High intensity ultraviolet light source

A high intensity ultraviolet light-source comprises an arc tube having a pair of electrodes, filled with certain amounts of mercury, tantalum halide and rare gas, and provided with a tube loading of at least 13 Watt/cm.sup.2.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved metal halide lamp of the type 
filling metal halide within an arc tube, and more particularly, to a high 
radiance ultraviolet light-source with enhanced wavelength range and 
intensity in the ultraviolet range used in apparatus for physics and 
chemistry. 
One of the most widely used ultraviolet light-sources in an apparatus for 
physics and chemistry is the deuterium lamp. Apparatus for physics and 
chemistry have detection limit which depends on the radiance of the 
light-source. Recent demands in ultra-fine analysis require ultraviolet 
light-sources having high radiance. In order to meet such requirements, 
there has been proposed, as a high radiance ultraviolet light-source in 
place of the deuterium lamp, a metal halide lamp filled with tantalum 
halide as disclosed in Japanese Patent Laid-open No. 52-45391 laid open on 
Apr. 9, 1977. This lamp has a continuum spectrum in wavelengths ranging 
220-450 nm, but has a low radiance in shorter wavelengths below 245 nm, 
and it cannot be used for the measurement in shorter wavelengths below 245 
nm. For example, when a spectrophotometer is used for measuring sugar or 
organic acid which do not absorb the light in the near ultraviolet range, 
they need to be measured in wavelengths around 210 nm or shorter where the 
above tantalum halide lamp cannot be used. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
ultraviolet light-source having high radiance in the wide ultraviolet 
range. 
In order to achieve the above-mentioned object, the basic feature of the 
present invention resides in a high intensity ultraviolet light-source 
having a pair of electrodes and filled with certain amounts of mercury, 
tantalum halides and rare gases within the arc tube, where the arc tube is 
provided with a tube loading of at least 13 Watt/cm.sup.2. 
The foregoing arrangement causes the efficient radiation from molecules of 
mercury with wavelengths around 190-245 nm, and consequently, an 
ultraviolet light-source having high radiance over a wide range of 
wavelength covering 190-450 nm is made feasible.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The principle of the present invention will be described first. In the 
description, the term "tube loading" is defined as a total power of 
discharge divided by the inner surface area of the arc tube neighbouring 
the discharge. For a cylindrical arc tube, the tube loading is defined as 
P/2.pi.R, where P is power in Watt/cm for the unit length of discharge 
between electrodes and R is the radius in cm inside of the arc tube. 
In the metal halide lamp, it is a general convention to add mercury to 
components filled in the tube in order to gain the radiation efficiency or 
to obtain the desired electrical characteristics. The inventors of the 
present invention have manufactured a metal halide lamp having two 
tungsten main electrodes and filled with TaI.sub.5 by 2 mg/cm.sup.3, Hg by 
6 mg/cm.sup.3 and Ar by 25 Torr. FIG. 1 shows the result of measurements 
for the radiance of the lamp. In the graph of FIG. 1, the relative values 
of radiance for each wavelength with the radiance of the conventional 
deuterium lamp being set to 1 are plotted on the ordinate for various tube 
loadings as a parameter. The spectra for wavelengths of above 245 nm are 
formed by superimposing of the radiation from atoms of Hg on the radiation 
from molecules of TaI.sub.5, and they provide a sufficient radiance. 
Whereas, the continuum spectra for wavelengths below 245 nm are formed by 
the radiation from molecules of Hg.sub.2. It was found that by increasing 
the tube loading (W/cm.sup.2), the radiation from molecules of Hg.sub.2 
can have a sufficiently effective radiance as shown in FIG. 1. That is, 
the radiation from molecules of Hg.sub.2 increases significantly as the 
tube loading increases. This property is shown in FIG. 2, where a curve a 
shows the radiance of radiation from molecules of Hg.sub.2 for the range 
225-230 nm, a curve b shows the radiance of radiation from molecules of 
TaI.sub.5 for the range 320-325 nm and a curve c shows the radiance of 
radiation mainly from atoms of Hg for the range 280-285 nm, each relative 
to the radiance of the deuterium lamp. It can be seen from FIG. 2 that the 
radiation from Hg.sub.2 molecules (curve a) increases sharply as the tube 
loading increases as compared with the radiation from TaI.sub.5 molecules 
(curve b) and the radiation from Hg atoms (curve c). On this account, the 
most significant factor for obtaining a light-source having a continuum 
spectrum with sufficiently high intensity in the wavelength range above 
190 nm is the determination of the value of tube loading. In order for a 
light-source to have a higher radiance than that of the conventional 
deuterium lamp, the lamp needs to have a higher radiance in the wavelength 
range 225-230 nm than that of the deuterium lamp. This condition is 
satisfied by setting the tube loading to 13 W/cm.sup.2 or more, as can be 
seen from FIG. 2. 
Further experiments in high intensity ultraviolet light-sources with tube 
loadings of 13 W/cm.sup.2 or more led to the following facts. The 
radiation from molecules of tantalum halide can be enhanced by increasing 
the ratio of the quantity of tantalum halide and the quantity of mercury 
filled in the tube. However, this causes a fall in the temperature of 
discharge plasma, resulting conversely in a weaker radiation from 
molecules of Hg.sub.2 on the spectrum below 245 nm. Several lamps filled 
with TaI.sub.5, Hg and Ar by 25 Torr. were manufactured, and their 
radiance from molecules of TaI.sub.5 and Hg.sub.2 were measured for lamps 
with various molar ratio of TaI.sub.5 to Hg. FIG. 3 shows the result of 
the measurement, where a curve d shows the relative radiance from 
molecules of TaI.sub.5 at 340 nm, and a curve e shows the relative 
radiance from molecules of Hg.sub.2 at 220 nm with a tube loading of 46 
W/cm.sup.2. It can be seen from FIG. 3 that in the range of the molar 
ratio below 20%, the radiation from molecules of Hg.sub.2 (curve e) has a 
sufficient radiance to realize a high intensity ultraviolet light-source 
with a continuum spectrum for wavelengths ranging 190-450 nm. 
Further experiments led to the following facts. Tantalum halide has a high 
saturated vapor pressure; e.g., TaI.sub.5 has a saturated vapor pressure 
of around 20 Torr. at 300.degree. C. With this high saturated vapor 
pressure, the radiation from TaI.sub.5 can be obtained sufficiently. On 
the other hand, the lower the temperature of the arc tube, the longer is 
the service life of the metal halide lamp. Accordingly, a lamp filled with 
tantalum halide can be a long life light source with a sufficiently high 
vapor pressure by maintaining the coldest point temperature below 
600.degree. C. 
One embodiment of the present invention will now be described with 
reference to FIG. 4. In the figure, main electrodes 2 and 2' made of 
tungsten are sealed at two points on arc tube 1 which is formed of fused 
silica or the like for transmitting ultraviolet rays. The tungsten main 
electrodes 2 and 2' are connected through molybdenum foils 3 and 3' to 
lead wires 4 and 4' made of molybdenum, respectively. The arc tube 1 is 
filled with filler 5 and Ar gas as will be described later. In some 
applications, the arc tube 1 is fixed within an outer tube, at least a 
part of which wall transmits ultraviolet rays, so as to stabilize the 
discharge. In most of this case, the outer tube is evacuated to a vacuum. 
The arc tube 1 is filled with TaI.sub.5 by 2 mg/cm.sup.3, Hg by 6 
mg/cm.sup.3 and Ar by 25 Torr. TaI.sub.5 may be filled in such a way that 
metal Ta and HgI.sub.2 are filled in the tube 1 so that they will react to 
become TaI.sub.5. The arc tube 1 was supplied with power with a tube 
loading of 13 Watt/cm.sup.2 or more, and a high intensity ultraviolet 
light-source providing a sufficient high radiance for wavelengths ranging 
190-450 nm could be achieved. 
It will be appreciated that an auxiliary electrode may be added to the main 
electrodes 2 and 2' for easy start of discharging or other purposes. 
According to the present invention, as described above, an ultraviolet 
light-source with high intensity for the wide ultraviolet range can be 
realized, which can be used for much universal-type apparatus for physics 
and chemistry, allowing high sensitivity analysis. In addition, the high 
intensity ultraviolet light-source according to the present invention has 
a longer service life as compared with the deuterium lamp, and it is also 
advantageous economically.