Low-pressure Mercury Vapor Discharge Lamp

Abstract

A low-pressure mercury vapor discharge lamp, having a glass envelope, whose inner side is provided with a layer of luminescent material and a light-transmitting layer of conducting tin oxide provided between this luminescent layer and the glass envelope. The tin oxide layer is provided with a light-transmitting protective layer of one or more of the colorless oxides of the elements from the secondary groups in column 4 and 5 of the periodical system, e.g. titanium dioxide or zirconium dioxide.

Description

The invention relates to a low-pressure mercury vapor discharge lamp and particularly to such a lamp which converts the ultraviolet radiation generated in the discharge into visible radiation by means of a luminescent material.

Lamps of the kind described above have been known for a long time in many forms. Many of these lamps are used for general illumination purposes, but special types are also used for making photo copies.

One of the problems occurring in all low-pressure mercury vapor discharge lamps is that the operating voltage is generally lower than the ignition voltage. This in itself would not be a drawback if the ignition voltage were lower than the supply voltage of the installation, for example, of the mains from which the lamp is fed. However, this is generally not the case. Therefore auxiliary means are almost always used for igniting the lamp. One of the well-known auxiliary means is a so-called starter which generates a voltage for a short period, which voltage is so much higher than the ignition voltage that an ignition of the lamp is ensured. Sometimes also so-called ignition strips are used which are provided either on the outer side or on the inner side of the envelope of the lamp and which extend from one to the other electrode. These ignition strips may optionally be connected to the electrodes.

In a special type of the low-pressure mercury vapor discharge lamps, the inner side of the glass envelope of the lamp was coated with a light-transmitting layer of conducting tin oxide on the entire surface or on a large part of the surface between the electrodes. In this connection and in the following description the expression "light-transmitting" is understood to mean that the useful radiation to be emitted by the lamp is transmitted. In lamps for general illumination purposes this means that the layer satisfactorily passes the light generated with the aid of the luminescent layer. The luminescent layer is always provided on the side of the tin oxide layer facing the discharge.

Since the discharge space between the electrodes must of course not be short-circuited, because otherwise no discharge would be able to occur, the resistance of the tin oxide layer is bound to given values. The resistance measured between the ends located near the electrodes is generally between 5,000 and 50,000 ohms. Since tin oxide in a pure form often has an insufficient conductivity, a so-called dope, for example, fluorine, is sometimes added to the tin oxide.

The above-described lamps provided with a tin oxide layer are distinguished by a very satisfactory ignition behavior over a wide temperature range and in this respect they are preferred to the lamps provided with ignition strips. In fact, lamps having a strip provided on the outer side of the envelope ignite satisfactorily at low ambient temperatures; lamps having a strip on the inner side only ignite at comparatively high ambient temperatures. On the other hand lamps having a tin oxide layer have a satisfactory ignition behavior over the entire temperature range which is covered by the two ignition strip types combined. The use of a tin oxide layer has, however, a great drawback, for it has been found that after some time, sometimes already after 50 operating hours, black stains having a diameter of up to several mms are formed on the inner side of the discharge tube. In these areas the tin oxide layer and the luminescent layer are damaged. As a result the emitted luminous flux decreases, while in addition the appearance of the lamp is detrimentally influenced in an aesthetical respect. An object of the present invention is to prevent this drawback as much as possible.

According to the invention a low-pressure mercury vapor discharge lamp comprises a glass envelope whose inner side is provided with a layer of luminescent material and with a light-transmitting layer of conducting tin oxide located between the luminescent layer and the glass envelope and is characterized in that the tin oxide layer on the side remote from the glass envelope is provided with a light-transmitting protective layer of one or more of the colorless oxides of the elements from the secondary groups in the columns 4 and 5 of the periodical system of the elements and that the protective layer has a thickness of from 0.02-1/.mu. m.

Experiments have shown that protective layers having a thickness of from 0.02-1/.mu. m. are sufficiently light-transmitting for all radiation to be passed. However, the protective layer preferably has a thickness of from 0.05-0.15/.mu. m., because at this thickness a satisfactory adhesion of the layer is ensured and there is no risk of cracks occurring. Furthermore, interference effects are avoided as much as possible at the preferred thicknesses.

Particularly the oxides of titanium, zirconium, hafnium, niobium and tantalum are suitable for the protective layer, because these oxides do not absorb the useful radiation to be emitted by the lamp. A layer of titanium dioxide is preferably used, because this material is not expensive and can be easily provided in the form of a homogeneous layer on the tin oxide layer by means of a method explained with reference to the following examples.

In certain cases, namely when greater layer thicknesses are used, zirconium oxide is preferably used for the protective layer, for zirconium oxide has a lower refractive index than titanium dioxide. Thus the layers may be made thicker without the interference effects being increased.

The oxides to be used for the protective layer are substantially insulating and constitute a dielectric layer. As experiments have shown, the protective layers influence the conductivity of the tin oxide layer only to a very slight extent. It has been found that the adhesion of the luminescent materials to the protective layer is satisfactory.

A low-pressure mercury vapor discharge lamp according to the invention is preferably manufactured by means of a method in which both the tin oxide layer and the protective layer are provided on the inner side of the glass envelope by means of identical spray processes. Due to the uniformity of the two coating processes, this method is very economic and renders the formation of very uniform layers possible.

In order that the invention may be readily carried into effect, it will now be described in detail by way of example with reference to the accompanying diagrammatic drawings.

In the drawing the reference numeral 1 denotes the glass envelope of a low-pressure mercury vapor discharge lamp the ends of which are provided with electrodes 2 and 3. The inner side of the envelope 1 has a layer of conducting tin oxide 4, a protective layer 5 of one or more of the colorless oxides of the elements from the secondary groups in the columns 4 and 5 of the periodical system of the elements and a layer 6 of luminescent material. The thicknesses of the tin oxide layer 4 and the light-transmitting layer 5 are indicated greatly exaggerated as compared with the thickness of the glass envelope 1.

The two examples below describe the manufacture of a lamp shown in the drawing with reference to the application of the tin oxide layer 4 and the protective layer 5. The other manufacturing stages of the lamp do not at all deviate from those of the known lamps.

EXAMPLE I

The glass envelope to be coated of a low-pressure mercury vapor discharge lamp was heated in an electric tube furnace to a temperature of approximately 450.degree.-500.degree. C., preferably 480.degree. C. A solution of 20 percent of SnC1.sub.4 in butanol to which a few percents of HF were added was sprayed with the aid of a spray nozzle and passed in vapor form through the envelope.

A similar method was used for the formation of the protective layer. The spray mixture was a 20 vol. percent solution of tetrabutyl titanate in butanol. The spray gas supplied to the spray nozzle must in this case be dry in order that the tetrabutyl titanate which is sensitive to hydrolysis is not hydrolized before the drops fall on the hot glass of the tube. Consequently, dry air or dry oxygen was used. Also the air transported from the ambience, which air is essential for the formation of a fine spray must be as dry as possible. To this end the spray nozzle was provided in the rear wall of a metal pot which was connected through an aperture to a plastic bag. During spraying enough dry air or dry oxygen was present in the plastic bag to maintain the supply to the nozzle. The layer was sprayed on in an intermittent process so that during the spraying intervals the initial temperature of approximately 480.degree. C. on the glass wall was reached again. The tetrabutyl titanate was decomposed pyrolytically on the hot glass of the tube while forming a titanium dioxide layer, the organic radicals burning completely. It was found that the flow rate of the spray could best be chosen in the boundary range where laminar flow changes into a turbulent flow.

EXAMPLE II

A glass tube for a low-pressure mercury vapor discharge lamp was provided with a tin oxide layer in a similar manner as described in example I. This tin oxide layer was coated with a protective layer of zirconium oxide by means of the method described with reference to example I. To this end a 20 percent solution of tetrapropylzirconate in propanol was used during spraying.

It was found that the use of alcoholates of alcohols having comparatively long chains is more favorable as regards the sensitivity to hydrolysis. Bright layers of titanium dioxide were manufactured with the aid of a solution of tetrapentyl titanate in butanol or pentanol without special steps being taken with reference to the dryness of the spray gas.

Experiments showed that for the formation of the protective layer alternatively chlorides, for example, titanium chloride or zirconium chloride dissolved, for example, in butanol may be used. The spray process may then be performed in exactly the same manner as for the formation of the tin oxide layer.

Low-pressure mercury vapor discharge lamps were made from the glass tubes coated in accordance with example I. It was found that the ignition behavior of these lamps as compared with the known lamps having a tin oxide layer was not influenced. The lamps according to the invention had no black stains after 2,500 operating hours.

Claims

What is claimed is:

1. A low-pressure mercury vapor discharge lamp having a glass envelope whose inner side is provided with a layer of luminescent material and a light-transmitting layer of conducting tin oxide provided between this luminescent layer and the glass envelope, characterized in that the tin oxide layer on the side remote from the glass envelope is provided with a light-transmitting protective layer of one or more of the colorless oxides of the elements from the secondary groups in the columns 4 and 5 of the periodical system of the elements and that the protective layer has a thickness of from 0.02-1 .mu.m.

2. A low-pressure mercury vapor discharge lamp as claimed in claim 1, characterized in that the thickness of the protective layer is 0.05-0.15 .mu.m.

3. A low-pressure mercury vapor discharge lamp as claimed in claim 1, characterized in that the protective layer comprises titanium dioxide.

4. A low-pressure mercury vapor discharge lamp as claimed in claim 1, characterized in that the protective layer comprises zirconium dioxide.