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The present invention relates to a lighting unit having a lamp and a reflector, and especially to a lighting unit with both the lamp and the reflector provided with an interference multi-layer filter.
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A lighting unit having a lamp and a reflector, each of which is provided with an interference multi-layer filter
is already known in the art. The lamp of the conventional lighting unit is a halogen lamp coated with a first interference multi-layer filter (film) which enables visible light to pass (transmit) therethrough and infrared-ray to reflect thereon. The reflector of the conventional lighting unit has a second interference multi-layer filter (film) which enables visible light to reflect thereon and infrared-ray to pass (transmit) therethrough. The second interference multi-layer filter is coated on the reflecting surface of a reflecting base of the reflector and is called a dichroic mirror film. The reflector is shaped into a paraboloid of revolution and surrounds the halogen lamp. Both the first and second interference multi-layer filters are composed of multiple layers of two kinds of different refractive index layers which are alternately disposed on the surfaces of the lamp and the reflecting base of the reflector.
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This lighting unit is used for spot-lighting in stores since this lighting unit has high efficiency and high color temperature, as compared with a lighting unit not having an interference multi-layer filter. Because infrared-rays reflected at the first interference multi-layer filter return to the filament of the lamp and heat the filament, the lamp has a high efficiency. Furthermore, light reflected at the second interference multi-layer filter does not include a high amount of infrared-rays since the second interference multi-layer filter enables visible light to reflect thereon and infrared-rays to pass therethrough, and therefore this lighting unit can prevent heat damage to abjects to be illuminated and can emit light of high color temperature, for example 3050-3600°K.
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However, this lighting unit has a shortcoming in that a color pattern having a ring shape appears on the surface of the illuminated objects. In this color pattern, the color of green is strong in the periphery of the lighting area. The interference multi-layer filters are thought to have a problem in that they emit a greenish light to the periphery of the lighting area. An additional shortcoming is that light of sufficiently high color temperature is not obtained.
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Although the structure described below is not believed to constitute prior art with respect to the present invention, it is described to enable a better understanding of the present invention. It has been proposed that the second interference multi-layer filter may have a thickness which varies according to its position on the reflecting base of the reflector, in order to overcome the above mentioned shortcomings. However, this lamp unit also has the disadvantages of generating an unwanted color pattern and does not emit light of sufficiently high color temperature.
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Accordingly, the present invention seeks to eliminate a color pattern of light emitted from the lighting unit and appearing on the surface of a lighting object. The present invention further seeks to increase the color temperature of a lighting unit.
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Thus, the lighting unit of the present invention comprises;
a lamp having
a hollow tube having a surface,
a filament included in said tube for generating multiple wavelength light, and
a first film on the surface of said tube for passing light of a specified range of wavelengths and for suppressing other wavelengths of light from passing therethrough, said first film having first means for suppressing a variation, at different points of the film, of the wavelength of said light which passes therethrough; and
a second film surrounding said lamp for reflecting said light of specified range of wavelenghts thereon and for suppressing other wavelengths of light from reflecting thereon, said second film having second means for suppressing a variation, at different points of the film, of the wavelength of said light which reflects thereon.
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For better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the following drawings, in which:
- Fig.1 is a partial sectional side view of a lighting unit according to the first embodiment of the present invention;
- Fig. 2 is a partial sectional view of the first interference multi-layer filter of the first film and the tube of Fig. 1;
- Fig. 3 is a partial sectional view of the second interference multi-layer filter of the second film and the reflecting base of Fig. 1;
- Fig. 4 is a partial modified sectional side view of Fig. 1, for explaining the interference multi-layer filter of the first and second films varying their thickness;
- Fig. 5 is a schematic diagram for explaining the relation between the incident angle of light and the incident position of light for interference multi-layer filters;
- Fig. 6 is a partial sectional view of the interference multi-layer filter of the first film and the tube according to the second embodiment of the present invention; and
- Fig. 7 is a partial sectional side view of a lighting unit according to the third embodiment of the present invention.
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Referring to the accompanying drawings, embodiments of the present invention will be described. However, in the drawings, the same numerals are applied to the similar elements in the drawings, and therefore the detailed descriptions thereof are not repeated.
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Fig. 1 is a partial sectional side view of a lighting unit according to the first embodiment of the present invention. The lighting unit is composed of a halogen lamp 11 and a refletor 31. The halogen lamp 11 comprises a cylindrical hollow tube 13 which is about 12 mm in outer diameter and about 40 mm in length. The tube 13 is made of quartz glass. The tube 13 has a pinch seal portion 15 at one end thereof where a pair of molybdenum leaves 17a and 17b connect a pair of inner wires 19a and 19b with a pair of outer wires 21a and 21b The pair of inner wires 19a and 19b support a tungsten coil filament 23 which is about 1.5 mm in outer diameter and about 5 mm in length. The tungsten coil filament 23 is located in the tube 13 so that the central axis of the tungsten coil filament 23 coincides with the central axis O₁-O₁ of the tube 13. The tube 13 contains a designated amount of argon gas and halogen gas therein. The first film 25 made of a first interference multi-layer filter is provided on the outer surface of the tube 13 other than on the pinch seal portion 15 and on the end of the tube 13 opposite the pinch seal portion 15. Alternatively the first film 25 may be provided on the inner surface of the tube 13 instead of on the outer surface. The first film 25 is explained later in detail. The opposite end of the tube 13 has a shielding film 27 on the outer surface for shielding light emitted from the filament 23. The shielding film 27 is made of light absorption materials such as cobalt oxide (CoO), nickel oxide (NiO) and so on, which forms a black film. It may be made of fine particle materials such as titanium oxide (TiO₂), aluminium oxide (Al₂O₃) and so on, which reflect light.
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The halogen lamp 11 above described is surrounded by the reflector 31. The reflector 31 has a reflecting base member 33 and a second film 35 which is made of a second interference multi-layer filter. The reflecting base member 33 is made of aluminium, but it may be made of glass materials. The reflecting base member 33 forms a reflecting portion 331 and a base end portion 333. The reflecting portion 331 has an opening 32 at one end for emitting light and is connected with the base end portion 333 at the other end thereof. The inner surface of the reflecting portion 331 is formed in a shape of a paraboloid of revolution and is coated with the second film 35 made of the second interference multi-layer filter. The base end portion 333 of the reflecting base member 33 is formed in a cylindrical hollow shape and supports the lamp 11 therein. The reflecting portion 331 and the base end portion 333 have a common central axis O₂-O₂ and the lamp 11 is fixed to the , base end portion 333 with adhesives 41, such as cement, so that the central axis O₁-O₁ of the lamp 11 coincides with the central axis O₂-O₂ of the reflecting base member 33 and that the pinch seal portion 15 of the lamp 11 faces to the inner surface of the base end portion 333 of the reflecting base member 33. The adhesives 41 are filled between the inner surface of the base end portion 333 of the reflecting base member 33 and the outer surface of the pinch seal portion 15 of the lamp 11.
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The first film 25 coated on the outer surface of the tube 13 has the function of passing (transmitting) light of a specified range of wavelengths i.e., visible light andfor suppressing other wavelengths of light passing through, i.e., for reflecting infrared radiation The second film 35,coated on the surface of the reflecting base member 33, has the function lf reflecting the desired wavelengths of light, i.e. visible light passing through the first film 25, and for suppressing other wavelengths i. e. so that infrared radiation is not reflected thereon, but instead the infrared radiation passes through. In other words, the first interference multilayer filter of the first film 25 is a visible light transparency/infrared radiation reflective film and the second film 35 is a visible light reflective/infrared radiation transmissive film
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Both the first film 25 and the second film 35 are made of interference multi-layer filters which are composed of multiple layers, for example 9-17 layers, of two kinds of different refractive index layers which are alternately disposed on the surfaces of the tube 13 of the lamp 11 and the reflecting base member 33 of the reflector 31, as shown in Fig. 2 and Fig. 3. The layers 51 having high refractive index are made of amorphous metal oxide such as titanium oxide (TiO₂), tantalum oxide (Ta₂O₅), zirconium oxide (ZrO₂) , zinc sulfide (ZnS) and so on, and the layers 53 having low refractive index are made of amorphous metal oxide such as silicon oxide (SiO₂), magnesium fluoride (MgF₂) and so on.
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Each layer of the interference multi-layer filters of the first film 25 and the second film 35 has a predetermined thickness according to the desired wavelengths of light passing therethrough and to other wavelengths of light to be suppressed.
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Futhermore, the thickness of each layer of the interference multi-layer filters of the first film 25 and the second film 35 varies continuously according to the position thereof, as is shown in Fig.2-Fig.4. This is one feature of the present invention which is different from the conventional lighting unit. Each layer of the first interference multi-layer filter of the first film 25 is thin in an area close to the filament 23 and is thicker in an area further away from the filament 23.
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The thickness of each layer in the area close to the filament 23 is smaller than the thickness of each layer in the area further away from the filament 23. As a result, the thickness t₁ of the first film 25 in the area close to the filament 23 is smaller than the thickness t₂ of the first film 25 in the area further away from the filament 23, shown in Fig. 4. In other words, each layer of the first interference multi-layer filter of the first film 25 is thin in an area having a small incident angle ϑ₁ of light emitted from the filament 23 and is thickerin an area having a larger incident angle ϑ₂ of light emitted from the filament 23 since the area close to the filament 23 has a small incident angle ϑ₁ of light and the area far away from the filament 23 has a large incident angle ϑ₂ of light.
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In the same way as the first interference multi-layer filter of the first film 25, each layer of the second interference multi-layer filter of the second film 35 is thin in an area near the filament 23 and is thicker in an area far from the filament 23
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thickness of each layer in the area near the filament 23 is smaller than the thickness of each layer in the area far from the filament 23. As a result, the thickness t₃ of the second film 23 in the area near the filament 23 is smaller than the thickness t₄ of the second film 35 in an area far from the filament 23. In other words, each layer of the second interference multi-layer filter of the second film 35 is thin in an area having a small incident angle ϑ₃ of light emitted from the filament 23 and is thicker in an area having a larger incident angle ϑ₄ of light emitted from the filament 23 since the area near the filament 23 has a small incident angle ϑ₃ of light and the area far from the filament 23 has a large incident angle ϑ₄ of light. Furthermore, in this embodiment, the thickness t₃ of the second film 23 in the area far from the opening 32 of the reflector 31 is smaller than the thickness t₄ of the second film 35 in the area near the opening 32 of the reflector 31.
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The first film 25 and the second film 35 are obtained by the well-known dipping method. The dipping method includes steps for dipping the tube into the solution including alkoxide of titan, tantal, silicon and so on, pulling up gradually the dipped tube from the solution, and drying the coated liquid which forms a layer. These steps are repeated for as many times as the number of layers of the interference multi-layer filters. Finally the layers are baked to eliminate alkoxy and additive materials, and to form the layers of the interference multi-layer filters. In order to form a layer having varying thickness, the speed of pulling up the tube varies according to the thickness of the layer. The speed of pulling up the tube is slow for forming a thick portion and is high for forming a thin portion. Of course, the varied thickness of each layer of the interference multi-layer filters may be obtained by other methods, for example vacuum evaporation coating.
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As is described above, it is one feature of the present invention that the thickness of the first film 25 and the second film 35 varies according to the position thereof. The advantages of this feature is explained in the following. In contrast the conventional lighting unit has a predetermined constant thickness of the first film and the second film at any position.
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In the above described embodiment, the visible light emitted from the filament 23 of the lamp 11 passes through the tube 13 and the first film 25, and reflects on the second film 35 of the reflector 31, and finally emits through the opening 32. The light emitted from the filament 23 includes not only visible light but also infrared radiation Most of the infrared radiation, for example infrared radiation having a wavelength of 700 nm - 800 nm, is reflected by the first interference multi-layer filter of the first film 25, but the visible light and a small amount of infrared radiation passes through the first interference multi-layer filter of the first film 25. The reflected infrared radiation returns to the filament 23 and heats the filament 23. Therefore the energy supplied to the filament 23 is reduced and the efficiency of the lamp 11 is improved.
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The visible light and the small amount of the infrared radiation passing through the tube 13 and the first interference multi-layer filter of the first film 25 reaches the reflector 31. The visible light is reflected by the second interference multi-layer filter of the second film 35, but infrared radiation, having for example a wavelength of 700 nm - 800 nm,passes through the second interference multi-layer filter of the second film 35. The infrared rradiation passing through the interference multi-layer filter of the second film 35 reaches the reflecting base member 33 and is converted to heat. The heat is radiated from the reflecting base member 33. Finally the visible light reflected by the second interference multi-layer filter of the second film 35 emits through the opening 32 to illuminate objects. Therefore, the illuminated objects are not heated by the infrared-rays and heating damage is eliminated.
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Usually, interference multi-layer filters have a feature that a range of wavelengths of light passing therethrough varies according to the incident angle of light For example, the greater the incident angle of the light, the more the range of wavelengths of the light shifts in the direction of short wavelengths. The reason is explained, using Fig. 5 which is a schematic diagram for explaining the relation between the incident angle of light and the incident position of light to the interference multilayer filter. In Fig. 5, A and B indicate respectively a light source and a refractive layer which has a thickness of d and has a certain refractive index n. Incident light rays I₅ , I₆ and I₇ have incident angles ϑ₅ (=O), ϑ₆ (relatively small) and ϑ₁ (relatively large) to the refractive layer, and generate transmitted light rays T₅, T₆ and T₇ and reflected light rays R₅, R₆ and R₇ respectively. In this case, the phase shift δ of the transmitted light T₅, T₆ or T₇ or the reflected light R₅, R₆ or R₇ is obtained by the following equation;
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When the phase shift δ is constant, interference happens. According to this equation, the larger the incident angle ϑ becomes, the smaller the wavelength λ of the light that can be transmitted and reflected becomes, under the condition that the phase shift δ and the thickness d are constant. In order to decrease the variation of the wavelength λ across the multi-layer filter, under the condition that the phase shift δ is constant, it is necessary to vary the thickness d so that the multiplied value of d. cos ϑ is constant.
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To the contrary, the wavelength λ of the transmitted light or the reflected light becomes smaller according to the increase of the incident angle ϑ, when the thickness d is constant. Since in the prior art the thickness d of each refractive layer of the conventional lighting unit is constant at any position, regardless of varying of the incident angle ϑ, the wavelength λ of the transmitted light or the reflected light varies and the color pattern happens.
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According to the present invention, because the thickness t₁ of the first film 25 at the portion where the incident angle ϑ₁ is small is smaller than the thickness t₂ of the first film 25 at the portion where the incident angle ϑ₂ is large, as shown in Fig. 4, the multiplied value of d.cosϑ is kept constant, and therefore the variaton of the wavelength λ of the transmitted visible light and the reflected infrared radiation is suppressed.
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With regard to the second film 25, because the thickness t₃ of the second film 35 at the portion where the incident angle ϑ₃ is small is smaller than the thickness t₄ of the second film 35 at the portion where the incident angle ϑ₄ is large, as shown in Fig. 4, the multiplied value of d.cosϑ is kept constant, and therefore the shifts of the wavelength λ of the transmitted infrared radiation and the reflected visible light are suppressed.
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As the variation of the wavelengths of the transmitted light and the reflected light at both films 25 and 35 are suppressed, the color pattern is not generated.
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Fig. 6 is a partial sectional view of the first interference multi-layer filter of the first film 25 and the tube 13 according to the second embodiment of the present invention. In Fig. 6, the same numerals are applied to the similar elements. The thickness of each layer of the first interference multi-layer filter of the first film 25 of this embodiment varies step by step instead of the continuously varying thickness of each layer of the first interference multi-layer filter of the first film 25 of the first embodiment. In the same way, the thickness of each layer of the second interference multi-layer filter of the second film 35 (not shown) may vary step by step instead of the continuously varying thickness of each layer of the interference multi-layer filter of the second film 35 of the first embodiment.
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The structure of the interference multi-layer filters of the first film 25 and the second film 35 is not limited to the embodiments explained above. In the third embodiment as shown in Fig. 7, the refractive index of each layer of the interference multi-layer filters of the first film 25 and the second film 35 may vary according to the position of each layer of the interference multi-layer filters instead of the thickness of each layer of the interference multilayer filters of the embodiments explained above varying. According to the equation (I), the similar result can be obtained by varying the refractive index n of each layer of the interference multi-layer filters of the first film 25 and the second film 35 according to the position of each layer of the interference multi-layer filters.
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In the third embodiment, the interference multi-layer filter of the first film 25 has five sections 251, 253, 255, 257 and 259, and the interference multi-layer filter of the second film 35 has three sections 351, 353 and 355. Each layer of the sections 251 and 259 has the same two kinds of refractive index, and each layer of the sections 253 and 257 has the same two kinds of refractive index. Each section has the same range of wavelengths of light transmitting therethrough and reflecting thereby even if the position of each section is different from each other, because the refractive index n of each layer of the first interference multi-layer filter of the first film 25 varies according to the sections of the first film 25. In other words, because each layer of each section of the first film 25 has different refractive indexes according to each section of the first film 25, the first film 25 suppresses a variation of the range of wavelengths of light which passes (transmits) therethrough from the range of the desired wavelengths of light.
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In the same way, each layer of each section of the second film 35 has a different refractive index according to the position of each section of the second film 35 so that the second film 35 suppresses variation in the range of wavelengths of light which passes (transmits) therethrough from the range of the desired wavelengths of light and so that each section of the second film 35 has the same range of wavelengths of light passing (transmitting) therethrough and reflecting thereby.
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Furthermore, the interference multi-layer filters of the first film and second film are not limited to the visible light transparency/infrared-rays reflective film and the visible light reflective/infrared-rays transmissive film. For example, when yellow light is necessary as a specific light, the interference multi-layer filter of the first film may have a function that yellow light selectively transmits and the interference multi-layer filter of the second film may have a function that yellow light selectively reflects.
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In the summary, it will be seen that the present invention overcomes the disadvantages of the prior art and provides an improved layer for preventing glass pieces from scattering when the glass envelope of the lamp is broken. Many changes and modifications in the above described embodiments can thus be carried out without departing from the scope of the present invention. Therefore, the appended claims should be construed to include all such modifications.
