WO2008078241A1

WO2008078241A1 - White light emitting electric lamp assembly

Info

Publication number: WO2008078241A1
Application number: PCT/IB2007/055125
Authority: WO
Inventors: Ward Cottaar; Georg Henninger; Joseph Fey
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2006-12-20
Filing date: 2007-12-14
Publication date: 2008-07-03

Abstract

The present invention relates to an electric lamp assembly comprising an electric lamp, said electric lamp comprising a lamp vessel (10) comprising a lamp envelope (20), a light-emitting element (12) located within said lamp envelope and a color temperature increasing optical interference filter covering at least a portion of a surface of said lamp envelope, said coating being comprised of a plurality of alternating high and low refractive index layers, said coating having a spectrally broad high reflectance of at least 80 % average in the IR wavelength range between 800 and 1300 nm and of at least 80% average in the UV wavelength range between 300 and 400 nm, a steep decrease of reflectance between 380 nm and 400 nm to a reflectance of 0 to 10 % at a wavelength of 400 nm and a continuously steeper increase in reflectance between 400nm and 800 nm, the reflectance being defined by R 300 -400> 80% average at 300-400; 0 % > R400-500 > 10% at 400-500; 10 % > R500-600 > 20% at 500-600; 20 % > R600-700 > 40% at 600-700; 40 % > R700-800 > 80% at 700-800; R800-1300 > 80% average at 800-1300 and R1300-1800 > 60 % average at 1300-1800. According to the gist of the invention an optical interference filter of the broadband pass type is applied, which causes gradual reflection rather than absorbance of undesired amber to red parts of visible light back to the light-emitting element, thus greatly enhancing the efficiency of the lamp and providing a desirable color temperature. 5 Such an electric lamp assembly is suitable as a lamp having a natural white lighting color for general illumination purposes.

Description

White light emitting electric lamp assembly

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an electric lamp assembly for general illumination purposes, such as illumination of homes, offices, shops, etc., where white light illumination is needed.

BACKGROUND OF THE INVENTION

An electric lamp useful for the invention is in particular an incandescent lamp. Incandescent lamps generate light by means of thermal emission, generally by means of an incandescent element, such as a metal filament. Incandescent halogen lamps are conventionally used for general illumination of commodities as well as for other applications that require a high degree of color rendering.

Lamps for general illumination are designed to produce "white" light, i.e., their light emissions have a color spectrum or mix of colors that appear "white." White light in general is a collection of electromagnetic waves of different frequencies, each wavelength of which represents a particular "color" of the visible light spectrum. Visible light is generally thought to comprise those light waves with wavelength between about 400 and about 700 nm. These wavelengths combine additively to produce a resulting wave spectrum, which makes up a specific colored or white light.

In incandescent lamps, the filament is heated to a temperature of about 2800° K. in order to produce white light. Such an incandescent lamp gives out a continuous color spectrum, which blends together to give white light.

Because of the importance of white light, and since white light is the mixing of multiple wavelengths of light, there have arisen multiple methods for characterization of white light that relate to how human beings interpret a particular white light. The internationally accepted methods for standardizing and measuring the characteristics of light sources are set forth in the publications of the CIE (Commission International de l'Eclairage). According to said methods the total of the light from a light source is defined by a color point with corresponding color coordinates x and y on the 1931 CIE chromaticity diagram and a correlated color temperature (CCT). The spectral distribution of light is defined as a color rendering capability, measured by the color rendering index (CRI).

The white color in itself is defined as a small portion of the color space in the CIE chromaticity diagram. In FIG. 2 the x, y-chromaticity diagram of the CIE system is shown. As a general rule, any color, which falls within the area enclosed by the dashed line, will have a "white" appearance to the eye. The black body locus white line or Planckian locus gives the temperatures of whites from about 700 K, generally considered the first wavelength range visible to the human eye, to essentially the terminal point.

In Fig. 2 the various shades of white light are indicated by letters, which define the shades of white-emitting lamps. The shades are identified as follows: A Warm White; B Deluxe Warm White; C White; D Cool White; E Deluxe Cool White; F Soft White; G Bluish White and H Daylight.

The "correlated color temperature" CCT relates to the shade of the light within the white range. It is defined as the temperature of a blackbody source having the closest color match to the light source in question. The color match is typically represented and compared in a conventional CIE chromaticity diagram.

Generally, as the color temperature increases, the light becomes more blue. As the color temperature decreases, the light appears more red. A color image viewed at 2,000 K will have a relatively reddish tone, whereas the same color image viewed at 10,000 K will have a relatively bluish tone. All of this light is called "white" but it has varying spectral content.

The second classification of white light involves its quality measured as color rendering index CRI, which measures the color properties of a source in relation to the corresponding color temperature of a black body radiator or natural daylight. Generally the number 100 represents the reference illuminant (black body or daylight), so the closer the CRI to 100, the more accurate a match to the reference illuminant the light source has.

If a light yields a high CRI compared to full spectrum white light then it is considered to generate better-quality white light, light that is more "natural" and enables colored surfaces to be better rendered. Light sources having a relatively continuous output spectrum, such as incandescent lamps, typically have a high CRI, e.g. equal to or near 100. Light sources having a multi-line output spectrum, such as high pressure discharge lamps, typically have a CRI ranging from about 50 to 80. Fluorescent lamps typically have a CRI greater than about 60. Also of importance as a light characteristic is the device luminous efficacy, which is defined as the luminous flux output by the lamp in the unit lumens (Im), divided by the electric power input to the lamp in watts (W).

The quality of viewing light in general affects the way in which an observer perceives a color image. An observer will perceive the same color image differently when viewed under lights having different correlated color temperatures and CRIs.

For example, a color image which looks normal when viewed in early morning daylight will look bluish and washed out when viewed under overcast midday skies. Further, a white light with a poor CRI may cause colored services to appear distorted.

The preference for a particular color temperature by the consumer depends on a variety of psychological and evolutionary factors. People in northern latitudes favor warmer color temperatures, but tend towards the "cool white" for the work environment. Thus, in addition to human predisposition, color temperatures are kept different depending on the ambiance or mood of the living environment.

It is well known to those skilled in the art that the color temperature of a lamp is fixed at the time of manufacturing and the spectral distribution of an incandescent lamp is different from e.g. fluorescent lamps.

In principle, incandescent lamps have relatively good color rendering owing to their continuous emission spectrum, but are subject to limitations as regards the color temperature, i.e. specifically as regards the "warmth" or, in contrast, the "whiteness" of the light. Particularly incandescent lamps are known to generate an excessive amount of light in the yellow region of the spectrum, leading to "washing out" of the colors of objects illuminated by them.

But, while the adjustment of the color temperature chromaticity is no problem in discharge lamps equipped with trichromatic (RGB) phosphor mixtures, such easy measures are not available for incandescent lamps.

Unfortunately an increase in the color temperature in incandescent lamps in favor of a whiter light or an increase in the luminous efficiency by means of the electrical design results in reductions in life owing to the higher incandescent filament temperatures, which are necessarily associated therewith.

Or putting it the other way round: means for extension in lifetime will result in lower incandescent filament temperatures, thus shifting the chromaticity of the lamp into the red to yellow range.

Further, altering the color temperature or spectrum will usually alter other lighting variables in an undesirable way. For example, increasing the voltage applied to an incandescent light may raise the color temperature of the resulting light, but also results in an overall increase in brightness.

In response to this, methods have been developed to achieve an improved color characteristics by filtering the visible light emitted from the lamp through an absorbing -type color filter, which transmits only the desires components of the visible spectrum.

The known measures to adjust the color point of electric lamps therefore rely on color filters comprising pigments that absorb in the amber to red wavelength ranges and thus enhance the blue ranges and adjust the overall color point but are in conflict with the luminous output of the lamp. Placing an absorbing filter in front of a white halogen lamp will dramatically decrease the overall brightness of the light. Moreover, such an absorbing filter wastes the energy emitted from the lamp as it dissipates it as heat in the filter itself.

A second aspect with regard to recent developments in lamp technology besides the aspect of improved chromaticity is improved efficiency. Electric lamps normally generate substantial amounts of emission in the near ultraviolet and near infrared, respectively. This ultraviolet emission and infrared emission represents wasted energy, since it is invisible, which substantially decreases the luminous efficiency for such lamps for converting electrical energy to visible radiation.

Current lighting technology has looked out for ways of making adjustments to known lamps types.

It is known to improve the illumination efficiency or efficacy of electric, incandescent and arc lamps by reflecting infrared radiation emitted by a filament or arc back to the filament or arc while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament or arc. This lowers the amount of electrical energy required to be supplied to the filament or arc to maintain its operating temperature.

US 2005/0127836 discloses an incandescent lamp comprising an infrared- reflective filter and in addition a color filter, comprising pigments, that markedly reduce the yellow component in the light emitted by the lamp. The spectral filtering causes an improved circadian efficiency in the blue spectrum of the emitted light.

US 2005/0140292 touches the subject of an optical compensating filter comprising an interference filter for use with discharge lamps, wherein the light generated by the discharge is discolored by a condensate, but fails to provide any technical solution to the problem.

There remains a need for an improved incandescent lamp to provide white light of higher color temperature without wasting energy.

SUMMARY OF THE INVENTION

In accordance with this invention an improved energy efficient electric lamp assembly is provided. The electric lamp assembly comprises an electric lamp, said electric lamp comprising a lamp vessel comprising a lamp envelope, a light-emitting element located within said lamp envelope and a color temperature increasing optical interference filter coating covering at least a portion of a surface of said lamp envelope, said coating being comprised of a plurality of alternating high and low refractive index layers, said coating having a spectrally broad high reflectance of at least 80 % average in the IR wavelength range Δ λ between 800 and 1300 nm and of at least 80% average in the UV wavelength range Δ λ between 300 and 400 nm, a steep decrease of reflectance between 380 nm and 400 nm to a reflectance of 0 to 10 % at a wavelength of 400 nm and a continuously steeper increase in reflectance between 400nm and 800 nm, the reflectance being defined by R₃oo -4oo> 80% average at Δ λ_3Oo-4oo; 0 % > R400-500 > 10% at Δ λ 400-500; 10 % > R₅oo-6oo > 20% at Δ λ _5Oo-6oo; 20 % > R₆₀₀-TOo > 40% at Δ λ_6OO-7oo; 40 % > R₇oo-8oo > 80% at Δ λ_7OO-8oo; R₈oo-i3oo > 80% average at Δ λ _8OO-i3oo and Ri₃oo-i8oo > 60 % average at Δ λ 1300-1800.

According to the gist of the invention an optical interference filter of the broadband pass type is applied, which causes gradual and continuously increased reflection of the undesired part of visible light rather than absorbance, thus greatly enhancing the efficiency of the lamp and providing a desirable color temperature. It should be understood that the optical filter according to the invention might take one of several forms, each of which embodies the general principle that it is a selectively reflective optical filter, i.e. is substantially transparent to radiation in which it is desirable that the lamp emits radiation and it is substantially reflecting over the remainder of the spectrum. According to a preferred embodiment of the invention the increase in reflectance is further defined by [Δ (R₄₀₀ - 500)/ Δ λ_4OO-5oo] < [Δ (R500 - eoo)/ Δ λ_5OO-6oo] < [Δ (Rβoo - 700)/ Δ λ₆oo-7θo] < [Δ (R700 - 800)/ Δ λ₇oo-soo] -

The optical reflective broadband pass filter according to the invention reflects infrared radiation back to the light-emitting element - in a known way to improve thermal management of the lamp. Moreover by reflecting a certain portion of the UV radiation in the range from 300 to 400 nm superior UV-blocking is achieved.

In addition to solving the problems of thermal management and UV- blocking the optical interference filter is also designed to reflect a certain portion of the visible radiation emitted by the light-emitting element back to said element. By reflecting back to the light-emitting element, the radiation, which is not desired to be emitted from the lamp, conserves the energy otherwise required to maintain the light-element at operating temperature and thus reduces overall energy requirements for operating the lamp.

The optical filter of the present lamp assembly provides a number of further advantages over the prior art. For example, the "left hand edge" of the reflectance curve is characterized by a very steep step from 80 % average reflectance in the near UV range to less than 10 % average reflectance in the adjacent blue to violet range of the visible range.

The "right hand edge" of the reflectance curve excels by a continuous exponential increase of reflectance from less than 10 % average between 400nm and 500 nm and 20 to 40 % between 600 nm and 700 nm. Thus the amount of amber to red radiation in the emitted radiation is continuously decreased by reflecting it back to the light-emitting element.

The present electric lamp assembly efficiently generates white light and illuminates objects with a color appearance, which is reasonably representative of their color appearance under natural light.

The lamp will exhibit the very noticeable visual effect of cleaner (or whiter) light compared to the lamps according to the prior art. A whiter radiation is also commonly acknowledged as more efficient since it enhances visibility at night. It should be noted that the whiter light generated is not of higher intensity and therefore does not create a glare effect that is observed with lamps designed for higher temperatures and wattage

The color point of the lamp is shifted into the bluish range of the white shades. This shift leads to increased red-green color contrast when objects are viewed under the emission of this lamp in comparison to a regular lamp bulb. This has been found to be very attractive to customers.

According to a preferred embodiment of the invention the optical interference filter comprises a plurality of alternating high and low refractive index layers

(H, L).

In particular the optical interference filter comprises at least five multiperiods, spectrally adjacent stacks comprising a plurality of alternating high and low refractive index layers (H, L).

Example given the optical interference filter coating may have the following general structure AⁿB^xC^yA ^mD^z, wherein A denotes a stack of structure (H/aL/a'H/a' ')

A' denotes a stack of structure (L/aH/a'L/a' ')

B denotes a stack of structure [HZbLZb⁵HZV(N)LZb⁵⁵⁵HZb⁵⁵LZb⁵HZb]

C denotes a stack of structure [HZCLZC⁵HZC⁵ VLZC^{5 5} OZC⁵ XZC⁵ZHZC] and D denotes a stack of structure d* [LZ2HLZ2]; wherein H and L denote quarterwaves of the high and low index material, respectively, and 3.5 < a < 8.0; 1.0 < a⁵ < 3.5; 0.8 < a" < 1.0 and a"< a⁵ < a; 0.5 < b < 1.0; 2.0 < b⁵ < 3.0; 2.0 < b" < 4.0; 0.25 < b"⁵ < 0.6; 0.5 < c < 1.0; 4.5 < c⁵ < 9.0; 0.5 < c" < 6.0; 0.4 < c⁵" < 0.8 and 1.7 < d < 2.0; at a reference wavelength λ between 450 and 550 nm and 0 < x <5; 0 < y <5; 0<z <8; 0 <n < 3, and 0 <m < 3 and N = 1 or 2 in an individual period of the stack B.

According to a preferred embodiment the parameters are selected as 4 < a < 6.66; 1.55< a⁵ < 2.66; 0.85 < a" 0.91and a< a⁵ < a"; b=0.85; b'=2.66; b"=3.22; 0. 38< b⁵" < 0.46; c=0.88; c'=6.66; c"=1.33; c'"=0.54 and d=1.85; at a reference wavelength λ at 510 nm.

Typically the material of the first layer (L) having the first refractive index comprises silicon oxide or aluminum oxide or mixtures or composites thereof. The material of the second layer (H) having the second refractive index is chosen from the group formed by titanium oxide, zirconium oxide, hafnium oxide, niobium oxide, tantalum oxide or mixtures or composites thereof.

Said interference filter may be arranged on the inner or outer surface of the lamp vessel.

The invention is especially useful, if said electric lamp is an incandescent lamp, in particular if said electric lamp is an incandescent halogen lamp containing halogen gas or a halogen compound in the lamp vessel. The principles of the present invention may also be applicable for improving the light characteristics of a discharge gas lamp. The term "light-emitting element," as used herein, will thus be understood to include - besides of incandescent filaments - other light sources in lamps, such as the arc in a high pressure mercury discharge lamp which contains metal halide in the gas filling.

The electric lamp may comprise a reflector, in particular a reflector having a generally parabolic shape.

DETAILED DESCRIPTION OF THE INVENTION

An electric lamp to be used for the purpose of the present invention is preferably an incandescent tungsten halogen lamp comprising a filament in a lamp vessel, filled in a known way with an inert gas mixture, comprising a halogen additive. Such lamps are of conventional construction and are generally constructed of a quartz tube, which forms a lamp vessel that encloses an elongated, tungsten filament. Preferably the material of the lamp vessel consists of IR- transmissive glass, quartz glass or fused quartz.

One embodiment of the invention relates to an electric lamp assembly comprising a reflector. More particularly, it relates to a parabolic aluminized reflector lamp (PAR). Ideally, a PAR lamp comprises a completely parabolic-shaped reflector, which is coated with a reflective substance. The reflector coating typically comprises aluminum, though the reflector coating can also comprise silver, gold, white gold, chromium or any other suitable reflective material.

Fig. 1 shows a typical incandescent reflector lamp of the PAR 38 type constructed in accordance with the present invention, which produces an output, which matches the characteristics of natural daylight in the visible region. The incandescent tungsten halogen lamp includes a lamp vessel 10 having therein a tungsten filament 12 connected at both ends to current supply conductors 14 which each comprise an inner lead 16 connected to the element 12, a molybdenum foil 17 in the pinch sealed portion of the lamp vessel, and an outer lead 18. The lamp vessel 10 has an elliptically shaped mid-portion which is provided with an interference filter according to the invention on the outside (not visible), thereby reflecting radiation back toward the filament 12 to improve thermal efficiency and reduce the power necessary for incandescence.

The outer envelope 20 is shaped as a parabolic reflector having an integral base 21, which receives the conductive mounting legs 26, 28 there through. The lead 26 is connected to screw base 27, while the lead 28 is connected to the insulated central contact 29. The glass or plastic lens or cover 23 may be attached by adhesive when a hermetic seal is not necessary. However, when it is desired to maintain an inert gas environment in the fill space 24, the cover 23 would typically be glass, which is flame- sealed to the envelope 20.

If an incandescent electric lamp is designed as an durable and longliving white light source, radiation coming from the light-emitting filament inevitably comprises UV radiation and infrared radiation in the near, middle and far infrared range, together with continuous radiation in the visible range with an unwanted excessive amount of light in the yellow region of the spectrum, which deteriorates the quality of an electric lamp as a white emitting radiation source useful for general illumination . In order to produce the desired spectral characteristics according to the invention, the electric lamp according to the invention is provided with an optical interference filter designed so that UV- radiation and most IR-radiation is reflected back to the light-emitting element, while concurrently visible radiation in the range from 400 to 700 nm is transmitted through it and is toned down continuously between 500 and 700 nm.

Radiation reflected by the filter back to the light-emitting element is reconverted to radiation in the visible portion of the electromagnetic spectrum, thereby greatly increasing the luminous efficiency of the lamp.

The optical interference filter according to the invention is preferably designed in thin film technology. A thin film optical interference filter coating for selectively reflecting and transmitting different portions of the electromagnetic spectrum comprises a plurality of alternating layers of a low refractive index material (represented by L) and a high refractive index material (represented by H).

As material for optical interference filters use is often made of refractory metal oxides having high and low indexes of refraction. Refractory metal oxides are used because they are able to withstand the relatively high temperatures, example given 400° C to 900° C, which develop during lamp operation.

Materials that can be advantageously used for the highly refractive sublayers H include titanium oxide (TiO₂), zirconium oxide (ZrO₂), hafnium oxide (HfO₂), niobium oxide (Nb₂Os) and tantalum oxide (Ta₂Os) and physical mixtures and multilayer arrangements thereof. Silicon oxide (SiO2) or aluminum oxide (Al₂O₅) and physical mixtures and multilayer arrangements thereof may be preferably used for the lowly refractive sub-layers L.

For the coating process itself, one may rely on any coating method as yet known, such as, e.g., electron beam vaporization, sputtering, in particular magnetron sputtering or other.

Preferably the layers of the interference filter are formed by a microwave enhanced DC magnetron sputtering process using two targets, for example comprising Si and Nb, to deposit SiO₂ and Nb₂Os by alternatively energizing the targets in an atmosphere containing oxygen as the reactive gas; the working gas is typically argon. Alternatively applying a coating to the interior and/or exterior surfaces of incandescent lamp 10 is accomplished in a simple manner employing a low pressure vapor deposition (LPCVD) coating process for applying alternating layers of high and low refractive index materials. In an LPCVD process a suitable metal oxide precursor reagent or reagents for each material of the film is separately introduced into a decomposition chamber wherein it is decomposed or reacted to form the metal oxide on a heated substrate. Separate layers of, for example, silicon oxide and niobium oxide are applied onto the substrate in this fashion until the desired filter is achieved. Such chemical vapor deposition techniques are well known to those skilled in the art.

Another process that is possible to employ to apply an optical interference coating in a uniform manner to all of the interior surfaces of a lamp envelope is an aqueous process that is also known to those skilled in the art. However, in an aqueous process the coating materials must be alternatively applied by spraying or dipping along with spinning and baking or drying in order to achieve uniform coating thicknesses and to enable successive alternating layers to be built up to obtain the film without diffusion of one material into the other. This process is extremely difficult to apply uniformly to a lamp envelope and is very time consuming. Consequently, an LPCVD or chemical vapor deposition (CVD) process employing a suitable reagent in gaseous form that is decomposed on the surface of the substrate to be coated is the present state of technology most preferred as the method to apply the optical interference coating to the interior and/or exterior surfaces of the lamp. The thicknesses of the layers are determined by the "quarterwave stack" principle.

According to the "quarterwave stack principle", optical interference coatings are based on a reflectance stack consisting of alternating layers of high and low index films, each layer having an optical thickness of one Quarter- Wave Optical Thickness (QWOT). The optical thickness is defined as the product of the physical thickness times the refractive index of the film. The QWOT is referenced to a conveniently chosen design wavelength. For example, at a design wavelength according to the invention of 520 nm, a QWOT equals 65 nm.

The thickness of the sub-layers should be uniform and accurately held to achieve the effect of the invention.

Since a single reflectance stack reflects across only a portion of the visible region, five or more multiperiod, spectrally adjacent stacks each comprising a plurality of alternating high and low refractive index layers (H, L) are combined for an extendedly graded transmission band across the visible spectrum. Such a five multiperiod filter stack is represented in the following manner, which is known to those skilled in the art:

The structure is generally denoted by AⁿB^xC^yA ^mD^z, wherein A denotes a stack of structure (H/aL/a'H/a"); A' denotes a stack of structure (L/aH/a'L/a"); B denotes a stack of structure [H^/b'H/b''(N)L/b'''H/b''L/b'H/b]; C denotes a stack of structure [H/cL/c'H/c"/L/c' "H/c"L/c'/H/c]; and D denotes a stack of structure d* [L/2HL/2]; wherein H and L denote quarterwaves of the high and low index material, respectively, and 3.5 < a < 8.0; 1.0 < a' < 3.5; 0.8 < a" < 1.0 and a"< a' < a; 0.5 < b < 1.0; 2.0 < b' < 3.0; 2.0 < b" < 4.0; 0.25 < b"' < 0.6; 0.5 < c < 1.0; 4.5 < c' < 9.0; 0.5 < c" < 6.0; 0.4 < c'" < 0.8 and 1.7 < d < 2.0; at a reference wavelength λ between 450 and 550 nm and 0 < x <5; 0 < y <5; 0<z <8; 0 <n < 3, and 0 <m < 3 and N = 1 or 2 in an individual period of the stack B.

According to a preferred embodiment the parameters are selected as 4 < a < 6.66; 1.55< a' < 2.66; 0.85 < a" 0.91and a< a' < a"; b=0.85; b'=2.66; b"=3.22; 0. 38< b'" < 0.46; c=0.88; c'=6.66; c"=1.33; c'"=0.54 and d=1.85; at a reference wavelength λ at 510 nm.

Layers forming a period are surrounded by brackets, with the superscripts x, y, z, n and m being the number of times the period is repeated in the stack.

The values for the denominators a, a, a", b, b', b", b", c, c', c" and d are chosen based upon the required optical thickness T₀ of each layer according to the formula: T₀ =λ/(4x denominator), wherein λ is the reference wavelength. The physical thickness T_p of each layer is equal to the optical thickness T₀ divided by the index of refraction of the material.

Accordingly, the notation L/a represents a fraction of a quarterwave of "optical thickness" of the L material at the reference wavelength, i.e., one- half of a quarterwave (1/8 wave) for a.

The sequence of the stacks can be exchanged and further stacks can be added to narrow the bandwidth of the pass band.

Further stacks are preferably added in the IR region. For example, a long wave-pass stack with blocking features in the NIR region next to the visible region can be added. It shifts the transmission pass region to higher wavelengths. Otherwise a further short wave-pass filter can be added to increase the amount of FIR reflected by the filter.

According to one embodiment of the invention the highly refractive sub- layers in themselves are composed of a sub-stack of two layers of one highly refractive material and a thin intermediate layer of a second highly refractive material known from prior art. The thickness of the intermediate layer is preferably in the range from 1 to 25 nm. This intermediate layer will avoid extended crystal growth in the highly refractive layer. How many times the various stacks A, A', B, C and D are repeated, in other words the choice for the exponents n, m, x, y and z, is determined on the basis of an analysis of the maximum increase of the reflectance per thickness increase of the filter design. Analysis has shown that it is very favorable to choose values for x, y, z, n and m as 0 < x <5; 0 < y <5; 0<z <8; 0 <n < 3, and 0 <m < 3, if a high reflectance is desired. A further increase of the values for y and z generally has the disadvantage that the width of the transmission window decreases. Such increase is also undesirable because it causes the tolerance of the filter design with respect to variations in layer thickness occurring during the manufacturing process of the interference film to be reduced.

The physical layer thicknesses of the H-L interference filter coating in accordance with the invention are the result of computer optimizations, which are known per se. Computer optimization is used to balance the need for high visible transmission and minimum layer count. The necessary calculations are applied to the complete filter design. Table 1 shows the number of layers and physical thickness of each layer.

If another reference wavelength is chosen the denominators a, b and c must be adapted accordingly to reach the same result for every one of the stacks given.

In the case the order of the stacks is to be interchanged it is preferred that the outermost layer next to ambient is chosen to be of the low index material layer L and that in case the outermost layer is not a low index material layer L such a layer is added to the filter design. According to an specific embodiment an optical interference filter comprising alternating layers of Siθ2 and Nb₂Os were applied to the outer surface of the envelope of a tungsten-halogen incandescent lamps of the type illustrated in FIG. 1, employing an LPCVD coating process according to the computer optimization set forth in Table 1 for a total of forty-seven alternating layers of SiC>2 and Nb₂O₅. In a particularly preferred form of the invention, the high index of refraction material will comprise niobium oxide and the low index of refraction material is comprised of silicon oxide.

A particularly preferred structure is provided in Table 1 wherein thicknesses are set forth in nanometers. Table I shows the filter designs, i.e. layer structure, layer materials and layer thickness of the optical interference band pass filter with a broadband graded transmission bandwidth in the range from 400 nm to 700 nm. Table 1 :

Fig. 3 depicts the transmission characteristics of the optical interference filter of Table 1. Wavelength λ[nm] is plotted along the x axis and energy in terms of average microwatts per nm per lumen along the y-axis. The transmission characteristics covers the range from 300 to 2000 nanometers, which is generally the range from the near UV to the far IR range including the visible range from 400 to 700 nm.

The relative transmittance of the optical interference filter is such that when applied to an incandescent lamp the radiation is filtered in a way that there is produced white light of predetermined CIE coordinates.

The optical reflective coating according to the invention reflects infrared radiation back to the light-emitting element - in a known way.

Additionally the optical reflective coating according to the invention reflects unwanted UV-radiation in the range from 400 to 300 nm, which causes fading of illuminated items, such as fabrics, plastics and painted articles.

And last not least, the optical reflective coating according to the invention reflects part of the visible radiation that is not wanted back to the light-emitting element, again improving performance. Transmission in the visible range is adapted in a way that the unwanted radiation the red to amber range of the electromagnetic spectrum is increasing gradually in relation to the wavelength range. It should be noted that the reflectance shown in FIG. 3 ascends from near zero at the 380 nanometer mark smoothly to the peak at 750 nanometer mark. Aside from the slight jitter, the curves are smooth.

Referring to FIG. 3, it is interesting to note that in accordance with the present invention most yellow-appearing radiations are minimized as much as possible in order to provide a good color rendition of illuminated objects. To express this in another way, the available energy is concentrated in other regions of the visible spectrum in order to achieve the best possible efficiency of light generation (i.e. lumens per watt) commensurate with good color rendition of illuminated objects. The light generated is of different average chromaticity. The effect is a shift in direction of the bluish range of the chromaticity diagram.

Analyses of the spectral energy distribution according to the 1931 CIE (Commission International de l'Eclairage) determine a correlated color temperature of 2785 to 2950 degrees Kelvin and x and y coordinates of x = 0.0449 to 0.454 and y = 0.415 toθ.422 respectively on a standard CIE chromaticity diagram.

In average these lamps have a light output of 800 lumens and chromaticity coordinates of x = 0.451 and y =0. 417. They also have a CRI of 96, which is close to the natural daylight.

While it is apparent that changes and modifications can be made within the spirit and scope of the present invention, it is our intention, however, only to be limited by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding the invention is shown in the accompanying drawings in which:

FIG. 1 is a view in elevation of an incandescent PAR 38 reflector lamp constructed in accordance with the present invention;

FIG. 2 illustrates the x, y-chromaticity diagram of the CIE system; FIG. 3 is a graph of relative output versus wavelength illustrating the overall emission of a lamp according to the invention.

Claims

CLAIMS:

1. An electric lamp assembly comprising an electric lamp, said electric lamp comprising a lamp vessel (10) comprising a lamp envelope (20), a light-emitting element (12) located within said lamp envelope and a color temperature increasing optical interference filter coating covering at least a portion of a surface of said lamp envelope, said coating being comprised of a plurality of alternating high and low refractive index layers, said coating having a spectrally broad high reflectance of at least 80 % average in the IR wavelength range Δ λ between 800 and 1300 nm and of at least 80% average in the UV wavelength range Δ λ between 300 and 400 nm, a steep decrease of reflectance between 380 nm and 400 nm to a reflectance of 0 to 10 % at a wavelength of 400 nm and a continuously steeper increase in reflectance between 400nm and 800 nm, the reflectance being defined by R 300 -4oo> 80% average at Δ λ_3Oo-4oo; 0 % > R400-500 > 10% at Δ λ 400-500; 10 % > R₅oo-6oo > 20% at Δ λ _5Oo-6oo; 20 % > R₆₀₀-TOo > 40% at Δ λ _6OO-7oo; 40 % > R₇oo-8oo > 80% at Δ λ _7OO-8oo; R₈oo-i3oo > 80% average at Δ λ _8OO-i3oo and Ri₃oo-i8oo > 60 % average at Δ λ 1300-1800.

2. An electric lamp assembly according to claim 1, wherein the increase in reflectance is further defined by [Δ( R₄₀₀ - 5oo)/Λλ_4OO-5oo] < [Δ ( R500 - 6oo)/Λλ_5OO-6oo] < [Δ (

RδOO - 7Oθ)/ Δ λ6OO-7Oθ] < [Δ( R700 - 8Oθ)/ Δ λ7θO-8Oθ] -

3. An electric lamp assembly according to claim 1, wherein the optical interference filter comprises a plurality of alternating high and low refractive index layers (H, L).

4. An electric lamp assembly according to claim 3, wherein the optical interference filter comprises at least five multiperiod, spectrally adjacent stacks comprising a plurality of alternating high and low refractive index layers (H, L).

5. An electric lamp assembly according to claim 6, characterized in that the interference filter coating has the following general structure AⁿB^xC^yA ^mD^z, wherein

A denotes a stack of structure (H/aL/a'H/a");

A' denotes a stack (L/aH/a'L/a");

B denotes a stack [H/bL/b'H/b"(N)L/b'"H/b"L/b'H/b]; C denotes a stack [HZCLZCOZC⁵ VLZC^{5 5 5}HZC⁵ XZCVHZC]; and D denotes a stack d* [LZ2HLZ2]; wherein H and L denote quarterwaves of the high and low index material, respectively, and 3.5 < a < 8.0; 1.0 < a⁵ < 3.5; 0.8 < a" < 1.0 and a"< a⁵ < a; 0.5 < b < 1.0; 2.0 < b⁵ <

3.0; 2.0 < b" <

4.0; 0.25 < b"⁵ < 0.6; 0.5 < c < 1.0; 4.5 < c⁵ < 9.0; 0.

5 < c" < 6.0; 0.4 < c⁵⁵⁵ < 0.8 and 1.7 < d < 2.0; at a reference wavelength λ between 450 and 550 nm and 0

< x <5; 0 < y <5; 0<z <8; 0 <n < 3, and 0 <m < 3 and N = 1 or 2 in an individual period of the stack B.

6. An electric lamp assembly as claimed in claim 5, characterized in that the material of the first layer (L) having the first refractive index comprises silicon oxide or aluminum oxide or mixtures or composites thereof.

7. An electric lamp assembly as claimed in claim 5, characterized in that the material of the second layer (H) having the second refractive index is chosen from the group formed by titanium oxide, zirconium oxide, hafnium oxide, niobium oxide, tantalum oxide or mixtures or composites thereof.

8. An electric lamp assembly according to claim 1, wherein said band pass interference filter coating arranged on the inner or outer surface of the lamp vessel.

9. An electric lamp assembly according to claim 1, wherein said electric lamp is an incandescent lamp.

10. An electric lamp assembly according to claim 9, wherein said electric lamp is an incandescent halogen lamp containing halogen gas or a halogen compound in the lamp vessel.

11. An electric lamp assembly according to claim 1 , wherein said electric lamp is a gas discharge lamp.

12. An electric lamp assembly according to claim 1, wherein the electric lamp assembly comprises a reflector.

13. An electric lamp assembly according to claim 12 wherein said reflector has a generally parabolic shape.