JP3518151B2 - Incandescent light bulbs and reflective lighting devices - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small incandescent lamp in which a coil filament is housed in a translucent airtight container having an infrared reflective film, and a reflective illuminator using the same.

[0002]

2. Description of the Related Art Recently, incandescent light bulbs including halogen light bulbs, a lamp having a visible light transmitting infrared reflecting film formed of a multilayer interference film on the surface of a bulb has been developed, for example, JP-A-55-117861. Example 1) and Japanese Patent Laid-Open No. 63-16548 (hereinafter referred to as Conventional Example 2).

When the visible light infrared reflecting film is formed on the surface of the bulb, the visible light emitted from the coil filament is transmitted to the outside, however, the infrared ray is reflected by the infrared reflecting film and the reflected infrared ray is returned to the coil filament. To heat the coil filament. Therefore, there is an advantage that the temperature rise of the coil filament is promoted, the power energy for heating the coil filament can be reduced, and the power consumption can be saved.

The light bulb described in Conventional Example 1 is a spheroidal glass bulb having an infrared reflecting film formed thereon, and has a structure in which a coil filament is disposed between two focal points of the spheroidal surface. .

Further, the light bulb described in the conventional example 2 is constituted by a glass bulb having an infrared reflecting film formed thereon, and a cylindrical portion and hemispherical portions provided on both sides of the cylindrical bulb. The focus is on the center of the hemisphere.

[0006]

By the way, the coil filament has a temperature distribution where the temperature is originally high in the middle part of the filament and low at both ends when there is no infrared feedback. With such a temperature difference, the filament material evaporates quickly in the high temperature portion, and local thinning of the coil filament causes early disconnection. Therefore, it is desirable that the coil filament has a uniform temperature distribution over the entire length.

In the case of the prior art example 1, if the both ends of the coil filament are arranged so as to match the two focal points of the spheroid, the infrared rays reflected by the infrared reflecting film are returned to both ends of the coil filament. It will contribute to the temperature rise at the end of the filament. However,
Even if the valve has a simple spheroidal shape, it may be insufficient to make the temperature distribution of the coil filament uniform.

Further, in the case of the conventional example 2, since the infrared rays reflected by the cylindrical portion are returned to the intermediate portion of the coil filament which faces the infrared ray, the infrared rays do not work effectively for the temperature rise at the end portion, and Since the length of the cylindrical portion is considerably longer than the length of the coil filament, the temperature at the end of the filament cannot be increased.

The present invention has been made in view of the above circumstances. An object of the present invention is to effectively collect returned infrared rays at the ends of the coil filament, thereby increasing the temperature at the ends of the coil filament. As a result, it is intended to provide an incandescent lamp in which the temperature distribution of the coil filament is made uniform over the entire length and a reflection type illumination device using the same.

[0010]

According to a first aspect of the present invention, there are provided rotational quadratic curved surfaces having substantial focal points and facing each other, and these opposing rotational quadratic curved surfaces have two virtual focal points. Assuming a quadric surface, dividing the virtual rotation quadric surface between these two virtual focal points and bringing the divided curved surfaces closer to each other to move the virtual focal points of the divided curved surfaces inward, A translucent airtight container which is a substantial focal point and is formed by continuously arranging opposing rotary quadric surfaces; a coil filament disposed in the airtight container so as to extend between the two virtual focal points And a visible light transmitting infrared reflecting film formed on the surface of the airtight container.

According to the first aspect of the present invention, since the distance between the real focal points of the curved surfaces facing each other is shorter than the distance between the virtual focal points, the ends of the coil filaments installed so that both ends straddle the virtual focal points. The substantial focus comes to be located near the part. As a result, the infrared ray return rate in the vicinity of both ends of the coil filament becomes higher than the infrared ray return rate in the middle portion of the coil filament, and as a result, the temperature distribution of the coil filament is equalized.

The opposed quadratic curved surfaces may be ellipses or spheroids, respectively, and the opposed quadratic curved surfaces may have the same shape or different shapes. . Furthermore, the respective substantial focal points of the opposing quadric surfaces may be equidistant from their respective virtual focal points or different distances from each other.

Hard glass, quartz glass, or the like is applied to the airtight container. The visible light transmitting / infrared reflecting film is often formed on the outer surface of the airtight container, but may be formed on the inner surface. The visible light transmitting / infrared reflecting film is generally formed of a multilayer interference film, and includes a known material. The coil filament may be a single coil, a double coil or a triple coil. A rare gas or halogen is enclosed in the airtight container. A base is provided at one end or both ends of the airtight container.

The incandescent light bulb of the present invention can be applied to light bulbs for vehicles such as automobiles and airplanes, but is not particularly limited to this, and it is a light source for general lighting, projectors and projectors, etc. It is also applicable to.

The invention of claim 2 is characterized in that the quadratic surface of revolution is a spheroid. If the quadratic surface of revolution is a spheroid, its shape can be easily regulated. The invention of claim 3 is characterized in that the virtual focal points formed by the two virtual focal points approaching each other are separated from the virtual focal point by a distance of ⅕ or more of the outer diameter of the coil filament. To do.

When the distance between the two virtual focal points and the real focal point is less than 1/5 of the outer diameter of the coil filament, infrared rays return near the ends of the coil filament and the temperature distribution of the coil filament is equalized over the entire length. become unable.

In this case, the distance between one virtual focal point and the virtual focal point may or may not be equal to the distance between the other virtual focal point and the virtual focal point. The invention of claim 4 is
It is characterized in that the portions where the curved surfaces facing each other are connected to each other are smoothly continuous.

Since the curved surfaces facing each other are connected to each other by smooth surfaces, no local inflection is generated, the visible light transmittance and the infrared ray reflectivity are equalized, and the airtight container of the airtight container is lit. The temperature distribution is also uniform.

According to a fifth aspect of the present invention, the smoothly continuous surface is a cylindrical surface. Since the cylindrical surface is formed between the curved surfaces facing each other, the infrared ray return rate of the intermediate portion of the coil filament is improved.

The invention of claim 6 is characterized in that the axial length of the cylindrical surface is shorter than the length of the coil filament. When the axial length of the cylindrical surface is longer than the length of the coil filament, the amount of infrared rays returning to the vicinity of the end of the coil filament decreases.

The invention of claim 7 is characterized in that the airtight container has a sealing portion at one end and a thin tube at the other end. This provides a single-sealed light bulb.

The invention according to claim 8 comprises the incandescent lamp according to any one of claims 1 to 7; and a reflector incorporating the incandescent lamp; It is a device. According to the invention of claim 8, there is provided a reflective illuminating device having the advantages of the incandescent lamp according to any one of claims 1 to 7.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below with reference to the first embodiment shown in FIGS. This embodiment shows an example applied to a single-sealed halogen light bulb and a reflection-type lighting device using the same. FIG. 1 is a front view of a single-sealed halogen light bulb used as a light source, and FIG. 2 is the same halogen light bulb. FIG. 3, a cross-sectional view schematically showing the structure of a multilayer interference film forming the infrared reflection film, FIG. 4 is a characteristic diagram showing the temperature distribution of the coil filament, and FIG. 5 is a diagram explaining the shape of the valve. FIG. 6 is a cross-sectional view of a reflection type illumination device using a halogen bulb as a light source.

The one-side sealed halogen light bulb 1 shown in FIGS. 1 and 2 is an airtight container or bulb 2 made of quartz glass.
The valve 2 has a crushing sealing portion 3 formed at one end, and a cylindrical portion 4 is formed continuously with the sealing portion 3. Further, the valve 2 has a bulging diameter portion 5 continuous with the cylindrical portion 4. The bulging diameter portion 5 has the shape of the present invention, which will be described later.

At the other end of the bulging diameter portion 5, the valve central axis O _1-
A thin tube 6 is provided so as to project above O ₁ . This thin tube 6
Utilizes the protrusion that remains after the exhaust pipe is substantially cut off.

A visible light transmitting infrared reflecting film 7 is formed on the outer surface (or inner surface) of the bulb 2. As shown in FIG. 3, the infrared reflective film 7 is configured by alternately forming high refractive index layers 71 ... And low refractive index layers 72 ... As a multilayer film having a total of 6 to 80 layers. Such a multilayer laminated film has a function of reflecting infrared rays by a multilayer interference effect, but transmitting visible light. The high refractive index layer 71 ...
Is titanium oxide (TiO ₂ ), tantalum oxide (Ta ₂ O
₅ ), zirconium oxide (ZrO ₂ ), zinc sulfide (Zn
S) or the like, and the low refractive index layers 72 ... Are made of silicon oxide (silica = SiO ₂ ),
It is composed of magnesium fluoride (MgF ₂ ). And, such an infrared reflection film 7 has a thickness of 400 nm
It has characteristics that the transmittance of light rays including visible light in the 770 nm range is 90% or more and the reflectance of infrared rays in the 770 nm to 1100 nm range is 50% or more.

A coil filament 8 is housed in the bulb 2. The coil filament 8 is made of a tungsten wire into a single coil, a double coil or a triple coil (in this example, a double coil), and its coil axis O ₂ -O ₂ is the valve center line O ₁ -O _1. Are arranged to match. This coil filament 8 has a total length CL of mm to mm and an outer diameter df of the filament of mm or less.

The coil filament 8 is installed between a pair of internal lead wires 10 and 11. These internal lead wires 10 and 11 are metal foil conductors 1 made of molybdenum Mo or the like, the base end portions of which are sealed to the crush sealing portion 3.
2 and 13, and the tip portion is guided to the bulging diameter portion 5 through the cylindrical portion 4 of the valve 2. The pair of internal lead wires 10 and 11 are connected to each other by a bridge glass 9 in the cylindrical portion 4 in the middle thereof, whereby the internal lead wires 10 and 11 are kept at a mutual interval.

The pair of inner lead wires 10 and 11 extend to the bulging diameter portion 5, and one inner lead wire 10 extends to the other end side of the bulging diameter portion 5. The tip of the internal lead wire 10 is inserted into the thin tube 6, and the inserted portion is mechanically locked to the thin tube 6 to form a locking portion 10a. The tip of the one inner lead wire 10 is further extended from the locking portion 10a in the thin tube 6 to the valve center line O ₁
It extends to the other end side of the coil filament 8 along -O ₁ . The other inner lead wire 11 is a valve center line O ₁ −.
It is guided to one end side of the coil filament 8 along O ₁ .

The filament 8 has coil-shaped leg portions 8a and 8b at both ends, and these leg portions 8a and 8b are provided.
Are connected to the respective tip portions of the internal lead wires 10 and 11. As a result, the filament 8 is installed between these internal lead wires 10 and 11.

Molybdenum Mo sealed in the crushing seal portion 3
External lead wires 14 and 15 are connected to the metal foil conductors 12 and 13 made of, for example, as shown in FIG. The crushing sealing portion 3 of the valve 2 is made of a ceramic insulating base 16
, And the insulating base 16 is
Are bonded by an adhesive 17. The insulating base 16 has a screw-in base 18 and an external terminal 19, and the external lead wires 14 and 15 are connected to the screw-in base 18 and the external terminal 19.

Basically, in the halogen light bulb 1 having the above-mentioned structure, the bulging diameter portion 5 is formed as shown in FIG. That is, the bulging diameter portion 5 basically has the shape of a quadric surface of revolution, and has an elliptical spherical surface as a basic shape. However, the ellipsoidal sphere shown in FIG. 5A is a virtual ellipsoidal sphere 50, which has two virtual foci f1 and f2 having a predetermined distance. The coil filament 8 accommodated in the bulging diameter portion 5 has a length such that the end portion of the portion that contributes to light emission overlaps the virtual focal points f1 and f2.

However, the bulging diameter portion 5 of the present invention divides the virtual elliptical spherical surface 50 into right and left portions between the virtual focal points f1 and f2,
As shown in FIG. 5B, the first ellipsoidal spherical surface 5a and the second ellipsoidal spherical surface 5b which are divided are close to each other so that the virtual focal length becomes shorter. That is, the divided first ellipsoidal sphere 5a and second ellipsoidal sphere 5b are
The curvature is formed to be the same as the curvature of the virtual ellipsoidal sphere 50, but they are close to each other. As a result, the divided first and second elliptical spheres 5a and 5b are formed.
Of the virtual focus f1 of the virtual focus f1
, F2 closer to the inside than dimensions f1 and a2. The movement dimensions a1 and a2 may or may not be equal to each other (a1 = a2).

The moving dimensions a1 and a2 are set to ⅕ or more of the outer diameter df of the coil filament 8. If the moving dimensions a1 and a2 are less than ⅕ of the coil filament outer diameter df, the infrared rays are Will be returned near the end of the coil filament, and the virtual elliptical sphere 50
This is no different from using the valve of No. 1 and the temperature distribution of the coil filament cannot be equalized over the entire length.

With the bulging diameter portion 5 having such a structure, the substantial focal points F1 and F2 are formed at positions shifted to the center side from the end portions of the coil filament 8 as compared with the virtual focal points f1 and f2. Become.

The divided first and second elliptic spherical surfaces 5a and 5b are connected to each other by a smooth surface, and in this example, the divided elliptic spherical surfaces 5a and 5b are connected.
A cylindrical portion 5c is formed therebetween, as shown in FIG. 5 (c). The axial length of the cylindrical portion 5c is shorter than the entire length of the coil filament 8.

The halogen bulb 1 having such a structure is shown in FIG.
As shown in FIG. The reflector 20 shown in FIG. 6 has a reflector 22 having a reflecting surface 21 formed thereon. The reflector 22 is made of glass, metal,
The reflecting surface 21 made of a resin or the like and formed on the inner surface thereof may be formed by evaporating aluminum or the like. However, a visible light is formed by a multilayer interference film in which high refractive index layers and low refractive index layers are alternately laminated. However, it may be formed of a visible light reflecting film which reflects infrared rays but transmits infrared rays. Such a reflector 2
An insulating base 23 made of ceramic or the like is bonded to the element 2. A socket 24 is attached to the insulating base 23, and the screw-in base 18 of the halogen bulb 1 is detachably attached to the socket 24, and is housed in the reflector 20. . At the end of the insulating base 23, a screw-type base 24 and an external terminal 25 are provided.
The external terminal 25 is connected to the power supply. Therefore, the halogen bulb 1 is connected to the insulating base 23
When attached to the socket 24, the halogen bulb 1 is connected to the power source through the base 24 and the external terminal 25.

The operation of the single-sealed halogen light bulb having such a configuration will be described. When a power supply voltage is applied to the external lead wires 14 and 15, a current flows through the filament 8 and the filament 8 emits light. This light is emitted to the surroundings, passes through the bulging diameter portion 5 of the bulb 2, and is emitted to the outside. When this light passes through the infrared reflection film 7 formed on the outer surface of the bulging diameter portion 5, visible light is transmitted, but infrared light is reflected.

The infrared rays reflected by the infrared reflecting film 7 are returned to the inside of the bulb 2, and the returned infrared rays heat the filament 8. In this case, the filament 8 is arranged so that its coil axis O ₂ -O ₂ coincides with the valve center line O ₁ -O _1, and the first elliptic spherical surface 5a.
Real focus F1 of F and the real focus F of the second ellipsoidal sphere 5b
Since 2 is arranged inside the end of the filament 8, the infrared rays reflected by the infrared reflecting film 7 on the first elliptic spherical surface 5a and the second elliptic spherical surface 5b respectively have a substantial focal point F.
By the principle of returning to the vicinity of 1 and F2, it returns to the vicinity of the end of the filament 8. Therefore, the temperature of the end portion of the filament 8 rises due to the feedback infrared rays.

In general, when the coil filament 8 is made to emit light nakedly, the intensity of infrared energy generated is high at the central portion of the filament 8 and low at the end portions thereof, as indicated by the characteristic A in FIG. If such a temperature difference is made and lighting is continued, evaporation of the filament material is accelerated in a high temperature portion, and local thinning of the coil filament occurs, leading to early disconnection.

On the other hand, the infrared rays reflected by the infrared reflecting film 7 formed on the outer surfaces of the first ellipsoidal spheres 5a and the second ellipsoidal spheres 5b have filaments whose substantial focal points F1 and F2 are on the ellipsoidal spheres 5a and 5b. Since it is arranged inside the end of filament 8, it returns to the vicinity of the end of filament 8. Therefore, the energy intensity of the feedback infrared rays is collected at the end of the filament 8 as shown by the characteristic B in FIG.

As a result, the infrared energy A due to the heat generated by the filament itself and the infrared energy B due to the feedback infrared rays together form a total infrared energy C, and the total infrared energy C is substantially equal along the length direction of the filament. It will exhibit energy intensity.

Therefore, since the temperature distribution along the length direction of the filament is made uniform and the local temperature difference is eliminated, the local thinning of the coil filament can be prevented and the early disconnection can be prevented.

From the above, the return infrared ray can be effectively used, and the halogen bulb 1 having excellent efficiency can be realized. The substantial focal points F1 and F2 of the first elliptic spherical surface 5a and the second elliptic spherical surface 5b are located inward of the virtual focal points f1 and f2 by the dimensions a1 and a2, respectively. It is desirable that the outer diameter is ⅕ or more. That is, when these moving dimensions a1 and a2 are less than ⅕ of the coil filament outer diameter df, the substantial focal points F1 and F2 approach the virtual focal points f1 and f2, respectively, and the coil filament of the virtual elliptical spherical surface 50 is accommodated. There is no big difference. Therefore, the reflected infrared rays come back to the vicinity of the end of the coil filament, the temperature of the end of the coil filament rises, and the total characteristic C of the infrared energy becomes a waveform, and the temperature distribution of the coil filament. Cannot be equalized over the entire length.

In the halogen bulb 1 of the above embodiment, the bulging diameter portion 5 has the cylindrical portion 4 continuously provided at one end thereof and the thin tube 6 continuously provided at the other end thereof. When the cylindrical portion 4 and the thin tube 6 are relatively larger than the bulging diameter portion 5, the effective area of the infrared reflecting film 7 is reduced and the amount of return infrared rays is reduced. Therefore, it is desirable to limit the sizes of the cylindrical portion 4 and the thin tube 6. In FIG. 1, the divergence angle when the boundary part between the cylindrical part 4 and the bulging diameter part 5 is facing from the center of the filament is θ1, and when the boundary part between the thin tube 6 and the bulging diameter part 5 is facing from the center of the filament. If the divergence angle of is θ2, these divergence angles are θ1 and θ2.
Is less than 30 ° (θ1 ≤ 30 °, θ2 ≤ 30 °)
Is desirable. In this way, the bulging diameter portion 5
The cylindrical portion 4 and the thin tube 6 do not become relatively large in comparison with the above, and therefore the first elliptic spherical surface 5a and the second elliptic spherical surface 5 are
A large area b can be secured, and the infrared reflection film 7
The effective area can be increased. Therefore, the amount of return infrared rays can be increased, which is effective in improving efficiency.

When the halogen bulb 1 having the above structure is housed in the reflector 20 shown in FIG. 6, the visible light transmitted through the bulb 2 and the infrared reflecting film 7 is reflected by the reflecting surface 21 and the front opening of the reflector 22. Irradiate from the front.

According to such a reflection type illuminating device, the efficiency of the halogen bulb 1 is improved, so that the advantage can be utilized and the illuminating device is excellent in efficiency. The present invention can also be configured as in the second embodiment shown in FIG. 7. That is, in the halogen bulb shown in FIG. 7, an elliptic spherical portion 5 is formed continuously with the sealing portion 3, and in the elliptic spherical portion 5, the coil filament 8 is intersected with the bulb center line O ₁ -O _1. Is housed. The elliptic spherical portion 5 is shown in FIG.
Similar to the structure shown in FIG. 2, the virtual ellipsoidal sphere 50 is divided and the divided first ellipsoidal sphere 5a and second ellipsoidal sphere 5 are separated.
b are brought close to each other, and the substantial focal points F1 and F2 of the first elliptic spherical surface 5a and the second elliptic spherical surface 5b are closer to the center of the filament 8 than the virtual focal points f1 and f2, respectively. , These first elliptical spheres 5a
The opposite ends of the second ellipsoidal spherical surface 5b are connected to each other. The connecting portions of the first elliptic spherical surface 5a and the second elliptic spherical surface 5b are connected by a cylindrical portion 5c.

In this embodiment, the sealing portion 5 and the exhaust thin tube 6 are formed at the position of the cylindrical portion 5c. Even in this case, the infrared rays reflected by the infrared reflection film 7 are returned to the vicinity of the end of the coil filament 8, the temperature of the end of the coil filament is increased, and the temperature distribution of the coil filament is distributed over the entire length. Equalize.

Moreover, since the sealing portion 5 and the exhaust thin tube 6 are located in the cylindrical portion 5c, it is possible to secure a large effective area of the first elliptic spherical surface 5a and the second elliptic spherical surface 5b.

[0050]

As described above, according to the first aspect of the present invention, infrared rays are effectively returned to the vicinity of the end of the coil filament, and the temperature distribution of the coil filament is equalized over the entire length. . Therefore, the local thinning of the coil filament is prevented, and the life characteristics are improved.

According to the second aspect of the present invention, since the quadratic surface of revolution is a spheroidal surface, its shape can be easily regulated. According to the invention of claim 3, since the distances between the two virtual focal points and the substantial focal points are each ⅕ or more of the outer diameter of the coil filament, the infrared rays satisfactorily return to the vicinity of the end of the coil filament, The temperature distribution of the filament can be made uniform over the entire length.

According to the fourth aspect of the present invention, since the curved surfaces facing each other are continuously formed by smoothly continuous surfaces, a local inflection portion does not occur, and visible light transmittance and infrared ray reflectivity are provided. And the temperature distribution of the airtight container becomes uniform during lighting.

According to the fifth aspect of the present invention, since the cylindrical surface is formed between the opposed curved surfaces, the infrared ray return rate of the intermediate portion of the coil filament is improved. According to the invention of claim 6, since the axial length of the cylindrical surface is shorter than the length of the coil filament, the amount of infrared rays returning to the vicinity of the end of the coil filament increases.

According to the invention of claim 7, there is provided a light bulb of one-side sealed type. According to the invention of claim 8, it is possible to provide a reflective illuminating device having the advantages of the incandescent lamp according to any one of claims 1 to 6.

[Brief description of drawings]

FIG. 1 is a front view of a halogen bulb showing a first embodiment of the present invention.

FIG. 2 is a perspective view of the halogen bulb of the embodiment.

FIG. 3 is a cross-sectional view schematically showing the structure of a multilayer interference film that constitutes an infrared reflective film.

FIG. 4 is a characteristic diagram showing an infrared energy distribution.

5A to 5C are views for explaining the shape of a bulging diameter portion.

FIG. 6 is a cross-sectional view of a reflective lighting device using the halogen bulb shown in FIGS. 1 and 2 as a light source.

FIG. 7 shows a second embodiment of the present invention and is a front view of a halogen bulb.

[Explanation of symbols]

1 ... Halogen bulb 2 ... Valve 3 ... Sealing part 4 ... Cylindrical part 5 ... Swelling diameter part 5a ... first split ellipsoidal sphere 5b ... Second split ellipsoidal sphere 5c ... Cylindrical part 6 ... thin tube 7 ... Infrared reflective film 8 ... Coil filament F1, F2 ... Real focus f1, f2 ... Virtual focus 50 ... Virtual ellipsoid 20 ... Reflector 21 ... Reflective surface 22 ... Reflector

Continuation of front page (56) Reference JP-A-63-16548 (JP, A) JP-A-60-77347 (JP, A) JP-A-7-45254 (JP, A) JP-A-6-338303 (JP , A) JP 6-310108 (JP, A) JP 6-290760 (JP, A) JP 6-290759 (JP, A) JP 6-283146 (JP, A) JP 3-283350 (JP, A) Actual development Sho 64-34753 (JP, U) Special table 10-501368 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01K 1/00 -1/70

Claims (8)

(57) [Claims] 1. A quadratic curved surface having a virtual focus and facing each other, and the quadratic curved surfaces facing each other are assumed to be quadratic curved surfaces having two virtual focal points. By dividing the curved surface between these two virtual focal points and moving the divided curved surfaces closer to each other, the virtual focal points of these divided curved surfaces move inward to become the substantial focal points, and these virtual rotations are opposed to each other. Two
A translucent airtight container configured by connecting a curved surface; a coil filament arranged in the airtight container across the two virtual focal points; and a visible light formed on the surface of the airtight container. An incandescent light bulb, comprising: a light transmitting infrared reflecting film; 2. The incandescent light bulb according to claim 1, wherein the quadric surface of revolution is a spheroidal surface. 3. The virtual focal points formed by the two virtual focal points approaching each other are closer to each other than the virtual focal point by a distance of ⅕ or more of the outer diameter of the coil filament. The incandescent light bulb according to claim 1 or claim 2. 4. The incandescent lamp according to claim 1, wherein the portions where the curved surfaces facing each other are connected to each other are smoothly continuous. 5. The incandescent light bulb according to claim 4, wherein the smoothly continuous surface is a cylindrical surface. 6. The incandescent lamp according to claim 5, wherein an axial length of the cylindrical surface is shorter than a length of the coil filament. 7. The incandescent lamp according to claim 1, wherein the hermetic container has a sealing portion formed at one end and a thin tube formed at the other end. 8. A reflective lighting device comprising: the incandescent lamp according to claim 1; and a reflector incorporating the incandescent lamp.