JPS62299901A - Optical element, shock resistance of which is adjusted - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION 3. DETAILED DESCRIPTION OF THE INVENTION This invention relates generally to optical elements, and more particularly to impact protection and reinforcement of optical elements.

As is known in the art, round imaging systems generally include externally mounted optical elements that isolate the remainder of the imaging system from the external environment. For example, infrared? flJCIR) Aerial Imaging System (1
nfrared airborne
system), the aviation system is equipped with a non-infrared transparent optical element, such as a window or dome, that blocks exposure of the rest of the infrared imaging system to wet, corrosive, and abrasive environments. installed. Prolonged exposure to such environments generally impairs the optical and physical properties of the material of the optical element. Generally, the most severe environmental exposure these external components encounter is likely to be the high velocity water droplet impact that occurs when aviation systems fly through rainy areas.

This water droplet impact problem is more commonly referred to in the art as rain erosion.

When flying through a rainy area, water droplets impinge on the surface of the external element, causing superficial destruction even at subsonic speeds.

For extremely brittle materials, these superficial fractures are due to pre-existing micro-defects near the surface of the optical element.
aw). Rain erosion damage to these optics occurs before material is significantly removed. The mere growth of these existing microscopic defects is sufficient to damage the optical element. In particular, these microscopic defects grow due to the tensile component of the surface stress waves generated when impacted by water droplets, and penetrate the optical element. Once formed, continuous growth of superficial fractures through the optical element will often result in large cracks in the optical element. Scattering and refraction of incident infrared energy occurs in the area of the crack, increasing internal reflection and infrared energy loss. When a large number of such cracks are included, the transmittance of the optical element is significantly reduced. Additionally, as the crack grows through the optical element, catastrophic failure of the element may occur. When an optical element shatters or breaks, the remaining optical elements of the infrared imaging system are exposed to the external environment, creating potential catastrophic damage to the imaging system.

There are generally a relatively small number of materials that provide the best mechanical durability and performance for infrared imaging systems (particularly in the 8-12 μm infrared band).Suitable materials include zinc sulfide, These include zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, and cadmium telluride. Ternary sulfide materials, such as imaged lanthanum calcium, are also currently available for infrared applications (8-12 μm in % developed/]
RI is there. Although these ternary sulfide materials offer some improvement in durability, even these materials are susceptible to the environmental exposures mentioned above. In general, all of the above materials are relatively brittle and have relatively low resistance to damage, particularly damage sustained during high velocity water drop impact.

As is also known in the art, if the refractive index of the material forming the optical element differs significantly from the refractive index of the energy generating medium, optical energy incident on the surface of the optical element will be reflected off this surface. will be done. Typically for aviation systems, the energy generating medium is air, which has a refractive index of about 1. Therefore, in the optical industry, in order to reduce this type of reflection loss, it is common practice to coat the entrance surface of an optical element with a material having an appropriate refractive index. At the deposited thickness, which generally relates to the original wavelength fraction, these coatings are transparent in the infrared range. However, these round-shaped coatings have hitherto had the role of reducing reflection loss caused by mismatching of refractive indexes, and have not been useful for increasing the ambivalence of optical elements.

It is known in the art that a hard carbon layer, i.e., a carbon layer with quasi-diamond bonds and substantial optical clarity, when applied over germanium, provides some protection to germanium optical elements from impact damage caused by rain erosion. It has been known.

Hard carbon on germanium is discussed in a miscellaneous paper entitled ``Liquid impact erosion mechanism in transparent materials'' (G.E.
Fields et al.), final paper September 30, 1982 - March 31, 1983, contrast genus AFO8R-
78-3705-D.

It is described in paper 16AFWAL-TR-83-4101. Hard carbon surfaces do not always adhere effectively to other infrared materials, such as zinc sulfide and zinc selenide. Moreover, hard carbon coatings, even on germanium, tend to peel off upon impact with high-speed water droplets, as described in the above-mentioned miscellaneous article. It is theorized that the shear forces created by the radial outflow of the water droplet impact cause the coating to detach from the germanium layer. This peeling phenomenon is thought to increase significantly as the thickness of the hard carbon layer increases (
・Ru. A thicker hard carbon coating layer should therefore provide more protection to the optical element against impact, but this has not been successful due to the peeling problems mentioned above. Another parallel with hard carbon is that its refractive index is about 2.45, which is substantially higher than the refractive index of many such optical materials, such as zinc sulfide and zinc selenide. Therefore, when an optical element is coated with a hard carbon coating, the reflection loss at the entrance face of the optical element will be higher than when the optical element is uncoated.

A third problem in the industry concerns the fracture strength of these materials. In this case as well, infrared transmittance (especially 8~
Most materials suitable for windows in the 12 pm band have low breaking strength. This property is particularly important in applications of these devices where the device separates a high pressure region from a low pressure region, ie where the device is under some static or dynamic mechanical loading. Miscellaneous paper entitled "Impact Loss Threshold in Brittle Materials Shocked by Water Droplets", by N.G. Evans et al., Journal of Applied Physics 5) (5), pp. 2473-2482 (May 1980), p. 2481. It has been proposed that martensitic reinforcement (phase change) at the surface of brittle materials may be useful in tempering this type of brittle material. It has also been suggested that surface compressive stress may be beneficial. However, the authors of the above miscellaneous papers do not specifically explain what they mean by "surface compression." These materials undergo surface compression when an incident water droplet impacts the surface of the material.

According to the invention, the optical element resistant to high velocity water drop impact has a first predetermined modulus of elasticity (modsls
s of elasticity) Y: a base layer of a first material with a base layer of a first material and a coating of a second material with a higher second modulus of elasticity. This coating layer bonds to the material of the optical element and is highly resistant to delamination caused by shear stresses encountered during high velocity water drop impact. Preferably, the high modulus coating is made of a material having a refractive index lower than that of the material constituting the optical element. Still preferably, the material is substantially transparent to infrared radiation and substantially water-impossible. According to this configuration, the higher modulus second material insulates the base layer of the lower modulus material against impact damage, particularly impact damage caused by high velocity droplet impact.

In addition, sheathing materials that are highly resistant to delamination caused by shear forces remain on the optical element upon high velocity water droplet impact, thereby protecting the optical element from environmental exposures such as rain erosion.

According to another aspect of the invention, the coating comprises a composite coating of a mixture (preferably a homogeneous mixture) of a first material and a second material. Each of these materials has a modulus of elasticity at least twice that of the material forming the base layer. The second material has a substantially higher modulus than the first material, the first material is insoluble and inert to water, and the second material is reactive with water. According to this configuration, the composite coating provides a layer on the optical element with a higher modulus of elasticity than would be obtained with just one layer of the first material. However, the composite material may also have relatively low bathability and reactivity with water, especially if the first material is applied to separate the mixture from the water source.

According to another aspect of the invention, the base layer material is silicon, germanium, gallium arsenide, gallium phosphide, cadmium telluride, cadmium mercury telluride, zinc sulfide and zinc selenide, more preferably cadmium telluride. , zinc sulfide, zinc selenide or ternary sulfides such as lanthanum calcium sulfide. The high modulus infrared transparent material that makes up the first coating layer is oxidized)
IJum, scandium oxide, and homogeneous compositions of yttrium oxide and magnesium oxide, compositions of scandium oxide and magnesium oxide, and mixtures of scandium oxide and yttrium oxide. According to this embodiment, an impact-resistant anti-reflection coating for optical elements operating in the wavelength band of 8 to 12 microns is applied,
This makes this type of element more resistant to damage caused by rain erosion or high velocity water drop impact.

According to yet another aspect of the invention, the optically transparent element comprising a first infrared transparent material having a first modulus of elasticity has a second modulus of elasticity that is substantially higher than the modulus of the material of the optical element. ,
and a first layer having a refractive index less than that of the material of the optical element, which provides protection from high-velocity water droplet impact. The material of the first coating layer is substantially resistant to delamination from the material of the optical element in response to shear stress caused by radial outflow of the droplet upon high velocity droplet impact. The second layer of the composite coating is comprised of a third material having a relatively high third modulus. This elastic modulus is higher than that of the first material constituting the optical element layer, and preferably higher than that of the second material of the first coating layer (.0) of the third material constituting the second coating layer. The material is substantially transparent to infrared radiation and has a refractive index higher than the refractive index of the second material of the first coating layer.The third material constituting the second coating layer also has a refractive index that is higher than the refractive index of the second material of the first coating layer. may be substantially resistant to delamination from the second material of the optical element, but the delamination resistance of the optical element to the first material may be relatively poor. According to this configuration, the radial outflow induced delamination a first layer of material having substantial resistance to delamination of the material from the optical element and further having substantial resistance to delamination of a third material of the second coating layer induced by radial outflow; The interposition of the protective coating results in a composite coating that is substantially resistant to radial outflow-induced delamination and also has a modulus of elasticity greater than that of the first material. The effective physical thickness of the coating can be increased, and the optical properties Z of the combination of composite membrane rupture layer and optical element can be increased.
Still maintaining or perhaps improving the vine.

According to another aspect of the invention, the first material of the optical element is silicon, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride, zinc sulfide and zinc selenide, or ternary sulfides. selected from the group consisting of. Preferably, the material is selected from the group consisting of cadmium telluride, zinc sulfide and zinc selenide. The second material of the first coating is yttrium oxide, scandium oxide, yttrium oxide, or tapped sulfur/sulfur oxide.
selected from the group consisting of a mixture of dium and magnesium oxide. The third material of the second coating is selected from the group consisting of cerium oxide, titanium oxide, zirconium oxide or hard carbon. According to this type of configuration, generally a hard Adhesion problems associated with carbon on most 8-12 μm optics are eliminated. Additionally, since materials such as hard carbon do not have adequate refractive indices for antireflective materials, such as zinc sulfide, zinc selenide, or cadmium telluride, the hard carbon layer is combined with a low refractive index first coating layer. When used in combination, a composite material with an effectively reduced refractive index can be obtained.

According to yet another aspect of the present invention, a plurality of low refractive index, high modulus materials are alternately stacked together to form a multilayer antireflective impact coating. The optical element is protected by a composite layer consisting of: According to this type of configuration, the coating has the advantage of a large total physical thickness of the composite multilayer coating and increased impact resistance, as well as to provide broadband antireflection or other optical functions, such as filtering effects. can be designed. Preferably, the material with a low refractive index and high elastic modulus is selected from the group consisting of yttrium oxide, scandium oxide, or a mixture of yttrium oxide or scandium oxide and magnesium oxide, and the material with a high refractive index and high elastic modulus is cerium oxide. , titanium oxide, zirconium oxide or hard carbon. The material constituting the optical element is selected from the group consisting of silicon, germanium, gallium arsenide, gallium phosphide, cadmium telluride, cadmium mercury telluride, zinc sulfide and zinc selenide or ternary sulfides.

According to yet another aspect of the invention, an impact resistant tuning optical element includes a base layer of an optical material having an early failure strength. A coating of a compressed layer of material is disposed over the base layer of optical material. This compressed material layer has a substantially smaller overall thickness compared to the thickness of the base layer of optical material. According to this type of configuration, the compressed material layer tends to dampen the impact of the tensile stress wave components encountered during droplet impact on the surface micro-defects and prevent them from growing over the entire surface of the optical element. will show. This compressed region will respond to the tensile stress wave component and tend to close those micro-defects and prevent them from growing, thereby reducing and compensating for the tensile stress wave component. By reducing this tensile stress component, damage resulting from water droplet impact on the surface of the optical element is mitigated, thus resulting in a relatively brittle material.

A conditioning surface is provided that is resistant to damage caused by rainfall. This adjustment surface simultaneously increases the fracture strength of the optical element.

According to another aspect of the invention, an optical element tailored to resist damage encountered during high-velocity impact includes a base layer of an optical material, on the surface of which a compressed layer of the material is disposed. A layer of compressed material has a number of grooves disposed in the layer, these grooves are separated by adjacent regions of the compressed layer, and below the grooves a portion of the compressed layer material is disposed. ing.

The thickness of the compressed region of the compact optical material is 3 microns or less. The groove is generally 10-10.00 OA deep, Q width, 01-002 OA. This configuration provides a tuning optical element that is highly resistant to damage encountered during high speed propulsion shocks.

In accordance with yet another aspect of the invention, a method for adjusting an optical element comprises machining a plurality of grooves into the optical element, the grooves generally having a depth of 10 to 10,000 mm and adjacent grooves. Between and under these grooves are compressed regions of the optical material.

According to yet another aspect of the invention, a method for surface conditioning an optical material includes single-point machining an optical element to apply a compressed layer with a thickness of 05 to 3.0 m on the surface of the optical element. . The compressed layer includes a number of grooves with a depth of 10 to 1o, o o o i, with adjacent grooves separated by a portion of the compressed layer of material of the optical element. The tE-1 and I/-1 grooves are introduced into the optical material by rotating the optical element at a predetermined speed with a single point diamond tool in contact with the surface of the rotating optical element. This tool is the seat on which the compressed layer is applied to the surface of the optical element at a predetermined speed. According to this method, a compressed layer of optical element material is applied by single point machining of the surface of the optical material. This compressive layer will stiffen the optical element and help prevent damage to the optical element caused by high-velocity droplet impact by relieving or reducing near-surface tensile stresses that occur during high-velocity droplet impact.

According to yet another aspect of the invention, an impact-resistant adjustable optical element includes an optical element material having an initial fracture strength and a predetermined first modulus of elasticity. A compression layer is arranged. The total thickness of the compressed material layer is substantially small compared to the base layer of optical material, typically between 1 and
It is 3 microns. Disposed on this compression layer is a coating layer of a second material having a second modulus higher than that of the material forming the base layer of the optical element. This coating layer is bonded to the compressed layer of optical material and is highly resistant to delamination caused by shear stresses encountered during high velocity water drop impact. According to this embodiment, the combination of the compression layer and the coating layer provides an optical element with high impact resistance and strength properties. The outer coating layer provides a coating of material with a higher second modulus of elasticity, thereby protecting the underlying base layer of lower modulus material from impact damage, such as from high velocity droplet impact. . Additionally, compressed material debris will tend to buffer the effects of this tensile stress wave component encountered upon droplet impact. Therefore, the combination of these two techniques will provide an optical element with substantially improved impact resistance and fracture strength.

The above features of the present invention, as well as the present invention itself, will be more fully understood from the following detailed description in conjunction with the drawings.

FIG. 1 is an isometric projection of an optical element (here a plate) consisting of a base layer and a protective layer according to the invention.

FIG. 2 is an exploded cross-sectional view taken along line 2--2 of FIG. 1, illustrating a protective layer comprised of a single layer liquid film in accordance with one aspect of the present invention.

FIG. 3 is an exploded cross-sectional view taken along line 3--3 of FIG. 1 illustrating a pair of coating layers in accordance with another aspect of the present invention.

FIG. 4 is an exploded elevational view taken along line 4--4 of FIG. shows.

Figures 5A-5D are a series of graphs showing radial stress drop as a function of normalized distance from the center of drop impact for coatings with different moduli (prior art).

Figure 6 shows a rate of change of 25.4 fins/hour (1 inch/hour);
Speed 724, 450 mph, impact angle 90''
and uncoated Z exposed to an accelerated rain field with raindrop size 2.
This is a micrograph of the nS surface.

FIG. 7 is a photomicrograph of an anti-reflection (AR) coated surface according to the present invention exposed to the same accelerated rain field as shown in FIG.

Figure 8 shows the thickness of 5. (181111 (0,20 inch)
Figure 2 is a plot of percentage transmission versus wavelength for a coated ZnS plate.

FIG. 9 is a cross-sectional view of a portion of the dome.

FIG. 10 is an enlarged sectional view of the surface portion of the dome shown in FIG. 11.

FIG. 11 is a schematic cross-sectional view of a raindrop falling on the surface of a general optical element having microstructural defects.

FIG. 12 is a schematic cross-sectional view of a water droplet falling on a compressed layer according to another aspect of the invention.

FIG. 13 is an enlarged view of a water droplet impacting the compressed layer shown in FIG. 12.

Figures 14A and 14B are micrographs of a dome surface polished by a conventional method and a dome surface prepared by the method of the present invention (single point diamond processing), respectively.

FIG. 15 shows a plot of Knoop hardness number as a function of load (lhss microhardness) for a conventionally polished ZnS surface and a conditioned ZnS disk surface.

Figure 16 shows general hardness difference (Knoop) adjusted ZnE;
It is plotted as a function of penetration depth into the compressed surface of the disk.

Figures 17A and 117B show Zn polished by a conventional method.
S lens and Z n S lens adjusted according to the invention
These are micrographs of the surfaces of lenses (DPM, diamond point processing) after being exposed to an accelerated rain field.

Figures 18A and 18E are respectively a lapped Zr'l lens (irregular) polished by a conventional method and a lapped Zn lens adjusted according to the invention (diamond point processing) showing the distortion caused by the compressed layer.
A micrograph of an S lens (highly concave).

FIG. 19 is a cross-sectional view of a portion of an optical element, such as a plate or dome, with a compressed layer of optical material and a coating layer according to another aspect of the invention.

Referring to Figure 1, the optical element (here plate) 1
0 is shown as comprising a base layer 12 of a material with predetermined optical properties. Although the optical elements are specifically shown here as plates, it is understood that other types of optical elements, such as windows, domes, lenses, etc., having shapes other than planar may be employed in place of the plates described above. Typical base layer 12 is at least 1.3++
m (0,05 inch), generally 2.5 to 12.7 dragon (0
1 to about 0.5 inches) or more. The optical element may further include selective optical properties. For example, the optical element may be comprised of a material that is generally transparent to light energy in the infrared, visible, and/or ultraviolet spectrum. This material may be dielectric or semiconductor. For optical elements used in infrared imaging systems, particularly in the wavelength range of 8-12 μm, examples of preferred materials include silicon, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride,
Contains one of zinc sulfide, zinc selenide, or ternary sulfides. The selected materials making up #12 may be processed by any known method, such as powder compaction.
It can be processed by chemical vapor deposition or chemical vapor deposition. Particularly for infrared applications, the materials chosen for layer 12 are generally 3.
5 x 10' ~ 10.5 x 10 to 9/c+++
2 (5x 10' ~ 15X106 psi), high infrared energy transmission (typically 2.0
It is generally characterized by having a refractive index of 2.2 to 4.0 over at least a portion of the infrared wavelength band of ~3.0 microns (50 to 75 microns) and 10 microns. The relevant mechanical and optical properties for some of these materials are shown in Table 1.

R% is the reflection loss (per surface) caused by a quarter wave anti-reflective (AR) single layer coating of Y2O3 on the corresponding material. Disposed on the base layer 12 is an impact-resistant anti-reflection coating layer 11 . Suffice it to say here that layer 11 can be any of the structures discussed.

Referring now to FIG. 2, coating layer 11 is shown as including a first protective layer 14 disposed over (and preferably over) the material comprising base layer 12. As shown in FIG. Protective layer 14
has a substantially higher modulus of elasticity than the base layer 120, a high degree of infrared transparency over the selected wavelength band of the optical element at the deposited thickness, and preferably a refractive index lower than the refractive index of the material comprising the base layer 12. Made of material with a refractive index.

Furthermore, the deposited material has a high degree of adhesion to the material of layer 12 and is particularly highly resistant to delamination caused by shear stresses induced by the radial outflow of high velocity droplet impact (e.g. water droplet impact). . Layer 14 can be deposited by any method such as ion beam sputtering, diode sputtering, or evaporation. Additionally, layer 14 can be applied onto plate 12 by dipping layer 12 into a solution of an organic vehicle and a high modulus material. The plate is immersed in a solution of such material and then removed from the solution and placed in an oven where the organic vehicle is driven off. Alternatively, the coating can be applied by spray drying a mixture of vehicle and coating material onto the substrate 12 heated to a predetermined temperature. Each of these coating formats provides a relatively inexpensive method for forming a uniform coating layer 14 on base layer 12. Suitable coating materials for the base layer materials include yttrium oxide (Y2O2), magnesium oxide (MyO) and scandium oxide (Sc).
20a), as well as homogeneous mixtures of these materials. The relevant mechanical and optical properties for these materials are shown in Table 2.

The main factor to consider when choosing the material for coating layer 14 is that the selected material must have optical properties suitable for the intended use of the optical element at the thickness at which the coating layer 14 material is deposited. It must not happen. Additionally, the material of coating layer 14 should generally have a modulus of elasticity that is at least about twice that of the material of base layer 12. Furthermore, if the intended use of the optical element 1o is to expose the coating layer 14 to water,
The material of coating layer 14 must be stable and insoluble in water. To provide anti-reflection modification, the refractive index of the material of coating layer 14 is preferably less than the refractive index of the material of base layer 12. For antireflection modification between air and the substrate 12 material, which typically has a refractive index of about 1.00, the required refractive index (R,) of the coating is the refractive index of the substrate 12 material plus the surrounding medium. C%, which is approximately equal to the arithmetic mean of the refractive index)
o As is generally known for most materials, the refractive index varies as a function of wavelength dispersion. Therefore, this antireflection modification also varies as a function of wavelength.

Preferably, layer 14 is deposited on base layer 12 to a physical thickness corresponding to a quarter wavelength at the particular wavelength of interest for the optical element. Generally, the optical thickness to) of this type of element is defined as the product of the physical thickness 1) of the coating 14 and the refractive index (ttc) of the material of the coating 14 (t6
=t-nc). The physical thickness desired for optical thickness λ/4 is given by 1=(λ/4)/nc, where λ is the specific wavelength for which the optical element is intended and nc is the coating thickness at the desired wavelength. is the refractive index of As will be recognized by those skilled in the art, the optical thickness to> is a higher order thickness, e.g. 3λ/4 or 5λ/4, and thus the physical thickness t is t=[(2N+1) λ/a:)/n
It is given by c. In the formula, N is an integer 0.1.2.3
,...is. Thus, increasing the physical thickness tlr of layer 14 provides greater impact protection for base layer 12 while still maintaining good antireflection and light transmission. For example, the refractive index n at 10.6 microns,
For material y2o, with = 1.63, 10.3
The optimum thickness for a λ/4 monolayer in microns would be about 1.63 microns.

Referring now to FIG. 3, plate 10 is shown as including a base layer 12 and a coating layer 11 disposed on at least a first side thereof. In this case, the coating layer 11 is an antireflective, impact-resistant composite layer 15. Layer 15 is base layer 1
The coating layer 14 is shown to include the coating layer 14 of a high modulus material having a refractive index less than that of the material of the base layer 12 and having good adhesion to the material of the base layer 12. On this first coating layer 14 is a second coating layer comprising a second material having a substantially higher modulus of elasticity and a higher refractive index than those of the materials of both the substrate 12 and the first coating layer 14. 16 are arranged. Suitable materials for this second coating layer 16 include cerium oxide, titanium oxide, zirconium oxide and hard carbon. Of these examples, hard carbon is preferred because it has the highest modulus of elasticity. However, as mentioned above, hard carbon does not always adhere well to zinc selenide, zinc sulfide, etc., which are among the preferred materials for the base layer 12, especially at wavelengths of light of 8 to 12 microns.

The relevant mechanical and optical properties for these materials are shown in Table 3.

TABLE 3 Compatible Properties for Layer 16 The hard carbon film can be deposited by any suitable method. For example, ion beam A sputtering methods and chemical vapor deposition methods involving decomposition of hydrocarbon-containing vapors can be employed. Although hard carbon layers do not necessarily adhere well to most infrared materials as described above, it is believed that this type of hard carbon layer will adhere to materials suitable for use in the first coating layer 14 (ARN). As mentioned above, suitable materials for layer 14 include MgO, 5C203, and Y2O.Hard carbons generally adhere very well to various types of oxides, including these. By being interposed between the very high modulus layer 16 and the base layer 12, the impact resistance benefits of the very high modulus 16 material provide an anti-reflective, high impact resistant composite layer 15 and the base layer 16. 12.Protects from all high velocity water drop impacts.

Referring now to FIG. 4, the optical element is now shown in base layer 1.
2 and a coating layer 11. Here, coating layer 11 is a broadband antireflective impact coating layer 17 comprising a plurality of antireflective impact resistant composite coating layers 15 described above in connection with FIG. This configuration provides a very thick antireflective coating 11 with excellent adhesion to the base layer 12 and a high degree of impact resistance. Furthermore, by appropriately selecting the thicknesses of the plurality of composite layers 15 and the individual coating layers 14 and 16 according to optical design principles for multilayer coatings, a complete broadband antireflection coating can be obtained.

5A-5D, Field et al.
A series of graphs adapted from the paper entitled Mechanism of Drop Impact Erosion in Transparent Materials (AFWAL-TR-82-4022) depicting the radial stress drop as a function of normalized distance from the drop impact center. Each graph plots the typical radial stress that occurs on a protected surface that has a higher modulus of elasticity than that of the optical element material, compared to the radial stress that occurs on an uncoated surface. As shown in FIG. 5D, if the modulus of elasticity of the material of the coating is ten times that of the material of the base layer, the tensile stress induced in the base layer upon water drop impact is substantially equal to zero.

Next, looking at Figures 6, '- and 7, the rate of change is 25.1 mm per hour (1 inch/hour), and the velocity is 724 km per hour (1 inch/hour).
450 miles/hour), an incident angle of 90°, and an accelerated rain field with a droplet diameter of 2.
Micrographs of the surface (Fig. 9) and the torsion surface (Fig. 9) are shown.

As can be seen, the amount of damage exhibited by the uncoated zinc sulfide surface is substantially higher than that exhibited by the IJium oxide coated zinc sulfide surface.

With full reference to FIG. 8, a plot of percent transmission versus wavelength is shown for a RAYTRANl type zinc sulfide coated plate having a thickness of 5.1y++R (0.2 inches). The coating was a quarter wavelength thick yttrium oxide at 10.0 microns.

The coating was approximately 2.45 microns thick. The coating was chosen to give maximum surface transmission at 10 microns and was applied to both major surfaces of the plate.

9-12, a portion of the optical element (in this case dome 110) is shown as including a layer 112 of material having predetermined optical properties. Although the optical element is shown here as a dome, it is understood that other types of optical elements may be employed in place of the dome 110, such as windows, plates, lenses, etc.

Generally, base I 7112 will have a similar thickness to base layer 12. The optical element may further include selected optical properties. For example, the optical element may be comprised of a material that is generally transparent to light energy in the infrared, visible, or ultraviolet spectrum. The material of the optical element may be dielectric or semiconductor. Particularly for optical elements used in infrared imaging systems, examples of preferred materials include silicon;
Germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride, zinc sulfide, zinc selenide, or with the general formula MN2S, where M is a monovalent ion and N is selected from the lanthanide series. ion and 2S is a sulfide ion S-2). The selected material constituting layer 112 may be prepared by any known method, such as powder compaction.
essification) or by chemical vapor deposition. Generally, the material chosen for layer 112 is characterized by a relatively high overall transmittance for a particular energy, such as generally 50% or more for infrared energy over at least a portion of the infrared band;
Generally 3.5~105×10'' dirt/cTrL2(5~
Extremely brittle with a modulus of elasticity of 15x106p8i)
It is a fairly rigid material, generally 387-1055)
q/CIrL2 (5,500-15,000 psi
), it is generally a fairly soft material.

Dome 110 further includes a plastically deformed compression layer 114 (FIG. 10) applied over surface 112a of optical element 110.

Preferably, compressed layer 114 (thickness tc) is part of the material of layer 112. As shown in more detail in FIG.
oove) 113k, adjacent ones of these grooves 113 are separated by compressed regions 113α of the material of layer 112, and below these grooves a compressed layer 113b is arranged.

The degree of compression of layer 114 is a function of the amount of compressive force applied during processing of dome 110, as described below.

11 and 12, the mechanism by which the compression layer 114 strengthens the dome and protects it from damage encountered during high velocity water shock or mechanical loading will now be described.

As shown in FIG. 11, a water drop 1 impinging on the surface 112a of the layer 112, normal to the surface 112a, at a final velocity Vo, on the surface 112α polished by a conventional method.
There are 15. The surface 112α of the layer 112 has pre-existing micro-defects 116 that occur during processing of the polished surface or otherwise due to the morphology of the base layer 112. Water droplets 115 are formed on the surface 11 by a conventional method.
The tensile component of the surface stress wave generated when colliding with 2tL (
(represented by arrow 118) is given. In response to this tensile force 118 in the area of microdefect 116,
The microscopic defects grow as cracks 116'. If the tensile force is high enough, the crack 116' will cause the base layer 112 of the optical element 10 to
It may grow through the. If the number of these cracks is sufficient, the optical transparency of the element can be significantly reduced due to internal reflection and refraction in the area of the cracks. More importantly, optical elements containing sufficient cracks are more likely to break or break, resulting in damage to the optical system (
The remaining portion (not shown) is subject to catastrophic damage.

As shown in FIGS. 12 and 13, according to the present invention, a compression layer 114 is applied over the dome 110 so that existing micro-defects 116 in the base layer 112 are healed by compression of the material in the region 114. .

Opposing compressive forces 114 are applied during processing of the compressed layer 114, thereby reducing the size of the micro defects, resulting in smaller micro defects 117. Further, the material surrounding the fine defect 117 is moved by the arrow 12 pushing the material in the region '113b.
Still under compression, as indicated by 0. As described above, upon impact with water droplets, a tensile stress component 118 is applied to the fine defect site. Since micro-defects are smaller, the speed at which water droplets impact the surface layer without causing damage is faster.

This is because the velocity threshold increases as the size of micro defects decreases. Furthermore, even after the grooves 113 are formed, the material remains in a compressed state, and the reduced tensile stress component 118' generated at the microdefect 117 site is such that this tensile stress component 118' increases the compressive force representing the degree of compression of the layer 114. Unless it rotates, it will not grow through the compressed layer 114 into the base layer 112. Compressive layer 114 thus provides two mechanisms to increase the velocity threshold at which impact damage occurs. That is, this generally reduces the degree of microdefects present in the material, thereby reducing the resulting tensile component for a given water drop impact velocity; and slowing or reducing the development of the tensile force developed in the compressed layer 114. Provides a compressive force to

The preferred method for applying compressed layer 111 is single point diamond machining of the surface portion of the optical element. Generally, the surface can be processed in two steps. The first step, 6 Rough Cuts, has machining parameters chosen to remove a substantial amount of material 119 as shown in FIG.

This would be a material on the order of 25-127 microns (1-5 mils) or larger. The second cutting process, "finishing °" is 1
It may be a turning or several pass or cutting process, in which a small amount of material, typically about 2.5 to 12.7 microns (
01-05 mil) is removed, resulting in a substantially flat 11-1 but grooved surface.

General surface characteristics for the tuned compression layer 114 shown in FIG. 10 are as follows.

The groove 113 generally has a width Wf (generally 0.01 to 0.02y
sm) k will also be present. Sidewall portion 113a typically has a height A/ of 10-1o, o o o x.

The general processing parameters used to machine the compressed layer 114 into a 25.4 mm (1 inch) zinc sulfide disk are as follows.

Roughing: Depth of cut = 0.076 mm (0.003 inch) Rotation speed - 7507 pm Feed speed - 12° 7 mm 1 minute (0.5 inch/min) Tool radius = 3.175 mm (0.125 inch) Finishing Depth of cut = 0.005 mm (0.0002 inch) Rotational speed - 550 rp Regular feed rate = 6.35 mm/min (0.250 inch/min) Tool radius = 3.175 mm (0.125 mm) Sixty zinc sulfide disk samples were processed by this method. The mechanical parameters measured for these discs were Knoop microhardness (in/mm2) and fracture strength. The breaking strength is generally at least 1371#/Cln2 (19
, 5oop8i), which corresponds to the fracture strength of a sample polished by a conventional method of 1090 #/cm2 (15,500 ps
It is thicker than i).

This method therefore increases the breaking strength by about 25%.

As shown in Figures 14A and 114B, the conventionally polished surface is substantially smooth, uniform, and featureless. In contrast, surfaces prepared according to the present invention include substantially regularly spaced ridges or grooves.

15 and 16 show a typical microhardness plot of typical Knoop hardness number versus load for a conventionally ground zinc sulfide disk and a zinc sulfide disk containing a compressed layer 114. The Knoop hardness number for a disk containing compressed layer 114 is typically 50 to 100 values higher than the Knoop hardness for a conventionally ground disk for loads of 30 r or less. Further, as shown in FIG. 16, the hardness difference expressed as the depth of penetration into the compressed layer 114 by the difference in Knoop hardness numbers shows that the hardness increases significantly for penetration depths of 2 microns or less. By extrapolating this data, it is estimated that the hardness effect is limited to within 3 microns of the surface area of the machined sample.

Figures 177 and 17B, i.e. 25.4 mm 1 hour (724 km 1 hour) (450 miles/hour)
Valued zinc plates and diamond-pointed zinc sulfide plates were conventionally polished after 5 minutes of exposure to a rain field (2 droplet diameters) accelerated to 1 inch/hour). It shows that the sample suffered significantly more damage consisting mainly of surface fractures.

In contrast, a plate containing a compressed layer applied by single point diamond machining of a zinc sulfide sample (FIG. 17B) suffered substantially less damage.

A comparison of Figures 18A and 18B shows that layer 114 is a compressed layer of optical material. Two zinc sulfide lens blanks from the same lot of zinc sulfide samples were polished. One lens had its surface polished optically flat using conventional polishing techniques, and the other lens had its surface machined flat according to the present invention. After processing, each sample was placed face down on a lapping pad, and the entire lapped surface was coated to a thickness of approximately 5.1 to 0.2 inches.
．． Thinned by 1 to 25 mm (0.01 inch) and a quarter wave flatness in the visible spectrum. When the samples were removed from the lapping pad, each sample experienced some degree of distortion. This distortion is shown in the interference topographs of Figures 187 and 18B. As shown in No. 187, the distortion of the sample polished by the conventional method was small, indicating that an irregular final surface was obtained.

However, as shown in FIG. 18B, the compressed layer 1 according to the invention
The sample containing No. 14 could not be measured with an interferometer due to significant distortion. Furthermore, when the test piece shown in Figure 3 was removed from the wrapping pad, the sample was distorted and became significantly concave. This distortion is related to the inherent stress contained in the layers of the lapped specimen. It is therefore clear that virtually no compressive stress was applied to the conventionally polished blank (Figure 18A); in contrast, the sample shown in Figure 18B had a highly compressed layer 114'ff. : I was prepared.

The radius of the concave surface was optically measured, and this radius was used to estimate the ttk of the compressive stress existing on the machined surface (FIG. 18B). The relationship between the radius of curvature (R) and compressive stress (S) of the surface is as follows.

In the formula, E is Young's modulus, 7.6 x 10 J/c
m2 (10,8 X 106 psi); d is the thickness of the sample, estimated to be 0.229 mm (0,009 inch); R is the radius of curvature, 129.2 X 10
−2 m; t is the thickness of the compressed layer and I
10 = estimated to be normal; V is the Possio
Jsrαtio) and is estimated to be 0.28. By solving for S, S=7100A1i1/cIrL
2 (IXI O'psi) is obtained. The reinforcing and hardening effect evidenced in the above data is therefore clearly the result of the formation of a surface compression layer on the zinc sulfide blank during said operation. Therefore, by appropriately selecting machining parameters such as tool speed, tool type, depth of cut, feed rate, tool angle, etc., it is possible to select the size of the compressed layer applied to the zinc sulfide surface. The degree of reinforcing/hardening of the zinc sulfide can therefore be controlled.

Referring now to FIG. 19, a portion of the optical element (here plate 130) is shown as including a layer 12 of material with the predetermined optical properties described above. Disposed over layer 12 is a layer of compressed material 114, which is described in connection with FIGS. 9-18. compression layer 1
14, there is disposed a coating layer 11 consisting of one of the aforementioned single-layer or multi-layer coatings described in connection with FIGS. 1-8. According to this embodiment, the enhanced hardenability and rain erosion resistance provided by both of the above methods would serve to provide an optical element with substantially improved rain erosion resistance and fracture strength. Conceivable.

Although the preferred embodiments of the present invention have been described above, it will be apparent to those skilled in the art that other embodiments incorporating the concept may be employed. It is believed that the invention should not be limited to the form presented, but rather should be limited only by the spirit and scope of the claims.

[Brief explanation of drawings]

FIG. 1 is an isometric projection of an optical element (here a plate) consisting of a base layer and a protective layer according to the invention. FIG. 2 is an exploded cross-sectional view taken along line 2--2 of FIG. 1, illustrating a protective layer comprising a single layer coating in accordance with one aspect of the present invention. FIG. 3 is an exploded cross-sectional view taken along line 3--3 of FIG. 1 illustrating a pair of coating layers in accordance with another aspect of the present invention. FIG. 4 is an exploded elevational view taken along line 4--4 of FIG. shows. Figures 5A-5D are a series of graphs showing radial stress reduction as a function of normalized distance from the center of total drop impact for coatings with different moduli (prior art). Figure 6 shows a change rate of 25.4 mm/hour (1 inch/hour).
, speed 724 to 100 mph (450 ph), impact angle 9
1 is a micrograph showing the metallographic structure of an uncoated 5iZnS surface exposed to an accelerated rain field of 0° and 2 raindrop sizes. FIG. 7 is a photomicrograph showing the metallographic structure of a coated surface according to the invention exposed to the same accelerated rain field as shown in FIG. Figure 8 shows the thickness of 5. Figure 9 is a cross-sectional view of a portion of the dome. Figure 10 is the surface of the dome shown in Figure 11. FIG. 11 is a schematic cross-sectional view of raindrops falling on the surface of a general optical element having microstructural defects. FIG. 12 is a schematic cross-sectional view of raindrops falling on a compressed layer according to another aspect of the present invention. FIG. 13 is an enlarged view of a water droplet impacting the compressed layer shown in FIG. 12. FIG. Fig. 15 is a micrograph showing the metallographic structure of the adjusted (single point diamond processing) dome surface.
A plot of the Knoop hardness number as a function of load is shown for the nS disk surface (ZnS microhardness). FIG. 16 plots the typical hardness difference (Knoop) as a function of penetration depth into the compressed surface of the conditioned ZnS disk. Figures 17A and 17B show Zn polished by a conventional method.
S lens and ZnS lens adjusted according to the present invention,
These are micrographs showing the metallographic structure of the surface after being exposed to an accelerated downhill field. Figures 18A and 18B respectively show a lapped ZnS lens (irregular) polished by a conventional method and a lapped ZnS lens (diamond platinized) prepared according to the present invention (diamond platinum processing) showing the distortion caused by the compressed layer.
This is a micrograph (metallic structure) of a highly concave structure. FIG. 19 is a cross-sectional view of a portion of an optical element, such as a plate or dome, with a compressed layer of optical material and a coating layer according to another aspect of the invention. In each figure, the numbers represent the following: 10.110: Optical element 11.111: Anti-reflection coating layer 12.112: Base layer 14: First coating layer 16: Second coating layer 113: Groove 114: Compression layer 115: Water droplet 116.117: Fine defect 118.118 ': Tensile stress component 120: Compressive force 5AI2] Transport! 5CI￥J Ki 11 You Attachment 1 Tail 5D Diagram Beginner 1 Arrogance N'N? Inside

Claims (45)

[Claims] (1) a base layer consisting of a first material having a predetermined transmittance of light over a predetermined range of wavelengths of light and a predetermined resistance to damage caused by high velocity droplet impact; An optical element comprising means disposed on said base layer comprising a second material having a refractive index of less than or equal to 2.0 for increasing the resistance of the base layer to damage caused by impact. (2) An optical element according to claim 1, wherein said means comprises a compressed region of a second material. (3) The optical element according to claim 2, wherein the compressed region of the second material is a compressed region of the first material. (4) the compressed layer comprises a number of furrows arranged in a surface portion of the layer, a compressed layer of a first material is arranged below these furrows, and between adjacent grooves a compressed layer of a first material; 4. Optical element according to claim 3, in which a compressed region of is arranged. (5) The first material is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride, and the general chemical formula MN_
2S_4 (wherein M is a cation selected from group IA elements, N is a cation selected from lanthanide series rare earth elements, and S is a sulfide anion S^2^-) The optical element according to claim 4, which is made of a material selected from the group consisting of: (6) The optical element of claim 4, wherein the groove has a depth of 10 to 10,000 Å and a width generally of 0.01 to 0.02 mm. (7) the first material has a first predetermined modulus of elasticity, and the second material has a second modulus of elasticity that is higher than the modulus of elasticity of the first material, resulting in increased impact resistance of the base layer; The optical element according to claim 1. (8) The optical element according to claim 7, wherein the elastic modulus of the second material is at least twice that of the first material. (9) When the second material is 9. Optical element according to claim 8, having a physical thickness given by the index of refraction of the second material at the wavelength. (10) The first material constituting the base layer is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide,
Cadmium mercury telluride and cadmium telluride and the general chemical formula MN_2S_4 (where M is a monovalent cation selected from Group IA elements, N is a cation selected from lanthanide rare earth elements, and S is a sulfide anion). and the second material is yttrium oxide, scandium oxide, magnesium oxide, or various mixtures of yttrium oxide, scandium oxide, and magnesium oxide. The optical element according to claim 9, selected from: (11) a first layer of a second material, the first material having a predetermined first modulus of elasticity, and the means having a second modulus of elasticity higher than the modulus of the first material; The optical element according to claim 1, comprising a second layer of a third material having a higher elastic modulus than that of the second material and a refractive index of 2.0 or more. (12) If the composite layer is t = (2N+1)λ/4/n_c (where N is an integer 0, 1, 2, 3, ..., λ is the desired specific wavelength, and n_c is the wavelength 12. Optical element according to claim 11, having a physical thickness t given by t, which is the refractive index of the composite layer at . (13) Base layer is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride and general chemical formula MN
_2S_4 (wherein M is a cation selected from group IA elements, N is a cation selected from lanthanide series rare earth elements, and S is a sulfide anion S^2^-) the second material is selected from the group consisting of yttrium oxide, scandium oxide, magnesium oxide, or various mixtures of yttrium oxide, scandium oxide, and magnesium oxide; and the third material is selected from the group consisting of hard carbon, scandium oxide, and magnesium oxide; cerium,
The optical element according to claim 11, which is selected from the group consisting of titanium oxide and zirconium oxide. (14) the first material of the base layer has a predetermined first modulus of elasticity, the means further compressing the second material and the third material having a second modulus higher than the material of the first layer; The optical element according to claim 1, comprising a layer of. (15) The compressed region of the second material is a compressed region of the first material and is disposed in the substrate, and the layer of third material is disposed in the compressed region of the first material. The optical element described in . (16) The optical element according to claim 15, wherein the modulus of elasticity of the third material is at least twice the modulus of elasticity of the base material. (17) The first material is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride and the general chemical formula MN_2S_4 (wherein M is a monovalent selected from Group IA elements). ions, N is a divalent ion selected from the lanthanide series rare earth elements, and S is a sulfide anion S^2^-; and a third 17. The optical element of claim 16, wherein the material is selected from the group consisting of yttrium oxide, scandium oxide, magnesium oxide, or various mixtures of yttrium oxide, scandium oxide, and magnesium oxide. (18) The third material is t=(2N+1)λ/4/n_c(
where N is an integer 0, 1, 2, 3, etc., λ is the specific wavelength of interest, and n_c is the third wavelength at that wavelength.
18. Optical element according to claim 17, having a physical thickness given by the refractive index of the material. (19) A fourth material in which the means is further disposed on the third material.
The fourth material is the first or third material.
The optical element according to claim 14, having an elastic modulus higher than that of the material and a refractive index of 2.0 or more. (20) The thickness of the composite layer is t=(2N+1)λ/4/n_
c (where N is an integer 0, 1, 2, 3, ..., λ is the specific wavelength of interest, and n_c is the refractive index of the composite layer material at that wavelength), the patent The optical element according to claim 19. (21) The first material is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride and cadmium telluride and the general chemical formula MN_2S_4 (where M is a group IA element and N is a lanthanide). selected from the group consisting of ternary sulfides having a sulfide anion (S is a sulfide anion S^2^-); the third material is yttrium oxide, scandium oxide, magnesium oxide, or yttrium oxide, selected from the group consisting of various mixtures of scandium and magnesium oxide; and the fourth material is hard carbon, cerium oxide,
The optical element according to claim 20, which is selected from the group consisting of titanium oxide and zirconium oxide. (22) a base layer comprising a first material having a predetermined transmittance of light over a predetermined range of original wavelengths and a predetermined modulus of elasticity; at least twice the modulus of elasticity of the material of the first layer; an optical element consisting of a layer of a second different material having a modulus of elasticity of 2.0 and a refractive index of 2.0 or less. (23) Base layer is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride and general chemical formula MN
2_S_4 (where M is a monovalent cation selected from Group IA elements, N is a cation selected from lanthanide series rare earth elements, and S is a sulfide anion S^2^
- and the second material is made of yttrium oxide, scandium oxide, magnesium oxide, or a homogeneous mixture of yttrium oxide, scandium oxide, and magnesium oxide. 23. Optical element according to claim 22, selected from the group. (24) The layer of the second material is t=(2N+1)λ/4/n_
c (where N is an integer 0, 1, 2, 3, ..., λ is the particular wavelength of interest, and n_c is the refractive index of the second material at that wavelength) 24. Optical element according to claim 23, having a thickness t. (25) consisting of a material having a predetermined original transmittance over a predetermined range of wavelengths;
a base layer, the material having a first predetermined modulus of elasticity and a first predetermined index of refraction; (i) disposed on at least a portion of the base layer; (ii) having a modulus of elasticity at least twice that of the base material; , and a second layer made of a second high-modulus material having a refractive index of 2.0 or more. (26) Base layer is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride and general chemical formula MN
_2S_4 (in the formula, M is a monovalent ion selected from group IA elements, N is a divalent ion selected from lanthanide series rare earth elements, and S is a sulfide anion S^2
the first high modulus material is yttrium oxide, scandium oxide, magnesium oxide, or various mixtures of yttrium oxide, scandium oxide, and magnesium oxide; and the second high modulus material is selected from the group consisting of hard carbon, cerium oxide, titanium oxide and zirconium oxide. (27) The first layer is t=(2N+1)λ/4/n_c_1
(where N is an integer 0, 1, 2, 3, ..., λ is the specific wavelength of interest, and n_c_1 is the refractive index of the first layer material at that wavelength). and the physical thickness of the second layer is t=(2N+
27. The combination of claim 26, given by 1) λ/4/π_c_2, where n_c_2 is the refractive index of the second layer material. (28) a first material having a modulus of elasticity at least twice that of the first material on the area to be protected and responsive to shear stress caused by radial outflow upon droplet impact;
applying a layer of a second material that is resistant to delamination from the material, protecting an area of the optical element made of a first material having a first modulus from impact damage encountered in a high velocity droplet environment; How to protect. (29) The material constituting the optical element is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide,
cadmium telluride selected from the group consisting of mercury telluride and cadmium telluride; and the second material is yttrium oxide,
28. The method of claim 27, wherein the oxide is selected from the group consisting of scandium oxide, magnesium oxide, or homogeneous mixtures of yttrium oxide, scandium oxide, and magnesium oxide. (30) further comprising applying a layer of a third material over the layer of the second material, wherein the third material has an elastic modulus at least twice that of the first material and a refractive index greater than the second material. 28. A method according to claim 27, wherein the material also has a large refractive index. (31) The method according to claim 30, wherein the third material is selected from the group consisting of hard carbon, cerium oxide, titanium oxide, and zirconium oxide. (32) A layer of hard carbon of group II-V or group III-V consisting of a step of applying a first layer made of a material selected from the group consisting of yttrium oxide, scandium oxide, and magnesium oxide between the base layer and the hard carbon layer. A method of bonding to a substrate consisting of a material selected from the group consisting of substances of the group consisting of: (33) The substances of Groups II to VI are selected from the group consisting of zinc sulfide, zinc selenide, cadmium mercury telluride, and cadmium telluride, and the substances of Groups III to V are selected from the group consisting of gallium arsenide and gallium phosphide. 33. The method of claim 32, wherein the method is selected. (34) The method of claim 33, wherein the base layer, first layer and hard carbon layer provide an optically transparent element. (35) The first layer is t=(2N+1)λ/4/n_c (where λ is the desired wavelength, n_c is the refractive index of the first layer material at the wavelength λ, and N is an integer 0, 1 ,2,3,
) has a physical thickness t given by
A method according to claim 34. (36) Consists of a base layer made of a material having a predetermined light transmittance, a surface portion of this optical material is compressed, and this compressed surface portion has a plurality of grooves arranged on the surface portion. an optical element having compressed regions of substrate material disposed below the grooves and between adjacent ones of the grooves. (37) The base layer is zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride, and the general chemical formula M
N_2S_4 (where M is a monovalent cation selected from group IA elements, N is a cation selected from lanthanide series rare earth elements, and S is a sulfide anion S^2
37. The optical element according to claim 36, wherein the optical element is made of a material selected from the group consisting of ternary sulfides having the following properties: (38) the compressed material region has a thickness of about 3 microns;
The optical element according to claim 37. (39) A method of applying a tailored reinforcing surface to a material, comprising the step of applying a layer having a breaking strength higher than the breaking strength of the material, converting the surface portion of the material into a compressed surface portion of the material. (40) Single-point turning the surface of the optical element,
A method for reinforcing an outer surface of an optical element, the method comprising the step of introducing a compressed surface layer portion of the optical element onto the outer surface. (41) The method according to claim 40, wherein the turning step is performed with a single point diamond tool. (42) the turning step turns the optical element to first remove from the outer surface a portion of the surface layer having a thickness of at least 25.4 microns (1 mil); and then turning the optical element to a thickness of at least 0.254 microns (1 mil);
41. Micron (0.01 mil) surface portions are removed from the outer surface and the outer surface of the optical element is provided with grooves in the surface portions, the grooves being separated by regions of compressed material. The method described in. (43) The method of claim 42, wherein the optical element is selected from the group consisting of a dome, a lens, a plate, and a window. (44) The material of the optical element is selected from the group consisting of zinc sulfide, zinc selenide, cadmium mercury telluride, cadmium telluride, germanium, gallium arsenide, and gallium phosphide. Method. (45) The method according to claim 44, wherein the material is selected from the group consisting of zinc selenide and zinc sulfide.