GB1585861A - Metal halide lamps - Google Patents

Description

(54) METAL HALIDE LAMPS (71) WE, THORN EMI LMITED (formerly known as Thorn Electrical Industries Limited), a British Company, of Thorn House, Upper Saint Martin's Lane, London, WC2H 9ED, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to metal halide electric discharge lamps, that is to say, high-pressure electric discharge lamps having fills including mercury and metal halides.

Metal halide lamps have been known and manufactured for several years, and may exhibit an efficacy of about 65 Lm/watt through life, and a CIE general colour rendering index of about 72. Although the quality of the emitted light is satisfactory for many applications, the relative lack of red to deep red radiation as compared with the other artificial light sources can make a conventional metal halide lamp unsuitable for certain critical applications. For example, satisfactory rendering of the colours of textiles and of fresh reddish foodstuffs, especially meat, demand a higher proportion of red radition than is given by a standard sodium iodide - scandium iodide - caesium iodide - thorium iodide lamp, even when the lamp is provided with the most efficient red emitting phosphor known.

We have now found that the addition of lithium iodide to the fill of a metal halide lamp of the sodium-scandium-whorium iodide system, with the optional inclusion of caesium iodide, can result in an increase in the proportion of red radiation without deleteriously affecting the efficacy of the lamp. This result is surprising since lithium iodide is corrosive and is known to attack the wall of the discharge vessel when employed in, for example, the sodium-indium-thallium iodide lamp described in British Patent No. 1,125,063 or, indeed, in a high pressure lamp having a fill of mercury alone and no metal halide, as discussed in British Patent No. 1,400,976.

Our experiments indicate that the lithium iodide should form between 10 and 50 mole % of the total iodides contributing the major light-emitting elements, to achieve the desired colour balance acceptable for the more critical applications. However, to maintain-the most satisfactory luminous efficacy (for example 15000 lumens through life from a 250W lamp) with the desired colour balance, it has been found that the lithium iodide should preferably lie in the restricted range of 10-30 mole % of the iodides contributing the major light-emitting elements, which in the present case are lithium, sodium and scandium.

By way of example, goods results have been obtained with 250W high pressure metal halide lamps having the proportions of major light-emitting components in the ranges shown below. it is nevertheless to be appreciated that it is not necessary to add all components as the compounds themselves.

LiI NaI ScI3 Mole % 10-50 81-43 9-7 If these proportions are expressed as molar percentages of the total metals (other than mercury) introduced into the lamp and present calculated as iodides, the values are somewhat different, namely 10-30% Lil, 76-60% NaI and 7.4-7.7% Scl.

Caesium iodide is an optional component added to broaden the spectrum. and thorium is included primarily to promote electron emission, being usually applied as metal on the electrodes. The function of the mercury is to obtain the required operating pressure and electrical characteristics.

In the experiments referred to, the mercury content of the lamps was 27.7 Rmole cm-3, thorium 0.3 ijmole cm-3 and caesium iodide 0.77 llmole cam~3. These components can, however, be varied as understood in the art, in order to achieve the desired operating characteristics.

Further improvements to the colour rendering of the lamp of this invention may be obtained by the use of mixtures of red and blue emitting phosphors on the inside of the outer bulb, in the manner known in the case of conventional lamps.

In the drawings accompanying the Provisional Specification: Figure 1 shows the colour rendering index RA, and the efficacy at 100 hours (Lm/watt) plotted against lithium content in a Na-Sc-Cs iodide lamp; and Figure 2 is a side elevation of a lamp embodying the invention.

A summary of the averaged photometric and colorimetric data taken after 100 hours life for the lamps in our experiments is given in Table 1. The variation of both luminous efficacy and general colour rendering index (RA) with lithium iodide concentration is shown by the fair-fit lines plotted in Figure 1. The lamps containing lithium iodide in the most preferred range of 10-30 mole % (Ll-Ln in Figure 1) have substantially improved colour rendering properties as judged by both the CIE figures and by the eye. Furthermore, it is to be noted that the luminous efficacy is only about 5-10% lower than that of the unmodified lamp.

TABLE 1 Summary of photometric nnd colorimetric data CIE Colour Mole Rendering Indices * Band 8 + Band 6 Lm/Watt General Special Cic Luminance % Luminance (100 Hr.) LiI RA R5 (660-760 nm) (620-760 nm) 0 74 37 0.25 5.63 76 10.2 75 45 0.47 5.8 74 15.2 77 49 0.53 6.0 73 20.1 76 49 0.54 6.0 72 29.9 82 57 0.62 6.1 70 48.8 84 63 0.68 5.7 66 *Cie (1948) 8 Band Svstem + Crawford 6 Band System.

An example of the lamp construction employed in our experiments is shown in Figure 2 of the drawings.

The lamp 10 has a power output of approximately 250 watts during operation and comprises a quartz glass discharge vessel 11 with pinches 12 and 13. one at each end of the vessel 11. through which current supply elements 14 and 15 are sealed. These current supply elements 14 and 15 are connected within the discharge vessel 11 to tungsten electrodes 16 and 17 between which the discharge takes place during operation. The discharge vessel 11 is placed in an envelope 18 which is filled with an inert gas. The envelope 18 is made of a hard glass and has a pinch 19 at one end through which current supply wires '0 and 21 have been passed in a vacuum-tight manner. The current supply wires 20 and 21 are connected to the current supply elements 14 and 15.The discharge vessel 11 has an internal diameter of 14 mm and an arc length of 23 mm and contains an inert gas. mercury and thorium. together with sodium scandium and optionally caesium iodides. as well as lithium iodide in accordance with this invention.

Two specific examples of the preparation and performance of lamps of the type shown in Figure 2 will now be described.

Example 1 The discharge vessel 11 of a lamp as described above with reference to Figure 2 is filled with Hg 22 mg NaI 7 mg LiI 3 mg Se 0.5 mg HgI2 4 mg Th 0.4 mg and with argon to a pressure of 60 torr. A phosphor was used comprising a mixture of 0.7 g europium activated yttrium vanadate (red emitting) and 0.9 g of strontium chloroapatite (blue emitting).

A light output of 68 Im/W, and a colour temperature T of 4500K for the emitted radiation, were measured on this lamp. The CIE general colour rendering index (1974 method) was found to be RA = 81 The CIE (1948) eight band % luminance data were Band No. 1 2 3 4 5 6 7 8 % Luminance 0.01 0.12 0.50 9.28 36.40 44.29 8.64 0.71 Example 2 The discharge vessel 11 of a lamp as described was filled with Hg 23 mg NaI 9 mg LiI 1 mg Sc 0.4 mg HgI2 4 mg Th 0.4 mg CsI 1 mg and with argon to a pressure of 60 torr. A phosphor comprising a mixture of 0.6 g of europium activated yttrium vanadate and 0.3 g of strontium calcium magnesium phosphate (SCMP) was employed.

A light output of 74 Im/W and a colour temperature T of 3900K were measured. For this lamp, RA = 75 Eight band Sc luminance data were Band No. 1 2 3 4 5 6 7 8 % Luminance 0.02 0.20 0.20 7.51 34.83 47.12 9.65 0.47 For comparison with Example 2 (but not constituting part of the Example). the results at 100 hours life for a typical Na - Sc - Cs halide lamp without lithium addition and with a mixture of 0.6 g of europium activated yttrium vanadate and 0.3 g of SCMP as phosphor were a light output of 76 ImiW and a colour temperature of 3900K. and an RA of 75.

Eight band Gc luminance data for this typical lamp were Band No. 1 2 3 4 5 6 7 8 % Luminance 0.02 0.19 0.26 7.59 34.48 48.5 8.69 0.25 Fills for other lamps embodying the invention can readily be formulated to give a desired performance. As will be apparent from the relationships demonstrated in Figure 1. the lithium iodide content can be selected to obtain a chosen balance between the colour rendering properties and the luminous efficacy. Moreover. modification of the RA value is possible by variation of the phosphor, in accordance with known techniques.

WHAT WE CLAIM IS: 1. A metal halide electric discharge lamp having a gas fill comprising mercury, an inert gas, thorium, sodium and scandium iodides and lithium iodide in a molar proportion of 10 to 50% of the total of lithium, sodium and scandium iodides.

2. A lamp according to claim 1 in which the lithium, sodium and scandium iodides are present in the molar proportions of 10 - 50% LiI, 81 - 43% Nal and 9 - 7% ScI3.

3. A lamp according to claim 1 or 2 in which the lithium iodide content is from 10 to 30 mole % of the total of lithium, sodium and scandium iodides.

4. A lamp according to claim 1, 2 or 3 in which the lamp additionally contains caesium iodide.

5. A metal halide electric discharge lamp substantially as described with reference to Examples 1 and 2 herein.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. possible by variation of the phosphor, in accordance with known techniques. WHAT WE CLAIM IS:

1. A metal halide electric discharge lamp having a gas fill comprising mercury, an inert gas, thorium, sodium and scandium iodides and lithium iodide in a molar proportion of 10 to 50% of the total of lithium, sodium and scandium iodides.

2. A lamp according to claim 1 in which the lithium, sodium and scandium iodides are present in the molar proportions of 10 - 50% LiI, 81 - 43% Nal and 9 - 7% ScI3.

3. A lamp according to claim 1 or 2 in which the lithium iodide content is from 10 to 30 mole % of the total of lithium, sodium and scandium iodides.

4. A lamp according to claim 1, 2 or 3 in which the lamp additionally contains caesium iodide.

5. A metal halide electric discharge lamp substantially as described with reference to Examples 1 and 2 herein.