Optical interference film and lamp having the same

An optical interference film formed on the surface of the bulb of a lamp includes high refractive index layers and low refractive index layers alternately stacked on each other to form more than ten layers in all. Each high refractive index layer includes titanium oxide containing at least one additive selected from the group consisting of antimony (Sb), silicon (Si) and tantalum (Ta). An amount of the at least one additive is the range from 0.1% to 30% of the titanium oxide in terms of the metal atomic ratio.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates, in general, to optical interference films. 
In particular, the invention relates to an interference film, formed on 
the outer or inner surface of a lamp, e.g., a halogen lamp, which 
selectively reflects light from the optical spectrum in a prescribed 
wavelength range. 
2. Description of the related art 
A halogen lamp which radiates light having a small amount of infrared rays 
is well known. In such a halogen lamp, a filament is disposed at the 
center of a glass bulb, and an optical interference film is formed on the 
outer surface of the bulb. The optical interference film transmits visible 
rays and reflects infrared rays Thus, infrared rays in the light radiated 
from the filament are reflected toward the filament by the optical 
interference film and heat the filament. As a result, a decrease in 
infrared rays in the radiated light and an improvement of the luminous 
efficiency are achieved by the interference film on the conventional 
halogen lamp. 
The Japanese laid-open patent publication 62-105357 discloses one example 
of such an optical interference film, which includes high refractive index 
layers and low refractive index layers alternately stacked one on the 
other, these being a total of nine to twelve layers, or more. Each high 
refractive index layer includes at least one metal oxide selected from 
titanium oxide (TiO.sub.2), tantalum oxide (Ta.sub.2 O.sub.5) and 
zirconium oxide (ZrO.sub.2) as a main component, and at least one additive 
selected from phosphorus (P), boron (B), arsenic (As), antimony (Sb), tin 
(Sn), zinc (Zn), lead (Pb), potassium (K), nickel (Ni) and cobalt (Co). 
Each low refractive index layer includes silica (SiO.sub.2) as a main 
component, and at least one additive selected from phosphorus (P) and 
boron (B). 
In the above-described conventional optical interference film, each 
inter-layer connection between high and low refractive index layers is 
strengthened by the additives. Distortion in the optical interference film 
caused by the difference in the heat expansion coefficient between the 
high and low refractive index layers is also reduced by the additives. 
Thus, cracking or peeling of the optical interference film can be avoided. 
However, the above-described additives adversely affect the heat-resisting 
ability of the optical interference film, so that luminous flux from such 
a lamp decreases significantly with time when the lamp is in use. 
SUMMARY OF THE INVENTION 
Accordingly , it is an object of the present invention to provide an 
optical interference film of desirably high transmitting without 
decreasing unacceptably the heat-resisting property of the optical 
interference film. 
To accomplish the above-described objects, an optical interference film 
includes at least two refractive layers of different refractive indices on 
the transparent substrate and at least one additive selected from the 
group consisting of antimony (Sb), silicon (Si) and tantalum (Ta) in the 
refractive layer having a higher refractive index. 
The refractive layer having the higher refractive index may include at 
least one compound, as a glass forming agent, selected from a phosphorus 
compound and a boron compound. 
The refractive layer having the higher refractive index may also include 
titanium oxide. An amount of the at least one additive to titanium oxide 
is 0.1% to 30% in terms of the metal atomic ratio. 
The optical interference film may be formed on at least either inner or 
outer surface of a halogen lamp to reflect infrared rays and transmits 
visible rays from the optical spectrum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention will now be described in 
more detail with reference to the accompanying drawings. In this 
embodiment, the present invention is applied to a well known type of 
halogen lamp. 
As shown in FIG. 1, a halogen lamp 11 includes a transparent bulb 13 made 
of a quartz glass or an aluminosilicate glass. One end of bulb 13 is 
closed, and the other end is pinched to form a sealed portion 15. A pair 
of molybdenum foils 17 and 17 is arranged in sealed portion 15. A coiled 
filament 19 acting as a light emitting member is arranged along the 
central axis of bulb 13. Each end of coiled filament 19 is electrically 
connected to a corresponding one of the foils 17 and 17 through a 
corresponding one of two inner lead wires 21 and 21. Sealed portion 15 of 
bulb 13 is fixed into a metalliccap 23. A fill including an inert gas such 
as argon and an amount of halogen is sealed in bulb 13. An optical 
interference film 25 is formed on at least either the inner or outer 
surfaces, e.g., outer surface, of bulb 13. Optical interference film 25 
transmits visible rays and reflects infrared rays from the optical 
spectrum. As shown in FIG. 2, optical interference film 25 includes high 
refractive index layers 27, the principal component of which is titanium 
oxide (TiO.sub.2) and low refractive index layers 29 the principal 
component of which is silica (SiO.sub.2). A first one of the high 
refractive index layers 27 is formed on the outer surface of bulb 13 and a 
low refractive index layer 29 is then formed on that first high refractive 
index layer 27. Further high and low refractive index layers 27 and 29 are 
then formed alternately to provide a stacked arrangement of a desired 
total number of layers. 
A method of forming the optical interference film will now be described. 
Firstly, titanium-alkoxide and alkoxide of at least one metal additive 
selected from the group consisting of antimony (sb) silicon (Si) and 
tantalum (Ta) are prepared in a vessel. Ethanol is added to and is 
uniformly mixed in the vessel. Either an acylating agent or a chelating 
agent is also provided in the vessel as the liquid therein is stirred at 
room temperature, and a reaction is caused to take place by heating the 
liquid, whilst maintaining it under reflux conditions, for about one hour. 
A glass forming agentis then added to the liquid resulting from that 
reaction and thus a first coating liquid whose density converted to a 
composite oxide is 4.5 percent by weight (wt%) is finally obtained. The 
above-described glass forming agent may include an inorganic or organic 
phosphorus compound or boron compound which has an organic solvent 
solubility. The above-described glass forming agent such as a phosphorus 
compound or a boron compound is added at less than ten percent by weight 
(wt%), preferably 0.1 to 5.0 percent by weight (wt%), to a total amount 
converted to a composite metal oxide on an oxide basis. 
In a first coating process, bulb 13 is dipped into the first coating liquid 
obtained by the above-described steps and is pulled up at a constant 
speed. A titanium oxide film (hereinafter referred to as TiO.sub.2 film), 
i.e. a first high refractive index layer 27, is then fixed on the outer 
surface of bulb 13 by baking bulb 13 for ten minutes at from four hundred 
to nine hundred degrees centigrade (.degree. C) in air. 
Secondly, a second coating liquid, including an organosilicon compound, 
e.g. alkoxysilane such as tetramethoxysilane, tetraethoxysilane, 
tetraisopropoxysilane, tetrabutoxysilane, diethoxydiisopropoxysilane and 
diohlorodimethoxysilane and/or a polymer thereof, is prepared. 
In a second coating process, bulb 13 on which the first high refractive 
index layer 27 has been formed is dipped into the second coating liquid 
and is pulled up at a constant speed. A silica film, i.e. a low refractive 
index layer 29, is then fixed on the first high refractive index layer 27 
by baking bulb 13 for ten minutes at from four hundred to nine hundred 
degrees centigrade (.degree. C) in air. The required complete optical 
interference film 25 is formed on bulb 13 by repeatedly executing the 
above-described first and second coating processes, preferably at least 
five times (giving at least 10 layers in all). 
The principles of the above-described embodiment will now be described in 
more detail. In general, the crystal structure of the TiO.sub.2 film 
formed by the above-described processes is amorphous, anatase or rutile. 
An amorphous-TiO.sub.2 film and an anatase-TiO.sub.2 film have high 
visible transmittance and have low refractive index, as compared with a 
rutile-TiO.sub.2 film. In addition, the crystal structure of the 
amorphous-TiO.sub.2 film or the anatase-TiO.sub.2 film is changed to that 
of the rutile-TiO.sub.2 film (a high temperature stable type) when it is 
heated at a high temperature for a relatively long period. The 
rutile-TiO.sub.2 film has a high thermal stability and a high refractive 
index, as compared with the anatase-TiO.sub.2 film. However, the 
transmittance of the TiO.sub.2 film for a range of visible rays is 
decreased when the crystal structure of the amorphous-TiO.sub.2 film or 
the anatase-TiO.sub.2 film is changed to that of the rutile-TiO.sub.2 
film. By appropriate control of the phase-change from the 
amorphous-TiO.sub.2 or the anatase-TiO.sub.2 to the rutile-TiO.sub.2 it is 
possible to produce a high refractive index layer, for an optical 
interference film, which layer has an advantageous combination of high 
refractive index and high visible transmittance. 
In the above-described embodiment, in which the high refractive index layer 
27 shown in FIG. 2 includes titanium oxide as a main component, the 
addition of at least one metal additive selected from the group consisting 
of antimony (Sb), silicon (Si) and tantalum (Ta), provides a means whereby 
the phase-change from the amorphous-TiO.sub.2 or the anatase-TiO.sub.2 to 
the rutile-TiO.sub.2, in the high refractive index layer 27, under 
influence of a high temperature can be controlled. Growth of crystal 
particles in high refractive index layer 27 can also be controlled. Thus, 
decrease in the visible transmittance of high refractive index layer 27 
can be controlled while achieving high heat-resistant ability for that 
layers. To a certain extent, the greater the included proportion of the 
above-described metal additive, the greater the above-described desirable 
effects. However, too much of the above-described metal additive can cause 
the refractive index of the composite oxide thin film layer 27 to be 
decreased unacceptably. A preferred range of the quantity of the 
above-described metal additive (M) compared with the quantity of titanium 
(Ti) in the high refractive index layer 27, in terms of the metal atomic 
ratio, is as follows: 
EQU 0.1(%)&lt;M/Ti&lt;30(%). 
To carry out experiments, a number of sample lamps were prepared, each 
provided with more than ten layers stacked one upon another and made 
alternately of high refractive index titanium oxide (TiO.sub.2) and low 
refractive index silica (SiO.sub.2) In a first sample of the 
above-described lamps, the high refractive index layers have no additive, 
and in second samples, the high refractive index layers are conventional 
high refractive index layers having a phosphorous additive (P) in the 
proportions 0.5 (%) and 1 (%) respectively. Third samples of the lamps 
have conventional high refractive index layers having a boron additive (B) 
amounting to 1 (%) and 0.5 (%) respectively. Fourth samples of the lamps 
include high refractive index layers having antimony (Sb) as an additive 
varing from 0.05% to 40% in amount. 
Results of the experiments are shown in TABLES I and II. In TABLES I and 
II, the luminous flux ratio of each sample is expressed by a relative 
value (%), with the initial luminous flux of the sample which includes no 
additive being taken as one hundred percent (%). 
TABLE I 
__________________________________________________________________________ 
AMOUNT OF LUMINOUS 
LUMINOUS FLUX 
BAKING 
METAL ADDITIVE 
TiO.sub.2 
FILM 
FLUX RATIO AFTER 
TEMP. (ATOM RATIO) 
(n) 
(T) RATIO 2000 hrs LIGHTING 
__________________________________________________________________________ 
600 (.degree.C.) 
none 2.10 
92.3% 
100% 44% 
10 (min) 
Sb 40% 2.09 
92.3% 
99% 67% 
Sb 25% 2.13 
92.5% 
108% 94% 
Sb 5% 2.20 
93.0% 
117% 95% 
Sb 1% 2.18 
93.2% 
118% 96% 
Sb 0.1% 2.17 
93.2% 
116% 95% 
Sb 0.05% 2.14 
93.0% 
111% 68% 
P 1% 2.09 
92.1% 
99% 53% 
P 0.5% 2.10 
92.0% 
100% 48% 
B 1% 2.07 
91.0% 
96% 33% 
B 0.5% 2.08 
91.5% 
98% 39% 
__________________________________________________________________________ 
n: initial refractive index 
T: transmittance 
TABLE II 
__________________________________________________________________________ 
AMOUNT OF LUMINOUS 
LUMINOUS FLUX 
BAKING 
METAL ADDITIVE 
TiO.sub.2 
FILM 
FLUX RATIO AFTER 
TEMP. (ATOM RATIO) 
(n) 
(T) RATIO 2000 hrs LIGHTING 
__________________________________________________________________________ 
900 (.degree.C.) 
none 2.17 
92.4% 
100% 56% 
10 (min) 
Sb 40% 2.25 
92.6% 
106% 82% 
Sb 25% 2.26 
91.4% 
118% 94% 
Sb 5% 2.35 
91.8% 
117% 96% 
Sb 1% 2.30 
91.6% 
116% 97% 
Sb 0.1% 2.29 
93.6% 
116% 96% 
Sb 0.05% 2.19 
92.8% 
104% 62% 
P 1% 2.15 
93.0% 
98% 49% 
P 0.5% 2.16 
92.8% 
99% 46% 
B 1% 2.13 
90.8% 
92% 30% 
B 0.5% 2.15 
91.4% 
96% 40% 
__________________________________________________________________________ 
n: initial refractive index 
T: transmittance 
As can be seen in TABLES I and II, a conventional optical interference film 
employing a high refractive index layer, i.e. titanium oxide (TiO.sub.2) 
layer, to which phosphorus (P) or boron (B) is added has a relatively low 
refractive index (n) and a relatively low luminous flux. In addition, the 
luminous flux of the above-described optical interference film is greatly 
decreased after 2000 hours lighting. However, optical interference films 
including high refractive index layers made of titanium oxide to which 
antimony (Sb) is added have relatively high initial refractive index, and 
the luminous flux thereof after 2000 hours lighting is maintained at a 
relatively high value, as compared with the conventional optical 
interference films. More desirable results are achieved when the high 
refractive index layer is baked at 900 degree centigrade (.degree. C), as 
shown in TABLE II. 
To some extent, the lower the proportion of antimony (Sb) added to the high 
refractive index titanium oxide layer, the higher the refractive index (n) 
of that layer. However, with a very low proportion of antimony (Sb) in the 
high refractive index layer the results achieved are poor. As stated 
above, a desirable proportional range of antimony (Sb) in the high 
refractive index layer is between 0.1% and 30%. Similar effects can be 
obtained when silicon (Si), or tantalum (Ta) is added to the high 
refractive index layer as a metal additive. 
As described above, in an embodiment of the present invention, by adding at 
least metal additive selected from the group consisting of antimony (Sb), 
silicon (Si) and tantalum (Ta) to a high refractive index layer made of 
titanium oxide, desirable optical characteristics such as a high luminous 
flux and a high heat-resisting ability can be achieved. 
The present invention has been described with respect to a specific 
embodiment. However, other embodiments based on the principles of the 
present invention should be obvious to those of ordinary skill in the art. 
Such embodiments are intended to be covered by the claims.