Metal halide discharge lamp

Abstract

A metal halide discharge lamp which is capable of reducing a color change when subjected to a variation in the lamp power and/or the voltage supplied to the lamp. The metal halide lamp has an arc tube filled with at least sodium halide and scandium halide. The arc tube is formed at its opposite ends with electrodes which gives an arc discharge therebetween. The lamp has regulator means for keeping a coldest spot temperature of the arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% or rated lamp power. It is found that when the lamp is configured to have a coldest spot temperature at 550° C. or more when operating the lamp at a lamp power which is 50% of the rated lamp power, the lamp shows much less color variation even subjected to the lamp voltage variation, thereby maintaining a desired color.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a metal halide discharge lamp, and more particularly a discharge lamp having an arc tube filled with metal halides.

2. Description of the Prior Art

Metal halide discharge lamps have been used in a wide variety of fields because of its superior performances, such as high luminance, high efficiency, and high color rendering properly. Among these, a metal halide lamp having an arc tube filled with sodium halide and scandium halide is preferred as it shows a less color change. That is, even when luminous intensity of reddish color from vapors of sodium halide varies to some extent, vapor of the scandium halide can provide a continuous color spectrum, thereby giving less change in color. Such discharge lamp is disclosed in the following listed prior art.

List of the Prior Art

a) Japanese Patent Early Publication No. 6-84496

b) Japanese Patent Early Publication No. 6-111772

c) Japanese Patent Early Publication No. 8-203471

d) Japanese Patent Early Publication No. 55-32355

e) Japanese Patent Early Publication No. 56-109447

Concise Explanation of the Listed Prior Art

Publication No. 6-84496 and No. 6-111772 disclose a metal halide lamp having an arc tube filled with sodium iodide, scandium iodide, and an inert gas but without mercury. It is described in this publication that due to the absence of mercury, color spectrum is substantially the same irrespective of a variation of an input power, causing no substantial change in color.

Publication No. 8-203471 discloses a metal halide lamp having an arc tube filled with sodium iodide scandium iodide, and a xenon gas. The arc tube is sealed within an envelope which is evacuated or filled with a lower pressure gas for thermally insulating the arc tube from outside of the envelope for limiting a cooling effect of the arc tube.

Publication No. 55-32355 discloses a metal halide lamp having an arc tube filled with sodium iodide, scandium iodide, mercury, and an inert gas. Scandium iodide is filled in a specific range of amount in relation to a rated lamp power, while a ratio of the filling amount of sodium iodide to that of scandium iodide is selected to a specific value, in order to improve lamp efficiency and operational life period.

Publication No. 56-109447 discloses a metal halide lamp having an arc tube filled with sodium iodide, scandium iodide, mercury, and an inert gas. The lamp is designed to satisfy a specific range as to a molar ratio of sodium iodide to scandium iodide, and at the same time to satisfy a specific relation between the molar ratio and cold spot temperature during a normal lamp operation at a rated power.

Problem of the Prior Art

However, the prior art discharge lamp is found still insufficient in keeping a uniform color when subjected to variations in a lamp power as well as in a voltage supplied to the lamp. Thus, dimming control of varying the lamp power may result in undesired color change of the lamp, and Thus, undesired color change may occur when dimming the lamp by varying the lamp power or when there is a variation in an output voltage from a ballast as a result of a variation in the line voltage, or in quality of the ballast, or even in quality of the lamp.

SUMMARY OF THE INVENTION

In view of the above, the present invention has been achieved to provide a metal halide discharge lamp which is capable of reducing a color change when subjected to a variation in the lamp power and/or the voltage supplied to the lamp. The metal halide lamp in accordance with a present invention comprises an arc tube filled with at least sodium halide and scandium halide. The arc tube is formed at its opposite ends with electrodes which gives an arc discharge therebetween. The lamp has regulator means for keeping a coldest spot temperature of the arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power. It is found that when the lamp is configured to have a coldest spot temperature at 550° C. or more when operating the lamp at a lamp power which is 50% of the rated lamp power, the lamp shows much less color change even subjected to the lamp voltage variation, thereby maintaining a desired color. The arc tube may be made of quartz or a transparent ceramic.

The lamp includes an envelope which forms a hermetically sealed space for accommodating therein the arc tube. The envelope is evacuated or filled with low pressure inert gas to define the regulator means. The envelope may be coated on its inner surface with a layer of reflecting an infrared radiation or with a phosphor.

Preferably, scandium halide is filled the arc tube in an amount of less than 4.08 mol/ml×10 −6 mol/ml to stabilize the arc discharge.

In a preferred embodiment, the lamp include a sleeve surrounding the arc tube to reduce a heat loss form the arc tube. Thus, the sleeve defines the regulator means alone or in combination with the envelope. The sleeve may be coated on its inner surface with a layer of reflecting an infrared radiation. The layer may be coated on the entire surface or partially on opposite ends of the sleeve corresponding to the electrodes.

Further, the lamp includes heat insulators formed on the arc tube at portions covering the respective electrodes so as to thermally insulate the portions of the arc tube adjacent the electrodes from the outside thereof. Thus, the heat insulators can define the regulator means alone or in combination with the envelope or the sleeve. The heat insulator may be a metal layer of reflecting the infrared radiation.

The arc tube may be formed to have reduced-in-diameter sections at opposite ends of the tube which have a diameter less than the rest and surround the electrodes, respectively. With the provision of the reduced-in-diameter sections, the opposite ends of the arc tube is kept at a relatively high temperature due to the heat from the adjacent electrodes. Thus, the sections can define the regulator means alone or in combination with the envelope, sleeves, or the heat insulators.

Formed at opposite ends of the arc tube are sealed ends for sealing the electrodes. The sealed ends are preferably made to have an outside diameter less than that of the arc tube for retarding the cooling of the arc tube around the electrodes. Thus, the sealed ends can also define the regulator means.

A molar ratio (R) of sodium halide to scandium halide is preferably between 2.8 to 22.7 in order to reduce color change when the lamp subjected to the variation in the voltage supplied to the lamp. For the lamp having a rated lamp power of less than 400 W, the molar ratio is preferably between 2.8 to 17.0. For the lamp having a rated power of 400W or more, the molar ratio is preferably between 5.7 to 22.7. The arc tube may additionally include cesium iodide or mercury.

For one lamp configuration where the envelope is evacuated, and the arc tube is made of quartz into a cylindrical shape and is formed on opposite ends with the heat insulators covering the electrodes, the arc tube is preferably designed to have an inside diameter of about 8 mm and a distance of about 80 mm between the electrodes, and is filled with about 2.32×10 −5 mol/ml of sodium iodide, about 2.04×10 −6 mol/ml of scandium iodide, about 1.2×10 −5 mol/ml of cesium iodide, and about 27000 Pa of xenon.

For another lamp configuration where the envelope is evacuated with its inner surface coated with a phosphor layer, and the arc tube is made of quartz into a cylindrical shape and is formed on opposite ends with the heat insulators covering the electrodes, the arc tube is preferably designed to have an inside diameter of about 8 mm and a distance of about 80 mm between the electrodes, and is filled with about 2.32×10 −5 mol/ml of sodium iodide, about 2.04×10 −6 mol/ml of scandium iodide, about 2.5×10 −5 mol/ml of mercury and about 6700 Pa of argon.

For a further lamp configuration where the arc tube is made of quartz into a ellipsoidal shape and is formed on opposite ends with the heat insulators covering the electrodes and with sealing ends for sealing the electrodes, and the correspondingly shaped envelope is evacuated, the ellipsoidal arc tube is preferably designed to have a maximum inside diameter of about 18 mm, an average inside diameter of about 14 mm, and a distance of about 48 mm between the electrodes, and is filled with about 1.35×10 −5 mol/ml of sodium iodide, about 1.15×10 −8 mol/ml of scandium iodide, about 2.14×10 −5 mol/ml of mercury and about 6700 Pa of argon. In this configuration, the sealed ends are also designed to be smaller in diameter than the arc tube.

For a still further lamp configuration where the arc tube is made of quartz into a ellipsoidal shape and is formed on opposite ends with the heat insulators covering the electrodes and with sealing ends for sealing the electrodes, and the correspondingly shaped envelope is evacuated, the ellipsoidal arc tube is preferably designed to have a maximum inside diameter of about 18 mm, an average inside diameter of about 14 mm, and a distance of about 48 mm between the electrodes, and is filled with about 1.35×10 −5 mol/ml of sodium iodide, about 1.15×10 −6 mol/ml of scandium iodide, and about 6700 Pa of argon, said envelope being filled with about 47000 Pa of nitrogen gas. Also in this configuration, the sealed ends are also designed to be smaller in diameter than the arc tube.

These lamp configurations are particularly advantageous for realizing the regulator means for maintaining the coldest spot temperature of the arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power, thereby reducing the color change even subjected to the variation in the voltage supplied to the lamp.

These and still other objects and advantageous features of the present invention will become more apparent from the following description of the embodiments when taken in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a cross section of a metal halide discharge lamp in accordance with a first embodiment of the present invention;

FIG. 2 is a front view of an arc tube utilized in the above lamp, showing cold spots of the tube;

FIGS. 3 and 4 are partial front views, respectively of modified end configurations of the arc tube;

FIG. 5 is a partial front view showing a sealed end of a modified arc tube;

FIG. 6 is a front view of the arc tube of FIG. 5 ;

FIG. 7 is a partial front view showing a sealed end of a modified arc tube;

FIG. 8 is a cross section of a metal halide discharge lamp in accordance with a second embodiment of the present invention;

FIG. 9 is a front view of an arc tube utilized in the above lamp, showing cold spots of the tube;

FIG. 10 is a partial front view showing a modified end configuration of the arc tube;

FIG. 11 is a partial front view showing a sealed end of a modified arc tube;

FIG. 12 is a graph showing characteristics of the lamp in accordance with examples 1 to 11;

FIG. 13 is a graph showing characteristics of the lamp in accordance with examples 12 to 17;

FIG. 14 is a cross section of the metal halide discharge lamp similar to the one shown in FIG. 1 with an infrared radiation reflecting layer;

FIG. 15 is a cross section of the metal halide discharge lamp similar to the one shown in FIG. 8 with a phosphor layer and an infrared radiation reflecting layer;

FIG. 16 is a cross section of the metal halide discharge lamp similar to the one shown in FIG. 1 with a phosphor layer and an infrared radiation reflecting layer applied to an arc tube; and

FIG. 17 is a cross section of the metal halide discharge lamp similar to the one shown in FIG. 1 with an infrared radiation reflecting layer applied to the arc tube.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to FIG. 1 , there is shown a metal halide discharge lamp in accordance with a first embodiment of the present invention. The lamp comprises a glass-made envelope 10 forming a hermetically sealed space therein, an arc tube 20 disposed in the space, and a base 30 attached to one end of the envelope 10 . The arc tube 20 is in the form of a cylinder having a uniform diameter and is supported to the envelope 10 through a pair of conductor props 32 and 33 extending commonly from a stem 31 fixed to the base 30 . The arc tube 20 is also of a cylindrical shape with a uniform diameter and has electrodes 22 at opposite lengthwise ends thereof. The arc tube is made of quartz or transparent ceramic to have at the opposite end sealed rends 23 for sealing the electrodes 22 . The electrodes 22 are connected respectively through molybdenum foils 24 to the conductor props 32 so as to develop an arc discharge between the electrodes 22 . As shown in FIG. 14 , a filler F fills the arc tube 20 and such fillers are sodium iodide, scandium iodide, and inert gas, for example. Additional metal halide or mercury M may be added in the tube.

Heat insulator layers 26 made of metal or zirconium oxide are formed respectively on the outer surfaces of the opposite ends of the arc tube to surround the electrodes 22 as well as the sealed ends 23 for reducing heat dissipation from around the electrodes 22 . A transparent sleeve 40 also of a cylindrical shape is disposed in the envelope 10 to surround the arc tube in an intimate relation thereto for reducing heat dissipation from the arc tube. The arc tube 20 is supported to the one conductor prop 33 by means of arms 34 . The conductor prop 34 carries at its one end adjacent the stem 31 a barium getter 36 and at the opposite end a zirconium-aluminum getter 37 .

The lamp is driven by a conventional magnetic ballast which includes a starter to apply a pulsating voltage to start the lamp and includes a dimmer function of varying a lamp power for dimming control of the lamp.

In the above lamp, the envelope 10 , the heat insulator layer 26 , and the sleeve 40 are either alone or in combination to define a regulator means which is responsible for keeping a coldest spot temperature of 550° C. or more when the lamp is operated at a lamp power which is 50% of a rated lamp power. The coldest spot temperature is determined to the temperature of the coldest one of spots that are chosen as indicated by (a), (b), (c), and (d) in FIG. 2 , where spot (a) is a tip-off, spot (b) is a root of the electrode, (c) is a bottom of the heat insulator at a horizontal lamp operation, and (d) is a point from which a bent arc is kept away or where unvaporized metal halides remain.

As shown in FIGS. 3 and 4 , the arc tube 20 may be configured to have its opposite ends shaped into reduced-in-diameter sections 28 around the electrodes 22 in order to narrower a spacing between the electrodes and the adjacent tube walls. The reduced-in-diameter section 28 is in the form of a tapered section which reduces the area of surface surrounding the adjacent electrode than the non-tapered end of the arc tube, thereby reducing a heat loss from the surface surrounding the electrode. Also, because of that the reduced-in-diameter sections are made close to the electrodes, the arc tube can have an increased wall temperature. In this sense, the reduced-in-diameter sections 28 is alone or in combination with at least one of the envelope, sleeve, and the heat insulator layer to define the above regulator means.

Further, as shown in FIGS. 5 and 7 , the sealed ends 23 may be shaped to have an outside diameter smaller than the arc tube 20 so as to reduce a heat loss by radiation and/or conduction from the sealed ends, thereby keeping the outer surface of the sealed end 23 at a relatively high temperature and therefore the adjacent ends of the arc tube around the electrodes. In this sense, the small-sized sealing ends 23 can additionally constitute the above regulator means either alone or in combination with at least one of the envelope, sleeve, heat insulator layer, and the reduced-in-diameter section for keeping the coldest spot temperature at a relatively high level when the lamp is operated at a reduced lamp power. The arc tube having the small-sized sealed ends 23 of FIG. 5 is preferred to have dimensions as shown in FIG. 6 .

FIG. 8 shows a lamp in accordance with a second embodiment which is similar to the first embodiment except that an arc tube 20 A and an envelope 10 A are both ellipsoidal in shape. Like parts are designated by like reference numerals with a suffix letter of ‘A’. Also in this lamp, the envelope 10 A is cooperative with at least one of the heat insulator layer 26 A and the sleeve 40 A to define a regulator means which is responsible for keeping a coldest spot temperature of 550° C. or more when the lamp is operated at a lamp power which is 50% of a rated lamp power. The coldest spot temperature is determine to the temperature of the coldest one of spots that are chosen as indicated by (a), (b), (c), and (d) in FIG. 9 .

As shown in FIG. 10 , the arc tube 20 A may be configured to have its opposite ends shaped into reduced-in-diameter sections 28 A around the electrodes 22 A in order to narrower a spacing between the electrodes and the adjacent tube walls, thereby reducing cooling effect of the tube walls. In this sense, the reduced-in-diameter sections 28 A can constitute the above regulator means.

Further, as shown in FIG. 11 , the sealed ends 23 A may be shaped to have an outside diameter smaller than the arc tube 20 A so as to keep the outer surface of the sealed end 23 A at a relatively high temperature and therefore the adjacent ends of the arc tube around the electrodes. In this sense, the small-sized sealing ends 23 A can constitute the above regulator means for keeping the coldest spot temperature at a relatively high level when the lamp is operated at a reduced lamp power.

The following examples further illustrate the nature and advantages of the present invention.

EXAMPLES 1 to 9

Lamps were fabricated in accordance with the first embodiment to have arc tubes of quartz which were dimensioned to have an inside diameter of 8 mm, and a distance of 80 mm between the electrodes. The arc tubes were filled mainly with sodium iodide and scandium iodide, with or without cesium iodide or mercury in listed amounts as shown in Table 1 below. The lamps were configured to have the regulator means defined by the envelope in combination with at least one of the sleeve, heat insulator layers, reduction-in-diameter sections, and the sealed ends, as shown in Table 1. For a comparative purposes, Comparative Example 1 were prepared which is identical to Example 1 except that the regulator means was not included.

EXAMPLE 10 and 11

Lamps were fabricated in accordance with the second embodiment to have arc tubes which were made of quartz and dimensioned to have a maximum inside diameter of 18 mm, and a distance of 48 mm between the electrodes. The arc tubes were filled mainly with sodium iodide and scandium iodide, and with cesium iodide or mercury in listed amounts as shown in Table 1 below. The lamps were configured to have the regulator means defined by the envelope in combination with at least one of the envelope, sleeve, heat insulator layers, reduction-in-diameter sections, and the sealed ends, as shown in Table 1. For a comparative purposes, Comparative Example 2 was prepared which is identical to Example 10 except that the regulator means was not included.

In order to evaluate the lamp characteristics for the Examples 1 to 11 and Comparative Examples 1 and 2, measurements were made to obtain a coldest spot temperature (CST) (°C.) at operating at 100% of rated lamp power and reduced lamp power as listed, as well as to obtain a variation (ΔT (K)) in color temperature when the voltage supplied to the lamp, i.e., the input source voltage to the magnetic ballast varies.

TABLE 1

Envelope

Envelope

Nal

Scl 3

with

with IR

Arc tube

(×10 −5

(×10 −6

Nal/Scl 3

Csl

Hg

Envelope

phosphor

reflection

Lamp

material

mol/ml)

mol/ml)

(molar ratio)

filled

filled

Envelope

evacuated

coating

coating

Example 1

Quartz

2.32

2.04

11.4

No

No

Yes

No

No

No

Example 2

Quartz

2.32

4.08

5.7

Yes

No

Yes

No

No

No

Example 3

Quartz

0.58

1.02

5.7

Yes

No

Yes

Yes

No

No

Example 4

Quartz

1.16

2.04

5.7

Yes

No

Yes

Yes

No

Yes

Example 5

Quartz

2.32

2.04

11.4

Yes

No

Yes

Yes

No

No

Example 6

Quartz

2.32

2.04

11.4

No

Yes

Yes

Yes

Yes

No

Example 7

Quartz

3.48

2.04

17.1

Yes

No

Yes

Yes

No

Yes

Example 8

Quartz

3.48

2.04

17.1

Yes

No

Yes

Yes

No

No

Example 9

Ceramic

2.32

2.04

11.4

Yes

No

Yes

Yes

No

No

Comparative

Quartz

2.32

2.04

11.4

No

No

No

—

No

—

Example 1

Example 10

Quartz

1.31

1.15

11.4

No

Yes

Yes

Yes

No

No

Example 11

Quartz

1.97

1.15

17.0

Yes

Yes

Yes

Nitrogen

Yes

Yes

filled

Comparative

Quartz

1.31

1.15

11.4

No

Yes

No

—

No

—

Example 2

Sleeve

With IR

reflection

ΔT (K) on

Sleeve

coating

Metal

input

with IR

only on

Heat

heat

Reduced-

Sealed

source

Rated

reflection

opposite

insulator

insulator

in-diameter

ends

Arc

Wla

CST

voltage

power

Lamp

Sleeve

coating

ends

layer

layer

section

size

bent

(%)

(° C.)

variation

(Watts)

Example 1

No

—

—

No

—

No

Normal

None

100

631

63

250

50

551

Example 2

No

—

—

Yes

No

No

Normal

Yes

100

628

42

250

50

589

Example 3

Yes

Yes

Yes

No

No

Normal

None

100

590

120

250

50

555

Example 4

No

—

—

Yes

No

No

Normal

None

100

601

65

250

50

566

Example 5

No

—

Yes

Yes

No

Normal

None

100

624

73

250

50

552

Example 6

Yes

Yes

—

Yes

Yes

No

Normal

None

100

663

55

250

50

622

Example 7

Yes

No

—

Yes

No

No

Normal

None

100

719

24

250

50

615

Example 8

No

—

—

Yes

Yes

Yes

Small

None

100

690

44

250

50

575

Example 9

No

—

—

No

No

Yes

Normal

None

100

650

34

250

50

579

Comparative

No

—

—

No

—

No

Normal

None

100

503

442

250

Example 1

63

459

Example 10

No

—

—

Yes

Yes

Yes

Small

None

100

752

85

400

50

645

Example 11

Yes

Yes

—

Yes

No

Yes

Small

None

100

697

64

400

50

612

Comparative

No

—

—

No

—

No

Normal

None

100

648

658

400

Example 2

50

500

In Examples 2 to 5, 7 to 9, and 11, cesium iodide was added in an amount of 1.25×10 −5 mol/ml. In Examples 6, 10, and 11, mercury was added in an amount of 2.50×10 −5 mol/ml. In Examples 11, mercury was added in an amount of 1.53×10 −5 mol/ml.

As to the column ‘envelope’ in Table 1, ‘Yes’ denotes the use of the envelope. As to the column ‘envelope evacuated’, ‘Yes’ denotes that the envelope is evacuated. Further, Examples 6 and 11 utilize the envelopes each coated on its inner surface with a phosphor coating, while Examples 4, 7, and 11 utilized the envelopes each coated on its inner surface with a coating capable of reflecting infrared radiation. Examples 2 to 4, 7, and 11 utilized the heat insulator layer made of zirconium oxide, while Examples 5, 6, 8, and 10 utilized the heat insulator layer of metal such as platinum or gold capable of reflecting infrared radiation to a large extent than zirconium oxide. In Examples 8 to 11, the reduced-in-diameter sections were formed on opposite ends of the arc tube. In Examples 10 and 11, the sealed ends of the arc tube were made to have a smaller diameter than the arc tube as shown in FIG. 6 . Arc bent was seen in Example 2.

As is seen from Table 1, Comparative Examples 1 and 2 show decreased coldest spot temperatures of 459° C. and 500° C., respectively when the lamp power (Wla) is reduced to 63% of the rated power, and large color temperature variation widths (ΔT) of 442K and 658K when the input source voltage varies by ±10%. On the other hand, all the Examples show the color temperature variation width (ΔT) of 120K or less in response to ±10% variation of the input source voltage to the ballast. This means that Examples are capable of reducing color change even subjected to source voltage variations.

FIG. 12 show curves plotting the coldest color temperatures (CST) changing with varying the lamp power for Examples 1 to 12, and Comparative Examples 1 and 2. The right end plot and the second one from the right of each curve was obtained when operating the lamp at 110%, and 100% of the rated power, respectively, while left and plots of curves for Examples 1 to 11 and Comparative Example 2 were obtained when operating the lamp at 50% of the rated lamp power. The curve for Comparative Example 1 has the left end plot which was obtained when operating the lamp at 63% of the rated lamp power.

EXAMPLES 12 to 17

Lamps were fabricated in accordance with the first embodiment to have arc tubes of quartz which were dimensioned to have an inside diameter of 8 mm, and a distance of 80 mm between the electrodes. The arc tubes were filled with sodium iodide and scandium iodide at varying molar ratio therebetween as listed in Table 2 below. Also, about 27000 Pa of xenon and 1.25×10 −5 mol/ml of cesium iodide were filled in the tube. For example lamp, the arc tube was contained in the evacuated envelope and is coated with the heat insulator layer of zirconium oxide. No sleeve was provided. Measurements were made to obtain the coldest spot temperature (CST) of each arc tube when operating the lamp at 100% and 50% of rated lamp power, respectively, and to obtain a width of color temperature change ΔT in response to ±10% variation in the source voltage.

TABLE 2
	NaI/ ScI 3			ΔT (K) on source
Lamp	(molar ratio)	WIa (%)	CST (° C.)	voltage variation
Example 12	17.0	100	655	59
		50	551
Example 13	14.2	100	645	47
		54	853
Example 14	11.4	100	646	12
		51	558
Example 15	8.5	100	669	45
		50	579
Example 16	5.7	100	618	66
		50	567
Example 17	2.8	100	638	44
		55	589

It is confirmed from Table 2 that the color temperature change (ΔT) can be reduced while the molar ratio of sodium iodide to scandium iodide varies from 2.8 to 17.0. FIG. 13 show luminous efficiency, color rendering index, and color temperature measured for Examples 12 to 17. As seen form FIG. 13 , it is known that Examples 12 to 17 show almost constant color rendering index of around 60, and efficiency of around 80 (lm/W), while showing varying color temperature as the molar ratio of sodium iodide to scandium iodide varies. With this result, it is found that a desired color can be chosen, yet reducing the color temperature variation ΔT against the variation in the source voltage.

EXAMPLES 18 to 21

Lamps were fabricated in accordance with the second embodiment to have arc tubes of quartz which were dimensioned to have a maximum inside diameter of 18 mm, an average inside diameter of 14 mm, and a distance of 48 mm between the electrodes. The arc tubes were filled with sodium iodide and scandium iodide at varying molar ratio therebetween as listed in Table 3 below. Also, about 6700 Pa of argon and 1.53×10 −5 mol/ml of mercury were filled in the tube. For each lamp, the arc tube was contained in the evacuated envelope and is coated with the heat insulator layer of zirconium oxide. No sleeve was provided. Measurements were made to obtain the coldest spot temperature (CST) of each arc tube when operating the lamp at 100% and 50% of rated lamp power, respectively, and to obtain a width of color temperature change ΔT in response to ±10% variation in the source voltage.

TABLE 3
	NaI/ScI 3			ΔT (K) on source
Lamp	(molar ratio)	WIa (%)	CST (° C.)	voltage variation
Example 18	5.7	100	645	60
		50	560
Example 19	11.4	100	752	85
		50	645
Example 20	17.0	100	697	64
		50	812
Example 21	22.7	100	759	79
		50	609

It is also confirmed from Table 3 that the color temperature change (ΔT) can be reduced while the molar ratio of sodium iodide to scandium iodide varies from 5.7 to 22.7.

EXAMPLE 22

Lamps were fabricated in accordance with the first embodiment to have arc tubes of quartz which were dimensioned to have an inside diameter of 8 mm, and a distance of 80 mm between the electrodes. The arc tubes were filled with scandium iodide at a varying mount between 1.02×10 −8 mol/ml and 4.59×10 −8 mol/ml and with sodium iodide at a varying molar ratio relative to scandium iodide from 0.0 to 19.8, as listed in Table 4 below. Also, about 27000 Pa of xenon was filled in the tube. For each lamp, the arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 550° C. or more when operating the lamp at 50% of its rated lamp power. No sleeve was provided. Three samples were prepared for each lamp. Observation was made to see whether an arc bent occurred or not for three samples of identical lamp configuration. The results are shown in Table 4 in which mark ‘◯’ denotes no arc bent occurred in any of the three samples, mark ‘Δ’ denotes arc bent occurred in only one or two of the three samples, and mark ‘X’ denotes arc bent occurred in all of the three samples.

TABLE 4
Scl 3	Nal/Scl 3
(×10 −6	(molar ratio)
mol/ml)	19.8	17.0	14.2	11.4	8.5	5.7	2.8	0.0
4.59	X	X	X	X	X	X	X	X
4.08	Δ	Δ	Δ	Δ	X	X	X	X
3.57	◯	◯	◯	◯	◯	◯	◯	Δ
3.06	◯	◯	◯	◯	◯	◯	◯	Δ
2.55	◯	◯	◯	◯	◯	◯	◯	◯
2.04	◯	◯	◯	◯	◯	◯	◯	◯
1.02	◯	◯	◯	◯	◯	◯	◯	◯

Also, measurements were made to obtain a width of color temperature change ΔT in response to ±10% variation in the source voltage. The condition range encircled by double-lines in Table 4 are found effective to reduce the color temperature change ΔT. Thus, it is known that the color temperature change in kept at a reduced level even when the arc bent occurs. Taking this into consideration, it is found possible to stabilize the arc and at the same time to reduce the color temperature change by suitably selecting the filling amount of the scandium iodide and the molar ratio of the sodium iodide to scandium iodide.

EXAMPLE 23

Lamps were fabricated in accordance with the second embodiment to have arc tubes of quartz which were dimensioned to have a maximum inside diameter of 18 mm, an average inside diameter of 14 mm, and a distance of 48 mm between the electrodes. In order to further investigate the relation between the arc bent and the filling amount of scandium iodide, the arc tubes were filled with scandium iodide at a varying mount between 1.15×10 −8 mol/ml and 5.73×10 −6 mol/ml and with sodium iodide at a varying molar ratio relative to scandium iodide from 0.0 to 28.4, as listed in Table 5 below. Also, the arc tube was filled with about 2.15×10 −6 mol/ml of mercury and about 6700 Pa of argon was filled in the tube. For example lamp, the arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 550° C. or more when operating the lamp at 50% of its rated lamp power. No sleeve was provided. Three samples were prepared for each lamp. Observation was made to see whether an arc bent occurred or not for three samples of identical lamp configuration. The results are shown in Table 5 in which the same marks as in Table 4 are utilized for evaluation of the occurrence of the arc bent.

TABLE 5
ScI 3	NaI/ScI 3 (molar ratio)
(×10 −8 mol/ml)	28.4	22.7	17.0	11.4	5.7	0.0
5.73	X	X	X	X	X	X
4.61	X	X	X	X	X	X
4.08	Δ	Δ	Δ	◯	Δ	X
3.45	◯	◯	◯	◯	◯	◯
2.31	◯	◯	◯	◯	◯	◯
1.15	◯	◯	◯	◯	◯	◯

Also, measurements were made to obtain a width of color temperature change ΔT in response to ±10% variation in the source voltage. The condition range encircled by double-lines in Table 5 are found effective to reduce the color temperature change ΔT. Thus, it is known that the color temperature change is kept at a reduced level even when the arc bent occurs. Taking this into consideration, it is found possible to stabilize the arc and at the same time to reduce the color temperature change by suitably selecting the filling amount of the scandium iodide and the molar ratio of the sodium iodide to scandium iodide.

EXAMPLE 24

A lamp was fabricated in accordance with the first embodiment to have the arc tube of quartz which was dimensioned to have an inside diameter of 8 mm, and a distance of 80 mm between the electrodes. The arc tube was filled with 2.32×10 −8 mol/ml of sodium iodide, 2.04×10 −8 mol/ml of scandium iodide (molar ratio of sodium iodide to scandium iodide is about 11.4), 1.02×10 −5 mol/ml of cesium iodide, and about 27000 Pa of xenon. The arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 586° C. when operating the lamp at 50% of its rated lamp power. No sleeve was provided.

EXAMPLE 25

A lamp was fabricated in accordance with the first embodiment to have the arc tube of quartz which was dimensioned to have an inside diameter of 8 mm, and a distance of 80 mm between the electrodes. The arc tube was filled with 2.32×10 −5 mol/ml of sodium iodide, 2.04×10 −8 mol/ml of scandium iodide (molar ratio of sodium iodide to scandium iodide is about 11.4), 2.50×10 −5 mol/ml of mercury, and about 6700 Pa of argon. The arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 569° C. when operating the lamp at 50% of its rated lamp power. No sleeve was provided, and the envelope was coated with a phosphor.

EXAMPLE 26

A lamp was fabricated in accordance with the second embodiment to have the arc tube of quartz which was dimensioned to have a maximum inside diameter of 18 mm, an average inside diameter of 14 mm and a distance of 48 mm between the electrodes. The arc tube was filled with 1.35×10 −5 mol/ml of sodium iodide, 1.15×10 −6 mol/ml of scandium iodide, 2.14×10 −5 mol/ml of mercury, and about 6700 Pa of argon. The arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 552° C. when operating the lamp at 50% of its rated lamp power. No sleeve was provided.

EXAMPLE 27

A lamp was fabricated in accordance with the second embodiment to have the arc tube of quartz which was dimensioned to have a maximum inside diameter of 18 mm, an average inside diameter of 14 mm and a distance of 48 mm between the electrodes. The arc tube was filled with 1.35×10 −5 mol/ml of sodium iodide, 1.15×10 −6 mol/ml of scandium iodide, 1.53×10 −5 mol/ml of mercury, and about 6700 Pa of argon. The arc tube was contained in the envelope filled with about 47000 Pa of nitrogen and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 551° C. when operating the lamp at 50% of its rated lamp power. No sleeve was provided.

For the lamps of Examples 24 to 27, measurements were made to obtain a width of color temperature change ΔT in response to ±10% variation in the source voltage. The results are shown in Table 6 below.

TABLE 6
		ΔT on ± 10% source
Lamp	WIa (%)	voltage variation	CST (° C.)
Example 24	100	22	692
	50		586
Example 25	100	12	642
	50		569
Example 26	100	128	612
	50		552
Example 27	100	105	638
	50		551

As seen in Table 6, the lamps of Examples 24 to 27 are found to show only reduced color temperature change ΔT. Particularly, the lamp of Examples 24 and 25 show a remarkably reduced color temperature change.

EXAMPLE 28

A lamp was fabricated in accordance with the first embodiment to have the arc tube of quartz which was dimensioned to have an inside diameter of 8 mm, and a distance of 80 mm between the electrodes. The arc tube was filled with 2.32×10 −5 mol/ml of sodium iodide, 2.04×10 −6 mol/ml of scandium iodide (molar radio of sodium iodide to scandium iodide is about 11.4), 1.20×10 −5 mol/ml of cesium iodide, and about 27000 Pa of xenon. The arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 550° C. or more when operating the lamp of 50% of its rated lamp power. No sleeve was provided.

EXAMPLE 29

A lamp was fabricated in accordance with the first embodiment to have the arc tube of quartz which was dimensioned to have an inside diameter of 8 mm, and a distance of 80 mm between the electrodes. The arc tube was filled with 2.32×10 −5 mol/ml of sodium iodide, 2.04×10 −6 mol/ml of scandium iodide (molar ratio of sodium iodide to scandium iodide is about 11.4), 2.50×10 −5 mol/ml of mercury, and about 6700 Pa of argon. The arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 550° C. or more when operating the lamp at 50% of its rated lamp power. No sleeve was provided.

EXAMPLE 30

A lamp was fabricated in accordance with the first embodiment to have the arc tube of quartz which was dimensioned to have an inside diameter of 8 mm, and a distance of 80 mm between the electrodes. The arc tube was filled with 2.32×10 −5 mol/ml of sodium iodide, 2.04×10 −8 mol/ml of scandium iodide (molar ratio of sodium iodide to scandium iodide is about 11.4), and about 27000 Pa of xenon. The arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 550° C. or more when operating the lamp at 50% of its rated lamp power. No sleeve was provided.

For the lamps of Examples 28 to 30, measurements were made to obtain luminous flux (lm), luminous efficiency (lm/W), color temperature (Tc (K)), cooler temperature change (ΔT), cooler rendering index (Ra), coldest spot temperature (CST). The results are shown in Table 7 below, in which source voltage ratio (%) is a ratio of the source voltage relative to the voltage for operating the lamp at 100% of the rated lamp power, and the luminous flux ratio (%) is a ratio of the luminous flux to that obtained at 100% rated lamp power. The color temperature change (ΔT) denotes a value relative to the color temperature obtained at 100% rated lamp power.

As seen from Table 7, the lamps of Examples 28 to 30 exhibit reduced color temperature change (ΔT) against the varying lamp power as well as against the varying source voltage. The lamp of Example 28 in which the arc tube additionally contain cesium iodide has a superior effect of reducing the color temperature change as compared to the lamp of Example 30 in which no cesium iodide is contained in the arc tube. From this, it is found that the addition of cesium iodide is responsible for providing a wide range in which the color temperature change is kept reduced, advantageous for dimming the lamp without causing no substantial color change. Also, it is noted that the lamp of Example 29 exhibits the reduced color temperature change against varying lamp power, irrespective of the fact that the arc tube additionally contain mercury. Further, it is confirmed that when the envelope of Example 29 is coated with the phosphor as is made in Example 25, the color temperature change against the varying lamp power can be still reduced.

TABLE 7
									Color
	Lamp	Source	Source		Luminous	Luminous	Color	Color	rendering	coldest spot
	power	voltage	voltage	Luminous	flux ratio	Efficiency	temperature	temperature	Index	temperature
Lamp	ratio (%)	Vs (V)	ratio (%)	flux (lm)	(%)	(lm/W)	Tc (K)	change ΔT	<Ra>	CST (° C.)
Example 28	100	510	100	25102	100	84	3998	0	55	636
	92	475	93	22774	91	83	4081	83	55	624
	84	440	86	19630	78	78	4115	117	55	615
	75	405	79	16352	65	73	4143	145	56	605
	67	370	73	13183	53	66	4165	167	56	594
	59	320	63	10141	40	58	4139	141	56	570
	50	262	51	7160	29	47	4145	147	57	561
	41	201	39	4652	19	37	4192	194	59	553
Example 29	100	440	100	23610	100	79	5204	0	62	618
	92	412	94	20140	85	73	5275	71	59	801
	84	386	88	18651	71	66	5238	134	56	595
	75	367	83	13301	56	59	5207	3	54	588
	67	340	77	10177	43	51	5167	−37	45	579
	58	328	75	6748	29	39	5055	−149	48	564
	50	312	71	3210	14	21	4998	−206	50	551
	42	305	69	1695	7	14	4980	−224	51	525
Example 30	100	590	100	23052	100	77	4557	0	59	644
	92	550	93	19143	83	70	4628	71	60	631
	83	512	87	16235	70	65	4643	86	60	618
	75	460	78	13395	58	60	4657	100	60	610
	67	410	69	10023	43	50	4477	−80	61	594
	58	359	61	7596	33	43	4201	−356	61	572
	50	292	49	3443	15	23	3952	−605	63	551
	41	215	36	1125	5	9	3562	−995	65	512

EXAMPLE 31

A lamp was fabricated in accordance with the second embodiment to have the arc tube of quartz which was dimensioned to have a maximum inside diameter of 18 mm, an average inside diameter of 14 mm, and a distance of 48 mm between the electrodes. The arc tube was filled with 1.35×10 −5 mol/ml of sodium iodide, 1.5×10 −6 mol/ml of scandium iodide, 2.14×10 −5 mol/ml of mercury, and about 6700 Pa of argon. The arc tube was contained in the evacuated envelope and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 550° C. or more when operating the lamp at 50% of its rated lamp power. No sleeve was provided.

EXAMPLE 32

A lamp was fabricated in accordance with the second embodiment to have the arc tube of quartz which was dimensioned to have a maximum inside diameter of 18 mm, an average inside diameter of 14 mm, and a distance of 48 mm between the electrodes. The arc tube was filled with 1.35×10 −5 mol/ml of sodium iodide, 1.15×10 −6 mol/ml of scandium iodide, 1.53×10 −5 mol/ml of mercury, and about 6700 Pa of argon. The arc tube was contained in the envelope filled with about 47000 Pa of nitrogen, and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 550° C. or more when operating the lamp at 50% of its rated lamp power. No sleeve was provided, and the envelope was coated with the phosphor. The lamp of Example 32 differs from the lamp of Example 31 only in that the envelope was filled with nitrogen and was coated with the phosphor.

EXAMPLE 33

A lamp was fabricated in accordance with the second embodiment to have the arc tube of quartz which was dimensioned to have a maximum inside diameter of 18 mm, an average inside diameter of 14 mm, and a distance of 48 mm between the electrodes. The arc tube was filled with 1.35×10 −5 mol/ml of sodium iodide, 1.15×10 −6 mol/ml of scandium iodide, 2.14×10 −5 mol/ml of mercury, and about 6700 Pa of argon. The arc tube was contained in the envelope filled with about 47000 Pa of nitrogen, and was coated with the heat insulator layer of zirconium oxide to give the coldest spot temperature of 550° C. or more than operating the lamp at 50% of its rated lamp power. No sleeve was provided. The lamp of Example 33 differs from the lamp of Example 31 only in the provision of nitrogen filled in the envelope.

For the lamps of Examples 31 to 33, like measurements as made for Examples 28 to 30 were done. The results are shown in Table 8 below in which the source voltage ratio (%) for Example 31 and 32 denotes a ratio of the source voltage relative to 200 V, the source voltage ratio (%) for Example 33 denotes a ratio of the source voltage relative to the voltage for operating the lamp at 100% of the rated lamp power, and the luminous flux ratio (%) is a ratio of the luminous flux to that obtained at 100 V source voltage.

Considering the results of Example 31 in which the envelope is not coated with the phosphor and the results of Example 32 in which the envelope is coated with the phosphor (emitting red light), both Examples show reduced color temperature change responsible for superior dimming characteristics although the phosphor coating can slightly lower the color temperature. Comparing the results of Example 31 having the evacuated envelope with the results of Example 33 having the envelope filled with nitrogen gas, it is confirmed that the lamp of Example 33 is also effective to reduce the color temperature change and is advantageous for making the dimmer control without causing substantial change in color.

As illustrated in FIGS. 14 and 15 , the envelope has its inner surface coated with an infrared radiation reflecting layer 14 and 14 A respectively. As illustrated by way of example in FIG. 14 , the arc tube is filled with mercury M as the filler F. As shown in FIGS. 15 and 16 , the envelope has its inner surface coated with a phosphor layer 12 A and 12 respectively. As shown in FIG. 17 , the sleeve 40 has its inner surface coated with an infrared radiation reflecting layer 44 .

Although in the above Examples, metal iodides are utilized as metal halides, the present invention is not limited to the metal iodides and should be equally applicable to metal bromides. Also, either when the lamp is operated at a horizontal position where the electrodes are spaced horizontally or at a vertical position where the electrodes are spaced vertically, the like results were obtained as demonstrated in the above Examples. Further, the like results were obtained to the lamps with the arc tubes having dimensions different from Examples and having rate gases of different filling pressures.

TABLE 8
									Color
	Lamp	Source	Source		Luminous	Luminous	Color	Color	rendering	coldest spot
	power	voltage	voltage	Luminous	flux ratio	Efficiency	temperature	temperature	Index	temperature
Lamp	ratio (%)	Vs (V)	ratio (%)	flux (lm)	(%)	(lm/W)	Tc (K)	change ΔT	<Ra>	CST (° C.)
Example 31	125	240	120	58190	140	116	3898	0	72	805
	119	232	116	53740	133	113	3900	2	71	800
	112	225	112	50584	125	113	3932	34	71	788
	106	218	109	47624	118	112	3951	53	70	778
	100	210	105	44648	111	112	3961	63	70	765
	93	202	101	41564	103	111	3973	75	69	760
	91	200	100	40406	100	110	3978	80	69	752
	88	195	96	38462	95	110	3984	86	68	741
	81	187	94	35197	87	108	3995	97	67	728
	75	179	90	31998	79	107	4017	119	66	714
	69	172	86	28664	71	104	4052	154	65	703
	63	165	83	25391	63	101	4123	225	63	689
	57	158	79	21823	54	97	4222	324	62	668
	50	152	76	18213	45	91	4377	479	61	645
Example 32	125	241	121	50500	140	101	3880	0	73	791
	118	233	117	48181	134	102	3895	15	72	779
	113	226	113	45801	127	102	3900	20	72	770
	106	218	109	42894	119	101	3907	27	71	760
	100	211	106	40107	111	100	3913	33	71	751
	94	203	102	37350	104	100	3920	40	70	740
	91	200	100	36072	100	99	3923	43	70	728
	87	195	98	34415	95	98	3927	47	70	728
	81	188	94	31900	88	98	3931	51	69	710
	75	180	90	28816	80	98	3934	54	68	700
	69	173	86	26019	72	94	3937	57	67	689
	63	165	83	22921	64	91	4035	155	66	680
	56	158	79	19605	54	87	4181	301	65	665
	50	153	77	16070	45	80	4367	487	65	650
Example 33	125	238	114	55500	131	111	4095	0	71	698
	119	232	111	52250	123	110	4100	5	71	689
	112	224	107	48287	114	108	4108	13	70	678
	106	217	104	45476	107	107	4107	12	69	667
	100	209	100	42386	100	106	4106	11	68	652
	94	202	96	39239	93	104	4110	15	67	645
	92	200	95	38415	91	104	4115	20	67	638
	88	194	93	36055	85	103	4134	39	66	629
	81	186	89	32630	77	100	4161	66	65	619
	75	179	85	29064	69	97	4231	118	64	611
	69	171	82	25712	61	93	4311	216	62	601
	63	164	78	22211	52	88	4439	344	61	592
	56	158	75	18249	43	81	4627	532	57	580
	50	153	73	14710	35	73	4707	612	53	568
	44	148	71	11032	26	63	4785	690	44	551

Claims

1. A metal halide discharge lamp comprising:an arc tube filled with at least sodium halide and scandium halide, said arc tube being formed at its opposite ends with electrodes which gives an arc therebetween; and a regulator for keeping a coldest spot temperature of said arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power of said lamp, wherein a molar ratio (R) of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 2.8≦R≦22.7.

2. The metal halide discharge lamp as set fort in claim 1, whereinsaid lamp has a rated lamp power less than 400 W, and a molar ratio (R) of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 2.8≦R≦17.0.

3. The metal halide discharge lamp as set fort in claim 1, whereinsaid lamp has a rated lamp power is 400 W or more, and a molar ratio (R) of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 5.7≦R≦22.7.

4. The metal halide discharge lamp as set forth in claim 1, whereinsaid regulator comprises an envelope which forms a hermetically sealed space within which said arc tube is disposed.

5. The metal halide discharge lamp as set fort in claim 1, whereinsaid lamp has a rated lamp power of less than 400 W, and said regulator comprises an envelope which forms a hermetically sealed space within which said arc tube is disposed, said space being evacuated.

6. The metal halide discharge lamp as set forth in claim 1, whereinsaid lamp has a rated power of 400 W or more, and said regulator comprises an envelope which forms a hermetically sealed space within which said arc tube is disposed, said space being evacuated or filled with a low pressure inert gas.

7. The metal halide discharge lamp as set forth in claim 1, whereinsaid regulator comprises an infrared radiation reflecting layer coated on an inner surface of an envelope within which said arc tube is disposed.

8. The metal halide discharge lamp as set forth in claim 1, whereinsaid regulator comprises a transparent sleeve surrounding said arc tube within an envelope.

9. The metal halide discharge lamp as set fort in claim 8, whereinsaid sleeve has its inner surface coated with an infrared radiation reflecting layer.

10. The metal halide discharge lamp as set fort in claim 8, whereinsaid sleeve being coated with an infrared radiation reflecting layer at opposite ends of said sleeve corresponding to said electrodes.

11. The metal halide discharge lamp as set fort in claim 1, whereinsaid regulator comprises heat insulators covering electrodes at the opposite ends of said arc tube.

12. The metal halide discharge lamp as set fort in claim 11, whereinsaid heat insulator comprises a metal layer reflecting an infrared radiation.

13. The metal halide discharge lamp as set fort in claim 11, whereinsaid heat insulator comprises a metal layer reflecting an infrared radiation, said metal layer covering said electrodes at the opposite ends of said arc tube.

14. The metal halide discharge lamp as set fort in claim 1, whereinsaid regulator comprises reduced-in-diameter sections formed at the opposite ends of said arc tube, said reduced-in-diameter sections surrounding said electrodes, respectively.

15. The metal halide discharge lamp as set fort in claim 1, whereinsaid regulator comprises sealed ends formed at opposite ends of said arc tube for sealing said electrodes, said sealed ends having an outside diameter less than that of said arc tube at a portion other than said sealed ends.

16. The metal halide discharge lamp as set fort in claim 1, whereinsaid arc tube is made of a transparent ceramic.

17. The metal halide discharge lamp as set fort in claim 1, whereinsaid scandium halide is filled in an amount of less than 4.08×10−6 mol/ml.

18. The metal halide discharge lamp as set fort in claim 1, whereinsaid arc tube is also filled with cesium halide.

19. A discharge lamp ballast for operating a metal halide discharge lamp, said lamp comprising:an arc tube filled with at least sodium halide and scandium halide, said arc tube being formed at its opposite ends with electrodes which gives an arc therebetween; and regulator for keeping a coldest spot temperature of said arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power of said lamp, said lamp having a rated lamp power less than 400 W, and a molar ratio (R) of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 2.8≦R≦17.0, said ballast comprising a dimmer for varying a lamp power to be applied to the lamp from 100% to 50% of a rated lamp power.

20. A discharge lamp ballast for operating a metal halide discharge lamp, said lamp comprising:an arc tube filled with at least sodium halide and scandium halide, said arc tube being formed at its opposite ends with electrodes which gives an arc therebetween; and regulator for keeping a coldest spot temperature of said arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power of said lamp, said lamp having a rated lamp power is 400 W or more, and a molar ratio (R) of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 5.7≦R≦22.7, said ballast comprising a dimmer for varying a lamp power to be applied to the lamp from 125% to 50% of a rated lamp power.

21. A metal halide discharge lamp, comprising:an arc tube filled with at least sodium halide and scandium halide, said arc tube being formed at its opposite sealed ends with electrodes which gives an arc therebetween; and a regulator for keeping a coldest spot temperature of said arc tube at 550° C. or more when operating the lamp at a lamp power which is 50% of rated lamp power of said lamp, wherein a molar ratio R of said sodium halide and said scandium halide filled in said arc tube satisfies a relation that 2.8≦R≦22.7, said arc tube being formed at its opposite sealed ends respectively with foils, each connected to each of said electrodes, said regulator including heat insulation layers respectively over said sealed ends in such a manner as to surround said electrodes as well as said foils entirely with respect to an axial length of said arc tube, said heat insulation layer being a metal layer reflecting an infrared radiation, said regulator also including a transparent sleeve which surrounds substantially the full axial length of said arc tube, said transparent sleeve being coated on its opposite axial ends with an infrared radiation reflection layer.