OPTICAL REFLECTING THIN FILM COATINGS

CROSS-REFERENCE TO RELATED APPLICATIONS Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT Not Applicable

REFERENCE TO SEQUENCE LISTING, A TABLE OR A COMPUTER PROGRAM

LISTING COMPACT DISK APPENDIX Not Applicable

BACKGROUND

[0001] The present invention relates broadly to optical thin film coatings and, more particularly, to a process for producing optical thin film coatings that function as interference filters reflecting and transmitting desired wavelengths of the electromagnetic spectrum, are resistant to the deleterious affects of ultraviolet radiation and/or high temperatures and, maintain their electrical resistivity. The optical thin film coatings of the present invention are used in a variety of applications, for example, to coat the reflecting inner-surfaces of high intensity reflector lamps. [0002] The background information discussed below is presented to better illustrate the novelty and usefulness of the present invention. This background information is not admitted prior art.

[0003] Presently available high performance lamps consist of a bulb or burner that provides the light source, a reflector, also known as mirror housings, to direct the light from the bulb, and an optical coating that is used to filter the spectrum emitted by the bulb. Typical high performance lamps currently available include high Intensity discharge (HID) lamps (FIG. 1a), tungsten-halogen lamps (FIG. 1b), and other lamps that have gas filled bulbs. FIG. 1a, illustrates a typical HID lamp assembly 1a, which commonly comprises a generally parabolic, elliptical, or dome-shaped reflector 2 having rearwardly extending neck section 2a in which bulb 5, also referred to as a burner, has electrodes 7 electrically connected for generating and emitting electromagnetic radiation. The lamp assembly also comprises forwardly located opening 6a covered by lens 6b through which light 16 is projecting. Optical reflective thin film coating 4

deposited on reflective inner surface 3, often referred to as a "mirror," is used to provide reflecting mirror-like surfaces required by high performance lamps. Electricity is provided to the bulb via first bulb contact 8b electrically connected to electrical lead 9a electrically connected to first electrode 7. The electrical circuit continues via second electrode 7 electrically connected to second electrical lead 9a electrically connected to second bulb contact 8b.

[0004] Compared to fluorescent and incandescent lamps, high performance lamps provide high light intensities (measured in lumens where a lumen is the amount of light emanating from a candle) and high color temperatures in a relatively small lamp assembly, but generate high levels of undesirable ultraviolet (UV) radiation. Radiation (as used herein radiation refers to the electromagnetic spectrum from the UV to the infra-red (IR) region) is generated in HID lamps by an arc discharge contained within a refractory envelope (arc tube). HID lamps are typically used wherever high levels of light and energy efficiency are desired, such as to provide lighting for gymnasiums, large public areas, warehouses, outdoor activity areas, roadways, parking lots, pathways, and recently they have found use as motor-vehicle headlamps. HID lamps, especially metal halide lamps, also are used in small retail and residential environments. Indoor gardeners frequently rely on HID lamps, especially for plants that require high intensity sunlight, such as vegetables and flowers. HID lamps are even being used for external lights on the super jumbo Airbus A380 and virtually all projection systems built to date use HID lamps as a light source, including projection television and high quality image projection systems.

[0005] FIG. 1b, illustrates a typical tungsten-halogen lamp assembly 1b, which commonly comprises a generally parabolic, elliptical, or dome-shaped reflector 2 having a rearwardly extending neck section 2a for receiving bulb 5b, also referred to as a burner, for generating and emitting electromagnetic radiation. Lamp assembly also comprises forwardly located opening 6a covered by lens 6b through which light 16 is projected. Optical reflective thin film coating 4 is deposited on reflective inner surface 3. Electricity is provided to coiled filament 7b of bulb 5b via first electrical lead 9a. The electrical circuit continues via second electrical lead 9a.

[0006] Tungsten-halogen incandescent lamps differ from HID lamps in that they have a coiled tungsten filament 7b that is enclosed within a transparent envelope. Tungsten-halogen lamps are used when there is a need for excellent lumen maintenance, compactness, and whiter light, such as in retail lighting, stage and studio lamps, medical, museums, and residential lighting. Although tungsten-halogen lamps are more effective and exhibit longer life, they generate more undesirable UV radiation than any other form of incandescent lamp.

[0007] Optical reflective thin film coatings are deposited on the inner surface of reflectors but may also be deposited on the backside of the reflector, or on both sides, to increase reflectivity. The inner surface of the reflector may be referred to as the substrate onto which the coatings are deposited. Thin film coatings may be designed to be totally reflecting so that substantially all of the electromagnetic spectrum emitted by the bulb is reflected forward. A well-known example of a typical coating design used in high intensity lamps is referred to as a "cold mirror" design. In a cold mirror coating design, as illustrated in FIG. 2, the optical interference coating acts as a wavelength filter so that selected portions of the visible spectrum generated by bulb 5 are reflected forward, out of and away from the lamp assembly, while most of the IR wavelengths are directed towards interior portions of the lamp to be transmitted through the coating. As the IR component of the electromagnetic spectrum is a source of heat, the visible radiation reflected forward is considered "cold" light.

[0008] More frequently, coatings are designed to selectively reflect or transmit desired regions of the electromagnetic spectrum. Design elements are dictated by the optical functions under consideration and the wavelengths desired to be reflected by the optical coating and typically include the chemical nature of the layers to be deposited, the number of each layer to be deposited, the order in which the different layers are deposited onto the substrate, the thickness of each layer, and the morphology of the layers. Such coatings frequently include multiple alternating layers of low refractive index (ηi_) materials, such as SiOx, and high refractive index (Ύ\H) materials, such as TiOχ. It is well known that coatings made of multiple alternating ηι_ and rm layers produce a "stop band" or area of high reflectivity centered on the design wavelength λ ₀ .

High reflectivity design requires each layer's thickness to be a quarter of the design wavelength. The design form resembles:

Medium/(HL) ^m H/Substrate Eq. 1

where: m is the number of periods of the multilayer stack,

Medium is the material the radiation is in before it is in contact with the outer surface of the coatings, in this case the medium is air or is equivalent to air,

HL is a pair of layers comprising a ημ layer and a η _L layer,

H is a layer of ηπ material,

L is a layer of η _L material, and

Substrate is the material onto which the coating is deposited, in this case the reflective inner surface of the reflector.

[0009] To form stacks that selectively filter a portion of the spectrum, coating design generally calls for multiple stacks of multiple layers where the coating's chemical composition, its thickness, and the position of each layer within a given stack are predetermined. For parabolic or elliptical shaped reflectors, where "wide band" or

"broadband" reflection is desired, the design of the optical coating includes multiple stacks of alternating η _L and ηπ layers where the high reflectivity regions overlap to include the entire wavelength spectrum band and the cone-angle oblique incidence of the light.

[0010] Available coatings, made according to Eq. 1 , however, suffer detrimental physical and chemical changes from the heat that is generated by high intensity lamp bulbs and from the high UV radiation flux produced by the lamps. FIG. 3 provides a graphical illustration of part of the flux intensity of part of the UV radiation spectrum (the part that includes wavelengths in UVA 400-315 nm region and are referred to as black light), UVB 315-280 nm and UVC 280-100 nm and visible light (400 - 700 nm wavelength region) generated by a typical HID high intensity lamp. The detrimental physical and chemical changes undergone by coatings include photo reduction and measurable decreases in electrical resistivity of the deposited coatings.

[0011] As lamp assemblies become smaller, as required in the design of large screen projection televisions and other projection display systems, the electrical leads of the bulb are attached to contacts that are positioned on the interior coated surfaces of the lamp. In this design, the electrical connectors of the leads will be in contact with the reflective coatings of the inner surface of the reflector. This means that the coating must be an electrical insulator and, thus, must be able to maintain acceptable levels of resistivity or resistance during operation. If UV photon flux, high temperatures, or other factors cause a drop in the resistivity of the coating, the electrical current used to operate the lamp's bulb will follow the path of least resistance and could short out the lamp. It is clear then that what is needed are optical thin film reflective coatings that can be designed to reflect or transmit desired wavelengths, that can withstand the affects of high temperature and exposure to UV energy, and that can maintain their resistivity, regardless of the magnitude of the UV photon flux and high temperatures that are typically generated by these lamps.

SUMMARY

[0012] The present invention satisfies currently unmet needs for optical coatings based on designs and materials that produce interference filters designed to manage desired wavelengths and to withstand the affects of high temperature and UV exposure, and, thus, to maintain acceptable levels of electrical resistivity. The coatings made according to the principles of the present invention are proving ideal for use in high intensity lamps where the lamp's bulbs routinely generate high levels of UV radiation and high temperatures. The coatings are also well-suited for use anywhere there is a need for a coating that will reflect and/or transmit desired wavelengths of electromagnetic energy, that is resistant to the detrimental effects of UV and high temperatures, that remains essentially electrically stable, where essentially electrically stable means that the coating maintains a sufficient level of electrical resistivity to dependably function as an electrical insulator when incorporated into lamps, and that selectively manages desired wavelength reflection and/or transmission. [0013] Reflective optical thin film coatings of the present invention are able to reflect visible light while transmitting infra-red radiation, remain resistant to the

detrimental effect of UV radiation, and maintain acceptable levels of its electrical properties. Such coatings comprise a stack comprising a plurality of alternating layers of predetermined chemical and physical configuration. One example of such an optical reflecting coating comprises a coating design that has a stack comprising a first layer of a low index material repeatedly alternating with a second layer of a high index material to manage to the spectrum. This part of the design is repeated for a desired number of stacks followed by a last to be deposited stack of a first layer of a high index material repeatedly alternating with a second layer of a low index material where, for example, the high index material comprises Ta ₂ O ₅ which is resistant to exposure to UV wavelengths, for a predetermined number of layers to produce desired optical activity, such as reflection and/or transmission of wavelengths.

[0014] Another design provides a reflective optical thin film coating, producing results similar to those described above, comprising a stacking sequence, including having first a plurality of coating stacks where each stack comprises repeating alternating layers of a high reflective index material and a low reflective index material, where the high index material is resistant to exposure to UV wavelengths, where, for example, the high index material comprises Nb ₂ O ₅ .

[0015] By being able to retain their high initial electrical resistivity, the coatings of the present invention are able to provide for a coated reflector where reflector and coating are able to dependably function as an electrical insulator when incorporated into lamps.

[0016] These and other advantages of the present invention are made possible by providing for an interference filter coating, comprising a coating comprising a plurality of a layer of a low refractive index material alternating with a layer of a high refractive index material, where a plurality of the high refractive index layers are resistant to the damaging effects of high temperature and/or ultraviolet radiation, and where the coating has essentially stable electrical resistivity properties when subjected to ultraviolet portions of the electromagnetic spectrum and/or high temperatures. [0017] Furthermore, the coating selectively reflects visible light, but transmits infrared radiation, and in particular, the coating selectively reflects at least 95% of visible

light having a wavelength between 400-700 nm and transmits at least 80% of infrared radiation having a wavelength greater than 870 nm.

[0018] Moreover, the low refractive index material of the interference filter coating has an index of refraction less than 1.8 and a melting temperature greater than 50O ⁰ C. and may include, but not be limited to, the group consisting essentially of SiOx, SiO ₂ , MgF ₂ , SiO, Si, Y ₂ O ₃ , AI ₂ O ₃ , BaF ₂ , CaF ₂ , CeF ₃ , Na ₃ AIF ₆ , NdF ₃ , YF ₃ , AIF ₃ , or, blends thereof. Comparatively, the high refractive index material has an index of refraction equal to or greater than 1.8 and a melting temperature greater than 500° C, and may include, but not be limited to, the group consisting essentially of Ta ₂ O ₅ or Nb ₂ O ₅ , HfO ₂ , ZrO ₂ , WO ₂ , Mo ₂ , In ₂ O ₃ or blends thereof.

[0019] In addition, the interference filter coating may further comprise high refractive index material that is not resistant to the damaging effects of UV radiation and/or high temperature, which may include, but not be limited to, the group consisting essentially Of TiO ₂ , TiχOγ, or blends thereof.

[0020] The invention may, alternatively be described as a coating, comprising: at least one coating stack comprising a plurality of i) a layer of a low refractive index material alternating with ii) a layer of an essentially electrically stable high refractive index material when subjected to ultraviolet portions of the electromagnetic spectrum * and/or high temperatures, wherein the coating stack provides for a reflective filter surface for reflecting predetermined portions of electromagnetic radiation and for transmitting predetermined portions of electromagnetic radiation. In some cases, the reflected predetermined portions of electromagnetic radiation comprise visible light, and the transmitted predetermined portions of electromagnetic radiation comprise infra-red radiation, whereas in other cases, the reflected predetermined portions of electromagnetic radiation comprise infra-red radiation and the transmitted predetermined portions of electromagnetic radiation comprise visible light. [0021] The coating may additionally comprise wherein an exterior (last to be deposited) stack consists of alternating layers of a high index material that is electrically stable and a low index material and at least one interior stack consisting of layers of: I) a low refractive index material alternating with

ii) a ultraviolet sensitive high refractive index material.

[0022] The invention also includes a process for making a reflective coating, comprising the steps of: a) providing for a plurality of: i) a layer of a low refractive index material alternating with ii) a layer of an essentially electrically stable high refractive index material, b) providing for a plurality of the high refractive index layers to be resistant to ultraviolet radiation and/or high temperature and having essentially stable electrical resistivity properties when subjected to high temperatures and/or ultraviolet portions of the electromagnetic spectrum, c) depositing the coating onto a reflective substrate surface, and d) tailoring the coating into a specific shape on the reflective substrate surface to project light having desired color temperature, intensity, and chromaticity.

[0023] The invention contemplates a lamp utilizing a reflector, the reflector having an inside reflecting surface and an outside reflecting surface where either the inside reflecting surface or outside reflecting surface or both surfaces are coated with an interference filter coating, comprising: a) a plurality of alternating layers of low refractive index material and high refractive index material, and b) a plurality of the high refractive index layers resistant to the damaging effects of UV radiation and/or high temperature, the coating having essentially stable electrical resistivity properties when subjected to UV portions of the electromagnetic spectrum and/or high temperatures. [0024] The reflector may be made of a glass, plastic, ceramic, glass-ceramics, metals or other useful material to comprise a front reflecting portion having a light reflecting surface for projecting reflected light forward of the reflector and a rear portion terminating in an elongated, rearwardly protruding cavity wherein the interior surface of the cavity does not form part of the forward reflecting surface, the reflector being coated on the light reflecting surface and on the inside surface or outside surface of the cavity

or both of the surfaces of the cavity with an interference filter coating having essentially stable electrical resistivity properties when subjected to UV portions of the electromagnetic spectrum and/or high temperatures.

[0025] The invention may also be referred to as an interference filter coating for reflecting predetermined portions of electromagnetic radiation and transmitting predetermined portions of electromagnetic radiation, that comprises: a) a plurality of alternating layers of low refractive index material and high refractive index material, and b) a plurality of the high refractive index layers resistant to ultraviolet radiation and/or high temperature, the coating having stable electrical properties when subjected to ultraviolet portions of the electromagnetic spectrum and/or high temperatures, wherein the coating is a coating of a reflector made of glass, metal, plastic, ceramic, glass-ceramic, or combination thereof, where the bulb is either detachably attached to the lamp or fixedly attached to the lamp to meet lighting, stage & studio, medical, projection lighting applications.

[0026] Still other benefits and advantages of this invention will become apparent to those skilled in the art upon reading and understanding the following detailed specification and related drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order that these and other features and advantages of the present invention may be more fully comprehended and appreciated, the invention will now be described, by way of example, with reference to specific embodiments thereof which are illustrated in appended drawings wherein like reference characters indicate like parts throughout the several figures. It should be understood that these drawings only depict preferred embodiments of the present invention and are not therefore to be considered limiting in scope. Thus, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which:

FIG. 1a is a cross-sectional view illustrating a HID lamp reflector that is presently available on the market.

FIG. 1b is a cross-sectional view illustrating a Tungsten-Halogen lamp reflector that is presently available on the market.

FIG. 2 is a diagrammatic sketch illustrating an interference thin film coating design that is presently known.

FIG. 3 is a graphical illustration of the intensity of the UV spectrum and visible light wavelengths generated by a typical HID high intensity lamp bulbs.

FIG. 4a is a cross-sectional view illustrating a HID lamp reflector with one coating design of the present invention deposited on the inner-surface of the lamp.

FIG. 4b is a cross-sectional view illustrating a Tungsten-Halogen lamp reflector with one coating design of the present invention deposited on the inner-surface of the lamp.

FIG. 4c is a cross-sectional view illustrating a HID lamp reflector with a second coating design of the present invention deposited on the inner-surface of the lamp.

FIG. 4d is a cross-sectional view illustrating a Tungsten-Halogen lamp reflector with a second coating design of the present invention deposited on the inner-surface of the lamp.

FIG. 5 is a graphical cross-sectional idealized view illustrating a UV resistant coating of the present invention deposited on a section of an inner-surface of a reflector wall, the coating comprising at least two coating stacks each with a plurality of coating layers.

FIG. 6 is a diagrammatic representation of the experiment that produced the results illustrated in GRAPHS 1 and 2

LIST OF REFERENCE NUMERALS AND THE PARTS TO WHICH THEY RELATE Reference Numerals Relating to Background Art

1a A presently available HID reflector lamp assembly.

1b A presently available tungsten-halogen reflector lamp assembly.

2 A generally parabolic, elliptical, or dome-shaped reflector (lamp housing). 2a A rearwardly extending neck section of 2.

3 Reflective inner surface of reflector lamp assembly 1.

4 Optical thin film coatings.

5 An arc-tube bulb or burner.

5b A coiled filament bulb or burner.

6a An opening through which light emanates.

6b A reflector lens for covering opening 6a.

7 Arc-tube electrodes.

7b A coiled tungsten filament.

8b Bulb contacts.

9a Electric leads.

Reference Numerals Relating to Present Invention

10 An HID reflector lamp of the present invention.

12 A generally parabolic, elliptical, or dome-shaped reflector housing. 12a A rearwardly extending neck section.

13 Reflective inner surface of reflector lamp assembly 10.

14 Type Two Coating Stack coating not deposited over any other stack

14a Type One Coating Stacks (first set of stacks to be deposited to form coating)

14b Type Two Coating Stack deposited over Type One Coating Stack(s).

15a An arc-tube bulb or burner.

15b A coiled filament bulb or burner.

16 Light projecting through lens of a presently available reflector. 16a Light projecting through lens of the present invention.

17 Arc-tube contacts.

17b A coiled tungsten filament.

18a A first lug or bulb contact

18b A second lug or bulb contact.

19a A first electric lead.

19b A second electric lead.

20 A tungsten-halogen lamp assembly of the present invention.

22 Single layer.

24 Stack of multiple layer.

H High refractive index layer ημ.

L Low refractive index layer η _L .

30 An HID reflector lamp assembly of the present invention.

32 Section of reflector wall.

40 A tungsten-halogen lamp assembly of the present invention.

60 Lamp.

62 UV shutter.

64 Reflector.

65 UV Sensor

[0028] It should be understood that the drawings are not necessarily to scale. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.

DEFINITIONS

Dielectric, as used herein, refers to inorganic oxides, fluorides, nitrites, borides, and similar materials that selectively transmit, reflect, or absorb at different wavelengths of the electromagnetic spectrum.

Dielectric coating, as used herein, refers to a high-reflectance, interference filter that is considered a coating made specifically to achieve reflectance of a desired wavelength, which may consist either of alternating layers of quarter-wave film comprising repeated alternating layers of a higher refractive index material and a lower refractive index material that is lower than the substrate or layers comprising varying layers' thicknesses or film indexes, spread over a wide wavelength interval.

Flux, as used herein, refers to a time rate of flow of energy or the radiant or luminous power in a beam.

High intensity discharge (HID) lamp, as used herein, refers to lamps that include mercury vapor, metal halide (also HQI), high-pressure sodium, low-pressure sodium and less common, xenon short-arc lamps. The light-producing element of HID lamps is a well-stabilized arc discharge contained within a refractory envelope (arc tube) with wall loading in excess of 3 W/cm ² (19.4 W/in. ² ).

Infrared (IR), spans three orders of magnitude of electromagnetic wavelengths between approximately 750 nm and 1 mm of the electromagnetic spectrum. Near infrared (NIR,

IR-A DIN), 0.75-1.4 μm in wavelength. Radiation in the near-infrared produces a sensation of heat.

Interference Filters (or optical filters) are multilayer thin film devices consisting of multiple layers of coatings deposited on a substrate having spectral properties are the result of wavelength interference.

Refractive index (or index of refraction) of a material is the factor by which the velocity of electromagnetic radiation is slowed in that material, relative to its velocity in a vacuum. It is usually given the symbol n, and defined for a material by: n = v^ Eq. 2 where ε _r is the material's relative permittivity, and μ _r is its relative permeability. For a non-magnetic material, μ _r is very close to 1 , therefore n is approximately V ⁶ ^ Resistivity (also known as specific electrical resistance), as used herein, is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electrical charge, and is described by a matrix defining the conduction of electricity in anisotropic materials which is generalized form of Ohm's Law, where

P = E I j or R = p ( i / A) Eq. 3 where: p = resistivity

E = electric potential

J = current density. i = current

A = cross sectional area of the coating

Ultraviolet (UV), as used herein, refers to that invisible region of the spectrum just beyond the violet end of the visible region where wavelengths range from 100 to 400 nm.

Ultraviolet A, as used herein, refers to the region of the electromagnetic spectrum typically from 315 to 400 nm

Ultraviolet B, as used herein, refers to the region of the electromagnetic spectrum typically from 280 to 315 nm

Ultraviolet C, as used herein, refers to the portion of the electromagnetic spectrum typically from 100 to 280 nm

DETAILED DESCRIPTION

[0029] The present invention is directed to reflecting optical thin film coatings that filter desired portions of the electromagnetic spectrum and to a method of making the coating. The coatings made according to the principles of the present invention are ideal for use wherever reflective optical coatings are required, especially when the coatings are required to reflect desired wavelengths and transmit other desired wavelengths, are able to withstand exposure to high levels of UV radiation and high temperatures, and are able to maintain their electrical resistivity. The coatings disclosed herein, surprisingly and unexpectedly, are able to withstand the effects of high temperature and/or UV radiation. Even more surprising and unexpected is that after being exposed to UV radiation and/or high temperature, the coatings are able to maintain their electrical resistivity (isolation resistance) regardless of the magnitude of the UV photon flux and the increasingly high temperatures that are routinely generated by the lamp bulbs. The present invention provides for reflective coatings that may be used, for example, to increase the reflecting inner surfaces of HID, tungsten-halogen, and/or other similar high performance lamps. Additionally, the design of the coatings provides for enhanced spectral performance and improved stability. The reflective coatings according to the methods of the present invention are custom-designable to reflect desired wavelengths of the electromagnetic spectrum while transmitting other desired portions, such as IR radiation.

[0030] The layers making up the reflective coatings of the present invention are each designed to have the selected chemical and physical configurations that will provide for the desired filtering of electromagnetic radiation. Layers are differentiated by their custom designed chemical and/or physical configurations. The coating as taught comprises at least one stack, where a stack may include one or many layers, and where each layer has predetermined chemistry and/or morphology. The present invention includes a stack design having a first layer alternating with a second layer repeatedly for a predetermined number of layers. Because the coatings of the present invention are able to withstand exposure to high levels of UV radiation and high temperatures due to the heat generated by IR radiation while maintaining their electrical resistivity, they are ideal for use wherever reflective optical filter coatings are required, such as for coating

reflective surfaces of high intensity reflectors. Thus the coating designs according to the present invention will be explained herein using as an example their application as high intensity reflector lamp coatings. This example is provided for clarity and conciseness without limiting the scope of the invention. Note that for viewing clarity, FIGS. 4a to 4d are limited to illustrating two coating stacks on the inner surface of the reflector lamp assemblies 10, 20, 30, and 40, although it is to be understood that any number of stacks may be used in any coating and that any number of layers may be used in a stack. The numbers of layers and stacks are dictated by the optical physics that will produce the desired results, which will be discussed below. [0031] Coatings are presently deposited by well-known deposition techniques including physical vapor deposition, resistance vacuum evaporation, E-gun, ion assisted, sputtering, cathodic arc, chemical vapor deposition, and low temperature solution deposition, although other less well-known or yet to be known techniques may be used. High intensity reflectors may be manufactured using well or lesser known materials, such as glass, ceramic, plastic, or metal or yet to be known materials that possess the required properties.

[0032] EXAMPLE 1: FIG. 4a, a cross-sectional view, illustrates an HID reflector lamp, which benefits from the coating of the present invention, where HID lamp assembly 10 comprises generally parabolic, elliptical, or dome-shaped reflector housing 12, rearwardly extending neck section 12a, reflective inner surface 13 of reflector lamp assembly 10, Type One Coating Stacks 14a (first set of stacks to be deposited) coated onto inner surface 13) and reflective thin film optical Type Two Coating Stack 14b coatings (exterior stack, last to be deposited, not affected by UV) coated onto Type One Coating Stacks 14a provide a reflective mirror surface coating resistant to damage from UV radiation and/or high temperatures and that maintains an essentially useful level of resistivity or resistance, reflector lens 16b covering the front opening of reflector housing 12, arc-tube contacts 17 of bulb or burner 15a that is the source of radiating light 16a, lug or bulb contact 18b supplying current to the bulb through first electric lead 19a and second electric lead 19b completing the bulb's electrical circuit through lug or bulb contact 18a.

[0033] The light-producing element of HID reflector lamps is an arc discharge contained within a refractory envelope that also contains various gases and metal salts. The refractory envelope is commonly referred to as an arc tube. These lamps produce relatively high pressures and high temperatures. It is, in part, the high temperature that damages presently available coatings. Types of HID lamps the will benefit from the coatings of the present invention include mercury vapor, metal halide, ceramic, low- pressure sodium, high-pressure sodium, and less commonly, xenon short-arc lamps, although the invention is not limited to use in the aforementioned lamp types. Any lamp that will benefit from the coatings of the present invention is contemplated for use with the invention

[0034] FIG. 4b, a cross-sectional view, illustrates tungsten-halogen incandescent reflector lamp 20 comprising generally parabolic, elliptical, or dome- shaped reflector housing 12, rearwardly extending neck section 12a, reflective inner surface 13 of reflector lamp assembly 20, Type One Coating Stacks 14a (first set of stacks to be deposited) coated onto inner surface 13 and reflective thin film optical Type Two Coating Stack 14b coated onto Type One Coating Stacks 14a provide the required reflective mirror surface coating that is resistant to damage from UV radiation and/or high temperatures and that maintains an essentially useful level of resistivity or resistance, bulb or burner 15b for generating the desired light, reflector lens 16b covering the front opening of reflector housing 12 coiled tungsten filament 17b the source of the radiating light, and first electric lead 19a and a second electric lead 19b. [0035] Tungsten-halogen incandescent lamps differ from HID lamps in that they have a coiled filament of tungsten that is enclosed within quartz or other special glasses. These bulbs are filled with a gas that is able to regenerate the filament by preventing the tungsten that evaporates from the coil from depositing on the enclosing glass tube. Tungsten-halogen lamps are used whenever there is a need for excellent lumen maintenance, compactness and whiter light. Although tungsten-halogen lamps exhibit higher efficiency and have a longer life, they generate more of the UV radiation that is known to degrade presently available reflective coatings than any of the incandescent lamps.

[0036] Referring now to the drawings, it should be noted that the disclosed invention is disposed to embodiments in various compositions, layer arrangements, thicknesses, chemistries, morphologies, sizes, shapes, and forms. It is to be appreciated that the embodiments described herein are provided with the understanding that the present disclosure is intended as illustrative and is not intended to limit the invention to the embodiments described herein.

[0037] For example, FIG. 4a and FIG. 4b illustrate HID reflector lamp and a tungsten-halogen reflector, respectively, where on the inner reflecting surface of each of the lamps a coating is deposited as follows. Deposited directly on the inner surface of the lamp is a low refractive index ηi_ material, such as Siθ ₂ , the next layer to be deposited in a layer of a high refractive index ηH material, such as TiO ₂ , the next is another layer of SiO ₂ , the next is another layer of TiO ₂ , and so on until a predetermined number of repeated alternating layers of a low index material followed by a high index material have been deposited to form a stack. This alternating layering pattern is referred to herein as a Type One Coating Stack. The number of layers in a Type One Coating Stack and the number of stacks deposited will depend on what the calculated coating design requires to achieve the desired optical properties. This is true also for the thickness of the layers, the positioning of the layers relative to each other, and to the morphology of the layers, all which are also predetermined by optical design theory. Each coating layer has a thickness defined as:

Thickness = Aj(λ/4ηi cosθ-i) Eq. 4 where: ηi is the refractive index of the material, λ is the wavelength of the center of the spectrum band, θ is the center angle of the cone-angle incidence, and

Aj is a coefficient defining the overlap of the spectrum band.

At this stage of coating formation, the two coating stacks produce a known "cold mirror" reflective coating. As explained above, a cold mirror coating reflects visible light forwardly out of the lamp while transmitting IR radiation rearwardly through the reflector coating and the reflector. After the last stack of Type One Coating Stack is deposited, Type Two Coating Stack is deposited. Type Two Coating Stack comprises alternate

coating layers of a low index material, such as Siθ2, and a high index material that is resistant to the detrimental effects of high temperatures and UV radiation, such as Ta ₂ O ₅ . The deposition of the Type Two Coating Stack over the Type One Coating Stack results in unexpected and surprising results. This coating design, requiring a high index material that is resistant to the detrimental effects of high temperatures and UV radiation, such as Ta ₂ O ₅ , provides for a coating that not only reflects nearly all the visible wavelengths of the electromagnetic spectrum produced by the bulbs of the high intensity lamps and transmits the IR heat producing wavelengths, but does so while resisting the detrimental effects of UV radiation and high temperatures and maintaining its electrical resistivity.

[0038] Table 1 illustrates one preferred embodiment of an optical coating design following the principles of the present invention, where the first part of the design, Type One Coating Stack, comprises two, or optionally several, stacks comprising repeatedly alternating layers of low refractive index material and high refractive index material, such as alternating layers of first dielectric material SiO ₂ with second dielectric material TiO ₂ , and the topmost stack or the last to be deposited, referred to herein as, Type Two Coating Stack, (which in Table 1 is shown at the bottom of the list of layers) is formed by the deposition of a repeated set of alternating layers of a high index material resistant to the affect of UV radiation and high temperatures, such as Ta ₂ O ₅ , with low index material SiO ₂ . As mentioned above, the surprising and unexpected result of this design is that the coating so produced is able to maintain its electrical resistivity resulting in a coating in which electrodes may be positioned without fear of shorting. In more particular, the coating described in Table 1 comprises a Type One Coating Stack having over forty layers, but could have a greater or fewer number of layers depending on the optical properties desired, made from a low index dielectric material, such as SiO _x , SiO ₂ , MgF ₂ , SiO, Si, Y ₂ O ₃ , AI ₂ O ₃ , BaF ₂ , CaF ₂ , CeF ₃ , Na ₃ AIF ₆ , NdF ₃ , YF ₃ , AIF ₃ , or blends thereof, repeatedly alternating with layers made from a high index dielectric material resistant to the deleterious effects of UV radiation and high temperature, such as TiO ₂ , Ti _x Oy, Nb ₂ O ₅ , Ta ₂ O ₅ , HfO ₂ , ZrO2, ZnS, WO ₂ , MO ₂ , In ₂ O ₃ or blends thereof, and where an additional topmost, or last to be deposited, Type Two Coating Stack comprises alternating layers of a low index dielectric material, such as SiO _x , SiO ₂ , MgF ₂ ,

SiO, Si, Y ₂ O ₃ , AI ₂ O ₃ , BaF ₂ , CaF ₂ , CeF ₃ , Na ₃ AIF ₆ , NdF ₃ , YF ₃ , AIF ₃ , or blends thereof and of a second dielectric material, such as Ta ₂ O ₅ Or Nb ₂ O ₅ , HfO ₂ , ZrO ₂ , WO ₂ , MO ₂ , In ₂ O ₃ or blends thereof, to produce a coating having an exterior that is not affected by UV or high temperatures.

Table 1 Reflector Multilayer Design

[0039] EXAMPLE 2: FIG. 4c and FIG. 4d illustrate a HID and a tungsten- halogen high intensity reflector lamp coated with an alternate coating of the present invention comprising multiple and identical Type Two Coating Stacks. As described above, Type Two Coating Stacks comprises repeated alternating layers of a low index dielectric material, such as SiO _x , SiO ₂ , MgF ₂ , SiO, Si, Y ₂ O ₃ , AI ₂ O ₃ , BaF ₂ , CaF ₂ , CeF ₃ , Na ₃ AIF ₆ , NdF ₃ , YF ₃ , AIF ₃ or blends thereof and of a second high refractive index dielectric material, such as Ta ₂ O ₅ or Nb ₂ O ₅ , HfO ₂ , ZrO ₂ , WO ₂ , Mo ₂ , In ₂ O ₃ or blends thereof, to produce a coating electrically stable and not affected by UV or high temperatures. In particular, FIG. 4c, a cross-sectional view, illustrates HID reflector lamp assembly 30 comprising generally parabolic, elliptical, or dome-shaped reflector housing 12, rearwardly extending neck section 12a, reflective inner surface 13 of reflector lamp assembly 30, multiple reflective optical thin film Type Two Coating Stacks 14 not affected by UV coated onto inner surface 13, surprisingly and unexpectedly, providing for required reflective mirror surface coating resistant to damage from UV radiation and/or high temperatures that that maintains an essentially useful level of resistivity or resistance, reflector lens 16b covering the front opening of reflector housing 12, arc-tube contacts 17 of bulb or burner 15a that is the source of the radiating light 16a, lug or bulb contact 18b for supplying current to the bulb through first electric lead 19a with second electric lead 19b completing the bulb's electrical circuit through lug or bulb contact 18a.

[0040] FIG. 4d, a cross-sectional view, illustrates a tungsten-halogen incandescent reflector lamp, which will benefit from the coating of the present invention, where tungsten-halogen lamp assembly 40 comprises generally parabolic, elliptical, or dome-shaped reflector housing 12, rearwardly extending neck section 12a, reflective inner surface 13 of reflector lamp assembly 40, multiple reflective optical thin film Type Two Coating Stacks 14 not affected by UV coated onto inner surface 13, surprisingly and unexpectedly, providing the required reflective mirror surface coatings resistant to damage from UV radiation and/or high temperatures and able to maintain an essentially

useful level of resistivity or resistance, reflector lens 16b covering the front opening of reflector housing 12, coiled tungsten filament 17b of bulb or burner 15b for generating radiating light 16a, first electric lead 19a electrically connected to coiled tungsten filament 17b electrically connected to second electric lead 19b to complete the electrical circuit.

[0041] FIG. 5, a graphical, cut-out, cross-sectional view, illustrates an example of this favored UV resistant coating design embodiment. In this, another favored coating embodiment, referred to as a Type Three Coating Stack coating design, the design consists exclusively of multiple and identical stacks of Type Two Coating Stacks. In particular, FIG. 5, for purposes of clarity, limits the illustration to two repeats of Type Two Coating Stacks where each stack 24 has a plurality of individual coating layers 22 where the individual layers comprise repeated alternating layers of low refractive index material L followed by high refractive index material H. According to the design, the first layer to be deposited directly on the inner-surface of reflector wall 32 comprises a low reflective index layer L. According to the principles of the present invention, one or more, additional stacks may be deposited over the two stacks illustrated in FIG. 5, to meet specific spectral response design goals.

[0042] Table 2 presents one example of a Type Three Coating Stack coating design comprising repeating alternating layers of low index dielectric material Siθ ₂ and of high index material Nb2θ ₅ that is resistant to the detrimental effects of UV radiation and high temperatures. Additionally, according to the invention, the stacks may be formed by deposition of any other low index dielectric material layer having an index of refraction or less than 1.8 and where the material has a melting temperature greater than 500° C, such as MgF ₂ , SiO, Si, Y ₂ O ₃ , AI ₂ O ₃ , BaF ₂ , CaF ₂ , CeF ₃ , Na ₃ AIF ₆ , NdF ₃ , YF _3, AIF ₃ or blends thereof, alternating with any other high index material that is not affected by UV radiation, having an index of refraction equal to or greater than 1.8 and where the material has a melting temperature greater than 500° C, such as Ta ₂ O ₅ , HfO ₂ , ZrO ₂ , WO ₂ , Mo ₂ , In ₂ O ₃ or blends thereof. Such coatings enhance the spectral properties of lamps, resist physical and chemical changes due to exposure to UV radiation and/or high temperature, and, even more importantly, unexpectedly, and surprisingly maintain their electrical resistivity at acceptable levels to enable the coating to maintain its

electrical resistance. To achieve these unexpected and surprising results, each of the dielectric layers is typically deposited to a thickness of one quarter of the wavelength corresponding to the center of the selected spectral band. The surprising and unexpected results of this design is that the coating is resistant to the effects of UV radiation, to the effects of the high temperatures produced by the lamp's bulb, and maintains its electrical resistivity, which results in a layer in which electrodes to accept the bulb's lead, or leads, may be positioned in the coating without fear of shorting. In this favored coating design over forty repeatedly alternating layers of high index ηπ coating material and low index T|L are deposited upon the substrate. This coating, ideal for deposition on the inner reflective surface of, for example, a high intensity lamp, selectively reflects or transmits predetermined portions of the spectrum, is resistant to the detrimental effects of UV and high temperatures, and maintains its electrical resistivity or resistance at levels that allow it to function as a insulator in these lamps.

Table 2 Coatin Multila er Desi n

[0043] THEORY: The optical coatings of interest herein are referred to as interference filters which are likely thin layers of materials such as silicon oxide and niobium oxide, or the many others mentioned in the lists of compounds above, which are deposited onto an optical substrate. By careful choice of the exact chemical composition, thickness, positioning of the layers relative to one another, their morphology, and the number of layers, it is possible to produce for the first time, coatings that are resistant to, and thus, not damaged by, the deleterious effects of UV radiation and high temperatures and, thus, that maintain nearly 100 percent of their electrical resistivity, in addition to tailoring both the reflectivity and transmissivity of the coatings to produce many desired optical behaviors.

[0044] Reflection coefficients of surfaces can be reduced to less than 0.2% with the proper selection of materials configured in a design specific for the reflector and spectrum from the bulb. Reflectivity can be increased to levels approaching 99.99% to produce a high-reflector (HR) coating. The level of reflectivity can also be tuned to any particular value, for instance to produce a mirror that reflects 90% or more of visible light and transmits nearly all the IR wavelengths as shown in GRAPHS 1 and 2. Alternately, the coating can be designed such that the reflecting surface reflects light only in a narrow band of wavelengths, producing an optical filter. The versatility of dielectric coatings leads to their use in many optical and photonic applications, as well as

consumer devices where the management of the spectrum is important and UV wavelengths are present.

[0045] It is understood that reflectance is at its maximum at the wavelength for which both the high index layers and the low index layers of the multilayer are exactly one-quarter-wave thick. Peak reflectance "f" is a function of the refractive-index ratio of the two materials used and the number of layers actually included in the stack. The peak reflectance can be increased by adding more layers, or by using materials with a higher refractive index ratio. Amplitude reflectivity at a single interface is given by:

where

where n _s is the index of the substrate,

HH is the index of the high index layer, r?ι_ is the index of the low index layers, and

N is the total number of layers in the stack.

The width of the high-reflectance part of the curve is also determined by the film index ratio. The higher the ratio is, the wider the high-reflectance region will be.

[0046] Alternatively, the transmission properties of a coating are dependent upon the wavelength of light being generated, the index of refraction of the substrate, which in the example used herein is the reflective inner surface of the lamp, the index of refraction of the coating layer or layers, the thickness of the coating, the angle of the incident light or shape of the interface surface, and the adsorption of the coating and the substrate.

[0047] As illustrated in Graph 1 and Graph 2, the calculations predict that coatings deposited according to the principles of the present invention will reflect nearly, if not all, wavelengths in the visible region and transmit most of the IR wavelength radiation and experiment verifies the prediction.

Graph 1 : Reflectance vs. Wavelength of Coating Made Following Example 1 Coating Design

Reflectance vs. Wavelength

- Design ^■ Experiment

350 450 550 650 750 850 950 1050 Wavelength (nm)

Graph 2: Reflectance vs. Wavelength of Coating Made Following Example 2 Coating Design

[0048] EXPERIMENT: FIG. 6 presents a diagrammatic representation of the experimental procedure used to test the efficacy of the coatings made according to the

principles of the present invention. The experimental set up included lamp 60, lamp shutter 62, UV sensor 65, reflector 64, a resistance meter, and a computer having a display screen to display the results. The first step of the experimental procedure was to insert multiple test probes into various locations of a lamps coated reflector inner surface to measure the resistance of the surface. After the resistance of the coated surface was measured under ambient conditions, the coated reflected surface was subjected to high intensity UV light consisting of ultraviolet A (315 to 400 nm), ultraviolet B (280 to 315 nm), and ultraviolet C (100 to 280 nm), and IR radiant heat. Resistance measurement continued to be made from the first measure at ambient conditions until changes in resistance became asymptotic. Measurements were electronically recorded continuously and displayed on a log plot to record the effects of UV radiation and heat on the electrical resistance of the coating. The results, as illustrated in GRAPHS 3, 4, and 5 indicate that even after exposure to the detrimental effects of UV radiation and elevated temperatures the coatings of the present invention retain sufficient resistance to provide stable electrical properties that is, the coatings maintain nearly all of their resistivity.

[0049] One curve in GRAPH 3 illustrates a significant permanent drop in electrical resistance of a coating that was subjected to UV radiation and the heat produced from IR radiation. The coating comprising only alternating layers of high refractive index TiO2 and low refractive index SiO ₂ was coated onto the interior surface of a reflector and measured according to the experimental procedure described above. As is illustrated on GRAPH 3, in about two seconds after the UV was turned on, the drop in electrical resistance was nearly three orders of magnitude, from a starting value of about 2 x 10 ⁷ Ohms to 3 x 10 ⁴ Ohms. A permanent electrical resistivity loss of this magnitude would likely contribute to an electrical short if two high intensity lamp bulb electrodes were in contact with the coating. Thus, this presently available and widely used coating is not best suited as a reflective optical coating for today's smaller high intensity lamps. The second, relatively flat, curve shown in GRAPH 3, also comprising layers of high refractive index TiO ₂ and low refractive index SiO ₂ , but of a slightly different formulation, exhibited unacceptable levels of resistivity before exposure to UV

radiation and high temperatures and a further drop in resistivity after exposure to UV radiation and high temperatures.

GRAPH 3 illustrates the electrical resistance of coatings of alternating layers of high reflective index TiO ₂ and low reflective index SiO ₂ .

[0050] A coating made following the design given in Table 1, where the first part of the design comprises two stacks of repeatedly alternating layers of low refractive index material SiO ₂ and high refractive index material TiO ₂ , followed by a topmost, or last to be deposited, stack formed of layers of a high index material resistant to the effects of UV radiation and high temperatures, which in this case is Ta ₂ θ ₅ alternating with low index material SiO ₂ , was subjected to UV radiation and heat produced from IR radiation according to the experimental procedure described above. As mentioned above, the novel feature of this design is that the coating so produced and treated maintains essentially all of its electrical resistivity, as illustrated in GRAPH 4. Essentially all of its electrical resistively means that the level of resistively required for the coating to act as an insulator is retained, resulting in a coating in which electrodes may be positioned in contact with the coating without risk of electrical shorting. The level of electrical resistance that the coating is able to maintain even after exposure to

the damaging effects of UV radiation and/or high temperatures ensures protec tion against an electrical short if two high intensity lamp bulb electrodes were in contact with the coating. Thus, this coating would likely be an excellent candidate for use as a reflective optical coating for today's smaller high intensity lamps.

Time in Seconds

GRAPH 4 illustrates the very small drop in electrical resistance when a coating made following the design illustrated in Table 1 was subjected to UV radiation and the heat produced from IR.

[0051] A coating made according to a second coating design of the present invention, comprising repeating alternating layers of low refractive index dielectric material SiO ₂ and of high refractive index material Nb ₂ O ₅ that is resistant to the detrimental effects of UV radiation and high temperatures, as illustrated in Table 2, was subjected to UV radiation and the heat produced from IR radiation according to the experimental procedure described above. GRAPH 5 illustrates that this coating not only inherently has an excellent level of resistivity but surprisingly and unexpectedly this appears to be able to maintain substantially all of its electrical resistivity when exposed to UV radiation and high temperatures, resulting in a coating that will function as an insulator with little if any changes in properties and in which electrodes may be positioned in contact with the coating without risk of electrical shorting. Thus, this

coating would also likely be an excellent candidate for use as a reflective optical coating for today's smaller high intensity lamps.

GRAPH 5 illustrates no substantial change in electrical resistance when a coating made according to the design of Table 2 was subjected to UV radiation and the heat produced from IR radiation.

[0052] A companion experiment, designed to look at the effects of high temperatures alone on the stability of the coatings, deposited a SiO ₂ /Nb ₂ θ ₅ coating made according to the design given in Table 2 onto a reflecting inner surface of a glass substrate and then heated the lamp in an oven to 1000 ⁰ C After cooling, the coating clearly suffered no physical damage, but the glass showed clears signs of softening and deforming, and in some experiments fractures appeared as the result of thermal induced stresses. It should be noted that the coating remained bonded to the distorted substrate and showed no evidence of separation from the substrate. This kind of durability has not been evident in reflectors produced with traditional materials. [0053] CONCLUSION: In conclusion, experimental results show that optical coatings manufactured according to the design principles of the present invention can manage electromagnetic radiation, as desired. That is, the coatings of the present

invention are designed to reflect and/or transmit desired portions ot the electromagnetic spectrum while providing unexpected and surprising resistance to the detrimental effects of UV radiation, in addition to providing overall greater durability and better retention of reflectivity over the life of the lamp, and even more surprising and unexpected providing for the coatings to maintain their electrical resistivity, thus making the coatings more applicable for use in smaller high intensity lamps, which also operate at higher temperatures. It should be understood that it is within the scope of the invention to apply the coating, if desired, to the inside of a reflector, to the outside, or to both. In particular, it has been shown that coatings have been made that transmit nearly all IR wavelength radiation (above 800 K) and to reflect nearly all visible light (400 - 700 λ), while exhibiting resistance to UV radiation and thermal degradation. Coating design, as taught herein, incorporates determining coating layer thickness, number of layers, the morphology of the layers, the arrangement of the layers relative to each other, and importantly, the chemistry of the layers to achieve thin film coatings with desired optical properties.

[0054] The foregoing description, for purposes of explanation, uses specific and defined nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. Thus, the foregoing description of the specific embodiment is presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Those skilled in the art will recognize that many changes may be made to the features, embodiments, and methods of making the embodiments of the invention described herein without departing from the spirit and scope of the invention. Furthermore, the present invention is not limited to the described methods, embodiments, features or combinations of features but include all the variation, methods, modifications, and combinations of features within the scope of the appended claims. The invention is limited only by the claims.