Metal halide high-pressure discharge lamp

A high-pressure metal halide lamp which is particularly suitable for inclon in optical systems is run at specific power between 100 and 180 W per mm arc length. The lamp includes, per cm.sup.3 chamber volume, between 0.3 and 3 .mu.mol dysprosium, hafnium and lithium respectively and between 0.2 and 2 .mu.mol indium, whereby luminance of between 25 and 75 kcd/cm.sup.2 can be generated at color temperature of between 4500 and 7000 K. Light spots with a diameter of about 4 mm and a color reproduction index Ra of 80 are achieved by means of a special reflector. This makes it possible to use the lamp in combination with thin glass-fiber bunches for illumination purpose, e.g. in endoscopy.

FIELD OF THE INVENTION 
The invention relates to a metal halide high-pressure discharge lamp having 
a mean arc power of between 100 and 180 W/mm of arc length. 
BACKGROUND 
Metal halide high-pressure discharge lamps of this type are used 
particularly for fiber-optic illuminating systems in medicine (endoscopy) 
and technology (boroscopy), where light at color temperatures between 4500 
and 7000 K. and good to very good color rendition in all color temperature 
ranges, along with high lighting intensities, are needed. 
Low-loss coupling of the light into the fiber-optic bunch necessitates good 
focusing, or in other words a focusing diameter that is less than or at 
most equal to the usable diameter of the fiber-optic bunch. To produce a 
corresponding light spot, the arc core is essentially projected by a 
reflector or other optical system. If the light emitted by the arc core 
does not include all the spectral components of the total light emitted by 
the lamp, then the color rendition property of the focused light can 
worsen compared with that of the unfocused light. It is therefore highly 
important, with a view to use in the aforementioned focusing systems, to 
purposefully find fill ingredients that emit at the hot arc core and not 
only at the cooler arc edge. Moreover, for good focusing and high light 
intensities at the entry to the fiber-optic bunch, especially compact lamp 
dimensions and a very short light arc (only a few millimeters) with 
maximum light densities (on average, several tens of kcd/cm.sup.2) must be 
sought. 
From European Patent Disclosure EP 0 193 086, to which U.S. Pat. No. 
9,686,419, Blook et al. corresponds, assigned to the assignee of this 
application, metal halide high-pressure discharge lamps with similar short 
light arcs and correspondingly high light densities are known that produce 
light with good color rendition properties. 
However, their disadvantage is that the fills of these lamps contain 
cadmium. For the sake of environmental protection, at the end of the lamp 
life the toxic heavy metal, cadmium, must be returned to the raw material 
cycle or be properly disposed of, which in both cases involves attendant 
costs. Moreover, the lamps with a Cd filling have an irritating greenish 
tinge, and the color location is located above Planckian locus. 
It is an object of The Invention to create a metal halide high-pressure 
discharge lamp that has a very short light arc with a very high light 
density as well as a color temperature between 4500 and 7000 K. at a color 
location near the Planckian locus, good color rendition, and especially in 
combination with a strongly focusing reflector or other optical system, 
and that attains this object with a cadmium-free fill. 
Briefly, the fill of the lamp according to the invention comprises mercury, 
at least one noble gas and at least one halogen, and metals that form 
halides, namely dysprosium (Dy), hafnium (Hf), lithium (Li) and indium 
(In). The fill quantities, in micromoles per milliliter (.mu.mol/ml) of 
vessel volume, are advantageously between 0.3 and 3 each for Dy, Hf and 
Li, and between 0.2 and 2 for In. 
The metal halide high-pressure discharge lamp is operated at specific arc 
powers between 100 and 180 W per millimeter of arc length. Given the 
compact geometrical dimensions of the lamp--very short electrode spacing 
(a few millimeters) and small vessel volume (a few tenths of a 
millimeter)--this is equivalent to wall loads of 70 to 120 W/cm.sup.2 of 
wall area of the discharge vessel. By means of the fill components, 
according to the invention, of the discharge vessel, mean light densities 
of 25 to 75 kcd per cm.sup.2 of arc area are attained, which can be 
focused with the aid of a reflector or other optical system onto a light 
spot whose diameter is less than 10 mm. The particular value of the 
invention is that the good to very good color rendition (Ra.gtoreq.75) is 
preserved even after focusing, and the color location is near Planckian 
locus, and this is achieved with a fill that does without the toxic 
cadmium used until now. 
Dysprosium, with its multiple-line spectrum, assures a high radiation flux 
in the visible range of the electromagnetic spectrum and additionally 
contributes to the continuous spectrum. Hafnium also produces a 
multiple-line spectrum and moreover reduces the tendency to 
devitrification, by building up a reinforced halogen jacket on the bulb 
wall. Because of the high vapor pressure of hafnium halides, the tendency 
to bulb blackening is also reduced, and consequently the usable light flux 
during the lamp life is increased. 
By means of lithium and indium, the radiation flux especially in the red 
and blue portions of the optical spectral region is reinforced. Overall, 
the light emitted has a spectral composition that is quite close to that 
of Planckian radiation, or in other words has good to very good color 
rendition properties. Depending on the proportion of fill quantities of 
the various components, light can be generated with a color temperature 
between 4500 and 7000 K. 
The lamp according to the invention is preferably used in dichroitic 
special reflectors, which essentially project the inner arc core. By the 
purposeful selection of the two atomic radiators, lithium and indium, 
which radiate preferentially in the hot arc core, it is achieved that the 
good color rendition properties are preserved even at the focal point of 
this reflector. Moreover, by the use of lithium in combination with 
hafnium, high color stability is attained; that is, the color temperature 
varies only slightly over the lifetime of the lamp. 
For arc stabilization, the discharge vessel can contain in addition up to 3 
.mu.mol of cesium per cm.sup.3 of vessel volume. To maintain the halogen 
cycle process, iodine and bromine are preferably used in a molar ratio 
between 0.3 and 1.5. The lamp also contains mercury, in an amount of 
typically a few tens to a few hundreds of .mu.mol per cm.sup.3 of vessel 
volume and a noble gas, such as argon, as the basic gas. The fill pressure 
of the noble gas in the cold lamp is less than atmospheric 
pressure--typically a few tens of kPa--so that in this case risk-free 
manipulation is possible. On the other hand, the pressure range is high 
enough that upon ignition an undesired evaporation of the tungsten 
electrodes with an attendant blackening of the discharge vessel is largely 
prevented. 
The metal halide high-pressure discharge lamp according to the invention, 
while preferably used in a reflector securely joined to the lamp, can 
nevertheless also be used without an integrated joined reflector.

DETAILED DESCRIPTION 
FIG. 1 shows a metal halide high-pressure discharge lamp 2, built into a 
reflector hose combination assembly 1, with a power consumption of 270 W. 
The lamp 2 has its axis located in the axis of the reflector 1. While an 
electrode shaft 3 is secured by means of cement 4 to the ceramic base 5, 
the other electrode shaft 6 is retained on the ceramic closure ring 8 of 
the reflector 1 by copper bands 7 that at the same time act as power 
supply leads. The metal halide high-pressure discharge lamp 2 has a 
discharge vessel 9, whose volume is 0.35 cm.sup.3. The electrodes 10, 11 
are joined, at a spacing of 2.2 mm, via vacuum-tight-sealed molybdenum 
foils 12, 13 to the power supply leads 14, 15. One power connection 16 is 
mounted in the base 5, and the other (not visible here) is mounted on the 
closure ring 8 of the reflector 1. 
The reflector 1 produces a substantially circular light spot in the focal 
plane with a light power .phi. of virtually Gaussian spatial distribution 
of lighting intensity E(r). In polar coordinates, it is therefore 
approximately true that 
##EQU1## 
where r is the radial coordinate and r.sub.0 is the radius of the light 
spot. The radius r=r.sub.0 accordingly indicates the radial spacing from 
the center of the light spot at which the lighting intensity is less, by 
the factor 1/e.sup.2, than the maximum lighting intensity E.sub.max 
(r=0)=2.PHI./.pi.r.sub.0.sup.2 in the center of the light spot. The 
thus-defined diameter d=2.r.sub.0 of the light spot is approximately 4 
mm--within this dimension, 1-1/e.sup.2 =86.5% of the total light power of 
the light spot (in reliance on the tentative standard DIN V 18730) is 
located. The opening angle of the caustic surface of the beam in the 
region of the focus is approximately 60.degree. . Thus virtually the 
entire light flux can be efficiently coupled into thin fiber-optic 
bunches, and the useful diameter of the fiber-optic bunch can be as small 
as 4 mm, as long as the acceptance angle of the bunch is at least 60%. 
From the following table, a fill according to the invention of the 
discharge vessel 9 of the lamp 2 of FIG. 1 and the technical lighting data 
of this lamp that are attained (color rendition index Ra for lamp 2 
including reflector 1) can be seen. 
TABLE 
______________________________________ 
Quantity of fill ingredients in .mu.mol: 
______________________________________ 
Dy: 0.5 
Hf: 0.45 
Li: 0.35 
In: 0.22 
Cs: 0.32 
J: 2.8 
Br: 3.9 
Hg: 42.5 
Fill pressure of the basic gas (Ar): 
45 kPa 
Discharge vessel volume: 
0.35 cm.sup.3 
Electrode spacing: 2.2 mm 
Power consumption: 270 W 
Arc drop voltage: 40 V 
Specific arc power: 125 W/mm 
Wall load: 82 W/cm.sup.2 
Light yield: 70 lm/W 
Mean light density: 35 kcd/cm.sup.2 
Ra (lamp including reflector): 
80 
Color temperature 5400 K. 
Lifetime: &gt;250 h 
______________________________________ 
The balanced spectral composition of the light emitted from the arc 
core--which is the prerequisite for good color rendition when a focusing 
reflector is used--is documented in FIG. 2. This shows two emission 
spectra, measured with the aid of a spectrometer, of the lamp described in 
FIG. 1 in the spectral range between 250 and 925 nm. 
They originate from the light from the arc core A and from the lower arc 
edge B, respectively, and clearly illustrate the location dependency of 
the spectral composition of the emitted light. The relative light 
intensity is plotted in relative units on the ordinate, and the wavelength 
is plotted in nanometers (nm) on the abscissa. The spectral resolution of 
the spectrometer used is approximately 1.5 nm. Its spectral transmission 
function was corrected with the aid of the spectrum of a halogen 
incandescent bulbs for wavelengths &gt;350 nm. The strongest lines of the 
mercury are not shown completely, so that the structure of the remaining 
spectra can be more clearly seen (the maximum values of the aforementioned 
lines are approximately 67,000 in relative units). The two most striking 
characteristics of both spectra are the background and the great number of 
spectral lines that show up against it. The background comprises continuum 
radiation (recombinant radiation of unbound electrons), molecule bands 
(such as halide molecules), and closely spaced resonance lines of atomic 
radiators (such as Dy, Hf), which are not resolved into individual lines 
by the spectrometer used. 
Because of the fill ingredients according to the invention, the light 
emitted from the arc core and then focused by the reflector has, as 
desired, a balanced spectral composition, which is similar to a Planckian 
distribution, within the entire visible range (approximately 380 to 780 
nm). As can be clearly seen, filling out of the spectrum A in the 
green-blue and the red range is attained in particular by indium and 
lithium, so that finally good to very good color rendition of the light 
emitted from the arc core is attained. The light emitted from the arc 
edge, conversely, does not have any good color rendition properties, since 
the blue-green spectral component is markedly underrepresented (see 
spectrum B).