Halogen incandescent lamp with heat transfer by conduction

An incandescent halogen lamp is operated under convection-free conditions. In this way, light yields of about 15 to 20 lm/W with service lives of about 2000 hours can be attained. The important parameters are the bulb dimensions, luminous element dimensions, and fill gas properties.

FIELD OF THE INVENTION 
The invention relates to incandescent halogen lamps closed on one end, 
particularly for general lighting (AB), but also for photographic or 
projection purposes (FO) or other applications. In particular, it is 
applicable to low-voltage lamps with low power, but it can also be used 
for high- and medium-voltage lamps. 
BACKGROUND 
Lamps of the type to which the invention relates are known for instance 
from German Patent Disclosure DE-OS 22 31 520. Their cold fill pressure is 
typically approximately 5 to 15 atm of an inert gas, predominantly noble 
gases (argon, krypton, xenon), which optionally have a slight proportion 
(5 to 10%) of nitrogen. Traces of one or more halogen compounds are also 
added, whose partial pressure amounts to only a few millibars. 
In such lamps, predominantly for general lighting purposes, the attainment 
of a relatively long service life (AB: 2000 hours, FO 200 hours and more) 
is a major goal. To achieve this, the assumption is generally made that 
the evaporation rate of the tungsten material of the luminous element must 
be damped by means of the highest possible fill pressure (in halogen lamps 
approximately 2.7 to 10.7 bar (approximately 2000 Torr), according to H. 
Lohmann, Electrotechnik Electrical Engineering!, 1986, pp. 33-36 and 
particularly page 35. At the same time, the halogen compound present as a 
fill component, with the aid of the convection occurring in the bulb, 
supports a cycle process for the tungsten particles evaporating from the 
luminous element (S. M. Correra, Int. J. Heat Mass Transfer 30, p. 663, 
1987). However, the convection also causes a considerable heat loss, which 
is on the order of magnitude of 10% of the lamp power. 
In general, it has until now been considered indispensible to maintain such 
operating conditions, in order to be able to achieve a high light yield 
(up to 25 lm/W) with a simultaneously high lamp life (at least 2000 
hours). 
For special reasons, it is true that special lamps with a low cold fill 
pressure have been individually developed (European Patent Disclosure DE-A 
295 592), but on the precondition that nevertheless the convection that 
drives the halogen cycle takes place. A further example of this is French 
Patent Disclosure FR-A 2 436 495. It describes a photographic lamp pinched 
on both ends, in which the cold fill pressure is lowered to about 0.2 bar 
in order to reduce the risk of explosion. This is achieved at the usual 
cost of reducing the service life. Moreover, in U.S. Pat. No. 4,463,277, 
an incandescent halogen lamp with a cold fill pressure of about 0.8 to 
0.93 bar is described. The low pressure is intended to enable the use of 
hard glass. 
An essential role in the theoretical discussion of incandescent lamps is 
played by the concept of the Langmuir layer, which has been explained in 
detail, for instance in Techn. Wissenschaftl. Abhandlungen der OSRAM GmbH 
(TWAOG) Industrial Science Papers published by OSRAM GmbH!, Vol. 9, pp. 
125-136, 1967, Springer-Verlag, Berlin. Here the existence of a horizontal 
stationary gas layer, in fact the Langmuir layer, adjacent to the luminous 
element, which is considered to be a cylinder, is assumed, and is observed 
to be homogeneous and of constant diameter. In it, heat removal takes 
place solely by heat dissipation, while outside this layer the heat losses 
are determined by free convection. In incandescent halogen lamps, the 
Langmuir layer thickness is on the order of magnitude of a few millimeters 
(see also the aforementioned German Patent Disclosure DE-OS 22 31 520). It 
is dependent on the fill pressure. 
From European Patent Disclosure EP-A 0 241 911, an incandescent halogen 
lamp with a luminous element with a wire diameter on the order of 
magnitude of 100 .mu.m is known. 
Special conditions pertain in soffit lamps, that is, elongated incandescent 
lamps pinched on both ends with an axially arranged luminous element. 
Here, if there is a deviation from the horizontal position (and 
particularly in a vertical burning position), severe problems arise in 
operation, having to do with the demixing that occurs between the fill gas 
and the halogen additive inside the bulb. In U.S. Pat. No. 3,435,272 and 
in an article in Illum. Engin., April 1971, pp. 196-204, the cooperation 
of diffusion and convection at a fill pressure of 0.5 to 15 bar is; said 
to be responsible for this thermally-caused demixing effect. This effect 
is suppressed by means of a glass tube introduced into the bulb and 
surrounding the luminous element. A similar concept is also presented in 
U.S. Pat. No. 4,703,220. Here the glass tube is intended to partially 
suppress convection. 
The invention describes an entirely new way to furnish incandescent halogen 
lamps as defined by the preamble to claim 1, which makes possible a 
relatively high light yield and at the same time a long lamp life. 
This novel technical teaching is based on the characteristics of the body 
of claim 1. Especially advantageous features are found in the dependent 
claims. 
While the prevailing teaching was based on a linear relationship between 
the fill pressure and service life, entirely surprisingly, it has also 
been demonstrated that under certain peripheral conditions, namely in 
lamps with thin wires as the luminous element material, values for the 
light yield and service life can be attained that are even equivalent to 
those at high pressures. The wires preferably have a maximum diameter of 
200 .mu.m, and especially preferably-less than 100 .mu.m. Excellent 
improvements can be attained in lamps with wire diameters of below 50 
.mu.m. The cause for this performance is that with small wire diameters, 
instead of the evaporation mechanism, some other failure mechanism 
determines the service life of the wire, namely the migration of the 
tungsten along the wire. At very small wire diameters, the grain structure 
of the wire makes itself strongly felt, since only from one to two grains 
are present across the wire diameter. The migration of the tungsten makes 
itself felt at the grain boundaries in the form of constrictions between 
the grains. This process is known as "grain boundary corrosion". This 
failure mechanism is independent of the fill pressure; on the other hand, 
it is highly dependent on the surface temperature of coiled wire (typical 
temperature values are approximately 2300 to 3200K). Under the operating 
conditions assumed here, this failure mechanism is surprisingly sharply 
reduced. 
A decisive consequence of the altered failure behavior is the 
correspondingly altered failure characteristic. Typically, the failure 
behavior of a number of lamps follows a modified Gaussian distribution, 
which is known as a Weibull distribution. It is characterized by the mean 
value (time period until failure of 63.2% of a set of lamps) and a certain 
deviation range (variance). Until now, for a mean value of typically 7000 
hours, this variance was typically 5000 hours. 
In other words, the variance was very large in comparison with the mean 
value. By comparison, in lamps according to the invention, quite a 
different failure behavior can be observed. Although the mean value of the 
Weibull distribution is markedly low-voltage lamps (up to 60 V), the field 
of use of the invention is practically unlimited. In high- and medium- 
voltage lamps (more than 60 V operating voltage), the invention can be 
used without restriction only for tubular lamps. 
With lamps pinched on one end, however, care must be taken to assure that 
sparkovers between current-carrying parts be avoided. 
The maximum cold fill pressure that is attainable without convection is 
generally markedly higher in high-voltage lamps than in low-voltage lamps, 
specifically being between 1 and 5 bar. This is due to the generally 
larger dimensions of these lamps. In the8case of high-voltage lamps, the 
intended "convection-free", mode of operation is therefore advantageous 
only if certain disadvantages can be overcome. In general, it has in fact 
been demonstrated that lamps operated convection-free exhibit a markedly 
lesser dependency on the mounting location (for instance, this is of 
interest for tubular lamps). The background for this is the improved 
uniformity of temperature distribution at the bulb found in 
convection-free lamps (a 50% improvement in variance is typical) and in 
general the trend to lower luminous element and bulb temperatures. Both 
effects lead to an improvement in the service life. 
As a consequence, such lamps can also especially advantageously be used in 
reflectors or light fixtures. This is true for both high-voltage and 
low-voltage versions. A reduction in the temperature load by 10%, measured 
at the lamp pinch, over "high-pressure lamps" is typical. 
By suitable geometrical dimensioning of the lamp it is also possible in the 
mode of the convection-free fill pressure range, instead of a simple 
linear dependency of the power losses (when plotted logarithmically), to 
create one region of linear dependency at low pressures and one region 
(plateau) at higher pressures, in which the power loss is virtually 
independent of the pressure a longitudinal axis of the luminous element is 
defined. The luminous element may be located either parallel or vertical 
to the end of the bulb, which is typically closed with a pinch seal. The 
bulb shape may be cylindrical but may also assume some other shape. 
Typical internal dimensions (the inside diameter in the case of the 
cylindrical shape, for instance) are between 3 and 15 mm, but larger 
values are possible as well. Examples of photoelectric data are from 10 to 
more than 20 lm/W (minimum value 1 m/W) for a service life of 2000 hours. 
The advantages of the invention are expressed particularly markedly in 
low-voltage, low-power lamps. The fill volume of the bulb in low-voltage 
lamps is on the order of magnitude of 0.05 to 1 cm.sup.3 and in 
high-voltage lamps up to 15 cm.sup.3. A noble gas, optionally with the 
admixture of nitrogen, is used as the fill gas. A typical cold fill 
pressure is 0.5 to 1.7 bar in low- voltage lamps and up to 5 bar in 
high-voltage lamps. Halogenated hydrocarbons are examples of suitable 
halogen compounds. 
The luminous element dimensions themselves also have an influence on 
operating performance. For instance, the enveloping cylinder of the 
luminous element should advantageously be shaped such that its length is 
at least equal to the diameter; in particular, it can be more than 1.5 
times and preferably more than 2 times greater than the diameter. 
The core factor and pitch factor of the luminous element also have an 
influence on the convection behavior. A point of departure for the pitch 
factor is a value of less than 2.0. The specific value in an individual 
case, however, must be arrived at empirically. 
The color temperature of the lamps of the invention is in a range from 
about 2400 to 3400K. In general, it is true that the present invention is 
advantageously applicable both to lamps closed on one end and lamps closed 
on both ends. The closure is typically done by pinching, but can 
optionally also be done by fusing. In lower (typically 4000 hours), the 
variance in the mean service life is so sharply reduced that the 3% value 
(that is, the service life until the first 3% of the lamps fail) can 
nevertheless be equally favorable or even more favorable than in lamps 
according to the known prior art. This means that the time period between 
the failure of 3% of the set of lamps and the failure of 63.2% of the set 
is sharply reduced, corresponding to a substantially steeper slope of the 
Weibull distribution. Thus despite a poorer mean service life, an 
approximately equivalent or even more- effectively usable service life 
(hereinafter called the rated service life) can be attained (defined as 
the time period defined by the service life of the first 3% of a set of 
lamps). 
A further particular advantage of the invention is that under the operating 
conditions according to the invention, faceting of the surface of the 
coiled wire can be avoided. Faceting describes the phenomenon in which the 
individual grains of the wire material in lamp operation begin to grow in 
accordance with their actually cubic space-centered lattice structure. As 
a result, for one thing the wire surface becomes uneven, and for the other 
the radiant wire surface area is enlarged. This process generally leads to 
a reduction in the residual light flux that is normally measured after 75% 
of the rated service life. Lamps according to the invention now exhibit 
the astonishing phenomenon that the residual light flux is markedly 
greater than in comparison lamps. An increase in the light flux over the 
initial value can even occur. The cause is suspected again to be the 
tungsten migration along the surface, which has a smoothing and equalizing 
effect on the surface profile. 
The incandescent halogen lamps according to the invention normally have a 
high-temperature-proof bulb of quartz glass or hard glass. The luminous 
element contained in it is shaped (typically as a single or double coil) 
cylindrically or at least approximately cylindrically (a slightly curved 
cylinder, for instance), so that range. This plateau behavior is 
especially suitable for attaining the operating state at relatively low 
pressure according to the invention even in high-voltage lamps. This is 
because the plateau makes it possible to choose the "operating point" not 
solely preferentially just below the transition point at which the 
convection ensues but rather to adjust the pressure to a markedly lower 
value within the plateau or also at the beginning of the plateau. 
In general, the risk of sparkover in high-voltage lamps can be varied by 
means of a fill gas mixture of noble gas with a small (up to about 10%) 
addition of nitrogen. 
Further advantages of the invention are reduced gas consumption (which is 
especially significant when expensive xenon is used) and increased 
security against bursting. 
Finally, in terms of the strain on the coil, it should be noted that 
high-pressure lamps (that is, lamps impinged upon by convection) not only 
have a markedly higher coil temperature but also a smaller radiant surface 
area than corresponding low-pressure lamps (that is, operated 
convection-free) having the same photometric data. 
Low-pressure halogen lamps overall exhibit quite a different failure 
behavior, similar to that of conventional incandescent lamps, from 
high-pressure halogen lamps. "Low pressure" here generally always means 
the pressure that assures the freedom from convection, in comparison with 
the higher pressure ("high pressure,") involving convection, for a 
particular lamp type. While in one lamp type a cold fill pressure of 2 bar 
can still belong to the "low-pressure" range, in another lamp type a cold 
fill pressure of 0.8 bar can already be considered part of the 
"high-pressure range". A reliable statement about this for each lamp type 
can be made by measurement of the power loss as a function of the fill 
pressure and by determination of the transition point.

In FIG. 1, an incandescent halogen lamp with a rated voltage of 6 V and a 
power of 10 W is shown. It comprises a bulb 1, pinched on one end, which 
is shaped as a cylinder with an outer diameter of approximately 7.0 mm 
(previously 8.2 mm) and a wall thickness of approximately 0.8 mm 
(previously 1.2 mm). It is closed with a pinch 2 and has a pump tip on the 
opposite end from the pinch. The bulb is made of quartz glass. The fill 
comprises 1000 mbar of xenon (or krypton) with an addition of 1800 ppm of 
iodine ethane (C.sub.2 H.sub.5 I) (in another exemplary embodiment, the 
halogen addition comprises 400 ppm of dibromomethane) for a lamp volume of 
0.15 (previously 0.22) cm.sup.3. A cylindrical light emittins, or luminous 
element 6 whose dimensions are 2.4 mm in length and 0.9 mm in diameter is 
located axially in the bulb interior. It is retained by two power supply 
leads 3, which are joined with foils 4 in the pinch 2, and the foils 4 are 
in turn connected to external base prongs 5. 
The filament which forms the luminous element is manufactured from tungsten 
wire having a diameter of 104 .mu.m with an effective total length of 32 
mm, so that its total surface area is approximately 10.0 mm.sup.2. The 
power supply leads are formed directly by the coiled wire. 
The luminous element comprises 12 windings with a pitch of 188 .mu.m, 
corresponding to a pitch factor of 1.8. The core diameter is approximately 
730 .mu.m, corresponding to a core factor of 7.0. The service life is more 
than 5000 hours at a color temperature of 2500K, a light flux of 110 lm, 
and a light yield of 10.5 lm/W. 
A second exemplary embodiment is a 12 V/5 W incandescent halogen lamp, 
which is shown in FIG. 2. The cylindrical hard glass bulb 1, with an outer 
diameter of about 9 mm and a wall thickness of 1.15 mm, contains as its 
fill xenon with a cold fill pressure of about 1000 mbar and a halogen 
additive of 3000 ppm CH.sub.2 clI. The lamp volume is 0.32 cm.sup.3. A 
singly coiled luminous element 6' is located approximately transversely to 
the lamp axis in the bulb interior; its originally cylindrical shape has 
been bent into approximately an annular segment. It is retained by two 
separate pronglike power supply leads 3'. Otherwise, the structure of the 
lamp is similar to that of the first exemplary embodiment. 
The luminous element is made of tungsten wire with a diameter of 41 .mu.m 
and an effective total length of 48 mm. The coil surface area is 
approximately 5.7 mm.sup.2. Originally, the luminous element comprises a 
singly coiled cylindrical body with the following dimensions: length, 3.9 
mm; diameter, 0.32 mm. It contains 54 windings with a pitch of 75 .mu.m 
and a pitch factor of 1.8. The core diameter is 240 .mu.m, corresponding 
to a core factor of 5.7. The service life is 3100 hours at a color 
temperature of 2625K and a light yield of 12 lm/W, derived from a light 
flux of 63 lm. 
A third exemplary embodiment is a 12 V/10 W lamp, which substantially 
corresponds to the second exemplary embodiment and is therefore also 
represented by FIG. 2. Unlike the second exemplary embodiment, a tungsten 
wire of 65 .mu.m in diameter is used, which is originally coiled into a 
cylinder whose dimensions are 4.2 mm in length and 0.58 mm in diameter. 
The total length is 58 mm, so that the wire surface area is approximately 
11.8 mm.sup.2. The pitch factor is 1.75, corresponding to a pitch of 115 
.mu.m. The core factor is 6.9, corresponding to a core diameter of 450 
.mu.m. The number of windings is 36. The service life is approximately 
3100 hours at a color temperature of 2700K, a light flux of 140 lm, and a 
light yield of 14 lm/W. A comparison with the previously known 
high-pressure version is shown in table 3. A fourth exemplary embodiment 
is a 12 V/20 W incandescent halogen lamp, which substantially corresponds 
to the first exemplary embodiment. The fill comprises 1000 mbar of xenon 
or krypton with 3000 ppm of iodine ethane. 
The luminous element has 22 windings and its pitch is 167 .mu.m, for a 
pitch factor of 1.65; the core diameter is 737 .mu.m for a core factor of 
7.2; the color temperature is 2700K; the light flux is 320 lm and the 
light yield is 15.4 lm/W. In this lamp, when a bulb 8 mm in diameter is 
used, the mean bulb temperature drops from 360.degree. C. to 310.degree. 
C., so that the use of a 7 mm bulb at a mean bulb temperature of 
335.degree. C. has become possible. 
Table 1 hereinafter shows a comparison of the operating data of the above 
first four exemplary embodiments, compared with values that are referred 
to lamps without the properties according to the invention, or in other 
words in particular without convection. 
At a rated service life of 2000 hours each, the deviation (variance) in the 
mean service life and the residual light flux at 75% of the rated service 
life are shown. In all lamp types operated convection-free, a drastically 
reduced variance in the service life is exhibited. This reduced variance 
is a direct consequence of the halogen cycle process that has been reduced 
to the absolute minimum necessary. No less impressive is the fact that the 
residual light flux practically remains virtually constant for 75% of the 
rated service life, which can be ascribed to the elimination of the 
faceting effect. 
An additional table, Table 2, for the same lamp types, shows how the light 
flux (expressed in lumens) can be varied on the basis of the invention. 
Column 1 shows the lamp type and column 2 the light flux when a high 
pressure involving convection is used (approximately 8 bar of krypton or 
13.3 bar of xenon in the prior art); column 3 shows the reduction in light 
flux that can be expected in the prior art if the fill pressure is lowered 
to approximately 1 bar. Column 4 shows the gain in light flux compared 
with column 3 resulting from the purposeful use of the means according to 
the invention (optimization of the lamp for convection-free operation by 
means of optimizing the bulb and the luminous element and by means of fill 
gas parameters) while preserving the same rated service life. 
It can be seen that a drop in the light flux of the high-pressure version 
(column 2) of between 20 and 33% (column 3) is expected as a result of the 
fill pressure reduction. As a result of the invention, these losses can 
largely be contained and sometimes even entirely compensated for (column 
4). A breakthrough in the positive sense has thus been achieved for the 
first time with respect to the relationship among fill pressure, service 
life and light flux that is familiar to anyone skilled in the art; 
particularly the drastically reduced variance in the lamps of the 
invention plays a major role. 
FIG. 3 shows a schematic illustration of the loss factor .beta., or in 
other words of the power loss .DELTA.L from heat dissipation as a function 
of the fill pressure (p), on the assumption of a constant coil temperature 
T.sub.w and hence a constant light flux, standardized for lamp power 
L.sub.o in a vacuum. The pressure is plotted (in millibar) logarithmically 
on the abscissa. The lamps according to the invention all have this basic 
pattern; depending on lamp type, the break in the curve (transition from 
range I of pure heat conduction through diffusion at low pressures, for 
instance below about 1 bar, to range II, in which convective heat 
conduction predominates) may be at some different value for the cold fill 
pressure. Typically, it fluctuates between 0.1 and 2 bar for low-voltage 
lamps; in high-voltage lamps (and occasionally in low-voltage lamps as 
well), however, higher values (such as 5 bar) can also be found. The 
preferred operating range according to the invention is just below the 
transition point. 
In FIG. 3, this is indicated by an arrow. Moreover, the usual pressure 
range for incandescent halogen lamps (5 to 10 bar) is designated by 
shading. A striking feature of this graph is that the basic behavior in 
range I is virtually independent of the fill gas, while the 
convection-impinged losses in range II depend strongly on the fill gas. 
The heavier the fill gas, the slighter the losses--which agrees with the 
known teaching meant for range II. The ratios for the noble gases argon, 
krypton and xenon are given as examples. 
Often a possibly heavy fill gas, such as krypton or xenon, is preferable in 
lamps according to the invention, because under otherwise identical 
conditions it better prevents the evaporation of the tungsten material. 
It is a further advantage of the present invention is that because of the 
low fill pressure, considerable cost savings for the fill gas (xenon) 
become possible. Limiting the losses to the pure heat conduction 
phenomenon also makes it understandable why the attainable improvements 
are especially pronounced in luminous bodies, which are made of thin, long 
wires and thus have a relatively large coil surface area. This is because 
the larger the coil surface area, the greater are the attendant heat 
losses. 
In FIG. 4a, to further illustrate the situation, the tungsten loss per unit 
of time (.DELTA.m/.DELTA.t) is calculated as a function of the fill 
pressure for a model lamp. It is especially strongly pronounced at low 
pressures, below approximately 1 to 2 bar, and above that decreases only 
slightly. This behavior justifies the choice of a relatively slight fill 
pressure of about 1 bar, since the still-attainable improvement at high 
pressures is slight compared with the situation at very low pressures. 
In FIG. 4b, the relationship illustrated in FIG. 3 is shown again for this 
model lamp, but without standardization to the power attained in a vacuum 
but instead in the form of an absolute value for the power losses. 
A look at FIGS. 4a and 4b together shows that it is quite possible, in the 
sense of a mathematical convolution, to consider attaining long service 
lives even at relatively slight pressures of about 1 bar, in other words 
by operating at the upper limit of the convection-free pressure range. At 
the transition point, both the pressure dependency of the power loss and 
that of the tungsten mass loss change abruptly. In FIG. 5, to illustrate 
the significance of the different variances, two Weibull distributions are 
shown as an example, with the same rated service life T.sub.N of 2000 
hours (defined by the first 3% of failures in a set of lamps). A first 
distribution (FIG. 5a) along the lines of the prior art shows wide 
variance (curve 1); the mean service life T.sub.M is about 9000 hours. The 
second distribution (FIG. 5b), using the technology of the invention 
(curve 2), discloses a substantially shorter mean service life T.sub.M of 
5100 hours, but because of the narrow variance it suffices to attain the 
same rated service life T.sub.N of 2000 hours. 
A striking measure of this subject matter is the slope (shown in dashed 
lines) of the rising (left-hand) branch of the Weibull distribution. 
According to the invention, it is much steeper (S2) than in previously 
known lamps (S1). A comparison of these slope values is shown in FIGS. 6 
and 7, respectively, for lamps of the 12 V/5 W and 12 V/10 W type. 
A 12 V/5 W lamp with a previously known high-pressure fill (FIG. 6, curve 
S1) attains a 3% failure rate of approximately 1900 hours; the mean 
service life T.sub.M1 (corresponding to a 63.2% failure rate) is not 
reached until not quite 10,000 hours. A light yield of 12.0 lm/W is 
measured. The freedom of design in the invention is represented by two 
versions. In a first version (curve S2), by means of a low-pressure fill 
according to the invention, which is optimized by varying the coil data 
for light yield, an even higher light yield (12.4 lm/W) is attained, at 
almost the same rated service life (1700 hours). The mean service life 
T.sub.M2 is approximately 2500 hours. 
In a second version (curve S3) of the low-pressure lamp, which is optimized 
for service life (mean service life T.sub.M3 of 6500 hours, the light 
yield is somewhat less (10.9 lm/W), but the rated service (4000 hours) is 
more than twice as long. 
FIG. 7 shows a similar relationship for a 12 V/10 W lamp. The previous 
high-pressure version with a rated service life of 1900 hours (curve S1) 
and a mean service life (T.sub.M1) of 5500 hours attains a light yield of 
14.1 lm/W. By comparison, a lamp of this type according to the invention, 
at a light yield of 13 lm/W, attains a markedly longer rated service life 
of 2500 hours, with a mean service life T.sub.M2 of 3400 hours (this is 
the version optimized in terms of service life). 
This accordingly demonstrates the fact not only that it is now possible to 
furnish incandescent halogen lamps with a low cold fill pressure, on the 
order of magnitude of 1 bar, which (virtually) without sacrifices of light 
yield attain the same rated service life as lamps with a considerable 
overpressure (approximately 8 to 13 bar), but also that there is a 
potential for optimization in various directions. 
The unexpectedly slight sacrifices of light yield are offset by the 
following decisive advantages: the elimination of the risk of bursting, 
and economies of material and fill gas. 
Further exemplary embodiments are low-voltage lamps (12 V) with relatively 
high power (20 W, 35 W, 50 W), in which light yields of 15.2 lm/W, 17 
lm/W, and 18 lm/W are attained. The cold fill pressure was approximately 
800 mbar. Purely in terms of calculation, according to the teaching 
prevailing until now, light yields of 13.5 lm/W (for 20 W), 14.6 lm/W (for 
35 W) and 15.2 lm/W (for 50 W) would have been expected. 
TABLE 1 
__________________________________________________________________________ 
Prior art Per invention 
Residual light Residual light 
flux at 75% of flux at 75% of 
Variance (relative 
rated service 
Variance (relative 
rated service 
Lamp type 
standard deviation) 
life standard deviation) 
life 
__________________________________________________________________________ 
12 V 5 W 
0.35 83% 0.076 96% 
6 V 10 W 
0.3 95% 0.11 100% 
12 V 10 W 
0.35 90% 0.112 96% 
12 V 20 V 
0.3 90% 0.18 100% 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
Light flux reduction 
Light flux gain (lm) 
Light flux (lm) 
(lm) from fill level 
from optimization 
Lamp type 
per prior art 
reduction per invention 
______________________________________ 
12 V 5 W 
60 -20 +20 
6 V 10 W 
120 -35 +25 
12 V 10 W 
140 -40 +40 
12 V 20 W 
350 -70 +40 
______________________________________ 
The preferred choice of the "operating point" just below the transition 
point (see FIG. 3) will be explained for example in terms of the 
aforementioned 12 V/10 W lamp. Here, the transition point is fairly 
precisely at 1 bar of cold fill pressure, corresponding to an operating 
pressure of approximately 3 bar (FIG. 8). The drop in light flux from the 
heat dissipation has been studied in the critical range (FIG. 9). It is 
found that at a cold fill pressure of 0.5 and 1 bar, the light flux 
remains virtually constant, since the heat losses are very slight (FIG. 3, 
range I) and increase only slightly. Upon a further increase to 2 bar of 
cold fill pressure (corresponding to an operating pressure of about 6 
bar), the light flux drops markedly, in agreement with the heat 
dissipation that sharply increases in the convective range (range II of 
FIG. 3). 
On the other hand, FIG. 10 illustrates an investigation of the initial 
quality (expressed in % SCE); the SCE value describes the light yield of a 
lamp of equal current whose light yield has been standardized for 
comparison to a 1,000 hour service. The higher the initial quality in 
percent SCE, the greater the advantage in terms of the light yield (for a 
constant service life) or in the service life (for a constant light yield) 
when the lamps are compared with one another. It can be seen that upon an 
increase in the xenon cold fill pressure from 0.5 to 1 bar, a marked 
increase in the initial quality can be attained. Conversely, in the 
convective range (at 2.0 bar of cold fill pressure), no increase (and in 
other cases no significant increase) in the initial quality is possible 
any longer. The explanation can be said to reside in the pronounced 
dependency of the service life on the fill pressure below the transition 
point, while above the transition point the theoretically longer service 
life is rendered invalid in practice because of its great variance, caused 
by the loss mechanisms discussed above. 
The grain boundary corrosion, which is suppressed according to the 
invention, is especially impressive. While a 12 V/5 W lamp filled with 1 
bar of xenon and operated convection-free exhibits practically no damage 
after 1800 hours in operation (FIG. 11a), the coil of a 
convection-impinged comparison lamp (13 bar of xenon), after the same time 
in operation, is already severely affected (FIG. 11b). 
Particular advantages are exhibited by lamps according to the invention in 
conjunction with reflectors or light fixtures, because of their reduced 
temperature strain. 
FIG. 12 shows measurements of the bulb temperature (MP stands for 
measurement point) in .degree.C., in comparison with a 12 V/10 W lamp of 
the prior art (column 2) and a lamp of the invention (column 3). The 
mounting location is shown in the first column in each case. A greatly 
improved isothermia is exhibited, because the variance for a different 
mounting position drops sharply. In the invention, it is 55.degree. C., 
compared with 120.degree. C. in the prior art. Moreover, the absolute 
temperature strain also drops. The maximum value drops from 315.degree. to 
240.degree. C., and the minimum value remains at about 10.degree. C. 
A reflector lamp of this kind, using a cold light reflector 21 known per 
se, is shown in FIG. 13. The cold light reflector 21 comprises an 
ellipsoidal glass dome or bowl 23 with a formed-on reflector neck 27. An 
interference filter 24, known per se, is applied to the inside of the 
glass dome 23; it has a high degree of reflection over the entire visible 
spectral range and is transparent to infrared radiation. The light source 
is a 12 V/10 W incandescent halogen lamp 22, whose pinch is fixed in the 
reflector neck 27 with the aid of cement 28. The light outlet opening of 
the cold light reflector 21 has a diameter of about 48 mm. Because of the 
low fill pressure of the lamp (see above), it is possible to dispense with 
a glass disk to cover the opening. In this reflector lamp, which has a 
coiled coil, the temperature at the pinch drops from 350.degree. C. (in 
the earlier high-pressure version) to 320.degree. C., if the lamp is 
operated convection-free. 
Table 3 shows a summary of essential comparison data of 12 V/5 W and 12 
V/10 lamps, each in a high- and low-pressure version with the same light 
yield. A point especially to be considered is the different radiant coil 
surface area and the different wire diameter for the luminous element. The 
coil temperature (in K) is lower (by 70K) for the lamps of the invention. 
Accordingly, the color temperature is also markedly lower, which has a 
favorable effect on the service life. A further factor is a substantially 
improved mechanical strength, because the wire is approximately 10% 
thicker. 
The present invention is significant not only in the low-voltage range, 
however, but in the high-voltage range as well. In the final analysis, the 
fundamental advantages are even more pronounced here. This is because in 
the high-voltage range (or medium-voltage range), the luminous element 
wire to be used is thinner and its total length is substantially longer. 
Such phenomena as the grain boundary corrosion discussed above therefore 
play a substantial role. 
For instance, the above-presented 12 V/50 W incandescent halogen lamp 
requires a luminous element with a wire diameter of about 120 .mu.m and a 
total length of 30 mm. In comparison, a similar lamp designed for high 
voltage (230 V), also with 50 W of power, has a double-coiled luminous 
element approximately 20 .mu.m in diameter with a total length of about 1 
m. Because of this great length, the coil, bent into a W or V, is located 
in a bulb of 14 mm in diameter pinched on one end. 
FIG. 14 shows measurements of the gas loss factor .beta. (see FIG. 3) for 
the 25 W version of a 230 V incandescent halogen lamp, structurally 
identical except for the correspondingly modified wire diameter, in a 
vertical mounting position. Once again, the fundamental dependency on the 
fill pressure known from the low-voltage range, can be seen. The 
transition point between the convection-free and the convection-impinged 
operating state of the lamp is clearly apparent. Surprisingly, this 
transition point here is markedly higher, at approximately 5 bar of 
operating pressure, corresponding to a cold fill pressure of approximately 
1.9 bar. 
An especially important property of the lamps according to the invention, 
which is expressed clearly especially in the high-voltage versions, is 
shown in FIGS. 15 and 16. This involves a 230 V/50 W reflector lamp 31, 
similar to the 25 W lamp just discussed above, which is secured via two 
long power supply leads in the apex of a pressed glass reflector 33, which 
by way of example has a diameter of 63 mm ( 20) or 95 mm ( 30). The 
reflector has a neck 34, which is secured in a screw base 35. The inner 
bulb 32, filled with halogen and noble gas, has a luminous element 37 bent 
into a W, which is retained by five frame wires 38 that are anchored in 
the pinch. The segments 39 of the luminous element are oriented 
approximately axially parallel; they are inclined by a maximum of 
10.degree. from the reflector axis. The reflector opening is covered by a 
lens 36. 
This lamp, if operated according to the invention, is also distinguished by 
improved isothermia. Once again, FIG. 16 confirms that the total loss 
factor .beta. fundamentally depends in the manner according to the 
invention on the operating pressure (or cold fill pressure). The 
transition point is at approximately 4 bar. The trend to a transition, not 
occurring until at a higher pressure, from the convection-free to the 
convection-impinged mode depends on the generally larger bulb diameters. 
FIG. 16 is of particular interest, however, for yet another 
characteristic. It is in fact demonstrated that the linear relationship 
between the gas loss factor and the logarithmically plotted operating 
pressure in the convection-free range (see FIG. 3) represents a more or 
less correct approximation. Depending on individual lamp parameters, a 
course deviating from this occurs--preferentially in lamps operated with 
medium and high voltage. At low operating pressures (below 400 mbar), a 
linear relationship exists, and then a steep rise occurs to approximately 
800 mbar. A plateau is reached there, within which the gas loss factor is 
virtually independent of the fill pressure, until it rises again highly 
sensitively with the pressure at the transition to the convective mode (at 
about 4 bar). The reason for this behavior is not yet entirely clear; 
however, it is probably linked to the ratio between the bulb and luminous 
element dimensions and the free path length existing in the fill gas. 
It has surprisingly been found that the invention is also applicable to 
tubular lamps, that is, tubular incandescent halogen lamps pinched on both 
ends. Typical power stages range between 25 and 1000 W. It is especially 
astonishing that the behavior of these elongated lamps depends relatively 
little on the mounting position. Especially when bulbs with glass ribs 
formed of the material of the bulb are used for mounting the luminous 
element (see U.S. Pat. No. 5,146,134, for instance), which ribs divide the 
fill volume into individual, loosely bounded portions, the operating 
behavior is practically independent of the mounting position. 
FIG. 17 shows that the gas loss factor of a 120 V/40 W tubular lamp with 
glass ribs is practically identical in the horizontal mounting position 
(measurement points represented by circles) and the vertical mounting 
position (measurement points represented by squares). The dependency on 
the pressure again exhibits the course already known for lamps pinched on 
one end. The transition point is at about 8 bar of operating pressure. The 
corresponding high-voltage version (230 V/40 W), which differs from the 
medium-voltage version in having different luminous element dimensions, 
exhibits a similar behavior (FIG. 18). The transition point is somewhat 
lower, namely at about 5 bar of operating pressure. The dependency on the 
fill gas (N.sub.2, Ar, Kr, Xe) was also investigated. As expected, it was 
found that the heavy noble gases (Kr, Xe) have the lowest gas loss factor. 
In the convection-free range, its dependency on the type of gas used is 
relatively slight. 
Finally, FIG. 19 shows the operating performance for a 230 V/150 W lamp. 
The cylindrical quartz glass bulb with a total length (including pinch) of 
about 110 mm and with a tube diameter of about 12 mm contains an axial 
double coil approximately 60 mm in length. The specific coil dimensions 
depend on the optimization desired, that is, whether the service life or 
the light yield is optimized. The transition point is relatively high in 
this case, at approximately 15 bar of operating pressure, corresponding to 
about 5 bar of cold fill pressure. The plateau behavior already described 
above is especially pronounced here, so that between 2 and 15 bar, the gas 
loss factor stays at a value of about 1.05, practically regardless of the 
fill pressure. The basic design of such lamps is described for instance in 
U.S. Pat. No. 5,146,134 and in European Patent EP 0 143 917, whose content 
is hereby expressly incorporated by reference. 
Typical inside diameters of the bulb for low-voltage lamps are on the order 
of magnitude of from 3 to 12 mm and for mediumand high-voltage lamps 
between 6 and 16 mm. 
The production of the lamps according to the invention is done as known per 
se, but the cold fill pressure of the inert gas (for low-voltage lamps 
this is typically a noble gas or a mixture of noble gases and for 
high-voltage lamps, especially with lamps pinched on one end, a slight 
admixture of nitrogen may be advantageous) is set in the vicinity of the 
upper limit of the convection-free pressure range. In low-voltage lamps, 
this "operating point" should usually be chosen to be just below the 
transition point (see FIG. 3); in high-voltage lamps, it is often in the 
region of the plateau, which in terms of FIG. 19, for instance, extends 
from the transition point to lower values of the fill pressure. 
TABLE 3 
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Radiant coil Color 
Tungsten wire 
surface area 
Light yield 
Coil temperature 
temperature 
Type diameter (.mu.m) 
(mm.sup.2) 
(lm/W) 
at mid-coil (K) 
(K) 
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12 V 5 W 
High pressure 13 bar 
37.61 4.34 12 2675 2700 
Low pressure 1 bar 
41.04 5.73 12 2605 2625 
12 V 10 W 
High pressure 13 bar 
60.02 8.95 14 2760 2800 
Low pressure 1 bar 
65.52 11.79 14 2710 2700 
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