GB2302208A - Electric incandescent lamps - Google Patents

Abstract

A tungsten-halogen low-voltage capsule lamp has an infra-red reflective filter (8) for reflecting infra-red radiation onto a filament (6) extending axially of the cylindrical envelope (1) of the lamp. The filter is transparent to visible light.

Description

ELECTRIC INCANDESCENT LAMPs The present invention relates to electric incandescent lamps.

Electric incandescent lamps comprising a tungsten filament within a glass envelope are well known.

Incandescent lamps are inherently inefficient because the bulk of the radiated power occurs in the infrared region of the spectrum. Only a small fraction of the radiated energy is radiated in wavelengths visible to the human eye; most of the energy is radiated in the near infrared.

One approach to increasing the efficacy (Lumens/Watt) of an incandescent lamp is to provide a halogen doped gas atmosphere surrounding the tungsten filament. A conventional incandescent lamp loses filament material by evaporation, much of the filament material being deposited onto the envelope. When a halogen is added to the filling gas, a reversible chemical reaction can be established between the tungsten and halogen. The tungsten is evaporated from the filament and some portion of this diffuses towards the bulb wall. The tungsten combines with the halogen and the tungsten halogen molecules diffuse towards the filament where they disassociate, the tungsten being deposited back onto the filament while the halogen is available for a further reaction cycle.

In addition, lamp makers have long been aware of the potential of improving the efficacy of incandescent lamps by returning the infrared radiation to the filament.

For example GB-A-497880 proposed in 1937 a lamp having an incandescent body, or filament, to which the infra-red portion of the radiation from the body is thrown back with minimum loss. GB-A-497880 observes "a geometrico-optical image of the incandescent body on it itself is not necessary but it is of paramount importance that the radiation density of the rays thrown back shall attain a sharp maximum at the place where the incandescent body is located.In other words, the optical arrangement must deflect back on to the incandescent body, with as little loss as possible, as large a proportion as possible of the undispersed rays which fall upon it from the said incandescent body." GB-A-497880 proposes, inter alia, "a rotationally symmetrical arrangement (in the simplest case a cylinder) along the axis of which the incandescent body (preferably in the form of a single coil or a multiply coiled coil), extends in a straight line, while the devices which throw back the rays are arranged at the surface of the rotation body or cylinder.The said devices may consist of reflecting prisms or pyramids, with plane or, preferably, curved reflecting surfaces and they may be so arranged that they direct certain of the rays sent out by the incandescent body back on to the said incandescent body again, by double total reflection.

In order, however, to produce the requisite separation between the visible radiation and the infra red radiation, either the reflecting members (prisms or pyramids) may themselves be made of suitable opal glass or opal quartz." Such arrangements have, as far as is known, never been put into practice and indeed appear to be impracticable.

In US-A-4160929, US-A-4,535,269, US-A-4,949,005 and US-A-4,839,559 there are disclosed practical tungsten halogen incandescent lamps in which the infrared radiation is returned to the filament. In all of these U.S. patents, the filament is in focused relationship with an infrared reflective coating on an envelope surrounding the filament, with the centre of curvature of the reflector within the filament. Furthermore, it is necessary to keep the filament at the focus of the reflective arrangement throughout the life of the lamp avoiding filament offset due for example to sag.

In US-A-4,535,269 a filament is held in place within an envelope having a substantially ellipsoidal shape with an infrared reflective coating on the outer surface of the ellipsoidal envelope. The filament extends between the two foci of the ellipsoidal envelope such that almost all of the infrared radiation emitted by the filament is reflected from the walls of the ellipsoidal envelope and returned to the filament at the first reflection.

US-A-4,839,559 discloses a tungsten halogen lamp having a filament within an envelope with an infrared reflective coating on the envelope. The envelope is a right circular cylinder with the filament extending along the axis of the cylinder and thus at the focus of the cylinder. As stated in US-A-4,839,559 "For infrared reflective coatings on such incandescent lamps to be effective, it is essential for the reflected energy to be focused back upon the lamp filament. For this optical criterion to be satisfied, it becomes necessary that the lamp filament be precisely located with respect to the reflective film deposited on the lamp envelope." US-A4,839,559 discloses various structural configurations for maintaining the filament position regardless of the lamp spatial orientation.

GB-A-2,044,993 proposes a practical improvement to a lamp disclosed in DE-A-2,811,037. The lamp of DE-A2,811,037 has a spherical envelope provided with a lightpervious infra-red radiation reflecting filter and a spherical filament at the centre of the filter; thus providing a focused relationship between the filter and filament. GB-A-2,811,037 observes it is impossible to manufacture a spherical filament and so provides, instead, in the lamp of DE-A-2,811,037, a filament which is a flat folded filament of helically wound wire and is situated within a square having a side length between 0.25 and 0.04 times the inner diameter of the lamp envelope. The filter of GB-A-2,811,037 may be a metaldoped metal oxide filter or a layer of silver between two layers of Ti02.

EP-A-0,588,541 discloses a tungsten halogen incandescent lamp comprising a quartz envelope, an end cap and terminal pins. The envelope contains a biplanar grid form filament having a plurality of filament sections. The outer surface of the envelope is coated with an infrared reflective filter which allows visible light to be transmitted through the envelope and filter.

The grid fills as much of the cross-section of the lamp as practicable. The filament sections are disposed within the envelope in a combined manner so as to avoid the requirement of precisely aligning the filament sections relative to the infrared reflective filter.

The filament sections thus intercept the reflected infrared radiation increasing the efficacy of the lamp, even though the filter is not in focused relationship with the grid.

EP-A-0,588,541 discloses a comparative example of a type CP77 lamp. A CP77 lamp is a single ended lamp having a coiled coil filament extending parallel to the axis but offset therefrom. Under experimental conditions simulating the effect of an infra-red reflective filter, a negligible increase in efficacy was achieved.

Thus the prior art teaches that the filament must be in precisely focused relationship with the infra-red reflective filter or in as nearly precisely focused relationship as is practicable. Alternatively, the filament must be constructed to fill as much of the cross-section of the lamp as possible to intercept reflected infrared radiation. Experiments made by applicants have indicated that if these conditions are not met, the effect of an infra-red reflector is negligible.

The applicants have now discovered contrary to the established practice that an infra-red reflective filter significantly improves the efficacy of single-ended capsule lamps having axial filaments without measures to focus the reflective filter on the filament.

According to the present invention, there is provided a single-ended capsule lamp comprising: a generally cylindrical envelope, a tungsten filament extending along the axis of the envelope, and an infrared reflective filter on the envelope. The pitch ratio of the filament is greater than one and preferably is less than or equal to 3, more especially less than or equal to 2. The filament may be a singly coiled or be a coiled-coil.

No measures are provided to focus the reflective filter on the filament and/or to maintain that focus with varying operating conditions. The lamp without the filter is preferably a known, standard production lamp.

Although in such a capsule lamp the reflective filter is in substantially unfocused relationship with the filament a useful increase in efficacy of about 12% is achieved.

According to conventional expectations in the art, because of the absence of focusing the effect of an infra-red reflector would be negligible because the chance of a ray of infra-red radiation intersecting the filament at first reflection would be small.

Such a capsule lamp may be a tungsten halogen lamp and may be used in a halogen display lamp in which the capsule is combined with a reflector which may be a glass-dichroic mirror or an aluminised mirror.

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings, in which: FIGURE 1 comprises front and side views of an illustrative capsule lamp in accordance with the invention.

FIGURE 2 comprises front and side views of another such lamp, FIGURE 3 is a graph which illustrates the reflectance characteristics of a multi-layer interference filter useful in the present invention, and FIGURE 4 illustrates a portion of the coiled coil of the lamp of Figure 1 or 2.

Figures 1 and 2 show by way of example a low-voltage tungsten-halogen capsule lamp. Low-voltage lamps are designed to operate at a voltage in the range 6 to 36V.

Individual types of lamp operate at e.g. 6V, 12V and 24V.

Referring to Figure 1, the single-ended capsule lamp comprises an envelope 1 made from a circular tube of quartz. A pinch seal 2 is formed in one end of the tube. The pinch seal includes molybdenum foils 3 to which are attached pins 4 extending out of the lamp and the leads 5 of a filament 6. A seal 7 is provided at the end of the envelope remote from the pinch 2. A Halogen fill is introduced into the envelope prior to the seal 7 being formed.

Figure 2 shows a lamp similar to that of Figure 1 but made of glass. The envelope differs in shape from that of the lamp of Figure 1. However, the envelope 1 is generally cylindrical having an approximately circular cross-section, and the filament 6 is generally coaxial with the axis of the envelope.

The shapes of the envelopes are not precisely controlled during manufacture of the lamps of Figures 1 and 2.

In accordance with the invention, an infra-red reflective filter is provided on the envelope of the capsule lamp. The filter is schematically indicated by dashed line 8 in Figures 1 and 2.

Preferably the filter is a non-diffusing filter which specularly reflects infrared radiation from the filament whilst allowing visible light from the filament to pass through the envelope and filter. An example of a suitable filter is disclosed in US-A-4,949,005. The filter of US-A-4,949,005 consists of alternate layers of tantala and silica suitable for high temperature use on electric lamps. FIGURE 3 shows the reflectance of a 46 layer Ta205/Si02 interference filter for infrared reflection. FIGURE 3 shows that the reflectance of the filter is negligible for visible light.

The filter is on at least to the cylindrical portion of the lamp as indicated by the dashed line 8. The filter may be on the whole of the lamp except for the pinch seal 2 which is free of the filter.

The filament of the lamp of Figure 1 or 2 is a coiled coil as shown in Figure 4. The filament has a pitch ratio. Pitch ratio is the ratio S/D of the space S between adjacent turns or coils of the coiled filament to the diameter D of the coiled filament to the diameter D of the coiled filament.

The pitch ratio is preferably in the range 1 to 2, preferably approximately 1. The pitch ratio cannot be one or less than one because the adjacent turns would then touch.

The pitch ratio is not more than about 3 because the larger the pitch ratio, the greater the space between adjacent coils, and the less likelihood that reflected infra-red radiation will intersect the filament.

Although the invention has been described in relation to tungsten halogen lamps of voltage in the range 6 to 36V, it is also applicable to: (a) incandescent lamps without a Halogen fill and/or (b) capsule lamps of higher voltage with similar geometrical arrangement.

The invention may be applied to capsule lamps used in conjunction with reflectors, such as dichroic-coated glass reflectors and aluminised glass reflectors.

Claims (8)

CLAIMS:

1. A single-ended capsule lamp comprising a generally cylindrical envelope, a tungsten filament extending along the axis of the envelope and an infra-red reflective filter on the envelope.

2. A lamp according to claim 1 wherein the pitch ratio of the filament is greater than one but less than 3.

3. A lamp according to claim 1 wherein the pitch ratio of the filament is greater than one but less than or equal to 2.

4. A lamp according to claim 1, 2 or 3 wherein the filament is singly coiled or is a coiled-coil.

5. A lamp according to claim 1, 2, 3 or 4 which is a tungsten-halogen lamp.

6. A lamp according to claim 1, 2, 3, 4 or 5 in combination with a reflector.

7. A lamp according to any preceding claim which is arranged to operate at a voltage in the range 6V to 36V.

8. A capsule lamp substantially as hereinbefore described with reference to Figure 1 or Figure 2 optionally together with Figure 3 and/or Figure 4.