Low-pressure mercury vapor discharge lamp

A low-pressure mercury vapor discharge lamp comprising a glass discharge envelope (1) in which during operation of the lamp a discharge is present and which contains mercury and a rare gas, at least a part of the inner wall of the discharge envelope being provided with a thin at least substantially homogeneous continuous transparent layer (4) which is resistent to the action of the discharge. According to the invention, this transparent layer (4) contains an oxide of at least one of the groups comprising yttrium, scandium, lanthanum, gadolinium, ytterbium and lutetium.

BACKGROUND OF THE INVENTION 
The invention relates to a low-pressure mercury vapour discharge lamp 
comprising a glass discharge envelope in which a discharge is present 
during operation of the lamp and which contains mercury and a rare gas, at 
least a part of the inner surface wall of the discharge envelope being 
provided with a thin at least substantially homogeneous continuous 
transparent layer, which is resistant to the influence of the discharge. 
It is known to take measures in low-pressure mercury vapour discharge lamps 
to prevent greying of parts of the inner wall of the discharge envelope 
which are in contact with the discharge. Such a greying, which is due to 
interaction of mercury and glass, is undersirable and not only gives rise 
to a reduction of the light output but also results in an unaesthetic 
appearance of the lamp, in particular due to the fact that the greying 
occurs irregularly, for example, in the form of dark stains and dots. 
It is proposed in the U.S. Pat. No. 3,377,494 to provide the inner wall of 
the discharge envelope with a thin substantially homogeneous continuous 
transparent layer of, for example, titanium dioxide or zirconium dioxide 
in order to prevent greying of the glass inner wall of the discharge 
envelope. 
In contrast to a protective granular layer consisting of a refractory metal 
oxide (such as aluminum oxide or silicon dioxide) which is composed of a 
large number of small particles and should be comparatively thick in order 
to prevent the occurrence of any interaction such a interaction between 
mercury and the glass wall. However in the lamp according to the 
aforementioned United States Patent Specification, a direct contact 
between the glass wall and the mercury discharge is avoided due to the 
presence of the thin homogeneous continuous transparent layer. 
Due to the presence of the transparent layer, according to said Patent 
Specification, it is sufficient to use in the discharge envelope a 
comparatively thin luminescent layer, as a result of which a considerable 
saving in the required quantity of luminescent material is obtained 
compared with lamps not provided with a transparent protective layer. 
It has been found that, especially in a transparent layer containing 
titanium dioxide, ultraviolet radiation having a wave length of 
approximately 350 nm is absorbed to a considerable extent; it has further 
been found that resonance radiation of mercury having a wave length of 354 
nm is even absorbed to a substantially complete extent. This is especially 
disadvantageous when such a layer is employed in lamps which emit 
substantially exclusively radiation of the this wave length. Examples of 
such lamps are germicide lamps and lamps emitting ultraviolet radiation of 
comparatively long wave lengths, such as lamps intended for solarium 
arrangements. 
SUMMARY OF THE INVENTION 
The invention has for its object to provide a lamp in which greying and 
discolouring of the glass wall of the discharge envelope are limited to a 
minimum, the light or radiation output of the lamp remaining at the 
highest possible level during lamplife. 
According to the invention, a low-pressure mercury vapour discharge lamp of 
the kind mentioned in the opening paragraph is therefore characterized in 
that the transparent layer is formed of an oxide of at least one of the 
elements in the group comprising yttrium, scandium, lanthanum, gadolinium, 
ytterbium and lutetium. 
The aforementioned oxides can be applied in a simple manner as a very thin 
continuous and homogenous transparent layer to the glass wall of a 
discharge envelope. This is effected, for example, by rinsing the 
discharge envelope with a solution of a suitable metallo-organic compound 
(such as an acetyl acetonate) in an organic solvent, the desired layer 
being obtained after drying and sintering. Alternatively, the layer may be 
applied by means of a method, in which a metal compound is introduced into 
a discharge envelope by means of a carrier gas (such as air) whilst being 
heated and is deposited on the wall. It has been found that layers of the 
aforementioned oxides are highly resistant to the action of the mercury 
and rare gas containing atmosphere in the discharge envelope of a 
low-pressure mercury vapour discharge lamp. They also satisfy very well 
the requirements of light or radiation transmission. The oxides according 
to the invention are selected especially from a plurality of oxides of 
rare earth metals. Layers provided with oxides of the metals according to 
the invention are particularly suitable for use in low-pressure mercury 
discharge lamps because they are colourless and substantially do not 
exhibit any absorption of useful radiation (such as UV radiation and 
visible light). 
It should be noted that the German Patent Specificaton No. 1,764,126 
discloses a low-pressure sodium vapour discharge lamp comprising a 
discharge envelope, the inner surface of the wall of which is provided 
with a homogeneous layer which is transparent to sodium light and 
resistant to sodium vapour and which, according to the said Patent 
Specification, may consist of one of the oxides of yttrium and/or the rare 
earth metals. Such a lamp, however, emits light only with a specific wave 
length in the visible range. Special problems which are due to the action 
of short-wave ultraviolet radiation on the layer do not occur in this 
lamp. 
It has been found that absorption of the resonance radiation of mercury 
having a wave length of 254 nm produced in the discharge envelope hardly 
occurs in the transparent layer in a lamp according to the invention. In 
low-pressure mercury vapour discharge lamps for irradiation purposes, in 
which mainly radiation of a wave length of 254 nm is emitted (such as 
germicide lamps) and in which the inner wall of the discharge envelope is 
only coated with the transparent layer according to the invention, it has 
been found that greying and discolouring of the glass wall rarely occurred 
even after a large number of operating hours of the lamp. The radiation 
output of the lamp then remained at a high level as compared with known 
lamps, without a transparent layer. 
The invention can also be used advantageously in lamps comprising a tubular 
discharge envelope, the inner wall of which is provided with a reflecting 
layer in which a longitudinal slot is formed. In such lamps, a luminescent 
layer is present at least on the reflecting layer. In a particular 
embodiment, however, this luminescent layer extends throughout the 
periphery of the inner wall of the discharge envelope. In these lamps it 
was a surprise to find that with the use of a transparent layer according 
to the invention on the glass wall at least at the area of the 
longitudinal slot a very high light or radiation output was obtained for a 
long operating time. 
The invention can also be used advantageously in lamps, the whole inner 
wall of which is coated with a luminescent material. The transparent layer 
is then present between the luminescent layer and the glass wall. The 
glass wall is then protected in an effective manner from the influence of 
the discharge. This has proved to be so especially in lamps provided with 
a curved discharge envelope (for example, a lamp as described in the Dutch 
Patent Specification No. 80011833, in which the luminescent layer is not 
continuous at the area of the curved parts of the discharge envelope and 
in which a comparatively high wall load occurs. 
Experiments have shown that during operation of the lamp the light output 
remained at a high level. 
The transparent layer in a low-pressure mercury vapour discharge lamp 
according to the invention preferably contains an oxide of yttrium and/or 
gadolinium. Such a layer has a comparatively high transmission coefficient 
for ultraviolet radiation and visible light. It has further been found 
that a layer containing these oxides is not very hygroscopic and adheres 
satisfactorily to the inner wall of a discharge envelope. Moreover, the 
layer can be applied in a comparatively simple manner (for example, with 
yttrium acetyl acetonate), which results in a reduction in cost especially 
in a mass production process for low-pressure mercury vapour discharge 
lamps. 
The invention will be described more fully with reference to the drawing, 
in which embodiments of a low-pressure mercury vapour discharge lamp 
according to the invention are shown diagrammatically in longitudinal 
sectional view are shown by way of example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a low-pressure mercury vapour discharge lamp comprising a 
tubular discharge envelope 1, at the the ends of which the electrodes 2 
and 3 are arranged. During operation of the lamp, a discharge is 
maintained between these electrodes. The discharge envelope contains 
mercury vapour and a rare gas, such as argon (pressure approximately 400 
Pa). The glass inner wall surface of the discharge envelope is provided 
with a thin substantially homogeneous continuous transparent layer 4 which 
is resistent to the influence of the discharge. The lamp shown in the 
drawing is a lamp for irradiation purposes (a germicide lamp) which mainly 
emits resonance radiation having a wave length of 254 nm. Such lamps are 
generally used in a room for destroying undesired bacilli, bacteria and 
the like as in hospitals. The said transparent layer in practical 
embodiments of the lamp has a thickness of approximately 5 nm to 
approximately 200 nm. With a thickness of more than 200 nm, an excessively 
large absorption takes place of the radiation produced in the discharge 
envelope. With a layer thickness of less than about 5 nm, interaction 
nevertheless occurs between the discharge and the glass wall. 
A number of experiments have been carried out on lamps (15 W, inner 
diameter discharge envelope 25 mm, length discharge envelope approximately 
50 cm, argon 400 Pa), the discharge envelope of which is provided with a 
transparent layer containing an oxide according to the invention. The 
transparent layer was obtained by rinsing the inner wall of the discharge 
envelope with a liquid containing a metallo-organic compound (for example, 
yttrium acetyl acetonate) in an organic solvent (for example, ethylene 
glycol monoethyl ether). The layer is formed after drying and sintering 
(for example, to approximately 600.degree. C.). The results of the said 
experiments are shown in the following Table I. The thickness of the oxide 
layer in all cases was 50 to 150 nm. Table I indicates the radiation 
output in (UV Watts) as well as (between brackets), the relative radiation 
output per lamp with respect to 100 operation hours. Table I further 
indicates the results with a known lamp which is free from a transparent 
protective layer. 
TABLE I 
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lamp 
oper- without 
ating transparent 
time layer with Y.sub.2 O.sub.3 
with Gd.sub.2 O.sub.3 
with Sc.sub.2 O.sub.3 
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0 h 4.6W (117%) 
4.6W (107%) 
4.5W (107%) 
4.5Q 
(110%) 
100 h 
3.9W (100%) 
4.3W (100%) 
4.2W (100%) 
4.1W 
(100%) 
1000 h 
3.4W (88%) 4.1W (95%) 4.0W (95%) 
3.9W 
(95%) 
2000 h 
2.9W (74%) 3.9W (90%) 3.8W (90%) 
3.8W 
(93%) 
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lamp 
operating 
time with La.sub.2 O.sub.3 
with Yb.sub.2 O.sub.3 
with Lu.sub.2 O.sub.3 
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0 h 4.5W (107%) 4.5W (107%) 
4.5W (107%) 
100 h 4.2W (100%) 4.2W (100%) 
4.2W (100%) 
1000 h 3.9W (93%) 4.1W (98%) 4.0W (95%) 
2000 h 3.7W (88%) 3.9W (93%) 3.8W (90%) 
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It appears from this Table that the radiation output of lamps according to 
the invention remains at a high level even after a long operating time. In 
the lamps according to the invention, attack of the glass wall by the 
mercury and as a result decrease of the radiation output substantially do 
not occur. 
The lamp shown in FIG. 2 likewise comprises a tubular discharge envelope 1, 
electrodes 2 and 3 and a transparent layer 4. This layer is provided on 
its side facing the discharge with a layer of luminescent material 5. This 
layer extends throughout the surface of the transparent layer. In a number 
of experiments, this luminescent layer 5 consisted of a mixture of three 
phosphors, i.e. green luminescent terbium-activated cerium magnesium 
aluminate, blue luminescent barium magnesium aluminate activated with 
bivalent europium and red luminescent yttrium oxide activated with 
trivalent europium. In the presence of a transparent layer according to 
the invention between the said luminescent layer 5 and the glass wall of 
the discharge envelope, it was a surprise to find that with a small powder 
weight of the luminescent material (as compared with lamps without a 
transparent layer) only a small reduction of the light output occurs. The 
powder weight is to be understood herein to mean the overall weight of the 
luminescent material in the whole discharge envelope. As compared with the 
known lamp (without a transparent layer), it has proved possible to limit 
in the lamps according to the invention the powder weight of the said 
luminescent material by approximately 25% to approximately 2 mg/cm.sup.2, 
while a reduction of the light output substantially did not occur. 
Experiments have been carried out on a number of low-pressure mercury 
vapour discharge lamps (power 36 W, length 1.20 m, inner diameter 25 mm, 
argon 400 Pa) provided with a transparent layer consisting of yttrium 
oxide and with a luminescent layer 5 consisting of a mixture of the 
aforementioned phosphors. Of the lamps, the light output (lumen) has been 
measured and compared with the light output of a known lamp having the 
same dimensions, the same power and a luminescent layer composed of the 
same phosphors, the latter lamp, however, not being provided with a 
transparent layer. The results of the experiments are indicated in Table 
II. The experiments have been carried out on lamps of different powder 
weights (i.e. with 2.8 g and 2.1 g, respectively, of luminescent material. 
The results are indicated in the second and the third column (2.8 g) and 
in the fourth and the fifth column (2.1 g). 
TABLE II 
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lamp 
without with Y.sub.2 O.sub.3 
without with Y.sub.2 O.sub.3 
transparent 
transparent 
transparent 
transparent 
layer layer layer layer 
operating 
2.8 gr. 2.8 gr. 2.1 gr. 2.1 gr. 
time Lum. mat. Lum. mat. Lum. mat. 
Lum. mat. 
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0 h 3460 Lm 3460 Lm 3405 Lm 3435 Lm 
100 h 3410 Lm 3440 Lm 3370 Lm 3410 Lm 
1000 h 3380 Lm 3410 Lm 3290 Lm 3380 Lm 
2000 h 3310 Lm 3380 Lm 3245 Lm 3370 Lm 
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It appears from this table that the light output of a lamp according to the 
inventon is high even after a large number of operating hours. It further 
appears from the table that, even with a small powder weight (3.5 
mg/cm.sup.2) the light output of the lamp provided with the transparent 
layer of Y.sub.2 O.sub.3 is comparatively high for a long operating time. 
Furthermore, experiments have been carried out on a plurality of lamps (15 
W, inner diameter discharge envelope 25 mm, length 50 cm, argon pressure 
400 Pa), in which only a transparent layer comprising yttrium oxide was 
present on the inner wall of the discharge envelope. For a number of layer 
thicknesses, the measured radiation output (UV-Watt, 2000 operating hours) 
is indicated in Table III. 
TABLE III 
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layer thickness 
radiation output 
(nm) (UV-Watt) 
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0 3.91 
8 5.17 
20 5.11 
40 5.27 
80 5.22 
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It appears from this table that the radiation output of the lamps provided 
with a transparent layer comprising yttrium oxide having a thickness of 
more than 8 nm was considerably higher than for lamps without a 
transparent layer. The comparatively low radiation output of the lamp 
without an Y.sub.2 O.sub.3 layer was due to the occurrence of greying of 
the wall of the discharge envelope.