GB1564260A - Low-pressure mercury vapour discharge lamp - Google Patents

Description

(54) LOW-PRESSURE MERCURY VAPOUR DISCHARGE LAMP (71) We, N. V. PHILIPS GLOEI LAMPENFABRIEKEN, 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 tubular lowpressure mercury vapour discharge lamps having a tubular glass envelope, two electrodes, a luminescent coating and a light transnissive conductive layer disposed between this coating and the glass envelope which layer is not connected to an electrode, to facilitate starting.

Lamps of the above-mentioned type are, for example, known from United States patent specification 2,733,371.

It is known that conductive layers for the above-mentioned lamps can advantageously be made of the oxides of tin or indium as the specific conductivity of these oxides can be varied within wide limits by so-called "doping" with other elements, for example, fluorine, oxygen, indium (for tin) or tin (for indium).

So the total conductivity of these layers (from end to end) can be easily varied within wide limits by the quantity of doping material.

Therefore it is possible to satisfy also with layers of a varying thickness the requirements which must be imposed on the value of the starting voltage. The required starting voltage of the lamp closely depends on the voltage supplied by the power supply unit (which is usually standardized).

Experiments proved that conductive layers which originally satisfy the requirements imposed as regards light-transmission and conductivity show greying after a comparatively low number of operating hours. Of course this greying results in loss of light and an unaesthetic appearance particularly because the greying occurs irregularly, for example as stains and dots. These stains and dots are found partly in the luminescent coating and partly in the conductive layer. The conduct ivity of the layer is hardly changed by it.

The invention provides a low-pressure mercury vapour discharge lamp having a tubular glass envelope, a luminescent coating, and a light-transmissive conductive layer disposed between the coating and the glass envelope, which layer is not connected to an electrode, -and wherein the conductive layer has a length which is approximately equal to the length of the lamp and a width which is less than one third of the internal circumference of the tubular glass envelope.

Since, as remarked above, the conductivity of the conductive layer can be easily varied, it is possible to give the conductive layer in a lamp according to the invention the desired conductivity value.

As the conductive layer in a lamp according to the invention is relatively narrow and, in addition, can be applied at the side of the lamp which faces away from the observer, stains and dots and, in general, greying are aesthetically less disturbing than with a fully coated inner circumference. The loss of useful radiation caused by the greying is small in most applications as it takes place at that side which contributes relatively less to the useful illumination.

If desired a light-reflecting coating of, for example, titanium dioxide can be applied between the luminescent layer and the conductive layer. This often delays the occurrence of greying. Furthermore, placing the lamp in the correct position in a lighting fitting is facilitated as the reflecting coating is clearly visible even when the lamp is not alight, and the lamp is always placed with the reflecting coating at the correct side, facing away from the observer. The conductive layer is hard to see and so lamps without a reflecting coating should preferably be provided with a special mark to give the necessary indication of how the lamps are to be disposed in light fittings.

Besides, the reflecting coating may cover more than one third of the circumference of the cross-section. In practice it usually coats approximately two thirds thereof.

As only a part of the lamp circumference is coated with conductive material, the con ductive layer may be thicker and, consequently, stabler than a layer (of the same total conductivity) which coats the entire circumference.

An embodiment of the invention will now be described with reference to the accompanying drawing, in which Figure 1 is a diagrammatic longitudinal section of a 40 Watt low-pressure mercury vapour discharge lamp, and Figure 2 is a cross-section on an enlarged scale of the envelope of the lamp shown in Figure 1 demonstrating the dispositions of coatings on the envelope. Figure 1 shows the glass envelope 1 whose inner side is coated with a luminescent material coating 2, for example, consisting of calcium halophosphate activated by antimony and manganese. Electrodes 3 and 4 are disposed in the discharge space of the lamp. On the stem which supports the electrode 3 a quantity of mercury vapour-pressure controlling amalgam 5 is applied, consisting for example of an alloy of indium and mercury.Across substantially the full length of the discharge space there is disposed between the envelope 1 and the luminescent coating 2 a conductive starting strip 6 consisting of tin oxide which has been doped with such a quantity of indium that the resistence of the strip 6 is 2000 ohms per square. A thin coating of light-reflecting titanium dioxide 7 (see Figure 2) is disposed between the strip 6 and the luminescent coating 2. As clearly appears from the cross-section of the lamp shown in Figure 2, the coating 6 covers approximately one sixth of the tube circumference. The total resistance of the strip 6 which in the 40 Watt lamp shown has a length of approximately 110 cm is 60,000 ohms.After 2000 operating hours a slight greying of the lamp in situ of the strip 6 could indeed be observed but the total loss of light of the lamp amounted to 5 to 8 ' relative to the 0-hour value.

WHAT WE CLAIM IS:- 1. A tubular low-pressure mercury vapour discharge lamp having a tubular glass envelope, a luminescent coating, and a lighttransmissive conductive layer disposed between the coating and the glass envelope, which layer is not connected to an electrode, and wherein the conductive layer has a length which is approximately equal to the length of the lamp and a width which is less than one third of the internal circumference of the tubular glass envelope.

2. A discharge lamp as claimed in Claim 1, wherein a light-reflecting coating is disposed between the luminescent coating and the conductive layer.

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

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. ductive layer may be thicker and, consequently, stabler than a layer (of the same total conductivity) which coats the entire circumference. An embodiment of the invention will now be described with reference to the accompanying drawing, in which Figure 1 is a diagrammatic longitudinal section of a 40 Watt low-pressure mercury vapour discharge lamp, and Figure 2 is a cross-section on an enlarged scale of the envelope of the lamp shown in Figure 1 demonstrating the dispositions of coatings on the envelope. Figure 1 shows the glass envelope 1 whose inner side is coated with a luminescent material coating 2, for example, consisting of calcium halophosphate activated by antimony and manganese. Electrodes 3 and 4 are disposed in the discharge space of the lamp. On the stem which supports the electrode 3 a quantity of mercury vapour-pressure controlling amalgam 5 is applied, consisting for example of an alloy of indium and mercury.Across substantially the full length of the discharge space there is disposed between the envelope 1 and the luminescent coating 2 a conductive starting strip 6 consisting of tin oxide which has been doped with such a quantity of indium that the resistence of the strip 6 is 2000 ohms per square. A thin coating of light-reflecting titanium dioxide 7 (see Figure 2) is disposed between the strip 6 and the luminescent coating 2. As clearly appears from the cross-section of the lamp shown in Figure 2, the coating 6 covers approximately one sixth of the tube circumference. The total resistance of the strip 6 which in the 40 Watt lamp shown has a length of approximately 110 cm is 60,000 ohms. After 2000 operating hours a slight greying of the lamp in situ of the strip 6 could indeed be observed but the total loss of light of the lamp amounted to 5 to 8 ' relative to the 0-hour value. WHAT WE CLAIM IS:-

1. A tubular low-pressure mercury vapour discharge lamp having a tubular glass envelope, a luminescent coating, and a lighttransmissive conductive layer disposed between the coating and the glass envelope, which layer is not connected to an electrode, and wherein the conductive layer has a length which is approximately equal to the length of the lamp and a width which is less than one third of the internal circumference of the tubular glass envelope.

2. A discharge lamp as claimed in Claim 1, wherein a light-reflecting coating is disposed between the luminescent coating and the conductive layer.