High intensity discharge lamps with uniform color

The amount of mercury used in a metal halide-mercury arc discharge lamp is controllably selected to produce a voltage gradient along the arc discharge of between approximately 2 v/cm and approximately 10 v/cm. When such a lamp is operated vertically, the usually observed color separation does not occur. Even though this voltage gradient is substantially less than the conventional gradient employed in such lamps, no significant loss of efficacy occurs.

BACKGROUND OF THE INVENTION 
This invention relates to high intensity arc discharge lamps and 
particularly to metal halide-mercury lamps operated in a vertical 
position. 
One of the serious problems that occurs in halide-mercury high intensity 
discharge lamps is the separation of color which results when the arc of 
the lamp is positioned in a vertical or nearly vertical position. 
Typically, in the higher wattage, physically longer lamps containing a 
sodium halide as one of the ingredients, the orange color of the sodium 
radiation appears predominately at the bottom of the discharge arc, 
leaving the radiation of other ingredients, including mercury, which is 
generally blue or blue-green, in the upper portion of the discharge arc. 
These lamps are typically operated at a positive column arc gradient of 
between approximately 25 and 35 v/cm. The color separation results in 
uneven distribution of color in the area being illuminated and in addition 
is accompanied by substantial loss of luminous efficacy. 
SUMMARY OF THE INVENTION 
In accordance with a preferred embodiment of the present invention, the 
amount of mercury used in the discharge tube of metal halide-mercury arc 
lamps is controllably selected so that the positive column arc gradient is 
between approximately 2 v/cm and approximately 10 v/cm. Within this range, 
the range from approximately 5 v/cm to approximately 10 v/cm is preferred 
so as to optimize the combination of luminous efficacy and color 
uniformity along the arc discharge. The lamps of the present invention may 
be operated in a vertical or near vertical position without the previously 
observed color separation.

DETAILED DESCRIPTION OF THE INVENTION 
The FIGURE illustrates a typical metal halide-mercury arc discharge lamp to 
which the present invention is directed. Light-transmissive discharge tube 
10 possesses electrodes 11 and 12 disposed at opposed ends thereof, said 
electrodes 11 and 12 protruding into the gaseous discharge medium 14 
typically comprising a mixture of a metal halide vapor and mercury vapor. 
The mercury vapor pressure within the discharge envelope 10 occurs as a 
result of the presence of mercury globule 15. A quantity of one or more 
metal halides is also introduced which condenses on the envelope walls 
when the arc is turned off. The amount of mercury present controls the 
positive column discharge voltage gradient along the electrical discharge 
flowing between electrodes 11 and 12. 
Also, typically present in such lamps is starting electrode 13 electrically 
connected to one side of starting resistor 17, the other side of which is 
electrically connected to the electrode 11 through supporting members 19, 
21, 18, and 22. Electrode 11 is connected by means of supporting members 
22, 18, and 19 to one side of the electric power supply for the lamp. The 
discharge envelope 10 is supported within an outer envelope 16 by means of 
support rods or wires 19 which are typically spot welded to metal band 
member 21 at the base end of the lamp. At the end of the lamp, opposite 
the base 24, the discharge tube 10 is supported by rod or wire members 18 
and metal band member 20. Metal strap 22 typically connects electrode 11 
electrically with one side of the power supply for the lamp, the other 
side of the power supply being connected by means of wire or rod 23 which 
is connected to electrode 12, the base-end electrode. Support members 18 
are typically supported by spot welding to metal band 25 surrounding a 
dimple in the end of the lamp distal from the base 24 which is often an 
Edison type base or a bayonet type base. The Edison type base is shown. 
The connections of the supporting members are typically made with one 
another by means of spot welds. A more detailed description of such lamps 
is found in U.S. Pat. No. 3,781,586, issued Dec. 25, 1973 to Peter D. 
Johnson, the applicant herein, and assigned to the same assignee as this 
invention. This patent is hereby incorporated herein by reference as 
background material. 
Conventional high intensity discharge lamps employing metal halide-mercury 
vapor operate with a voltage gradient along the arc discharge of between 
approximately 25 v/cm and approximately 35 v/cm. As seen in the FIGURE, 
the distance d between electrodes 11 and 12 is divided into the voltage 
difference between electrodes 11 and 12 to determine this voltage 
gradient, commonly referred to as the positive column voltage gradient. 
However, it is observed in such conventional lamps that color separation 
occurs along the arc discharge. That is, when the lamp is operated in 
vertical position, the lower portion of the discharge exhibits a 
predominately orange radiation while the upper portion of the discharge 
emits blue or blue-green radiation. This problem is most prevalent in the 
higher wattage lamps having a relatively long discharge length, d. Also, 
in conventional higher wattage lamps, the arc is often unstable and swirls 
when operated vertically. 
The positive column voltage gradient is controllable by selecting the 
amount of mercury to be used in the discharge envelope 10. As the 
conventional amounts of mercury employed are reduced, the voltage gradient 
likewise is reduced but the discharge current increases. Conventional 
metal-halide mercury arc discharge lamps are not operated with these 
reduced amounts of mercury, and therefore lower mercury vapor pressure 
because the increased current that results used to be very difficult to 
regulate with standard ballasts and the high current also had a severely 
deleterious effect upon the discharge electrodes. However, improvements in 
electrode construction and ballast now permit the operation of such lamps 
at relatively high currents. 
In accordance with a preferred embodiment of the present invention, the 
amount of mercury employed in the discharge envelope 10 is selected so 
that the positive column arc voltage gradient is between approximately 2 
v/cm and approximately 10 v/cm instead of between the usual 25 to 35 v/cm. 
Within the aforementioned range of operation, the preferred range of 
voltage gradients for optimizing the combination of luminous efficacy and 
color uniformity is between approximately 5 v/cm and approximately 10 
v/cm. Below this range, loss of efficacy occurs and above this range, 
color separation is observed when the lamp is operated in a vertical 
position. Even though the lower mercury vapor pressure results in higher 
current in the discharge, this is no longer a serious problem considering 
the present state of the art in electrode construction and materials and 
ballast components. The optimum arc potential gradient depends to some 
extent on the arc tube dimension, but nonetheless, the range from between 
approximately 2 v/cm and 10 v/cm is preferred to achieve color uniformity 
along the entire arc discharge. 
When constructed and operated in accordance with the present invention, 
high intensity discharge metal halide-mercury arc discharge lamps exhibit 
a uniform color of radiation along the entire discharge length. 
Additionally, lamps of the present invention have a stable arc which is 
not subject to the swirling phenomenon observed in conventional discharge 
lamps when they are operated vertically. 
By way of example, and not limitation, a metal halide-mercury arc discharge 
lamp may be constructed in accordance with the following specifications. 
For example, the arc discharge tube may comprise a light-transmissive 
quartz envelope with electrodes disposed therein at a separation distance, 
d, of 9 cm. The inside diameter of the discharge envelope is approximately 
2.2 cm and the internal volume of the envelope comprises approximately 35 
cm.sup.3. Within the volume, various metal halides and metal halide 
mixtures may be included. A particularly useful halide mixture may 
comprise, for example, approximately 25 mg of a mixture of sodium iodide 
(NaI), thallium iodide (TlI) and indium iodide (InI), said iodides being 
included in a proportion giving good color rendition in the absence of 
color separation. Another useful halide mixture for example comprises 
approximately 25 mg of a mixture containing 5 mole percent of NaI and 95 
mole percent of scandium iodide (ScI.sub.3). A starting gas is also 
included within the envelope and comprises, for example, argon at a 
pressure of approximately 20 torr. The amount of mercury to be added 
depends upon the desired voltage gradient. Thus, for a voltage gradient of 
approximately 10 v/cm, 60 mg of mercury are included in the discharge 
envelope (that is, approximately 1.7 mg/cm.sup.3). For a voltage gradient 
of approximately 5 v/cm, 25 mg of mercury are included (that is, 
approximately 0.7 mg/cm.sup.3). This level of mercury is appropriate for 
either of the aforementioned metal halide mixtures. Such a lamp would 
typically be operated at a power level of approximately 1,000 watts. 
Accordingly, from the above, it will be appreciated that the arc discharge 
lamp of the present invention and the method of operating it described 
above provides a simple and inexpensive method of achieving color 
uniformity along the arc discharge when the discharge lamp is operated in 
a vertical or near vertical position. 
While this invention has been described with reference to particular 
embodiments and examples, other modifications and variations will occur to 
those skilled in the art in view of the above teachings. Accordingly, it 
should be understood that the appended claims are intended to cover all 
such modifications and variations as fall within the true spirit of the 
invention.