Transparent heat mirror coatings of three discrete films of titanium dioxide-silver-titanium dioxide

(Ti0 ₂ /A /Ti0 ₂ ) have been proposed to be used in a unique manner on the wall of the lamp glass envelope of an incan¬ descent lamp for the purpose of transmitting the visible light produced by the incandescent filament and reflecting the infrared energy produced by such filament. The envelope is shaped so as to reflect infrared (IR) radia- tion back to the filament to thereby raise its tempera¬ ture. The end result of this is that the filament is at least partially heated by the reflected IR energy thereby resulting in a reduction in the amount of electrical energy needed to heat the filament. This results in an energy-saving lamp.

A Ti0 ₂ /Ag/Ti0 ₂ coating as applied to an incandescent lamp has several novel features and advan¬ tages. First of all, silver is unique among the common non-alkaline metals in having the lowest absorption to visible light and infrared radiation. A thin film of silver can be considered an almost lossless material to visible light. The use of the two layers of dielectric material, e.g., TiO _? , improves the visible transmission. Theoretically, a three-film insulator-silver-insulator (ISI) coating, can be designed to maximize the transmis¬ sion of visible energy wavelength at the peak of the luminous output of an incandescent lamp (about 585 nm for a 3,000° K tungsten filament). This requirement uniquely determines the optimum thickness of the silver and insu- lator films for any given insulator material.

One of the disadvantages of the Ti0 ₂ /Ag/Ti0 ₂ combination is that the reflectivity of the energy in the near infrared region is not as high as optimally desired for certain applications, for example, in an incandescent electric lamp. In addition, Ti0 ₂ is a fairly refractory substance and where it is deposited on a substrate by sputtering, it sputters slowly in film preparation (the

sputtering rate of Ti0 ₂ is only about 3-5% of that of

Ag) . The latter disadvantage makes the time of production relatively lengthy and, consequently, results in increased cost. Accordingly, it would be desirable to have a coating which uses silver with another less refractory insulator material and which will produce a higher IR reflectivity than the Ti0 ₂ /Ag/Ti0 ₂ coating while approaching the ideal of one hundred percen transmittance to visible energy, neglecting the absorption of the metal itself.

The foregoing advantages are accomplished in accordance with the subject invention by providing ^' an incandescent lamp utilizing a multi-film coating designed following some of the teachings of the etalon principle. An etalon device utilizes a layer of insulating material, for example air, between two metal reflective layers, for example, silver.

The present invention applies an etalon type device to an incandescent lamp in that a composite metal- insulator-metal coating of thin films is formed on the wall of an incandescent lamp envelope. The thickness of the indiyidual films of the coating and their inter¬ relationship are selected so as to maximize the coating transmission characteristics to energy produced by the filament in the visible range and also to maximize the reflecting properties to energy in the infrared range.

Fig. 1 is a side elevational view in section showing a heat-transparent mirror utilizing an etalon film in accordance with the present invention;

Fig. 2 is a diagram illustrating the general response of an etalon film;

Fig. 3 is a schematic diagram showing the response characte istic of a preferred etalon film utiliz- able with an electric lamp and the overlay of the response curves of the eye and a typical incandescent filament; Fig. 4 is a view of an electric lamp made in accordance with the invention;

Fig. 4A is an enlarged f agmentary ^* view of a portion of the lamp envelope and the coating; and

Fig. 5 is a diagram of an electric lamp made in accordance with the invention utilizing an additional coating for protection effect.

Fig. 1 shows a fragment of a substrate 10, for example, of lime glass or Pyrex, on which the etalon coat¬ ing is deposited. The etalon coating has three discrete film components. The first of these is a film layer 11 of a reflecting metal, such as silver, which is deposited on one surface of the substrate 10, a film layer of an insulating material 12 (to be discussed in detail below) which is deposited on the metal film layer 11, and an outer film layer 13 of a reflecting metal, which also can be silver, which is deposited on the insulator. Any conventional and suitable techniques can be used for depositing the three layers, some of these being, for example, chemical deposition, vapor deposition,, sputtering, etc. The three film layers are preferably made separate and discrete from each other. That is, it is preferred that there be no inter-diffusion of the layer materials. However, the film layers cooperate to produce the desired transmission and reflection characteristics and they are designed as a composite. Incident radiation R, assumed to have components in the visible .spectrum as well as a component in the infrared spectrum, is shown as impinging upon the layer 13 most remote from substrate 10. In accordance with the invention, the etalon coating is designed to have charac- teristics such that it will transmit a maximum amount of energy in the visible wavelength range and will reflect a maximum amount of energy in the longer wavelength range, including the IR band.

Fig. 2 shows a typical response curve for an etalon coating. The ordinate shows the transmission characteristic of the coating to incident radiation and the abscissa shows the wavelength. Characteristically, there are a number of energy transmission passbands at

different wavelengths, these wavelengths ^' being integrally multiply related. The etalon coating has a last transmis¬ sion passband at the longest wavelength, this, shown as the third from the right. The number of transmission pass- bands depends upon the coating design. In the present invention, the coating is designed to use the last passband to transmit visible energy and to block, or reflect, IR energy. Also, the etalon is designed so that the last transmission peak of Fig. 2 (located at largest wavelength) falls at the peak of the luminous output of the filament used for the lamp.

In the design of the etalon-type coatings, the nature of the insulating film layer, that is, its index of refraction and thickness, controls the width and shape of the passband characteristic and, in conjunction with the metal layers, the slope of the passband cutoff, that is, the sharpness at which the mirror makes the transition from transparent to reflective at the desired wavelength. The metal layers provide the IR energy reflectance. An optimum design, insofar as an incandescent electric lamp is concerned, has a high transmission in the visible range with little absorption and a high reflectance in the IR range. The design of the optimum filter balances several considerations. A small amount of visible absorption re- quires a thin metal film while high IR reflection requires a thick metal film. In addition, the location of the transmission peak in the visible range and a rapid rise in reflection as the IR is approached demands that the di¬ electric film 12, in thickness and index, must be properly designed in conjunction with the metal films 11,13.

It should be understood that conventional quarter wave theory can not be used in the design of an etalon operating as disclosed in the subject invention. Conventional quarter wave theory considers phase changes induced by the metallic film as those due to a very thick film. For example, the phase change upon reflection from one metal layer of the etalon is taken in conventional quarter wave theory as -180°. In the thin metal films

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used in this invention, reflection and transmission phase changes depart completely from conventional quarter wave practice and design according to that practice give composite filters which are far inferior to the filters of this invention. The rapid rise in IR reflectivity dis¬ played by the filters employed here cannot be predicted by conventional quarter wave theory. In addition, conven¬ tional quarter wave theory demands a thickness of the dielectric layer which can, when employed in practice, place the peak in light transmission far away from the portion of the visible wavelength region desired.

Fig. 3 shows a graphical representation of cer¬ tain criteria for the design of an efficient etalon-type filter for an incandescent lamp. Curve 30 shows the response of the eye, generally from about 400-700 nm, with a peak at 550 nm, in the yellow color range. Curve 32 represents the output of an incandescent tungsten fila¬ ment. As the tungsten filament is heated to a higher temperature, the curve 32 would shift upward and further to the left. At all practical operating temperatures the tungsten filament produces a preponderant amount of IR energy.

Dotted line curve 50 represents an idealized transmission bandpass curve for the transparent heat mirror as applied to an incandescent lamp. That is, an idealized coating will transmit all visible energy from 400 to 700 nm and reflect all other energy, particularly all IR energy above 700 nm. Such a curve as 50 with a vertical cutoff line is not practically realizable. The dash-dot line curve 34 represents the desirable bandpass transmission characteristic of a coating, which is obtain¬ able with an etalon type coating. As explained previously, the thickness of the metal layers controls the width of curve 34 and the thickness of the dielectric film, in cooperation with the metal films, the wavelength of its peak. On curve 34, A_ _Q represents the wavelength at which maximum possible energy is transmitted and the two points designated/^ on each side of the curve each

represent the point at which one-half of the peak of the visible energy is transmitted.

Fig. 4 shows an incandescent lamp in accordance with the invention, a fragment of the lamp being shown enlarged in Fig. 4A to show the details of the transparent mirror coating. The lamp includes a glass envelope 40 of a conventional PYREX, lime glass, or other suitable refractory material. A metal base 42 is provided at the bottom of the envelope for sealing it. The base also has a lower button contact 44 to provide, with base 42, the electrical current to a filament 46 mounted on electri cally conductive leads 47,48. The leads extend through a stem 49 of insulating material, such as glass, which contains an exhaust tubulation 52 through which the envelope is exhausted. The leads 47,48 are connected to the base and contact ^* button.

Upon application of current to filament 46, it incandesces and produces radiant energy in both the visible and IR range, as shown in Fig. 3. Typical oper- ating temperatures for the filament are in the range from about 2300°K to about 3300°K. As the filament temperature decreases, the peak (s) o ) of the light output decreases in wavelength (becomes more reddish) and as the filament operating temperature increases, the wavelength of the light output increases (becomes more greenish).

A transparent heat mirror coating, generally designated as 56, is located on the inner wall of envelope 40. The purpose of coating 56 is to pass as much-.of the visible light as possible from filament 46 through envel- ope 40 and to reflect as much of the filament's IR energy as possible back toward and onto the filament. An ideal transparent heat mirror for an incandescent lamp would pass all energy within the visible range of the eye and reflect all IR energy. As explained below, the etalon is designed to approach or be at maximum transmission to visible energy at the wavelenth A)c of maximum lumen (visible output of the filament). This peak shifts by a relatively small amount over a fairly wide operating

filament temperature range, for example, from about

550-590 nm over a temperature range of from about 3300°K] down to 2300°K. This range overlaps the wavelength of maximum eye sensitivity. Filament 46 and envelope 40 are preferably designed to be optically related so that the maximum amount of IR energy produced by the filament 46 and incident on the inner wall of the envelope is reflected back toward the filament. One way of doing this is to 0 select the interrelationship of the shape and design of both the filament and envelope. These relationships are explained in greater detail in the aforesaid application Serial No. 781,355. However, a reduced energy consumption benefit can be obtained even if the filament is not 5 optically centered.

The coating 56 is preferably placed on the inner wall of- envelope 40 by any suitable and conventional technique, e.g., chemical or vapor deposition, RF sput¬ tering, etc. Coating 56 also can be placed on the outer ⁰ wall of the envelope, although this is less preferable since it would be more subject, to abuse and damage.

Coating 56 is formed by the outer and inner metal film 11,13 which sandwiches the dielectric film 12. In the preferred embodiment, the metal films 11,13 are of

²⁵ silver since this metal, of the more common ones, has the highest transmission to visible light. Other metals (e.g. gold,, copper, aluminum), alloys, and indeed, two different metals, may be used as the metal layers of the sandwich. The insulator, or dielectric, film 12, can be any suitable ® insulating material which is compatible with silver.

Preferred insulators are silver chloride (AgCl) and magnesium fluoride (MgF ₂ ). Silver chloride has the advantage of being highly compatible with a silver metal film. The coating system is hereafter referred to

-" as S-I-S (silver-insulator-silver) to distinguish it from I-S-I systems of the Ti0 ₂ /Ag/Ti0 ₂ type.

The thickness of insulator 12 is chosen so that the phase angle due to two way travel in the insulator

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between the silver film 11,13 plus two reflections off the silver films, is 0 ^β at the visible wavelength chosen for maximum transmission. In the visible region the phase angle due to reflection off the silver does not commonly follow quarter wave practice. The relative silver film thicknesses are chosen to give the same individual reflec¬ tivities. With this arrangement, constructive interfer¬ ence occurs and the overall transmission of the combina¬ tion is 100% at the chosen visible wavelength as with the I-S-I combination, neglecting absorption in the silver film.

To find the thickness of the silver and dielec¬ tric films, the following steps are taken:

(1) First, the peak transmission is located at a chosen visible wavelength,λo, where the overall phase angle,δ, is set to zero. The wavelength λo is generally selected as the peak of the luminous output of the fila¬ ment. For example, a filament operating at about 3000°K has a peak at about 585 nm. Here,

^δ " ^Q i2,GLASS ^Qi a,AIR ^{+ 2Q}

λ A = insulator thickness. n = insulator index of refraction at λ . •-_>ii = reflectivity phase change from insulator i,j film 12 to either metal film 11 on glass side or metal film 13 on air side. This quantity is determined from the equation:

Hi ² -[2η. ( + — ^~ — ) coshα.. sinhα• • ] tanQi = - ^■ —12 ±2

[( _H i ² - . _H i ² ) -(κ ² -η _i ² nj2) sinh ² α _i j]

K'

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A ., wipo

i •= 12, j = glass or air.

N. = dielectric index of dielectric 12. ND. = dielectric index of either glass or air. K = imaginary part of complex refractive index of silver. Real part is taken as zero.

A _i; . K2π

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&■ - - silver film thickness between dielectric 12 and either glass or air.

(2) Second, the wavelength at which the trans¬ mission falls to one-half its peak value is chosen at 0.5 (see Fig. 3) by setting

Here assume RM _M ≤ R,.2. , g_l,a ₌ s _c? s _o ^* ~=■R1, _2 _> , A,i.r where: _ 2 cosh"? (X.J + (_K+ ^η i ^η - ^J i) ^~ sinh 2cu .

(n- _L +ri ) cosh α^ _j + (K- i j) sinh α^j

K

The quantityδ _{0 5} along with the e~ . ■ and the indices η . , η . and K are evaluated as before, but at the wavelength λo. ₅ . ^' The λ _{0 5} dealt with is the higher one, since it is the action of the etalon coating in the wave¬ length region from visible to IR which is of particular interest. In an incandescent lamp according to the inven¬ tion operating on the last etalon peak,λ ₀ _ ₅ is typically

set at about 800 nm. It is at the wavelength the the eye rapidly begins to lose visual response to the energy and the IR energy starts to become effective. By designing the high wavelength skirt wall of the bandpass trans- mission properly, the IR reflectivity can approach the range of about 90% or better at 1000 nm. where the IR energy is effective, and continue to increase with in¬ creasing wavelength.

These prescriptions determine the three variable quantities of the etalon film: two silver film thick¬ nesses and the dielectric layer thickness, once them's and K are known as functions of wavelength. The method can be applied to any suitable metals, metal alloys or combinations of metals or metal alloys. The reflectivity in the near IR increases rapidly with wavelength by a mechanism outside conven¬ tional quarter wave theory. At longer wavelengths of the infrared the phase difference in the insulator decreases toward zero, while the phase shift on each reflection decreases toward -180°, the conventional value taken in the quarter wave practice. At some near IR wavelength the overall phase angle decreases from 0° to -180° (on its way to -360° at very large wavelength) and destructive etalon interference occurs giving an overall reflectivity of 2 4R-./(l+R _M ) . This overall reflectivity is very close to unity for R >0.5, where R _M is the IR reflec¬ tivity of one silver film. Thus, a particular silver- insulator-silver combination can be designed to give a high IR reflectivity, say at a wavelength of 1 micron. As wavelength increases further, the reflectivity in¬ creases uniformly toward unity.

Films of many common insulators will produce higher IR reflectivities in the S-I-S coating than does i0 ₂ in an I-S-I, Ti0 ₂ /Ag/Ti0 ₂ coating for the same overall thickness for the silver films, i.e., one film for I-S-I and two for S-I-S. Materials having lower indices of refraction, and less refractory materials are favored i the S-I-S coating, so that rapid sputtering, evaporation,

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vapor or chemical deposition may be possible for deposit¬ ing the insulator film. In addition to the freedom in choosing an insulator, the overall thickness of the silver film can also be varied to produce a wider design range for the bandpass characteristics in the S-I-S combination than in any I-S-I combination since the I-S-I combination does not allow freedom in choosing prescription (3).

Table I below compares calculated values of transmission and reflection for two S-I-S coatings with a Ti0 ₂ /Ag/Ti0 ₂ coating, all coatings designed for 100% nominal visible transmission at 600 nm.

In Table I the light absorption has been neglec¬ ted. In a good silver film it will probably be less than 5% in the visible and less than 1-2% over most of the IR range. For comparison purposes, the overall silver thickness in the S-I-S coatings has been selected to be the same as in the coatings of prior application Serial No. 781,355 so that the absorption of visible light in both coatings should be essentially identical. Thicker or thinner silver films can be used in S-I-S coatings. In Ti0 ₂ /Ag/Ti0 ₂ coatings, the silver thickness is not readily adjustable.

TABLE I Ex.: Ti0 ₂ /Ag/Ti0 ₂ Ex: Ag/MgF ₂ /Ag

325A - 240A - 325A 120A - 1770A - - 120A designed for transmission to visible

(T) = 100% at 600 nm

IR Reflectivity

SYSTEM 600 nm 1,100 nm 1,700 nm

ISI: Ti0 ₂ /Ag/Ti0 ₂ 0% 84% 94.5%

SIS: Ag/AgCl/Ag 0% 89% 95%

SIS: Ag/MgF ₂ /Ag 0% 93- 98%

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In all cases, the total Ag thickness is chosen so as to yield a similar absorption (5%, at 600 nm, for a good silver film). Further, in Table I, the ideal reflec¬ tivity is calculated at 3 wavelengths and absorption is neglected.

Variations of the above-described S-I-S coating are possible over the visible band. In typical coatings, the silver film can be in the range of from about 105A to about 135A and the magnesium flouride film in the range o from about 1605A to about 1935A.

Other readily available insulators which can be used with silver are aluminum oxide, titanium dioxide and chromium oxide. A typical coating of Ag/Ti0 _? __./Ag would have the two silver films each in the range of from about o o 105A to about 135A and the TiO-, dielectric film in the ^Δ range of from about 600A to about 830A with a preferred o coating having silver films of about 120A and the dielec- o trie film of about 720A.

For satisfactory S-I-S etalon coatings for an incandescent lamp, the etalon should be designed for maxi mum transmission at the peak of the luminous output of th filament. For conventional tungsten filaments operating in a conventional temperature range (e.g. 2300°K to 3300°K), the filament peak luminous output will be close to the point of maximum eye sensitivity. The ^ ₎ . point is selected to pass the useful visible energy and to bloc the IR energy.

Fig. 5 shows a further embodiment of the inven¬ tion wherein the coating on the interior of the lamp envelope has been overcoated with a film layer 70 on the face opposite the filament to protect the coating. The coating 70 should be compatible with the metal, here silver, and also protect the coating against deleterious materials, such as tungsten evaporated from the filament. Suitable coating materials are, for example any of the dielectric materials previously mentioned as well as others, the principal criterion being that their absorp¬ tion to light and near infrared be negligible.

Where a protective film such as 70 is used, in calculating the thickness of the S-I-S films, the protec¬ tive film must be accounted for in designing the S-I-S coating. As explained previously, the S-I-S film can be coated on the exterior of the envelope. Calculations can be made for the thicknesses of the film and the index of the dielectric in the manner described above, using appropriate optical coefficients. Comparing the arrange- ment with the coating on the exterior of the bulb previ¬ ously described, the thicknesses of the films without overcoat would be the same as when coated on the inside of the bulb. For exterior S-I-S films, overcoats of a pro¬ tective layer can include, for example, plastic and organic materials which are transmissive to visible light.