GB1565129A - Low-pressure mercury vapour discharge lamp - Google Patents

Description

(54) LOW-PRESSURE MERCURY VAPOUR DISCHARGE LAMP (71) We, N.V. PHILLIPS' GLOEILAMPENFABRIEKEN. a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands, 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 invention relates to low-pressure mercury vapour discharge lamps.

Low-pressure mercury vapour discharge lamps (called for convenience "fluorescent lamps"), are very well-known light sources which are frequently used for general illumination purposes as well as for special applications. Most fluorescent lamps used in practice have discharge tubes having nominal lengths of from 60 to 150 cm and during operation consume a power of from 0.25 to 0.50 Watt per cm of tube length. The standard types of fluorescent lamps which have been in existence for many years are the 20 Watt lamp having a nominal length of 60 cm, the 40 Watt lamp having a nominal length of 120 cm and the 65 Watt lamp having a nominal length of 150 cm.

Optimisation of lamp properties, such as luminous flux, lamp voltage. lamp current and starting voltage resulted for these lamps in a choice of the internal diameter of the discharge tube of approximately 36 mm.

United Kingdom Patent Specification 1,458,700 describes low-pressure mercury vapour discharge lamps which contain a red-luminescing, trivalent europium-activated rare earth metal oxide, and these lamps emit in comparatively narrow spectral ranges, situated in the blue, in the green and in the red portion of the spectrum. With these lamps it is also possible to obtain both a high luminous flux and a proper rendition of object colours. The red emission in these known lamps is obtained from a rare earth metal oxide activated by Eu3+.

The invention provides a low-pressure mercury vapour discharge lamp having a vacuum-tight, radiation-transmissive discharge tube having a nominal length of 60-150 cm, an inside diameter of from 20 to 28 mm, and intended to consume during operation a nominal power of 0.25-0.50 W per cm length of the discharge tube, which tube comprises a quantity of mercury, a quantity of rare gas, and a luminescent layer comprising a red-luminescing europium-activated rare earth metal oxide having a composition defined by the formula Ln203:pEu, wherein Ln represents at least one of the metals Y, Gd and Lu, and wherein 0.01SpA0.20.

It was found that fluorescent lamps according to the invention could be made which have higher luminous fluxes than comparable known lamps having discharge tubes with an inside diameter of 36 mm. The invention is based on the recognition that these rare earth metal oxides have a quantum efficiency which increases with an increasing density of the exciting ultraviolet radiation.

It was found that the excitation density of ultraviolet radiation in lamps according to the invention is considerably greater than in the known lamps having discharge tubes with an inside diameter of 36 mm. This is caused on the one hand by the smaller inside diameter of the discharge tubes and on the other hand bv the increase in the ultraviolet radaition produced in the discharge. A great advantage is that the rare earth metal oxides used are excited substantially equally well by thr 185 nm -mercury line as by the 254 nm-mercury line. Said increase in the ultraviolet production is mainly found in the 185 nm line.

An advantage of the lamps according to the invention is that the decav in the luminous flux during the life of the lamp is of the same order of magnitude or even less than that of comparable lamps having discharge tubes with an inside diameter of 36 mm.

These lamps according to the invention furthermore yield a great economic advantage because during fabrication they require considerably smaller quantities of raw materials such as material for the discharge tube and luminescent material.

Lamps according to one aspect of the invention have an inside diameter of the discharge tube between 23.5 and 25.5 mm. Very good results are obtained with these lamps as regards a gain in luminous flux and a saving in materials.

In another aspect of the invention, the fluorescent lamp comprises a mixture of luminescent materials as mentioned in United Kingdom Patent Specification 1,458,700, wherein the red-luminescing material is defined by the formula Ln2O:Eu3+. An advantage is that the aluminates with a crystal structure related to the structure of the hexagonal ferrites used for the blue-and green- luminescing materials have a low decay of the luminous flux during the operational life, just as the red-luminescing material. In such lamps, a given ratio of the three luminescent materials must be chosen for each desired colour temperature of the emitted radiation.It was found that a given mixture intended for a lamp with a discharge tube having an inside diameter of 36 mm need substantially not be changed to obtain the same colour temperature in lamps with a discharge tube having an inside diameter of 24 mm. The explanation is that in the narrow lamps, not only the 185 nm line but also the visible blue and green mercury lines increase in intensity. These additional contributions in the blue and in the green parts of the spectrum then support the gain in the red derived from the rare earth metal oxide so that substantially the same colour point is attained.

An advantage of the lamps according to the invention is that they can be made with the same longitudinal dimensions as the known standard types, the electrical properties being substantially equal to those of the standard types. Consequently, the lamps according to the invention can replace the known lamps in existing light fittings without extra provisions.

Preference is given to lamps according to the invention wherein the discharge tube has a length of 60 - 120 cm and comsumes during operation a power of 0.3 - 0.4 Watt per cm. The frequently used 20 W - and 40 W-lamps belong to this group.

Some embodiments of the invention will now be described with reference to the single Figure of the accompanying drawing, which is a diagrammatic longitudinal section of a low-pressure mercury vapour discharge lamp according to the invention, and to the following Examples.

Reference 1 in the drawing indicates a glass discharge tube of a 40 Watt low-pressure mercury vapour discharge lamp. The discharge tube 1 is 120 cm long, has an internal diameter of 24 mm and a wall thickness of 0.7 mm. At the ends of the tube 1 electrodes 2 and 3 are disposed between which the discharge takes place during operation of the lamp.

The lamp comprises 0.5 to 5 Torr of a rare gas or a mixture of rar gases, serving as starting gas. In addition, the discharge tube comprises a small quantity of mercury. The inside surface of the discharge tube 1 is coated with a luminescent layer 4 which comprises a luminescent rare earth oxide. This luminescent layer 4 can be applied in the customary manner to the tube 1, for example by means of a suspension containing the luminescent oxide. In an manner which is known in itself, a reflector layer for visible radiation which extends over a portion of the tube 1 can be applied in a lamp according to the invention between the tube 1 and the luminescent layer 4. Furthermore it is possible to deviate from the straight tube form shown in the drawing and to construct the lamp as a bent tube, for example as a torus or in a U-shape.

Example I A lamp of the type shown in the drawing (40 W) was provided with a luminescent layer 4 consisting of a mixture of 38who by weight of Ce167Tb033MgA111O19; 12% by weight of BaO gEuo lMgAll0Ol7; 50% by weight of Yl gEu("03. besides mercury the lamp comprised 2.2 Torrs of argon so that when connected to a standard 40 W-ballast, the lamp appeared to consome a power of approximately 40 W at a lamp current of 390 mA. The white light emitted by the lamp had a colour temperature of approximately 4000 K and had a particularly good colour rendition possible. The initial luminous flux of this lamp amounted to 96.4 Im/W. After 100 hours the luminous flux had descreased only 1.5% to 95.01 mTW.

For comparison, a 40 W lamp with a discharge tube having an inside diameter of 36 mm was made with the same luminescent mixture. This lamp required a quantity of luminescent materials which was 50% larger. The rare gas in this lamp was a mixture of 75% A and 25% Ne at a pressure of 2.5 Torr. This lamp (consumed power 40 W, lamp current 425 mA) has an initial luminous flux of 89.5 Im/W which after 100 hours has decreased by 2.8% to 87.0 Im/w. It appears that the lamp according to the invention (at 100 hours) furnishes a luminous flux which is 9.2% greater than that of the known lamp. For further comparison it should be noted that a 40 W halophosphate lamp (4000 K) having a diameter of 24 mm has after 100 hours only an approximately 5% higher luminous flux compared with a comparable lamp having a 36 mm diameter discharge tube.

Example II A lamp provided with a discharge tube having a length of 150 cm and an inside diameter of 24 mm was coated with the same mixture of luminescent materials as was used in Example 1. A mixture of 25% A and 75% Kr at a pressure of 1.5 Torr was used as rare gas.

When the lamp was connected to a 65 W ballast, the lamp appeared to consume a power of approximately 59 W at a lamp current of 678 mA. The initial luminous flux of the lamp was 96.1 Im/W. After 100 hours this luminous flux is 94.1 Im/W (decrease 2.1%). A comparable lamp however, with a discharge tube having an inside diameter of 36 mm and filled with 2.4 Torr A appeared to consume a power of 64 W at a lamp current of 670 mA and had an initial luminous flux of 88.2 Im/W. After 100 hours the luminous flux had decreased approximately 4.1% to 84.7 Im/W. At 100 hours the lamp according to the invention has a luminous flux which was 11.1% greater than that of the said comparable lamp.

Example III A 40 W lamp of the type shown in the drawing was provided with a luminescent layer consisting of 5.3 grams of a mixture of 54.8 wt. % Y19Eu0.1Q 36.6 wt. % Ce067Tb0.33MgAl11 019 8.6 wt. % BaO.gEu0 IMgAll0Ol7 The white light emitted by the lamp had an initial luminous flux of 100.4 Im/W. After 100 hours the luminous flux had decreased by only 0.3% to 100.1 Im/W. After 1000 hours the luminous flux was 98.4 Im/W. For comparison, a 40 W lamp having a discharge tube with an inside diameter of 36 mm was made with 7.65 grams of the same luminescent mixture. This lamp had an initial luminous flux of 92.6 Im/W. After 100 hours the luminous flux had decreased to 92 Im/W and after 1000 hours to 89.9 Im/W.

Example IV A 40 W lamp of the type shown in the drawing was provided with a luminescent layer consisting of 3.05 grams of a red-luminescing Yl 9Eu).l03. The luminous flux of this lamp was initially 76.2 Im/W and after 100 hours 72.5 Im/W. For comparison, a 40 W lamp having an inside diameter of 36 mm was made with 4.65 grams of the same luminescent substance.

This lamp had initially a luminous flux of 70.2 Im/W and after 100 hours of 68.1 Im/W.

WHAT WE CLAIM IS: 1. A low-pressure mercury vapour discharge lamp having a vacuum-tight, radiationtransmissive discharge tube having a nominal length of 60-150 cm, an inside diameter of from 20 to 28 mm, and intended to consume during operation a nominal power of 0.25-0.50 W per cm length of the discharge tube, which tube comprises a quantity of mercury, a quantity of rare gas and a luminescent layer comprising a red-luminescing trivalent europium activated rare earth metal oxide having a composition defined by the formula Ln203:pEu, wherein Ln represents at least one of the metals Y, Gd and Lu and wherein 0.01p0.20.

2. A low-pressure mercury vapour discharge lamp as claimed in Claim 1, wherein the inside diameter of the discharge tube has a value between 23.5 and 25.5 mm.

3. A low-pressure mercury vapour discharge lamp as claimed in Claim 1 or Claim 2, wherein the discharge tube has a length of 60-120 cm and comsumes during operation a power of 0.3 - 0.4 Watt per cm.

4. A low-pressure mercury vapour discharge lamp having a discharge tube with an inside diameter of from 20 to 28 mm, substantially as herein described with reference to any of Examples I to IV.

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

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. after 100 hours only an approximately 5% higher luminous flux compared with a comparable lamp having a 36 mm diameter discharge tube. Example II A lamp provided with a discharge tube having a length of 150 cm and an inside diameter of 24 mm was coated with the same mixture of luminescent materials as was used in Example 1. A mixture of 25% A and 75% Kr at a pressure of 1.5 Torr was used as rare gas. When the lamp was connected to a 65 W ballast, the lamp appeared to consume a power of approximately 59 W at a lamp current of 678 mA. The initial luminous flux of the lamp was 96.1 Im/W. After 100 hours this luminous flux is 94.1 Im/W (decrease 2.1%). A comparable lamp however, with a discharge tube having an inside diameter of 36 mm and filled with 2.4 Torr A appeared to consume a power of 64 W at a lamp current of 670 mA and had an initial luminous flux of 88.2 Im/W. After 100 hours the luminous flux had decreased approximately 4.1% to 84.7 Im/W. At 100 hours the lamp according to the invention has a luminous flux which was 11.1% greater than that of the said comparable lamp. Example III A 40 W lamp of the type shown in the drawing was provided with a luminescent layer consisting of 5.3 grams of a mixture of 54.8 wt. % Y19Eu0.1Q 36.6 wt. % Ce067Tb0.33MgAl11 019 8.6 wt. % BaO.gEu0 IMgAll0Ol7 The white light emitted by the lamp had an initial luminous flux of 100.4 Im/W. After 100 hours the luminous flux had decreased by only 0.3% to 100.1 Im/W. After 1000 hours the luminous flux was 98.4 Im/W. For comparison, a 40 W lamp having a discharge tube with an inside diameter of 36 mm was made with 7.65 grams of the same luminescent mixture. This lamp had an initial luminous flux of 92.6 Im/W. After 100 hours the luminous flux had decreased to 92 Im/W and after 1000 hours to 89.9 Im/W. Example IV A 40 W lamp of the type shown in the drawing was provided with a luminescent layer consisting of 3.05 grams of a red-luminescing Yl 9Eu).l03. The luminous flux of this lamp was initially 76.2 Im/W and after 100 hours 72.5 Im/W. For comparison, a 40 W lamp having an inside diameter of 36 mm was made with 4.65 grams of the same luminescent substance. This lamp had initially a luminous flux of 70.2 Im/W and after 100 hours of 68.1 Im/W. WHAT WE CLAIM IS:

1. A low-pressure mercury vapour discharge lamp having a vacuum-tight, radiationtransmissive discharge tube having a nominal length of 60-150 cm, an inside diameter of from 20 to 28 mm, and intended to consume during operation a nominal power of 0.25-0.50 W per cm length of the discharge tube, which tube comprises a quantity of mercury, a quantity of rare gas and a luminescent layer comprising a red-luminescing trivalent europium activated rare earth metal oxide having a composition defined by the formula Ln203:pEu, wherein Ln represents at least one of the metals Y, Gd and Lu and wherein 0.01p0.20.

2. A low-pressure mercury vapour discharge lamp as claimed in Claim 1, wherein the inside diameter of the discharge tube has a value between 23.5 and 25.5 mm.

3. A low-pressure mercury vapour discharge lamp as claimed in Claim 1 or Claim 2, wherein the discharge tube has a length of 60-120 cm and comsumes during operation a power of 0.3 - 0.4 Watt per cm.

4. A low-pressure mercury vapour discharge lamp having a discharge tube with an inside diameter of from 20 to 28 mm, substantially as herein described with reference to any of Examples I to IV.