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(11) | EP 0 588 541 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Electric incandescent lamps |
| (57) A tungsten halogen incandescent lamp comprises a quartz envelope 1, an end cap 3
and terminal pins 4. The envelope 1 contains for example a bi-planar 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 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 intercept
the reflected infrared radiation increasing the efficacy of the lamp. |
[0002] Electric incandescent lamps comprising typically a tungsten filament within a glass envelope are very 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.
[0003] One approach to increasing the efficacy (Lu- mens/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.
[0004] 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.
[0005] 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.
[0006] 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."
[0007] Such arrangements have, as far as is known, never been put into practice and indeed appear to be impracticable.
[0008] 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.
[0009] 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.
[0010] 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-A-4,839,559 discloses various structural configurations for maintaining the filament position regardless of the lamp spatial orientation.
[0011] GB-A-2,044,993 proposes a practical improvement to a lamp disclosed in DE-A-2,811,037. The lamp of DE-A-2,811,037 has a spherical envelope provided with a light-pervious 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 metal-doped metal oxide filter or a layer of silver between two layers of Ti02.
[0012] According to one aspect of the present invention, there is provided an electric incandescent lamp comprising: an envelope; a refractory metal filament within the envelope for producing infrared radiation and light; and a filter on the envelope which filter reflects the infrared radiation back towards the filament and allows at least selected wavelengths of the light to be transmitted through the envelope and filter; wherein the filament and the infrared reflective filter are in substantially unfocused relationship, the filament being arranged to intercept the reflected infrared radiation.
[0013] In a preferred embodiment of said one aspect the filament is supported by a frame, the segments of the filament extending in one direction in the frame and being distributed in the frame transversely of said one direction.
[0014] In accordance with the present invention it has been found, contrary to the conventional expectations in the art, that infrared reflective filters can be applied with advantage to incandescent lamps, especially certain types of tungsten halogen lamps, which are not specifically designed to focus the infrared radiation back onto the filament. As discussed hereinbefore, the practical application of infrared reflective films to incandescent lamps has been based on the assumption that it is essential to maintain the filament substantially at the focus of a reflective envelope so that the maximum amount of infrared radiation returns to the filament at the first reflection. It was considered that if this criterion was not met then the infrared reflective filter would not significantly increase efficacy because the infrared would not return to the filament and/or the filament would be unevenly heated leading to hot spots and reduced filament life due to localized evaporation.
[0015] For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:
FIGURES 1 and 1A are a side view of a tungsten halogen lamp,
FIGURE 2 is a detailed front view of a lamp filament,
FIGURE 3 is a side view of the filament of Figure 2,
FIGURE 4 is a graph which illustrates the reflectance characteristics of one type of multi-layer interference filter useful in the present invention,
FIGURES 5 to 7 illustrate known tungsten halogen lamps which may benefit from the present invention, and
FIGURE 8 shows, by way of comparison, a tungsten halogen lamp forwhich the present invention has a minimal effect.
[0016] FIGURE 1 is a schematic illustration of a tungsten halogen lamp having a fused quartz envelope 1 enclosing a tungsten filament generally indicated at 2. The envelope 1 is generally cylindrical but has what is known as a "blown" form; i.e. it has a bulbous, but non-spherical, portion 5 around the filament. The lamp has a conventional end' cap 3 provided with terminal pins 4. As is known in the art the end cap 3 and pins 4 can take a wide variety of forms other than those shown in FIGURE 1.
[0017] FIGURE 2 shows an example of a bi-planar grid-form filament 20 which can be used as the filament 2 of the lamp 1. Filament 20 comprises segments of coiled tungsten wire connected together and supported by segment supports between the segments in a generally rectangular frame 21. The bi-planar nature of the filament is shown in FIGURE 3. As shown in FIGURES 2 and 3, filament 20 comprises a multiplicity of vertical segments 22 alternately arranged in two adjacent parallel rows 23 and 24, the segments 22 in row 23 being positioned so that they align with the gaps between segments 22 in the other row 24. This is done to present a source which appears as solid as possible to a fresnel lens when the lamp is used as e.g. a studio lamp.
[0018] The envelope 1 has on the outside thereof 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 4 shows the reflectance of a 46 layer Ta205/SiO2 interference filter for infrared reflection. FIGURE 4 shows that the reflectance of the filter is negligible for visible light.
EXAMPLES
[0019] Trials have been carried out on the effect of infrared reflecting coatings on the efficacy of various known tungsten halogen lamps. Examples of the lamps are shown in FIGURES 1A and FIGURES 5 to 7. All of the lamps are standard production lamps in which no special measures have been taken to provide focusing of reflected radiation on the filament.
FIGURE 1A
[0020] FIGURE 1A shows a lamp identified as type T19 in the GE Lighting Lamp Catalogue published in Europe in 1992. The T19 lamp is a single ended lamp having a bi-planar grid-form filament and, in the example shown, has a blown envelope although it may also be made with a straight-sided cylindrical envelope. A 46 layer tantala/silica interference filter for infrared reflection was provided on the outer surface of the envelope. Three such lamps were made. The three lamps with interference filters were tested and compared with three similar lamps without such filters. The tests showed that the lamps with filters produced an increased efficacy (lumens per watt) of about 20% compared to the lamps without filters, when run at the same filament temperature as measured by resistance as discussed below.
FIGURES 5 - 7
[0021] Lamps of the types shown in FIGURES 5 - 7 were also tested, but for the purposes of the lamps of FIGURES 5 - 7 the effect of an infrared reflective filter was simulated using the following experimental procedure.
[0022] The visible radiation emanating from a tungsten filament is dependent upon the temperature of the tungsten; the hotter the filament the more visible radiation emitted. The resistance of the filament is proportional to the average filament temperatures to the power 1.2. The temperature and light output of a specific filament can therefore be estimated from its operating resistance. In order to determine the potential gains in efficacy achievable via the application of an infrared reflective filter to a tungsten filament lamp, lamps were operated under standard conditions and measurements made as voltages were increased in equal steps from 0 up to the standard operating voltage of the lamp under test. To simulate the effect of the infrared reflective filter, the external walls of the lamps were then coated with aluminium to produce a wide spectral bandwidth reflective layer. The resistance of the filament was once again determined at the incremental voltages between 0 and the operating voltage. A comparison was then made between the power at which the resistance of the aluminium coated lamp is equal to the resistance of the uncoated lamp operating at its stated power.
[0023] The envelopes of the lamps were of quartz and the aluminium coating oxidised at high operating temperatures. The lamps were cooled using air flowing over them. However, in the experiments this did not prevent the aluminium coating degrading before full operating voltages were achieved. Thus the following results are based on the maximum experimental voltage achieved before the coating degraded.
RESULTS
[0024]
FIGURE 5 shows a Type CP70 lamp which is a single-ended lamp having a bi-planar grid form filament. Under the experimental conditions the coated lamp achieved a power saving of 26% compared to the uncoated lamp.
FIGURE 6 shows a T18 lamp which is a single ended lamp having a mono-planar grid form filament. Under the experimental conditions the T18 tamp achieved a power saving of 13% compared to the uncoated lamp.
FIGURE 7 shows a type M40 lamp which is a single ended lamp having a grid form bi-planar filament. Under the experimental conditions the coated M40 lamp achieved a power saving of about 20% compared to the uncoated M40 lamp. COMPARATIVE EXAMPLE: FIGURE 8 shows a type CP77 lamp. CP77 is a single ended lamp having a coiled coil filament extending parallel to the axis of the generally cylindrical envelope but offset from the axis. Under the experimental conditions, the coated lamp achieved only negligible power savings compared to the uncoated lamp.
[0025] The types of lamp achieving the best results were the lamps having grid form filaments in which the grid has an extent, orwidth, laterally of the axis of the generally cylindrical envelope, greater than 25% (e.g. 30%) of the diameter of the cylindrical envelope in the region of the grid and less than the internal diameter of the envelope, the maximum width being a matter of choice determined by practical considerations. As shown in the FIGURES 1A, 5, 6 and 7 of the drawings the width of the grid is about 50% of the diameter of the envelope. The length of the grid parallel to the axis of the envelope is a matter of choice. The maximum length of the grid is determined by practical considerations, mainly sag of the filament due to gravity. As shown in FIGURES 1A, 5, 6 and 7 of the drawings the length is about 1 to 1% times the width. It is believed that those results are due to the fact that such a grid form filament having e.g. a plurality of filament segments supported on a frame intercepts a large proportion of the infrared radiation on its first reflection from the envelope because the grid extends longitudinally and transversely of the axis of the generally cylindrical envelope providing effectively a nearly solid-looking surface. Unexpectedly, there is no evidence of uneven heating, e.g. hot spots, occurring in the grid form.filament. Because the envelope is generally cylindrical it might be expected that the reflected infrared would be concentrated along the axis of the generally cylindrical envelope producing excessive heating there, leading to premature failure of the lamp due to localised evaporation of the filament.
[0026] It is believed that the CP77 type lamp shown in FIGURE 8 produces no significant power saving because the coil is so shaped and positioned as to fail to intercept reflected infrared.
[0027] Lamps as shown in Figures 5 to 7 are, in accordance with the invention, provided with an infrared reflective filter which allows light to be transmitted on the envelope thereof.
[0028] The use of the infrared reflective filter allow trade-offs between efficacy, lamp life and power input. Furthermore, the interference filter can be designed to increase color temperature, block UVand/or provide a cooler beam.
[0029] The present invention provides benefits for studio lamps and for projector lamps because a lamp with an infrared reflective filter produces higher visible beam output for a given power than a similar lamp with no filter and also produces less infrared radiation in the beam without excessive heating of the fitting in which the lamp is used.
1. An electric incandescent lamp comprising:
an envelope;
a refractory metal filament within the envelope for producing infrared radiation and light; and
a filter on the envelope which filter reflects infrared radiation back towards the filament and allows at least selected wavelengths of the light to be transmitted through the envelope and filter;
wherein the envelope is generally cylindrical,
the filter comprises alternating layers of tantala and silica, the filament and the infrared reflective filter are in substantially unfocused relationship and the filament extends longitudinally and transversely of the cylindrical envelope to intercept the infrared radiation reflected by the filter.
2. A lamp according to claim 1, wherein the filament is supported by a frame, the
segments of the filament extending in one direction which is longitudinal of the envelope
in the frame and being distributed in the frame transversely of said one direction.
3. Alamp according to claim 2 wherein the filaments extend, transversely of said one
direction, over more than 25% of the diameter of the envelope in the region thereof
occupied by the filaments.
4. A lamp according to claim 2, wherein the filaments extend transversely of said
one direction over about 50% of the diameter of the envelope in the region thereof
occupied by the filaments.
5. A lamp according to any preceding claim, wherein the filament has a mono-planar
grid form.
6. A lamp according to any preceding claim, wherein the filament has a bi-planar grid
form.
7. Alamp according to any preceding claim, wherein the generally cylindrical envelope
has a bulbous zone around the filament, the bulbous zone being in unfocused relationship
with the filament.
8. A lamp according to any preceding claim wherein the envelope is glass.
9. A lamp according to any one of claims 1 to 7 wherein the envelope is of fused quartz.
10. A lamp according to any preceding claim which is a single-walled lamp, the single
wall of which is provided by the said envelope.
11. A lamp according to any preceding claim wherein the filter is on the outer surface
of the envelope.
12. A lamp according to any preceding claim which is a tungsten-halogen lamp.