped with vast experience and with our state-of-the-art facilities, you can be assured that our windows produced are high quality, measured, and tested with our comprehensive metrology.

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