Compact high efficacy fluorescent lamp

Low-pressure mercury vapor discharge lamp having a tubular discharge vessel arranged within a lamp envelope. The discharge vessel consists of two, preferably generally helically bent tube parts which are connected together by a coupling joint which intersects the tube parts at a location spaced from their sealed ends remote from the electrodes. A cool area is formed in the discharge vessel near this coupling joint to control vapor pressure.

BACKGROUND OF THE INVENTION 
The invention relates to a low-pressure mercury vapor discharge lamp 
comprising a tubular discharge vessel, which is arranged within a lamp 
envelope and curved at a number of areas. The vessel is sealed in a 
gastight manner and filled with mercury and a rare gas. Electrodes are 
arranged at the ends of the discharge vessel and are located at the same 
end of the space within the lamp envelope. A fluorescent lamp of this type 
is known from U.S. Pat. No. 4,383,200. 
The known lamp comprises a discharge tube located within the lamp envelope 
and bent into the form of a hook. The lamp has small dimensions and serves 
as an alternative to an incandescent lamp for general illumination 
purposes. 
The discharge vessel of this lamp is surrounded by the lamp envelope. 
During operation of the lamp the temperature easily increases to such a 
high value that the optimum mercury vapor-pressure for the conversion of 
supplied energy into ultraviolet resonance radiation of mercury in the 
discharge vessel (approximately 6.times.10.sup.-3 Torr) is exceeded. It is 
then necessary to take special measures in order to stabilize the mercury 
vapor-pressure at the optimum value, for example by providing a 
vapor-pressure controlling amalgam in the discharge vessel or by providing 
the discharge vessel with an appendix which is kept at a relatively low 
temperature (see U.S. Pat. No. 4,546,284). It is, however, complicated to 
provide such an appendix. 
Moreover, the discharge vessel of the known lamp is secured only on one 
side within the lamp envelope. This makes the lamp vulnerable to 
vibrations and shocks. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a compact low-pressure mercury 
vapor discharge lamp, such as a fluorescent lamp, having a high efficiency 
of energy conversion, which does not have the drawbacks of the known lamp. 
To this end, in a fluorescent or other low-pressure mercury vapor discharge 
lamp of the type described in the opening paragraph, the tubular discharge 
vessel is composed of two bent tube parts, each having an electrode at one 
end and being sealed in a gastight manner at its other, far end. At least 
one of the tube parts is bent in a generally helical shape. The tube parts 
are connected together by a coupling joint intersecting the parts at 
locations spaced some distance from the far ends, the coupling joint being 
located at the end of the lamp envelope remote from the electrodes. 
The tubular discharge vessel of the lamp according to the invention has a 
longer length than the discharge vessel of the known lamp having the same 
dimensions. The efficiency of the conversion to ultraviolet energy, and 
therefore the efficacy of a fluorescent lamp of this type, will thus be 
greater than that of the known lamp. 
The tube parts are initially formed from, for example, a straight tube 
which is provided with a luminescent layer and which is subsequently bent 
through approximately 180.degree. at positions one third and two thirds 
along its length. A stem with an exhaust tube and an electrode is provided 
at one end in each tube part, and the other or far end is sealed in a 
gastight manner. 
Preferably, the two tube parts are substantially identical. Each is 
generally helical; that is, the first bend is about an axis which may be 
defined, generally transverse to the central axis of the tubing. From the 
electrode end to the first bend, around that bend, and then to the second 
bend, the tubing is following a path around the defined axis in a given 
direction. This portion of the part may conveniently be formed as two 
substantially straight, parallel runs of tubing, lying in a plane 
perpendicular to the defined axis. Going through the second bend, and 
extending along a generally straight run to the sealed end, the tubing 
follows a path continuing the same given direction of rotation about the 
defined axis, but also advancing in an axial direction in a generally 
helical fashion. For a given configuration of lamp envelope, which usually 
has a circular cross-section about a lamp axis, the plane defined by the 
tubing of a part (i.e., a plane perpendicular to the defined axis) is 
parallel to the lamp axis. The exact angle of the second bend, which will 
involve bending through approximately 180.degree. about an axis which is 
skewed with respect to the axis of the first bend, is chosen so that the 
sealed ends of the two tube parts may be close to each other when the two 
parts are arranged with their electrode ends spaced diametrically with 
respect to the lamp axis and with the helices of the two parts advancing 
toward each other. 
Subsequently the two tube parts are connected together at a location near, 
but spaced from, the said sealed ends, by means of the coupling joint. The 
discharge vessel thus formed is exhausted and provided with the mercury 
and rare gas atmosphere and the exhaust tube is sealed in a gastight 
manner. A relatively cool space is then obtained between the sealed end of 
each tube part (that is, the end where there is no electrode) and the 
coupling joint, because no discharge passes through these spaces during 
operation of the lamp. The mercury vapor pressure is then stabilized at 
the value of 6.times.10.sup.-3 Torr during operation, so that the 
conversion of supplied energy into ultraviolet radiation is optimum. This 
effect is further enhanced because the cool space is located at the end 
within the lamp envelope remote from electrodes. 
Preferably the discharge vessel and the lamp envelope (which preferably 
consists of a synthetic material) are connected together at an area near 
the sealed far ends of the tube parts remote from the electrodes. This 
connection is provided by an inwardly extending collar formed in the 
envelope. The discharge vessel is then anchored between two ends of the 
envelope facing each other. The risk of breakage of the discharge vessel 
as a result of vibrations and shocks is thus smaller than in the known 
lamp. 
In a different embodiment, the end wall of the lamp envelope is provided 
with a recess at the area of the coupling joint and the cool space ends, 
the bottom part of the recess having an aperture accommodating the far 
ends of the tube parts. 
In this manner the far ends of the tube parts are exposed to and cooled by 
the atmosphere surrounding the lamp, but the risk of breakage of the ends 
of the tube parts protruding from the lamp envelope is small. 
The lamp according to this preferred embodiment of the invention is 
compact, and has a high efficiency and a remarkably homogeneous light 
distribution. In addition, the lamp can be manufactured in a relatively 
simple manner in mass production. 
It will be recognized that the "generally helical" construction of at least 
one of the tube parts is chosen so that one can obtain a maximum length of 
tube between the electrode end and the far sealed end, within a convenient 
outer envelope and with orientation of the far end at an end of the lamp 
remote from the electrodes. In the preferred embodiments the discharge 
vessel operates in conjunction with a ballast unit contained within the 
base of the lamp just beyond the electrodes. To minimize the outline 
dimensions of the lamp, it may be desirable that the two electrodes be to 
the same side of the lamp, as well as at the same end. With such 
construction, desirably one tube part is generally helical, but only the 
portion between the electrode and the second bend has a similar 
configuration in the second tube part. The second bend of this latter part 
is then so oriented that the far sealed end of the second part is at a 
side of the lamp opposite the side on which the two electrodes and the far 
sealed end of the first tube part are located. 
The invention will be further described with reference to the accompanying 
drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The lamp of FIG. 1 comprises a lamp envelope 1 of a transparent synthetic 
material within which a tubular discharge vessel is arranged. The vessel 
is sealed in a gastight manner and is filled with mercury and a rare gas. 
At the ends of the vessel electrodes 2 and 3 are arranged. The electrodes 
are located diametrically with respect to the longitudinal axis of the 
lamp envelope 1. A synthetic material plate 4, to which the discharge 
vessel is secured by means of the collars 4a and 4b, is provided at this 
end. The plate is also connected to the lamp envelope 1. 
The discharge vessel, whose inner wall is provided with a luminescent 
layer, is composed of two substantially identical, generally helically 
bent tube parts 5 and 6. Each of these tube parts supports an electrode at 
one end and is sealed in a gastight manner at its other, far end (7, 8). 
At some distance (for example, 1 to 3 times the internal diameter of the 
tube parts) from these sealed ends the two tube parts are connected 
together by a coupling joint 9 shown diagrammatically. This coupling joint 
(through which the discharge passes during operation) is formed by fusing 
collars facing each other and surrounding apertures in the walls of the 
tube parts. 
A method of fusing collars of this type is described in U.S. Pat. No. 
4,324,447 and is particularly advantageous for joining multiple bent tube 
parts because the glass wall of the discharge vessel is only locally 
heated for the purpose of providing the coupling joint. 
The ends 7 and 8 of the tube parts now constitute a relatively cool area in 
the discharge vessel, and determine the magnitude of the mercury vapor 
pressure in the discharge vessel during operation of the lamp. The ends 7 
and 8 are secured with the aid of a clamping joint in the form of an 
inwardly extending circular collar 10 forming part of the synthetic 
material lamp envelope 1. The discharge vessel is thus rigidly clamped 
between the envelope and the synthetic material plate 4. 
For simplicity in manufacturing, and maximum use of space within the 
generally cylindrical (slightly tapered) envelope 1, each tube part is 
only generally helically bent. See FIG. 4. The electrode is incorporated 
in a gastight manner at the end of a first straight portion 21. This 
straight portion is connected to a second straight portion 22 via a 
U-shaped bent portion 23. This second portion is in turn connected via a 
second, generally U-shaped bent portion 24 with a third straight portion 
25 which is sealed at its far end (for example, at 7). 
As is shown in the drawing the longitudinal axes of the two tube portions 
25 are skewed at an acute angle near the coupling joint. This angle is 
10.degree. at a maximum. The collar 10 then forms a reliable clamping 
joint with the outer wall of the far ends. 
Below the envelope 1 the lamp is provided with a conical synthetic material 
lamp tray 11 accommodating an electronic circuit 12 (diagrammatically 
shown) for operating and starting the lamp. A circuit of this type is 
described in the Netherlands patent application No. 8400923 laid open to 
public inspection, to which U.S. Pat. No. 4,647,820 issued 3/3/87 
corresponds. At the small end of the tray 11 the lamp has an Edison cap 13 
with which it can be screwed into an incandescent lamp holder. 
FIG. 2 shows a detail of the upper side of a synthetic material lamp 
envelope 31 of an alternative embodiment of the lamp according to FIG. 1. 
The same components as in the lamp according to FIG. 1 have the same 
reference numerals. The wall of the lamp envelope is bent inwards to form 
a collar 32 surrounding a recess 33. An aperture 35, through which the 
ends 7 and 8 of the tube portions 25 of the parts 5 and 6 extend, is 
formed in the bottom part 34 of the recess. Since the ends 7 and 8 
directly communicate with the ambient surrounding the lamp, the 
temperature of these ends is relatively low. The recess is formed and 
dimensioned in such a manner that the cool ends of the tube parts, located 
above the coupling joint 9, do not protrude beyond a plane defined by the 
wall of the lamp envelope adjoining the recess. 
In a practical embodiment of the lamp according to FIG. 1 the total length 
of the tubular discharge vessel (that is, the sum of the length of the 
separate tube parts 5 and 6) is 45 cm. A luminescent layer consisting of a 
mixture of two phosphors, namely green luminescing cerium magnesium 
aluminate activated with terbium, and red luminescing yttrium oxide 
activated with trivalent europium, was provided on the inner wall of the 
discharge vessel whose internal diameter is approximately 10 mm. The 
length of the total lamp (measured from the top of the lamp envelope to 
the end of the cap 13) was 14.5 cm, and the maximum width of the lamp was 
7.5 cm. The luminous efficiency was 1200 lm for an applied power to the 
lamp of approximately 20 W (including electronic circuit). 
The embodiment shown in FIG. 3 uses many of the same parts as that of FIG. 
1, similar or identical parts being shown with the same reference numeral 
as FIG. 1. Thus the generally helical bent tube part 5 may have a shape 
identical to that of the tube part 5 of FIG. 1. The tube part 40, however, 
is partially different in its directions of bending. The first straight 
portion 41, at whose end electrode 3 is located, is arranged to the same 
side of the axis of the lamp as the portion 21 of the tube part 5, rather 
than being diametrically opposite as in the FIG. 1 embodiment. This 
straight portion 41 is connected to a second straight portion 42 via a 
U-shaped bent portion 43. This second portion 42 is in turn connected via 
a second, generally U-shaped bent portion 44 with a third straight portion 
45 which is sealed at its far end 8. Unlike the bent portion 24, which 
continued the helical direction about an axis generally perpendicular to a 
plane defined by the first and second straight portions, the bent portion 
44 is bent about an axis which is approximately perpendicular to the axis 
of bending of the portion 43. Thus the third straight portion 45 does not 
continue in a generally helical pattern from the portions 41, 42 and 43. 
This embodiment provides substantially the same total arc path length as 
the embodiment of FIG. 1, but offers the advantage that the synthetic 
material plate 64, corresponding to the plate 4 of the FIG. 1 embodiment, 
has its collars 64a and 64b formed to the same side of the lamp, rather 
than being diametrical about the lamp axis. This may provide easier 
connections, or space to the other side of the lamp where the plate 4 
might even have a raised portion to provide clearance for components of 
the circuit 12. 
It will be clear to those of ordinary skill in the art that many variations 
from the disclosed embodiments fall within the scope of the invention. For 
example, in some applications a more spherical outer envelope is 
considered attractive, and in that situation the two tube parts may 
advantageously be more nearly helical in form. 
When the two tube parts are not identical, as in the embodiment of FIG. 3, 
the advantage of having a coupling joint connecting with one of the tube 
parts at a distance spaced from the sealed far end will be provided even 
if only one of the tube parts has a portion extending beyond the coupling 
joint. 
In another variation which might have advantage for surface uniformity of 
brightness, at some sacrifice in efficacy, each tube part may have a 
configuration generally like that of the tube part 40, so that in point of 
fact neither tube part would truly be generally helical. Such a 
construction could nonetheless permit the use of two identical tube parts, 
having the sealed far ends at the end of the lamp remote from the 
electrodes, either in the style shown in FIG. 1 or in FIG. 2, so that the 
pressure control of the mercury gas may still be obtained.