3.3.3.1.1  Temporal

Temporal instability is the change that takes place in a component as a function of time in a fixed environment. It is a permanent change. For example, two sets of nominally similar l in. gauge blocks were tested at the National Bureau of Standards (NBS) over a period of roughly 30 years.25 One set exhibited a positive and relatively constant rate of change of dimensions of as much as 10⁻⁶ m/m/year. That is a very small amount, and yet, it is totally unacceptable for optical applications. The other set typically changed only 25 nm in 22 years. This kind of dimensional instability is generally associated with relaxation of residual stress.

3.3.3.1.2  Hysteresis

Thermal/mechanical hysteresis is the change measured in a fixed environment after exposure to a variable environment, that is, measured in a laboratory environment before and after exposure to changes in temperature and/or mechanical loading. It, too, is a permanent dimensional change. A common example is the dimensional change that takes place in fiber-reinforced composites when subjected to thermal cycling over a wide temperature range. The behavior typically shows a substantial change in length of up to 1% on the first cycle, but the amount of change decreases with each succeeding cycle, approaching an asymptote. This kind of behavior is discussed later in this section for other materials. For composites, the cause for the dimensional changes is usually internal microcracking of the fibers, while in single-phase materials, it is usually some other form of internal stress relief. Similar behavior has been observed with mechanical cycling and vibration.

3.3.3.1.3  Thermal

Thermal instability is the dimensional change measured in one fixed environment after a change from another fixed environment, independent of the environmental path. This dimensional change is reversible upon returning to the original conditions. Figure 3.3 shows evidence of just such a change. This beryllium (Be) mirror was made from an experimental billet produced in the late 1960s which had a substantial amount of thermal expansion inhomogeneity.26 It was interferometrically tested many times over a period of almost 10 years and exhibited the same distortion shown in the figure when heated and always returned to the same optical figure at room temperature, within the 0.02 wave accuracy of the instrument. This behavior has been virtually eliminated in modern Be materials.

Figure 3.3   Optical interferograms of an electroless nickel-coated experimental beryllium alloy mirror (c. 1968) showing a reversible thermal instability of approximately two waves. (From Paquin, R. A., Workshop on Optical Fabrication and Testing, Technical Digest, Optic Society of America, Washington, DC. 1981.)

3.3.3.1.4  Other Instabilities

The other principal type of instability is the change measured in a fixed environment after being exposed to a variable environment where the change is dependent on the environmental path between the fixed environment measurements. This type of distortion can be permanent or reversible. For example, in Figure 3.4, the length of Zerodur on cooling from 300 to 20°C depends on the cooling rate.27 This is typical behavior for glasses containing MgO. But note that the curves are parallel below 150°C, indicating that the temperature range of sensitivity is between 150 and 300°C. This behavior has been eliminated in a new version of this material called Zerodur M. This type of behavior is rarely observed in metals. These are the major types of dimensional instability that can be encountered in optics and precision instruments. Many of the other commonly observed instabilities can be placed into one or more of these four categories.

Figure 3.4   Thermal length contraction of Zerodur for three cooling rates from 300 to 20°C illustrating a hysteresis of dimensional instability. (From Lindig, O., and Pannhorst, W., Applied Optics, 24, 3330, 1985.)

3.3.3.2.1  External Stress

When an external stress is applied to a component, if it behaves according to Hooke’s law, it should deform elastically no matter how long the stress is applied and return exactly to its original shape when the stress is removed. But this being an imperfect world, and most materials not being perfect, there are other responses to externally applied stress. If a load is suddenly applied, held for a length of time, and then released, the elastic response has exactly the same square wave shape as the applied load. An anelastic strain shows a time-dependent elastic response with respect to the applied load. For this type of behavior, there is no strain when the load is first applied, but it increases toward an asymptote with time; when the load is removed, the strain asymptotically returns to zero. Anelastic behavior is rarely observed in metals and ceramics, has been observed in some glass-ceramics at low temperature, but is more commonly observed in polymers. Plastic strain is permanent and does not decrease as the load is removed. The most common behavior for metals is a combination of elastic and plastic response to stress. Time-dependent plastic strain is called creep. Most of the time, many materials exhibit a combination of these elastic responses to externally applied loads.

There are a number of material properties that are important to dimensional stability. Among these are thermal properties such as the CTE, thermal conductivity, and mechanical properties: elastic modulus (Young’s modulus), a measure of stiffness and the slope of a stress vs. strain curve; Poisson’s ratio, the relationship between tensile (or compressive) and shear strain; yield strength (at 0.2% offset), the stress to cause 2 × 10⁻³ permanent or plastic strain; ultimate or fracture strength; MYS, the stress to produce 1 × 10⁻⁶ plastic strain (one unit microstrain); and microcreep strength, which has no acceptable definition other than that it is less than the MYS and is a constant stress that produces microstrain after some.

Microyield behavior cannot be directly inferred from the macromechanical properties of either yield strength or modulus. For example, when the behaviors of I-400 Be and 2024-T4 aluminum, metals with approximately the same yield strength, are compared, Be exhibits a MYS of approximately 50 MPa, but with increased stress, it yields little more. However, the Al alloy resists yielding for a high MYS of 250 MPa, but then continues yielding readily. Recent analyses have shown, however, that for any given family of alloys of the same base material, MYS is proportional to yield strength.

MYS is strongly dependent on the prior history of the material. If it has been annealed, the microyield will be lower than in almost any other condition. Conversely, if there has been prior straining, either through intentional or inadvertent applications, the MYS will be raised. While prestraining produces a stronger material, it also leaves a level of residual stress that may be detrimental. Residual stress is discussed further in the next section.

Since high MYS is a desirable property, and since many materials have relatively low MYS, it is important to know that there are methods for increasing it. Prestrain, as mentioned earlier, is one method, but it has its disadvantages. Many aluminum alloys, after rolling into plate form, are stretched to a few percent to both straighten and level the stress through the thickness of the plate, which also increases the MYS; but this process also seems to lower microcreep strength. Since the process of microyielding occurs, at least in the early stages, by the movement of dislocations, anything that pins or prevents dislocation movement will increase MYS. By reducing the grain size of a material, dislocations are more readily pinned, as they are when particle or fiber reinforcement is added to a single phase material. Multiphase materials almost always have higher MYS than similar single-phase alloys. Thermal treatments that precipitate a second phase or produce a metastable phase tend to increase strength, and alloying a pure material usually produces dislocations and lattice strains that likewise increase MYS.

3.3.7.1.1  Properties

The physical, mechanical, and thermal properties of selected refractive materials, which are most commonly used for optical and mechanical components, are covered in the subsequent sections. To keep the material property tables concise, only the nominal values at room temperature are listed and, therefore, must only be used for preliminary evaluation and comparison purposes. Since the mechanical and thermal properties of materials can vary from one manufacturer to another and even from lot to lot for the same material from the same manufacturer, it is advisable to contact the manufacturer for obtaining more exact values of these properties for critical applications.

The optical properties of materials such as refractive index, Abbe value, reflectivity and transmittance, and variations of these properties as function of wavelength and temperature have deliberately been left out of these tables to avoid duplication of property tables from other sources. Some excellent and comprehensive references for optical properties of materials are Handbook of Optics,3 The Infrared Handbook,4 and Yoder.34

3.3.7.1.2  Typical Requirements

The selection of refractive materials is most often driven by the material’s optical properties applicable to the optical system. Optical throughput is dependent in part on the transmission of its refractive materials in the wavelengths of interest. Materials with high transmission over broad regions of the electromagnetic spectrum such as sapphire, fluorides, and zinc selenide have wide applicability in optical systems. Materials are also carefully selected for their optical index and dispersion properties so that they support the desired optical imaging or light-gathering purposes when combined with the other refractive materials in the optical system. In general, high indexes of refraction materials are desirable as they accomplish increased light bending with reduced element curvature. Refractive materials are often selected in combination with other specific optical materials for their combined achromatic potential. For instance, in the visible wavelength, flints are combined with crowns. In the infrared wavelength, germanium is often used in conjunction with a chalcogenide material.35

Although the mechanical properties of the glass being used in a system may be of secondary importance, they do play a critical role in ensuring dependable performance during operation. The mechanical and thermal properties of the selected refractive materials such as density, elastic modulus, MYS, coefficient of thermal expansion, and thermal conductivity are of special significance if the designed optical system must be lightweight, rugged, and capable of retaining its performance over a large temperature range. Therefore, rather than selecting a particular glass merely on the basis of its optical properties, due consideration must also be given to its mechanical and thermal properties before finalizing the choice.

In addition to mechanical properties, a refractive element’s fabrication and robustness to its intended application environment must be considered when selecting a material. Lens and window refractive elements are most often cut from larger billets or boules and generated or ground to shape. Then, the optical surface(s) are generated through various techniques that often include polishing for visible glass materials and/or single-point diamond turning for IR crystals and plastics. For high-quantity manufacturing where the material is capable, molding may be used to provide a finished product at reduced reoccurring costs. Available fabrication techniques are greatly dependent on the material. For instance, silica-based glass materials cannot be single-point diamond turned. Chapter 14 provides a more detailed and comprehensive discussion of fabrication approaches applicable to specific materials.

The intended environment will also play a major role in the selection of the refractive material. Optical windows and domes may see very severe environments, including thermal extremes, rain, sand, and salt fog. For example, sapphire is often used because of its high surface hardness, good thermal conductivity, and resistance to acids and alkalis.36 Space environments can expose optical materials to high radiation, which can cause darkening or have other negative effects on the materials performance. Radiation-hard materials are often selected to eliminate or minimize these effects. Salts such as potassium bromide (KBr) are very useful with their very broad and long wavelength transmissivity. However, these salt materials are hygroscopic, which makes them vulnerable to environments that may expose them to water or excessive humidity.

3.3.7.1.3  Glasses

Glasses are the most commonly used class of refracting material in optical systems. Glass is an amorphous material primarily composed of silica with additional materials added to alter the optical properties, chemical reactivity, and manufacturability. Boric oxide, alumina, alkaline earths, and alkali oxides are commonly added to produce high optical transparency and chemical resistance with the desired optical index and optical dispersion. Glass with a relatively low index and low dispersion are referred to as crown glass. Higher density material such as lead, zirconium, titanium, or other metal oxides are added to produce higher-index glass with higher dispersion, which is categorized as flint glass. Additionally, glass with longer infrared wavelength application is achieved by replacing the oxygen element with other elements of the chalcogenide group such as sulfur, selenium, or tellurium.37 For a more detailed discussion and explanation of the various terms used in describing the properties of optical glasses, see the study by Marker38 and the catalog Schott Optical Glass.39

Most glass used in optical systems is rapidly quenched from liquid in order to accomplish solidification in an amorphous condition. This provides the most clarity by preventing additional transmission loss due to scattering along with the absorption. This also assures from a mechanical point of view that the material is completely elastic. The notion that glass is elastic is not always clear to an inexperienced designer. The elastic modulus is actually quite low and comparable to aluminum at about 70 GPa (10 Mpsi) for most glass. The obvious concern is although the glass is able to recover from any induced strain short of breaking, this amount of strain is very small. Thus, when the glass is anything short of perfectly smooth on the edges and even on the surfaces, the stress will be higher at defect sites resulting in premature fractures since the material does not yield plastically. The area under the stress–strain curve is very low. This value is the strain energy and is very important in the design of lightweight optical components.

The normal design stress limit for most silicate glass including quartz is only 7–14 Kpa (1 or 2 Kpsi). This is in spite of the fact that very smooth small fibers in bending modes may exhibit a tensile (and yield) strength of several hundred thousands of pounds per square inch. This in turn creates a dichotomy with regard to lightweight glass optics. It is possible to improve the resistance to breakage by using resilient mounts at the edges of the glass and by smoothing the edges of the lens or other component by firing the edges either with a flame or with a laser of proper wavelength for the energy to be absorbed and cause rapid localized heating of the edge.

In addition to the chemical composition of a glass, its properties are dependent on the fabrication process, including the thermal time history. For example, there is a difference between fused silica and fused quartz, or quartz glass.40 Fused silica is manufactured by the pyrolytic decomposition of reactive gases and usually has high water content and no metallic impurities. Fused quartz is made by fusing crystalline quartz to form a glass. Fused quartz has some level of metallic impurities that can cause UV fluorescence, and the water content depends on the firing method. Fused quartz can have some granularity, a residual of the original quartz crystal structure. Therefore, these nominally identical materials will have slightly different index and dispersion from different manufacturers.

Table 3.2 lists physical, mechanical, and thermal properties of selected optical and specialty glasses which are most commonly used in a majority of the optical systems.1 ^, 41

3.3.7.1.4  Crystals and Semiconductors

Optical crystals and ceramics are widely used in broadband optical systems and include both synthetic and naturally occurring materials. Optical crystal materials are available as single and polycrystalline forms. The polycrystalline form of a material consists of small, randomly oriented individual crystals and are manufactured by various methods such as by hot pressing of powders, sintering, and chemical vapor deposition. On the other hand, single crystals are typically grown from dissolved and molten materials. Polycrystalline materials, in general, have higher strength and hardness as compared to single crystal materials.16 Polycrystalline materials generally have isotropic properties while single crystals have directionally dependent anisotropic properties.

Optical crystals are widely used in IR applications, but there are a number of crystals which have good transmission over a wide band from ultraviolet to far-IR wavelengths; for example, CaF₂ and LiF are extensively used for achromatized lenses in far-UV to mid-IR (0.11–10 μm) applications. CaF₂ has the best strength and moisture resistance of all fluoride crystals and has a very low thermooptic coefficient.

Another very useful material for high-temperature application is sapphire, which is the single crystal form of aluminum oxide. It has high strength and hardness and excellent thermal shock resistance. It is widely used in specialized optical systems subjected to severe environments. Another useful naturally occurring optical crystal is quartz, which is commonly used for UV prisms and windows and in IR applications with up to 4 μm wavelength. Quartz and sapphire are also grown artificially to improve transmission by controlling the amount of impurities. The thermal properties such as the CTE of these materials are direction dependent and quite sensitive to thermal shock.42

Silicon and germanium (Ge) are extensively used in IR systems for lenses, windows, and domes. Silicon is very suitable for missile domes because of its good mechanical and thermal properties. Germanium is quite hard but is susceptible to brittle fracture. Both materials have high index of refraction and are therefore very suitable for making multiple lens assemblies to keep the thicknesses and weight within reasonable limits. Due to the high index of these materials, efficient antireflection coatings are required to minimize internal reflection losses. The optical properties of Ge, such as index of refraction and absorption, are quite sensitive to temperature. Table 3.3 lists optical, physical, mechanical, and thermal properties of selected crystalline materials, while Tables 3.4 and 3.5 list the same properties for IR-transmitting materials. A more extensive list of materials and properties can be found in the study by Tropf.43

3.3.7.1.5  Plastics

This section summarizes the application of plastics in optical application. More details on this topic are presented in Chapter 5. Optical plastics are used in a small fraction of optical systems compared to optical glasses and crystal materials. The use of plastic lenses is finding favor in systems of lower precision requirements. The plastic lens is not brittle like the glass, and some plastics have higher index of refraction and low density permitting thin and very lightweight designs. Plastic is not stable over a wide temperature range and typically exhibits high chromatic dispersion or wavelength-dependent changes. The index of refraction typically varies unacceptably with both temperature and wide band-pass for high performance systems. For selective wavelength or relatively narrow band-pass, however, one of the better substitutes for brittle glass optics is the use of polycarbonate plastics such as is used in millions of eyeglass lenses. These materials can be made into lightweight and shock-resistant optics. Low-end camera, microscope, and binocular lenses are also large markets for plastic optics.

The manufacturing methods for various plastic lenses vary, but usually consist of casting or injection molding principles starting with the liquid plastic monomer or dimer and heating to produce the polymeric solid plastic lens. This can even be performed in an optician’s office for a customer on the same day. The molds are typically made by electroforming nickel over a very carefully prepared glass master. The master can be used many times to produce the same lens mold for many vendors and the molds are used by independent lens fabricators many times over. Certain other plastics may be melted and directly injection molded.

Advanced designs in plastic optics are favoring the use of solid optics. Such designs use plastic or even plastic and glass lenses in contact with each other to eliminate the air space and provide very light compact optical systems with excellent durability. The plastic or glass lens, which is exposed to the environment, must usually be coated to prevent scratching and for antireflection purposes.

The number of optical plastics available is quite limited compared to the number of optical glasses. The optical plastics can be classified into two broad categories: thermoplastics and thermosets. The term thermoplastic means a “material which flows when heated,” but there are some thermoplastics that do not flow when heated. Thermoset plastics can be set by heating these materials. Thermoplastics, or linear plastics as they are sometimes called, do not undergo any chemical change during the molding process and, therefore, can be remolded several times without affecting their properties. On the other hand, thermosets, also known as cross-linked plastics, start with a linear polymer chain, which gets permanently cross-linked in the presence of heat during molding.44

The most widely used optical plastic is acrylic, specifically known as polymethyl methacrylate. It is a low-cost plastic that can be easily molded, machined, and polished and has the best combination of optical properties. It has a low thermal conductivity and a high linear coefficient of thermal expansion (70 ppm/K), which is 8–10 times greater than that of typical optical glass. It has a shrinkage of 0.2–0.6% and a good optical memory, which is the ability to return to its original shape after exposure to heat. Acrylic has very good transmission (92%) and low internal scattering, and its refractive index varies from 1.483 for λ = 1 μm to 1.510 for λ = 380 nm. The index of refraction varies from about 1.492 to 1.480 over a temperature range of 20–90°C.44

Polystyrene is a second common optical plastic that can be combined with acrylic to obtain highly corrected achromatic lens designs. Its index of refraction is 1.590, and transmission is about 90%. It can be easily injection molded, and it is the lowest-cost optical plastic. It has a lower moisture absorption than acrylic, but it is more difficult to machine and polish as compared to acrylic.

Polycarbonate is another widely used optical plastic for ophthalmic lenses, street lights, and automotive tail light lenses due to its high durability and impact resistance. It is more expensive than acrylic and styrene because it is more difficult to mold, machine, and polish, and it scratches easily. Its refractive index ranges from 1.560 to 1.654, and the transmission is about 85%. It retains its performance over a broad range of temperature (−137 to 121°C).

The only thermoset optical plastic used is allyl diglycol carbonate, commonly known as CR 39. It is extensively used in making cast ophthalmic lenses, which are subsequently machined and polished, which makes it more expensive. It cannot be injection molded. It has excellent optical and mechanical properties such as clarity, impact, and abrasion resistance. It can withstand continuous temperature of up to 100°C and up to 150°C for short periods, but it also has a high shrinkage rate of up to 14%.14

Other less commonly used optical copolymers of styrene and acrylic include methyl methacrylate styrene, styrene acrylonitrile, methyl pentene, and clear acrylonitrile butadiene styrene. Table 3.6 lists physical, mechanical, and thermal properties of some of the optical plastics discussed here. A more detailed discussion of optical properties and design and fabrication methods for plastic lenses can be found in the studies by Lytle43 and Welham.34

3.3.7.2.1  Properties

Selection of adhesive materials involves engineering trades. The adhesive must be adequately strong and stiff to effectively act as the joining method. However, these same properties result in the stressing of the optical element, which can produce optical surface distortion and optical index variation in transmissive optical media. Initially, stress results from the adhesives cure shrinkage with compounding effects created from elevated temperature curing. Adhesives can have very high coefficients of thermal expansion, especially above its respective glass transition temperature. Glass transition is the transition of the material from an amorphous rigid state to a more flexible state, which produces substantial property changes in strength, stiffness, and rate of thermal expansion. This nonlinear temperature-dependent change in properties must be accounted for in the design, analysis, and use of adhesives.

To reduce stressing the optical element, it is best practice to accurately control the volume and extent of the adhesives material. For instance, long effective lengths are inadvertently created with filets and full injection holes, increasing the stressing effects of the adhesive’s large CTE. Furthermore, the effectiveness of the adhesive to meet requirements is dependent on the process controlling the adhesive’s components, mixing, cure schedule, and application. Therefore, a number of the adhesives’ characteristics and properties must be carefully considered, including cure shrinkage, CTE, glass transition temperature, outgassing, shear and peel strengths, stiffness, and Poisson’s ratio.

Compatible application processes and appropriate cure schedules and temperature must also be considered. Adhesives and cements are nonlinear elastomer materials with properties that vary with temperature, stress, and cure temperature. Tables in the following sections list physical, mechanical, and thermal properties of a number of commonly used structural adhesives and optical cements at room temperature. These properties’ values are nominal values and, therefore, must be used for comparison and preliminary design purposes. For critical applications, it is advisable to obtain the actual data and specification sheets from the manufacturers and complete internal testing to better understand the expected properties for specific application processes, geometries, and environments.

3.3.7.2.2  Structural Adhesives

Structural adhesives and elastomers can be used to bond structural components to each other or to bond optical components such as mirrors and lenses to their cells or mounts. The three main classes of structural adhesives are epoxies, urethanes, and cyanoacrylate adhesives. The thermosetting epoxy adhesives have high bonding strengths and good thermal properties. The epoxies are available in one- or two-part types, and some are room-temperature curable. The urethanes or polyurethanes have fairly high strength and can be used to bond together a variety of materials. They are flexible and, therefore, susceptible to creep and not suitable for high temperature (>100°C) applications, but are well suited for cryogenic applications. The one-part cyanoacrylate adhesives have low viscosity and are suitable for bonding smooth surfaces with very thin bond joints. They have cure times of less than 30 s, so proper fixturing is a requirement, and care must be taken to protect the skin from accidental bonding. These materials outgas more than other adhesives and are suitable for applications where the humidity is low and the temperature stays below 70°C. The key physical, mechanical, and thermal properties of some commonly used structural adhesives in optical applications are listed in Table 3.7.34

The two-part room temperature vulcanizing rubbers (RTVs) available from GE and Dow Corning are extensively used to bond mirrors, lenses, filters, and optical windows to their mounts. These silicone rubber elastomers are chemically inert and can tolerate a temperature variation of −80 to 200°C or more. The two main reasons for their popularity are the low cost and ability to accommodate differential thermal expansion between high-expansion metal mounts and low-expansion optical elements. Since a fairly thick bond layer of RTV is needed, the edges or diameters of the optical elements and their mounts do not need to be machined to close tolerances, thereby reducing their fabrication cost. Moreover, retainers, clips, and screws for securing the optics in their mounts are eliminated, resulting in a much simpler design. RTV is resilient and allows for the differential expansion between the optic and its metal mount when the ambient temperature changes, without introducing any adverse stresses in the optic. The key physical, mechanical, and thermal properties of some commonly used silicone rubber-type elastomers in optical applications are listed in Table 3.8.34

3.3.7.2.3  Optical Cements

Optical cements are the adhesives used for bonding the refracting optical elements to each other. Therefore, these adhesives must have good transmission and homogeneity over the desired spectral wavelengths in addition to the desirable mechanical properties such as low shrinkage and outgassing; good strength and stability; and resistance to adverse environmental effects such as humidity, temperature variations, and UV exposure. The optical cements come in four basic types: solvent loss, thermoplastic, thermosetting, and photosetting cements.

The solvent-loss cements, such as Canada balsam, have a high viscosity and are heat cured by the elimination of solvent to a refractive index ~1.53. They have poor bond strength, can introduce distortion in the bonded optical surfaces due to high shrinkage on curing, and are therefore seldom used in precision optical systems. The thermoplastic cements, such as cellulose caprate with n = ~1.48, are colorless or lightly colored solids that liquefy when heated to about 120°C. Their principle advantage is that the bonded elements can be separated by applying heat, which is easy and risk free. The thermosetting cements are two-part adhesives, which can be cured at room temperature by the addition of an appropriate catalyst. The room temperature curing time for this type of adhesive varies from 3 to 7 days. The cure time can be reduced to a few hours with a low elevated temperature cure, typically 70°C. Summer’s C-59, M-62, F-65, RD3-74, Lens Bond’s, and MasterBond’s EP30LV-1 are some of the commercially available thermosetting cements with n of 1.55–1.57.34

Photosetting optical cements are generally one-part clear adhesives that are cured by exposure to UV light of 250–380 nm wavelength. These cements are suitable for bonding small low-mass optics that have transmission in this spectral region. Bondline thickness must be kept small to prevent excessive stress due to shrinkage. Norland’s NOA-61 (n = 1.56), Dymax’s OP-4-20632 (n = 1.55), and Summer’s UV-69 (n = 1.55) and J-91 are some of the UV-curing optical cements available. A two-step curing process, a short exposure for 20 s, followed by a long exposure of up to 60 minutes, is used for some of these cements. The bonded parts can be gently handled and cleaned, or debonded if needed, after the short exposure. Once the adhesive is fully cured after the long exposure, it becomes quite difficult to separate the parts. The entire area of the bond joint must be completely exposed to a uniform intensity UV illumination to obtain complete curing of the joint to prevent surface distortions. If feasible, the strength of the bond joint can be improved by heating the bonded parts to 40°C. Typical properties of optical cements are shown in Table 3.9. For specific properties, manufacturer’s data sheets should be obtained.