Frequently Asked Questions (FAQs)

Below you’ll find answers to the questions we get asked the most when looking for a required IR optics solution

Reduced SWaP Continuous Zoom Lens for MWIR 10μm pitch VGA Sensors

The Ophir LightIR 16-180mm f/3.6 continuous zoom lens is a compact, folded design that reduces size while maintaining a long effective focal length (EFL). It uses fewer optical elements than similar focal length lenses to reduce weight. It is designed with low power consumption to meet design criteria.

Almost any application can benefit from reducing size, weight, and power, but in some applications it is essential. Drone and UAV designers are constantly balancing the size and lift capacity of the aerial vehicles with the sophisticated technology needed in the payloads. The Ophir LightIR 16-180mm f/3.6 lens allows for a sophisticated, long-range MWIR sensor while limiting the payload size, weight and power consumption. This makes it ideal for airborne gimbles on SWaP constrained platforms. It is specifically designed to enhance imagery in MWIR 640×512 10µm pitch sensors preferred by engineers with SWaP constraints.

Even though SWaP constraints limit size, weight and power, the performance required of the systems cannot be compromised. Imaging systems must maintain the ability to produce clear, crisp images for the end user. Ophir’s LightIR 16-180mm f/3.6 maintains excellent MTF (modulation transfer function) as well as maintaining focus throughout the zoom. It also includes fast zoom and focus, tight boresight, low power consumption, wide operational temperature range, and high durability in harsh environmental conditions. Ophir has created a unique design that implements strict quality control to achieve the best images any given sensor can produce.

Maintaining high performance in any lens begins in the design phase, the key to the designs is to build in performance and manufacturability. Near diffraction limit MTF with a design that allows for some minor tolerances in the build cycle leads to a superior product that meets or exceeds its testing requirements. Ophir’s state-of-the-art manufacturing facilities maintain tight quality control over the entire manufacturing process from design to delivery, to ensure a superlative product.

Single- and dual-FOV (field of view) lenses are not well suited to SWaP constraints. Single FOV does not provide the flexibility required by systems engineers and dual-FOV cannot meet size and weight constraints. Ophir’s LightIR 16-180mm f/3.6 lens provides a wide FOV for detection and situational awareness and continuously narrowing FOV for identification of objects of interest. The folded optical design allows this in a smaller volume package perfect for SWaP design constraints. In order to withstand harsh environmental conditions, the lens is offered with high durability (HD) and low reflection hard carbon (LRHC) AR coatings.

Drone and UAV design engineers are perpetually balancing size and capability requirements for each aerial system. Larger aerial vehicles have larger lift capacities, but cannot be readily moved by the people who operate them. Smaller aerial vehicles have limited range and limited lift capability. In order to balance these constraints, design engineers detail size, weight and power requirements for each subsystem used on a particular design. Ophir works with sensor manufacturers as well as drone and UAV designers to provide the best optics to meet these rigorous needs. LightIR 16-180mm f/3.6 is the culmination of this teamwork between optics designers and the end users of complex aerial systems.

When it comes to developing optical components and optical systems for UAV payloads, three factors must be measured. These factors can be summarized by the acronym SWaP – size, weight, and power consumption. UAV payloads, especially for smaller commercial UAVs, impose strict size and weight restrictions. Power consumption must be reduced to minimize fuel usage, thus maximizing flight time.

The optical payloads for small autonomous aerial systems must be able to provide high optical performance despite their compact form. Detectors are increasing in both resolution (number of pixels) and format (size), while decreasing in pixel size, which puts pressure on optical manufacturers to make smaller, lighter optics with lens quality that will still allow for maximum imaging performance.

Various technologies are being used to meet these optical needs. These technological solutions include innovative optical and mechanical designs, free-form optics, and unique lens coatings.

Large mirrors for multispectral optical systems

Over the last 10 years, multi-spectral optical systems have been used by system integrators in the defense and aerospace industries, in surveillance and monitoring, and in certain commercial applications. Multi-spectral optics enable many new possibilities for defense and national security missions on land and sea. Multi-spectral imaging systems enable the combining of multiple cameras into one, to significantly improve performance without increasing size and weight. These systems generally have long focal length and are meant for long-distance surveillance, possibly tens of kilometers. The multi-spectral optics enable maximum error correction and allow for a wider field of view, performing to a high level across a wide range of wavelengths (Short-wave IR, Mid-wave IR, Long-wave IR, Visible and Near IR). They allow for high day/night performance in situations where vision is obscured, such as low lighting and adverse weather conditions.

For example, multi-spectral optics is integrated into the optical systems of large unmanned aerial vehicles (UAVs) for the long-distance aerial monitoring of agricultural field temperature using IR, as well as in the aerospace industry, for satellites and long-range telescopes.

A multispectral electro-optical system (EOS) combines multiple optical channels into one, allowing significantly improved performance while reducing size and weight – all thanks to the inclusion of large mirrors. An electro-optical day/night system built for aircraft payloads providing high performance in harsh weather conditions is a perfect example of a multispectral EOS.

Long range defence, surveillance and monitoring applications use multi-spectral optical systems. Certain commercial applications also use a multi-spectral EOS. For example, aircraft or large UAVs often carry a multi-spectral EOS with an integrated large mirror in the payload.

Unlike lenses that are often bandwidth limited, large mirrors combine several imaging bands in the visible, NIR, SWIR, MWIR, and LWIR, allowing significantly improved performance without increasing the size and weight of the EOS. Large mirrors perform to a high level across a wide range of wavelengths including laser-based applications such as laser range finders (LRF) and laser designators.

The mirror’s reflection angles are identical for all wavelengths and, therefore, all optical channels can be combined, creating the multispectral system.

The mirrors are also the key enabler for a folded optics design contributing to reducing the size of the multispectral system.

Large mirrors play an important role in long-range reflective systems, for long distance surveillance, such as reflective telescopes. These telescopes use a combination refractive and reflective optics to maximize error correction and allow for a wider field of view.

Furthermore, Large mirrors are often integrated in the optical systems of defense and surveillance systems, for example, in large aircraft. These mirrors allow for the production of high resolution imagery during long distance surveillance and monitoring. To produce high quality images from a distance, the mirrors must meet a strict set of requirements. Detectors are increasing in resolution all the time, necessitating mirrors with increasingly accurate surfaces, meeting tight tolerances in terms of shape and irregularities.  The multi-spectral nature of the optical systems means that mirrors must also be able to perform throughout a variety of wavelengths.  They must have minimum roughness, especially when they are used in VIS wavelength, under 40Å rms (root mean square), to prevent light from scattering.

The Narcissus Effect

With its name taken from Greek mythology, narcissus is one of the unwanted effects of thermal imaging, whereby a cold detector images the reflection of itself and displays it in the processed video output. Whenever the thermal detector senses variations in background radiation caused by reflections from lens surfaces, the phenomenon of narcissus may occur.

Typically, the narcissus effect comes from the detector cold shield. The cold shield is kept at cryogenic temperatures, while the lens’ temperature is usually near ambient temperature. Narcissus occurs when the image of the cold shield is reflected from a surface in the lens train and is focused onto the detector. In such cases, the image of the cold shield shows in the middle of the display as a dark or bright circle. Because of the difference in temperature between the cold shield and the lens assembly, every infrared zoom lens in a system with a cooled detector is prone to narcissus.

To test levels of narcissus when manufacturing a lens system, Ophir sets up a narcissus simulation. Every surface of the lens system is examined through the entire zoom sequence, at many intervals. If the narcissus effect occurs at any point, it is measured, analyzed, and methods are put into place to correct it.

Narcissus reduction should be addressed during the manufacturing of a lens system. Here at Ophir, this is done through unique lens designs and anti-narcissus coatings that will prevent the production of back reflections that could focus on the detector and cause the narcissus effect.

Thermal Imaging

It’s a tradeoff. Where customer’s requirements are for long range vision, high-sensitivity, a better signal to noise ratio, and atmospheric transmission – MWIR is the logical selection. The MWIR detector is more sensitive at a higher signal to noise ratio, and comes with a cooling mechanism, which makes it larger, and more costly. In other cases, when considering size, costs, and environmental harsh conditions (dust, smoke, sand, extreme temperatures), the LWIR is more appropriate.

Ophir’s MWIR lenses come with long Focal Length which makes them more suitable for longer range application. This is reflected in our product’s DRI capabilities with over 25km, thanks to the very long EFL (around 1,000mm or more).In comparison,

Ophir’s  LWIR DRI performance reaches approximately 13km.The following is a comparison table that captures the main differences between MWIR and LWIR optical performance:

MWIR 3-5µm wavelength LWIR 8-12µm wavelength
Daytime vs Nighttime observation Lower performance Good performance, as the sun emits low radiation in this spectral range, and therefore there is less spectral noise
Smoke screen Lower performance Good performance
Dust screen Lower performance Good performance, as the LWIR long wavelength is less disrupted by small dust particles
Haze Lower performance Good performance
Humidity Good performance Lower performance due to high water absorption at this wavelength
Temperature sensitivity High Low
High signal sensitivity to noise ratio High Low
Wide landscape picture Lower performance Good performance, since landscape objects emit more radiation at a long wavelength
Matrix implementation Easier None
Long Range vision Good performance Lower performance
Cost and size High due to cooling system needed Low

Zoom lenses

Ophir addresses boresight retention using tight tolerances for all optical and mechanical elements, as well as backlash-free design. Using Ophir’s zoom lenses, once the zoom position is changed, the boresight is maintained, meaning that no adjustments are needed during optical system operation. This is an integral feature in zoom lens design, which we consider as part of the standard requirements and quality tests.

Ophir’s boresight error ranges from 0.15mm to 0.35mm at the focal plane.

Long Range Zoom Lenses

The focal length determines the field of view (FOV) of the thermal imaging camera. The longer the focal length, the smaller the FOV, which translates into more pixels across a target at a fixed range (meaning, the target angle divided by the IFOV angle).

For example, consider a man at a range of 1km. The effective angle of the target is about 1mRad (1m/1000m). If we consider, for example, a 500mm focal length combined with a 15um pixel detector, the IFOV would be 30 uRad. Therefore, the number of pixels on a target is equal to the target angle divided by the aIFOV angle, which is about 30 pixels on the target 1000/30 (>8 pixels required for identification based on the Johnson’s criteria).

Extenders allow flexibility. A lens can be replaced easily, using the same basic lens body, with no need for hardware or software modifications. This lowers the complexity in the design and can also result in reduced NRE costs. Yet, reduced cost is not always achieved. Size and weight are mostly a disadvantage, as a second optical assembly is added to an existing one.

Most of Ophir’s lenses cover a wide range of temperatures, from -30°/40°C to 70°c, and even 85°c. Using proper coating and sealing techniques, the lenses are also resistant to high humidity. Dust and vibration are also environmental factors that are tested, to ensure resistance to harsh environmental conditions. Thanks to these features, Ophir’s lenses answer a wide range of application demands. In commercial applications, such as UAVs and drones, vibration management is crucial, as is the temperature range. Ophir’s key competitive advantage in IR zoom lenses is the ability to maintain a high level of optical performance, which is, for example, characterized by the modulation transfer function (MTF) throughout the temperature range, and in different environmental conditions. Ophir’s lenses provide outstanding performance, and an MTF close to the diffraction limit, over the entire temperature range, which is a critical demand for many applications.

Optical Components

Ophir have free-form capabilities, as part of components manufacture. These include highly complicated unique components, such as mirrors and lenses designed to answer customers’ requirements.

 

Optical Coatings

Optical coatings typically consist of thin films made up of single or multiple layers of either metallic or dielectric materials. When properly designed and fabricated, these coatings can dramatically modify the reflection and transmission properties of an optical component. The properties can be controlled from the deep UV to the IR with narrowband, broadband, or multi-band response, and can be polarization sensitive. Optical coatings can be applied directly to the surface of an optical component to tailor its reflectivity, as in the case of an optical mirror or beam splitter. For other components, such as lenses, the applied coatings may simply improve their overall transmission properties by reducing surface reflectivity. When optical coatings are integrated into a monolithic component for the express purpose of controlling the spectral transmission of light, the component is referred to as an optical filter.

Ophir® Optics Optical Coating Capabilities

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook

The individual layers that make up optical coatings are typically a few tens of nanometers to a few hundred nanometers in thickness, while a single optical coating can be comprised of several hundred layers. Consequently, the techniques used to deposit these layers require a high degree of precision. Generally, the process begins with surface fabrication to minimize surface roughness and sub-surface damage. It continues with surface cleaning and preparation and is followed by deposition of high-performance thin film designs. The deposition technologies include thermal evaporation, electron-beam, ion-assisted deposition, and advanced plasma deposition. The most appropriate coating technology for the intended product design depends on the operating environment, spectral requirements, physical characteristics, application requirements, and economic targets. The optical coating process is completed with comprehensive performance testing using sophisticated metrology tools.

Metal coatings used on optical mirrors typically consist of a single layer approximately 100 nm thick. This ensures that the broadband high reflectivity properties of the metal due to the complex index of refraction are present. In order to provide greater tuning of the reflectivity and over specific wavelengths of interest, dielectric coatings are used . These coatings consist of alternating high refractive index (nH = 1.8 – 4.0) and low refractive index (nL = 1.3 – 1.7)

dielectric layers (see Figure 3). The thickness of each layer is chosen such that the product of the thickness and the index of refraction of the layer is λ/4.


Figure 4. Scanning electron microscope image (top) and schematic (bottom) of an optical interference coating shown on left. Reflection and transmission of light by a filter consisting of an interference coating (right).

Dielectric optical coatings are used in a myriad of ways. In addition to highly reflective dielectric mirrors (see Figure 3), these coatings are incorporated in broadband beam splitters and IR wavelength lenses. When light is incident at an angle to a surface, i.e., not normal incidence, the reflectivity becomes polarization sensitive. This allows dielectric coatings to be polarization selective and such coatings are used in polarizing beam splitters (see Section III.A.5). In addition to enhancing the reflectivity, dielectric optical coatings can also be used to reduce surface reflections in the form of broadband anti-reflection coatings. These coatings can be applied to any optical component, e.g., lens, prism, beam splitter, window, to markedly improve its transmission efficiency. The reflection from an air-glass (n2 ≈ 1.5) interface gives a reflectivity of 4%, which can be reduced considerably with a broadband anti-reflection coating (see Figure 5).


Figure 5. Typical broadband anti-reflection coating in the UV and VIS spectral regions.

These reflectivities can be reduced even more to improve transmission in laser systems with multiple optical elements, saving valuable laser energy from being lost to surface reflections. This superior performance, however, is achieved at the cost of reduced wavelength range.

Ophir® Optics Optical Coating Capabilities

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook

Optical Lenses

Image quality is affected by all system components – lens, detector, electronics, image processing, and monitor.

The lens related image quality factors are:

  1. Diffraction limit
  2. Lens design
  3. Manufacturing tolerances

Diffraction limit is a physical limit, which depends on the wavelength and F# only, not on the lens design or manufacturing.

The other two factors, lens design and manufacturing tolerances, determine lens aberrations, which limit image quality. There are several types of aberrations, each of which deteriorates image quality in a different way. An ideal lens eliminates all aberrations completely. In reality, it is impossible and impractical to design and manufacture an aberration-free lens. In order to achieve high image quality, lens aberrations should be minimized. This is a challenging task, especially with the smaller pixel sizes of modern detectors, because there is a tradeoff between minimum lens aberrations and reasonable cost.

The optical design specifies the layout of the optical elements, the raw material, shape and dimensions of each optical element, as well as tolerances for manufacturing and mounting the optical elements. Theoretically, it is possible to minimize lens aberrations by adding more optical elements (to cancel aberrations) and specifying tight tolerances on lens manufacturing and assembly. This approach, however, would increase lens cost dramatically, making it too expensive for the market. The right approach would be to balance between lens performance and cost. Ophir has succeeded to achieve excellent lens performance together with competitive prices, by combining an innovative well experienced design team with excellent in-house manufacturing capabilities.

Lenses are the optical components that form the basic building blocks of many common optical devices, including cameras, binoculars, microscopes, and telescopes. Lenses are essentially light-controlling elements and so are exploited for light gathering and image formation. Curved mirrors and lenses can accomplish many of same things in terms of light collection and image formation. However, lenses tend to be superior in terms of image formation because they are transparent, which allows light to be transmitted directly along the axis to the detector whereas mirrors require an off-axis geometry. Mirrors are typically preferred in terms of light collection as they can be made significantly more lightweight than lenses and therefore can achieve larger diameters and light collecting ability.
This section discusses the mechanism of refraction that underlies the operation of a lens, issues that affect its performance, and the different lens types.

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook

In addition to light reflecting off a planar interface between two media, it can also be transmitted and then refracted in the second medium (see Figure 6). Refraction refers to the change in the angle of the incident light when it enters the second medium. Since the speed of light in a medium is inversely proportional to its index of refraction, it will either slow down or speed up when it enters a different medium, resulting in the light changing its direction. Figure 6 shows an example where the index of the second medium (n2) is greater than the first (n1), which results in a bending of the light toward the normal to the interface. This phenomenon of refraction is described by Snell’s law.

Figure 6. Illustration of Snell’s law of refraction at an interface between media of refractive indexes n1 and n2 [157].

A lens is typically made up of a transparent dielectric material like fused silica or optical glass with the front and back surfaces having a spherical curvature . Since the surfaces are curved, each ray of light that comes in parallel to the optical axis (as shown in Figure 7) has a different value of ?i with respect to the surface normal. Each ray then refracts according to Snell’s law. For a positive lens, this causes the light to converge toward its focal point on the right side of the lens while light will diverge from the focal point located on the left side of a negative lens. The ramifications of these operations are that lenses can be used for image formation as well as collection and collimation of light (see Figure 7). There are several important aspects to optical imaging with lenses, including the relationship between object and image distances and the resulting magnification as well as the quality of the resulting images. Details about these concepts can be found in . Similarly, important aspects of involving the light gathering ability of lenses including throughput and its relationship to numerical aperture (NA) or f-number (F/#) are described in.

Figure 7. Illustration of how a lens affects incoming parallel light rays (left). Applications of lenses (right) include creating a magnified image of an object (top), collimating light from a point source (middle), focusing a collimated light source (bottom).

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook

Ideal lenses would form perfect images (or exact replicas of the object being imaged) and would be able to focus collimated light to a spot size limited only by diffraction. However, real lenses are not perfect and induce optical aberrations, which cause degradation in the ability to form a high-quality image, collimate a beam, or focus it tightly. Monochromatic aberrations, i.e., no wavelength dependence, are common to both mirrors and lenses and come from the inability of spherical surfaces to focus light properly when it is far from the axis.

These aberrations include spherical aberration, coma, and astigmatism. Figure 8 demonstrates the impact of spherical aberration in a lens where rays with smaller angles are effectively collimated while rays with large angles converge instead. Unlike monochromatic aberrations, chromatic aberration only occurs in lenses. Due to dispersion of the index of refraction of the lens material, different wavelengths will refract with different angles according to Snell’s law (see Figure 8). This causes degradation in image quality or light gathering ability when broadband light is being used.

Figure 8. Effects of spherical aberration (left) and chromatic aberration (right) on collimation when a point source is at the focal point.

While spherical lenses do induce aberrations, choosing the proper lens shape can help minimize optical aberrations (see Figure 9). For instance, plano-convex lenses, where only one side is curved, are the best choice for focusing parallel rays of light to a single point. Bi-convex lenses (both sides have curvature that may not be equal to one another) are the best choice for imaging when the object and image are at similar distances from the lens. When a single spherical lens may be unsuitable due to spherical aberration, aspheric lenses may be used. These lenses have surfaces with tailored curvatures that help minimize the impact of aberrations but are typically expensive due to the complexities associated with fabrication. Alternatively, multiple spherical lenses can be used where one lens can cancel the aberration caused by another, as shown in Figure 9. In addition to correcting for monochromatic aberrations, an achromatic doublet can be used to minimize chromatic aberrations by choosing the dispersion of the materials in the two lenses to produce a focal length that is independent of wavelength. Microscope objectives are multi-element lens systems that can significantly reduce the impacts of aberrations but are more expensive due to the complexity of the design. All the aforementioned lenses are rotationally symmetric, that is, light focuses the same regardless of which transverse axis it passes through.

Figure 9. Using single and multiple lens systems to minimize optical aberrations for a specific imaging application.

There are many features to take into consideration when choosing a lens, including focal length, lens shape, F/#, lens material, transmission properties, wavefront distortion, scattered light, types of coating, and cost.

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook

Optical Mirrors

Mirrors are arguably the most commonly used optical components. They appear industrial applications, as well as large-scale optical systems. These components utilize reflection to redirect, focus, and collect light. Optical mirrors consist of metallic or dielectric films deposited directly on a substrate such as glass, differing from common mirrors, which are coated on the back surface of the glass. Consequently, the reflective surface of an optical mirror may be subject to environmental conditions. This means that durability and damage resistance must also be considered when choosing a mirror as well as how well it reflects light at the wavelength of interest. This section introduces the physical concept of reflection and discusses the important attributes of the mirror as an optical component.

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook

G Generally, when light reaches a planar interface between two media (see figure 1) a portion of it is reflected back into the original (incident) medium and a portion is transmitted and refracted into the second medium . Absorption of the light in either medium is also possible, but non-absorbing media will be assumed here. Reflection can occur from smooth surfaces such as those found on mirrors (referred to as specular reflection) or from rough, uneven surfaces (called diffuse reflection or scattering). Although both obey the same laws of reflection, specular reflection leads to rays that reflect as a group at the same angle, whereas diffuse reflection occurs at different angles off randomly oriented surfaces. This enables specular reflection to perform the useful operations of redirecting light.

Figure. 1. Illustration of the law of reflection at a planar surface

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook

Mirrors made up of planar surfaces, such as that shown in Figure 1, are important components for directing light through the proper path in an optical system. Such mirrors can be combined to form optical components known as retroreflectors or corner cubes. These components consist of three mirror surfaces all perpendicular to one another. Such a geometry enables 180 degrees reflection of the light, regardless of incidence angle, and therefore requires very little alignment compared to a single flat mirror.

Curved mirror surfaces also called concave reflectors, can be exploited with the goal of collecting, focusing, and imaging light as illustrated in Figure 2. These mirrors possess an advantage over lenses in that they perform satisfactorily across a broad-wavelength range without requiring refocusing. The reason for this is that reflection occurs at the surface of these optics, rather than passing through the optic as is the case with a lens, and so the dispersion of the index of refraction does not come into play. Simple spherical reflectors can be used to collect radiation from a source at the focal point (located at half of the radius of curvature of the mirror) and reflect it as a collimated beam parallel to the axis. Since spherical mirrors possess spherical aberration, a parabolic curved surface can be used instead to either collimate light from a focal point or focus light from a collimated beam (see Figure 2). Ellipsoidal surfaces can focus light from one focal point to another (see Figure 2).

Figure 2. Concave reflectors with different surface shapes allowing for light collection and focusing. A paraboloidal reflector reflects light from the focus into a collimated beam (left). An off-axis paraboloidal reflector refocuses a collimated beam off the mechanical axis (middle). Ellipsoidal reflectors reflect light from one focus to a second focus, usually external (right).

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook

Selecting the proper mirror for laser system requires consideration of a number of factors, including reflectivity, laser damage resistance, coating durability, thermal expansion of the substrate, wavefront distortion, scattered light, and cost. These mirror characteristics depend on the optical coating, the substrate, and the surface quality. The optical coating is the most critical component of a mirror as it dictates its reflectivity and durability. Optical mirror coatings are typically made up of either metallic or dielectric materials. By virtue of their conductivity, metals have a complex index of refraction with a large imaginary part over a very wide wavelength range. This gives rise to a large reflectivity that is relatively insensitive to wavelength, which gives metallic mirrors their shiny appearance. Metallic coatings are usually made of silver, gold, or aluminum and the resulting mirrors can be used over a very broad spectral range (see figure 3). Metallic coatings are relatively soft, making them susceptible to damage, and special care must be taken when cleaning. Mirrors with dielectric coatings are more durable, easier to clean, and more resistant to laser damage. However, as a consequence of their dispersive and predominantly real indices of refraction, dielectric mirrors have a narrower spectral reflectivity and are typically used in the VIS and NIR spectral region. There is greater flexibility in the design of dielectric coatings compared to metallic coatings. When compared with metallic mirrors, a dielectric mirror can offer higher reflectivity over certain spectral ranges and can offer a tailored spectral response (see figure 3).

Figure 3. Reflection spectra of silver metallic mirrors showing broadband reflectivity (left) and dielectric laser-line mirror showing two narrow reflection bands (right).

Most substrates upon which the coatings are deposited are dielectric materials and these substrates control the thermal expansion and transmission properties of mirrors. Certain materials have lower thermal expansion coefficients, e.g., fused silica, than others, e.g. N-BK7 optical glass, but the cost of the material and ease of polishing must also be considered. If light transmitted through the substrate is not required, the backside of the substrate is typically ground to prevent inadvertent transmissions. However, for transmissive mirrors, a substrate material with a homogenous index of refraction is important, e.g. fused silica.

Prior to depositing the optical coating, the substrate’s surface must be ground and polished to the proper shape (either planar or curved). The surface quality and flatness determine the fidelity of the mirror performance with the targeted application dictating the requirements for these parameters. Surface flatness is often specified in wavelengths, e.g. λ/10, over the entire usable area of the mirror. When preservation of the wave front is critical, a λ/10 to λ/20 mirror should be selected, while less demanding applications can tolerate a λ/2 to λ/5 mirror with the associated reduction in cost. Surface quality is usually dictated by the severity of random localized defects on the surface. These are often quantified in terms of a “scratch and dig” specification, e.g. 20-10, with a lower value indicating improved quality and therefore lower scattering. For high precision surfaces, such as those found within the cavity of a laser, a scratch-dig specification of 10-5 may be required since it would yield very little scattered light.

Surface polishing tolerances in terms of irregularity, surface roughness, and cosmetic imperfections are verified using state-of-the-art metrology equipment. These same parameters and procedures are used to assess the quality and flatness of other optical components such as lenses or windows.

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/2019. https://www.mksinst.com/mks-handbook