US3349274A

US3349274A - Low-pressure mercury vapor discharge lamp

Info

Publication number: US3349274A
Application number: US435162A
Authority: US
Inventors: Kolkman Dirk; Visser Adolf Jan De
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1964-03-06
Filing date: 1965-02-25
Publication date: 1967-10-24
Anticipated expiration: 1984-10-24
Also published as: NO118121B; DK111825B; GB1035593A; BE660651A; DE1989932U; NL6402292A; AT247971B; SE309452B; ES310103A1; CH461637A; FR1428299A

Description

Oct. 1967 D. KOLKMAN ET AL 3,349,274

' LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP Filed Feb. 25, 1965 45 -s Q5 +12% T ADOLF .1. DE VISSER BY ik/L fi' United States Patent Ofiice p 3,349,274 Patented Oct. 24, 1967 Claims priority, application Netherlands, Mar. 6, 19s4,

e r-2,292 8 Claims. (Cl. 313-109) ABSTRACT OF THE DISCLOSURE A low-pressure mercury vapor discharge lamp employing a starting strip between the envelope and the layer of luminescent material. A portion of the layer of luminescent material extending between an electrode and a point between the electrodes which point is not more than one-fifth the distance between the electrodes is either removed to expose the starting strip or a con- .ductive starting strip is provided over the layer in order to facilitate ignition at both high and low temperatures.

The invention relates to low-pressure mercury vapor discharge lamps having electrodes arranged at the ends of an elongate discharge space, a luminescent layer and a conducting strip extending between the electrodes.

One of the problems involved in the use of low-pressure mercury vapor discharge lamps is that for the ignition a voltage is required which is higher than the normal operating voltage, that is to say, the voltage measured between the electrodes of the normally operating lamp. This problem arises in particular when the length of the discharge space is so great that the ignition voltage is higher than the mains voltage for which the lamp is intended. Many methods have been tried to obviate this difiiculty. One of these methods consists in that a so-called ignition strip, that is to say, a comparatively narrow conducting strip, is provided in the lamp and extends between the electrodes. In the discharge lamps having a luminescent layer applied to a support, for example, the wall of the lamp, to which lamps the invention relates, the ignition strip may be provided on the luminescent layer and may consequently be exposed to the discharge, or it may be provided between the luminescent layer and the support thereof.

It is :known to connect such ignition strips to an electrode, This method has a grave disadvantage in that it is difficult to make the connection in practice, since in the manufacture of low-pressure mercury vapor discharge lamps, first the luminescent layer and the ignition strip are provided; subsequently, the electrodes, which are mounted on a so-called stem mount, are introduced into, I

the lamp, whereupon the mount is sealed to the envelope of the lamp. Consequently, a connection must be made between the electrodes and the ignition strip by means of long flexible 'wires or by means of resilient members. A discharge lamp having a non-connected strip does not suffer from this limitation and is therefore much more attractive; its manufacture is only slightly more complicated than that of a lamp without an ignition strip.

Investigations which have led to the invention have proved that there is a great difference between the behaviour of discharge lamps which have an internal nonconnected ignition stripwhich is disposed between the luminescent layer and the support, and that of lamps which have a non-connected internal ignition strip which is provided on the luminescent layer and is thus in electrically conducting contact with the discharge plasma. This is apparent inter alia from the form and the location of the characteristic curves which indicate the relationship between the ignition voltage and the temperature of the lamp after the electrodes have reached the emission temperature. One of the factors which determine this lamp temperature is the ambient temperature. It is known that the ignition voltage is a minimum at a given temperature and increases both at lower and at higher temperatures. Tests with ignition strips of equal dimensions have shown that with the use of strips disposed between the luminescent layer and the support the characteristic curves have approximately the same form as the characteristic curves obtained with the use of strips provided on the luminescent layer, but as a whole they lie at lower temperatures and often show a slightly higher minimum value of the ignition voltage. This means that lamps in which the ignition strip is provided on the luminescent layer ignite less readily at low temperatures but lamps in which the ignition strip is disposed between the luminescent layer and the support ignite less readily at high temperatures.

It is advantageous to construct the lamps so that they have a low ignition voltage over a wide temperature range, which guarantees their ignition and re-ignition at widely different ambient temperatures.

To this end, lamps have already been manufactured in which the whole inner side of the envelope is provided with a conductive coating. However, this necessitates the use of transparent conducting layers. In most cases tin ture range. This method has a serious disadvantage,

however; dark points and spots are produced on the wall of the tube and on the layers applied thereto already after a few hours operation. In order to mitigate this disadvantage, it has been suggested to remove part of the luminescent layer, for example, at the ends located near the electrodes. It has been found, however, that the improvement obtained in this manner is negligible.

In investigations which ultimately have led to the invention an ignition strip was provided both on and under the luminescent layer. If these two strips were provided one above the other, the lamp behaved like a lamp having an ignition strip provided on the luminescent layer; if the ignition strips were not disposed exactly above each other, the ignition voltage found at lower temperatures was always higher than that of a lamp having an ignition strip provided under the luminescent layer, while at higher temperatures this voltage was higher than that of a lamp having an ignition strip provided on the luminescent layer.

A low-pressure mercury vapor discharge lamp in accordance with the invention having electrodes arranged at the'ends of an elongate discharge space, a luminescent layer and a conducting strip from 1 to 10 mm. wide, which strip extends between the electrodes but is not connected thereto and is disposed between the luminescent layer and its support, is characterized in that part of the strip between at least one of the electrodes and a point spaced from this electrode by a distance which is smaller than one fifth of the distance between the electrodes is in electrically conducting contact with the discharge plasma.

There are various ways of establishing the electrically conducting contact between the ignition strip and the discharge plasma. For example, part of the ignition strip According to a particular embodiment of the invention, the conducting contact between the ignition strip and the discharge plasma may also be established in that a conducting material, in particular a conducting material identical with that of the ignition strip, is applied to the above described desired part of the luminescent layer which is located above the ignition strip. For it has been found that in this case during the thermal treatment to which the lamps are always subjected after the application of the luminescent layer, a sufliciently conducting contact is established between the ignition strip disposed under the luminescent layer and the conducting material applied along a short distance to the luminescent layer.

Since in a lamp in accordance with the invention the width of the conducting strip is limited, it is possible to use both transparent and opaque ignition strips. Although with wider ignition strips of otherwise equal structure the ignition voltage obtained is always lower, the use of the invention with strips having a width exceeding mm. is not expedient, since the reduction of the ignition voltage with respect to that of a lamp having a strip of 10 mm. width located under the luminescent layer is too small. Moreover, with transparent strips having a width exceeding 10 mm. the above-described formation of spots becomes inconvenient, while with the opaque strips the loss of light is excessive. When opaque material is used, the width of the strip preferably does not exceed 4 mm.

It is not necessary for both ends of the ignition strip located near the electrodes to be in contact with the discharge plasma. In some cases an electrically conducting contact at one end may be sufficient.

The length of the part of the ignition strip which is in electrically conducting contact with the discharge plasma is limited to one fifth of the distance between the electrodes. If the length of this part should be greater, one comes near to those constructions in which the whole strip is exposed to the discharge or in which ignition strips are provided above each other both on and under the luminescent layer. As stated hereinbefore, both constructions are unfavorable.

The length of the part of the ignition strip which is in electrically conducting contact with the discharge plasma preferably is between 4 mm. and mm.

The shortest distance between an electrode and the part of the ignition strip which is in electrically conducting contact with the discharge plasma preferably is less than 20 mm. If this distance is made greater, the form of the ignition characteristic curve is approximately maintained, it is true, but the ignition voltage increases at the same lamp temperature.

It has been found that in lamps in accordance with the invention it is advantageous when the ignition strip does not extend beyond the part which is in electrically conducting contact with the discharge plasma. The ignition voltage of these lamps is then mostly slightly lower (by about 5 v.) than with an ignition strip which extends beyond said part.

The invention will now be described more fully with reference to the accompanying drawing, in which:

FIG. 1 and FIG. 1a are respectively a longitudinal sectional and cross-sectional view of a discharge lamp in accordance with the invention;

FIG. 2 shows a particular embodiment of a lamp in accordance with the invention;

FIG. 3 shows a graph of the relationship between the ignition voltage and the lamp temperature for lamps in accordance with the invention which have different ignition strips.

In FIG. 1, reference numeral 1 denotes the envelope, made for example of glass, which encloses the elongate discharge space 2 of a low-pressure mercury vapor discharge lamp; in this space are disposed electrodes 3 and 4. Reference numeral 5 designates the luminescent layer which along the greater part of the circumference bears directly on the envelope. However, an ignition strip 6,

which is not connected with the electrodes 3 or 4, is locally interposed between the layer 5 and the wall 1. In the proximity of the electrode 3 a window 7, which extends from a point 8 to a point 9, is provided in the luminescent layer 5 so that at this area the ignition strip 6 is in electrically conducting contact with the discharge plasma in the space 2. The distance between the point 9 and the electrode 3 always is less than one fifth of the distance between the electrodes 3 and 4. The shortest distance between the electrode 3 and the window 7 is preferably less than 20 min. Consequently, the length from point 8 to point 9 and the location of the window are restricted within limits which are determined by the structure of the lamp and more particularly by the location of the electrodes. The distance between point 8 and point 9 preferably is between 4 mm. and 40 mm. With a shorter distance, the decrease of the ignition voltage generally is not sufficient at higher temperatures; for reasons of manufacture, a distance greater than 40 mm. is not particularly attractive and usually not necessary for attaining the object of the invention. The width of the window 7, measured along the circumference, may be greater than the width of the conducting strip 6. As is clearly apparent from the drawing, the ignition strip terminates at the point 8.

FIG. 2 shows a lamp which for the greater part corresponds to that of FIG. 1. Instead of the window of FIG. 1, a small surface 11 of conducting material is provided on the luminescent layer opposite part of the conducting strip 10. This material is preferably identical with that of the ignition strip 10. What was said with reference to FIG. 1 inter alia about the length, the width and the place of the window 7, with due alterations applies to the conducting surface 11 also.

The material of the ignition strip may be constituted by a great many substances or mixtures of substances which have already been suggested for this purpose. The ignition strip may also be provided in a known manner. A particularly suitable mixture of substances is obtained by mixing 680 gms. of graphite powder, 4500 mls. of butylacetate, 300 mls. of monoethylglycolether, gms. of nitrocellulose and 2040 gms. of powdered glass having a grain size of 0.075 mm. Part of the above-described suspension is applied to the wall in the form of a strip by means of a tool suitable for this purpose.

In the graph of FIG. 3, the ignition voltage of a lowpressure mercury vapor discharge lamp is plotted along the ordinate, while the lamp temperature in degrees centigrade is plotted along the abscissa. The characteristic curves shown in the graph were measured on a 40 w. lamp in which the distance between the electrodes was approximately cms. and the strip had a total resistance of approximately 4000 0.

Curve 1 gives the relationship between the ignition voltage and the lamp temperature with the use of an ignition strip 2 mm. wide provided on the luminescent layer (consequently not in accordance with the invention). As is apparent from the curve, the ignition voltage is lowest at approximately 45 C. and increases both at higher and at lower temperatures. Especially at lower temperatures, the ignition voltage rises steeply. Over the range between 205 v. and 220 v., which is the most important range in practice, ignition is obtained at ambient temperatures of from 25 to 15 C.

Curve 2 applies to a strip which is 2 mm. wide and is disposed entirely under the luminescent layer (consequently not in accordance with the invention). As is apparent from the figure, the ignition voltage in this case is considerably higher at higher temperatures than that of the lamp in which curve 1 was measured. At low temperatures such a lamp operates satisfactorily, but this is no longer the case at temperatures higher than approximately 20 C.

Curve 3 was measured on a lamp in which a 2 mm. w de ignit on strip is provided on the luminescent layer and a 2 mm. wide ignition strip is disposed under this layer. The distance between the strips was approximately 90 measured along the circumference. Contrary to expectation, especially at lower temperatures, such a lamp does not operate satisfactorily with regard to its igni tion voltage.

Curve 4 applies to a lamp which has an ignition strip 4 mm. wide and the structure of which is otherwise the same as that of the lamp on which the curve 2 was measured. The ignition voltage is generally lower, but the shape of the curve has remained substantially unchanged. At high temperatures, this lamp does not operate satisfactorily either.

The curve 5 was measured on a lamp in accordance with the invention having an ignition strip which is 2 mm. wide and is disposed under the luminescent layer, a part of 20 mm. length of this layer situated over the ignition :strip, measured between the point of intersection of the axis of the nearest electrode with the luminescent layer and the other electrode, being removed. Over a very wide temperature range the ignition voltage of this lamp remains within the important region of 205 v. to 220 v.

Curve 6 was measured on a lamp in accordance with the invention having an ignition strip 2 mm. wide, a part of the luminescent layer having a length varying from 20 mm. to 60 mm., measured between a point at a distance of 5 mm. from the point of intersection of the axis of the nearest electrode with the luminescent layer and the other electrode, being removed. Over a wide temperature range the ignition voltage of this lamp is lower than 220 v. and even lower than 205 v.

Curve 7 was measured on a lamp in accordance with the invention having an ignition strip 2 mm. wide, a part of 20 mm. length of the luminescent layer situated over this strip being removed, while the distance between the end of the resulting window nearest to an electrode and this electrode was 100 mm.

Curve 8 was measured on a lamp in accordance with the invention having an ignition strip 4 mm. wide the structure of which was otherwise equal to that of the lamp on which curve 5 was measured. The ignition as a whole is much lower but for practical purposes such low ignition voltages are not always necessary. If the ignition strip is opaque, which is the case, for example, with the strip made of the material of the above-mentioned composition, the strip intercepts a greater amount of the radiation emitted by the lamp.

Lamps in accordance with the invention are particularly suitable for use in systems operating without starters.

What is claimed is:

1. A low-pressure mercury vapor discharge lamp comprising an elongate envelope containing an ionizable medium, a p ir of electrodes positioned at opposite ends of the envelope between which a discharge plasma is formed, a layer of luminescent material supported by the inner surface of the envelope, and a conducting strip extending substantially between the electrodes but not connected thereto, said conducting strip having a width of about 1 to 10 mm. and being disposed between the luminescent layer and the envelope, said strip having a portion which is in electrically conducting contact with the discharge plasma, said portion being positioned. between at least one electrode and a point spaced from said electrode, the distance between said electrode and said point being less than one-fifth of the distance between said pair of electrodes.

2. A low-pressure mercury vapor discharge lamp as claimed in claim 1 in which the portion of the strip which is in electrically conducting contact with the discharge plasma is not covered by said luminescent material.

3. A low-pressure mercury vapor discharge lamp as claimed in claim 1 in which the portion of the conducting strip which is in electrically conducting contact with the discharge plasma is covered with said luminescent material over which a conducting material is applied.

4. A low-pressure mercury vapor discharge lamp as claimed in claim 3 in which the conducting material applied to the luminescent layer is identical with the material of the conducting strip provide-d under the luminescent layer.

5. A low-pressure mercury vapor discharge lamp as claimed in claim 1 in which the shortest distance between the electrode and the portion of the conducting strip which is in electrically conducting contact with the discharge plasma is less than 20 mm.

6. A low-pressure mercury vapor discharge lamp as claimed in claim 1 in which the conducting strip does not extend beyond the portion which is in electrically conducting contact with the discharge plasma.

7. A low-pressure mercury vapor discharge lamp as claimed in claim 1 in which the length of the portion of the conducting strip which is in electrically conducting contact with the discharge plasma is between 4 mm. and 40 mm.

8. A low-pressure mercury vapor discharge lamp as claimed in claim 1 in which the conducting strip is made of opaque material and has a width less than 4 mm.

References Cited UNITED STATES PATENTS 1/1956 Campbell 313-197 X 7/1964 Ray 313221

Claims

1. A LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP COMPRISING AN ELONGATE ENVELOPE CONTAINING AN IONIZABLE MEDIUM, A PAIR OF ELECTRODES POSITIONED AT OPPOSITE ENDS OF THE ENVELOPE BETWEEN WHICH A DISCHARGE PLASMA IS FORMED, A LAYER OF LUMINESCENT MATERIAL SUPPORTED BY THE INNER SURFACE OF THE ENVELOPE, AND A CONDUCTING STRIP EXTENDING SUBSTANTIALLY BETWEEN THE ELECTRODES BUT NOT CONNECTED THERETO, SAID CONDUCTING STRIP HAVING A WIDTH OF ABOUT 1 TO 10 MM. AND BEING DISPOSED BETWEEN THE LUMINESCENT LAYER AND THE ENVELOPE, AND STRIP HAVING A PORTION WHICH IS IN ELECTRICALLY CONDUCTING CONTACT WITH THE DISCHARGE PLASMA, SAID PORTION BEING POSITIONED BETWEEN AT LEAST ONE ELECTRODE AND A POINT SPACED FROM SAID ELECTRODE, THE DISTANCE BETWEEN SAID ELECTRODE AND SAID POINT BEING LESS THAN ONE-FIFTH OF THE DISTANCE BETWEEN SAID PAIR OF ELECTRODES.