Note: Descriptions are shown in the official language in which they were submitted.
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LUMINAIRE COMPRISING AN ELONGATE LIGHT SOURCE AND
A BACK REFLECTOR
The present invention relates to luminaires of the type comprising an elongate
light source
and a baclc reflector. The invention is especially, but not exclusively,
applicable to
luminaires employing fluorescent tubes as light sources.
Elongate light sources in the form of fluorescent tubes are widely used in
luminaires for
space lighting, both indoors and outdoors. They also have many other uses, for
example
1o in shelf lighting and in display lighting, including not only commercial
and emergency
signs but also electronic displays.
The distribution of light required from a luminaire depends on the use for
which the
luminaire is intended. For example, in the case of ceiling-mounted luminaires
employing
15 fluorescent tubes for general space lighting, a light output having a wide
angle transverse
distribution of the so-called "batwing" type is often preferred because that
enables a
relatively uniform level of illumination at floor level to be achieved even
when the
luminaires are comparatively widely spaced. If the space to be illuminated is
one in which
computer display screens are used, the "batwing" distribution preferably
exhibits a cut-off
2o at about 60° on either side of the downward vertical to reduce the
amount of glare from
the display screens experienced ~by the users. On the other hand, when a
luminaire
employing a fluorescent tube is used for edge illumination of a light guide,
for example to
provide a bacl~light system for a display, the light output of the luminaire
should have a
narrow transverse distribution so that as much light as possible is injected
into the guide.
A fluorescent tube will normally emit light generally uniformly in all
directions around its
axis and, in a luminaire, it is often provided with a baclc reflector for re-
directing
rearwardly-emitted light in a forwards direction. Spaced back reflectors are
widely used
with fluorescent tubes for space lighting, and are also frequently used with
fluorescent
3o tubes in backlights for electronic displays, and they can result in
luminaires that are very
efficient in their use of energy. The baclc reflectors are, however, often
very bullcy in
comparison with the light sources, and not always suitable for use in confined
spaces.
Moreover, when the luminaires are used for space lighting, front diffusers are
often also
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required to provide a uniforxri level of illumination at floor level and add
further to the
bulkiness of the construction.
Examples of luminaires comprising a linear light source provided with a spaced
baclc
reflector are described in US-A-4 642 741, US-A-4 514 793 and US-A-3 654 471.
In the
arrangement described in US-A-4 642 741, the back reflector can be wrapped
around the
linear 1ig12t source for shipment and handling.
As an alternative to luminaires comprising fluorescent tubes with spaced back
reflectors,
l0 so-called "aperture lamps" have been developed. In this type of elongate
light source, a
reflective material closely surrounds (or is integral with) part of the
circumference of the
fluorescent tube leaving an elongate aperture through which the light
(including light
reflected by the reflective material) can emerge. The reflective material can
be a sheet
material or a coating applied directly to the inside or the outside of the
fluorescent tube, or
15 applied to the inside or outside of a protective sheath that completely
surrounds the tube.
Depending on the reflective material employed and the size of the elongate
aperture that is
formed, the aperture can exhibit high levels of surface luminance but does not
always
provide a controlled light distribution.
2o Examples of aperture lamps are described in US-A-3 115 309, US-A-4 186 431,
US-A-4
991 070, US-A-5 036 436, US-A-5 510 965, WO 94/22160, and WO 99/60303. In the
arrangement described in US-A-5 510 965, a printed film is used as the
reflector, and the
printing pattern is selected to modify the output of the light source in a
required manner.
25 In some cases, back reflectors have been used that engage closely around a
lighting tube.
Examples of that type of arrangement are described in US-A-2 078 370, US-A-2
595 275,
US-A-3 140 055, and DE-A-195 28 962. In some of those examples, the reflectors
are
provided with portions that extend away from the lighting tube (see US-A-2 078
370, US-
A-2 595 275 and US-A-3 140 055).
Although a lighting tube with a spaced back reflector requires more space, it
will generally
be more energy-efficient than a comparable aperture lamp, because light will
undergo
2
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fewer reflections before being emitted in the forwards direction so that the
amount of light
lost on reflection is reduced.
US-A-4 933 821 and US-A-5 414 604 describe luminaires comprising spaced back
reflectors that are shaped to ensure that some of the light from a fluorescent
tube leaves
the luminaire at a sharp angle to the remainder of the light. In the luminaire
described in
US-A-4 933 021 that is achieved by shaping the edge of the reflector. In US-A-
4 418 378,
the output of a fluorescent tube used in a light box is modified by providing
the tube with
an apertured sleeve with cut-away ends.
1o
The problem with which the present invention is concerned is that of
providing, for an
elongate light source in a luminaire, a reflector that is comparatively
compact and will not
only re-direct light in a required direction but will also enable the
distribution of the light
from a luminaire to be tailored to meet the requirements of particular
situations in which
15 the luminaire is to be used.
The present invention provides a luminaire comprising a reflector that defines
an elongate
concave cavity in which an elongate light source is located in spaced
relationship to the
reflector whereby the latter surrounds the light source on its rearward side
to reflect light
20 from the source and cause it to be emitted from the cavity in a generally-
forwards
direction; the reflector being provided with a prismatic structure upstanding
from its imler
surface to intercept and deviate light that is emitted, in the generally-
forwards direction,
from the space in the cavity between the light source and the reflector.
25 The present invention also provides a luminaire comprising a reflector that
defines an
elongate concave cavity in which an elongate light source is located in spaced
relationship
to the reflector whereby the latter surrounds the light source on its rearward
side to reflect
light from the source and cause it to be emitted from the cavity in a
generally-forwards
direction; the reflector being provided, on opposite sides of the cavity, with
a respective
30 prismatic structure upstanding from its inner surface to intercept and
deviate light that is
emitted, in the generally-forwards direction, from the space in the cavity
between the light
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source and the reflector.
The elongate light source of a luminaire in accordance with the invention
should be one
that will not completely absorb light that is returned to it by the reflector
and will
preferably absorb substantially none of that light. A suitable light source is
a fluorescent
tube.
The term "light" as used herein refers to electromagnetic radiation in the
ultraviolet,
visible andlor infraxed regions of the electromagnetic spectrum.
The term "prismatic structure" as used herein normally refers to a structure
whose two
ends are similar, equal and parallel rectilinear figures, and whose sides are
parallelograms.
In its simplest form, a prismatic structure has a triangular cross-section.-
However, as used
herein, the term extends to structures having cross-sections with more than
three sides and
also to the limiting case in which the structure has a cross-section with a
multiplicity of
sides to the extent that at least some of those sides form a curve.
By way of example only, embodiments of the invention will be described with
reference to
the accompanying drawings, in which:
2o Fig. 1 is an exploded perspective view of a luminaire in accordance with
the present
invention;
Fig. 2 is a perspective view of the luminaire of Fig. l, in an assembled
condition;
Fig. 3 shows a transverse cross-section through the luminaire of Fig. 2;
Fig. 3A illustrates the output light distribution of the luminaire in the
plane of Fig. 3;
Figs. 4, 5, 6 and 7 show transverse cross-sections through respective
luminaires in
accordance with the invention;
Figs. 4A, SA and 6A illustrate the output light distributions of the
luminaires of Figs. 4, 5
and 6 respectively in the planes of those Figures; and
Fig. 8 is a diagrammatic illustration of a backlighting system incorporating a
luminaire in
3o accordance with the invention.
4
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The luminaire 1, shown in an exploded condition in Fig. 1 and in an assembled
condition
in Fig. 2, comprises a linear fluorescent tube and a reflector 3. The
reflector 3 is of
elongate form and, as shown in Fig. 3, has a generally-concave transverse
cross-section
that defines a cavity 4 in which the light source 2 is located so that it is
partially
surromided, on one side, by the reflector. The reflector 3 forms the rear of
the luminaire
which, in use, is intended to emit light in a forwards direction, out of the
cavity 4 (that is,
away from the reflector).
The reflector 3 comprises an elongate shell 5 with a transverse cross-section
that is
l0 approximately semi-circular, and three upstanding longitudinally-extending
ribs 6, 7 on its
inner surface. Two of the upstanding ribs, indicated by the reference 6, are
located at the
longitudinal edges of the shell 5, and the third upstanding rib 7 is located
at the centre.
The reflector 3, comprising the shell S and the ribs 6, 7, is formed from an
optically-
transparent (preferably polymeric) material and is preferably a moulded or
extruded
15 component. A suitable material for the reflector 3 is polycarbonate but it
could,
alternatively, be formed from an acrylic material. The ribs 6, 7 contact the
envelope of the
fluorescent tube 2 and serve to space the shell 5 from the latter and, in the
case of the ribs
6, to modify the distribution of the light leaving the luminaire 1 as will be
described in
greater detail later. A highly-efficient specularly-reflecting layer 8 is
formed on the outer
2o surface of the shell 5 (including, as shown, the edge portions opposite the
bases of the ribs
6) to reflect light passing through the shell from the light source 2.
The reflector 3 may be mounted on, or form a part of, a fitting for receiving
the fluorescent
tube 2. Alternatively, it may be mounted directly on the envelope of the tube
2, in a
25 manner that permits it to be removed and, possibly, adjusted relative to
the tube as
required. In a preferred arrangement, the reflector 3 extends around the
fluorescent tube 2
to an extent that enables it to be retained on the tube solely by the action
of the ribs 6, it
being necessary only to provide some means for securing the reflector relative
to the tube
in the desired circumferential location. In that case, the shell 5 of the
reflector 3 must be
3o sufficiently flexible to permit insertion and removal of the tube 2 when
required. Various
other arrangements for mounting a reflector directly on the envelope of a
fluorescent lamp
axe known, and examples are described in US-A-4 514 793 and 2 S95 275.
Alternatively,
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the reflector 3 may be mounted using the same arrangement as the reflector
available
under the trade designation "Clip-On Reflector" available from Mimiesota
Mining and
Manufacturing Company of St. Paul, Minnesota, USA..
The luminaire 1 functions generally as follows. The fluorescent tube 2 will
emit light
generally uniformly in all directions around its longitudinal axis. Light that
is emitted in
the rearwards direction, i.e. towards the reflector 3, will pass through the
shell 5 and be
reflected by the layer 8 back towaxds the fluorescent tube 2, where it may be
reflected
again and returned to the layer 8. Light may, in fact, undergo multiple
reflections in the
l0 cavity 4, in the space between the fluorescent tube 2 and the shell 5,
before it is finally
able to leave the cavity 4 (travelling in the forwards direction) through
either the tube 2 or
one of the ribs 6. As so far described, the reflector 3 functions in a
conventional manner.
To reduce the amount of light lost on reflection at the layer 8, the latter
should have a
reflectivity of at least 90%, preferably at least 98%, facing into the cavity
4. The layer 8
may comprise a reflective film that is laminated to the outer surface of the
shell 5, in
which case a preferred reflective film is a mufti-layer optical film of the
type described in
US-A-5 882 774 and WO 97/01774. A suitable alternative film is available,
under the
trade designation "Miro", from Alanod of Ennepetal, Germany. As an alternative
to the
2o use of a reflective film, the layer 8 could be a vapour-deposited layer. In
some cases, the
layer 8 may be primarily a diffusely-reflecting material although strips of
specularly-
reflecting material would be required opposite the bases of the ribs 6.
In a modification of the arrangement shown in Fig. 3, the reflective layer 8
is transferred
to the inner surface of the shell 5, although strips of specularly-reflective
material are
retained on the outer surface opposite the bases of the ribs 6.
Each of the ribs 6 is in the form of a prism having a triangular cross-
section, the base of
the prism being a continuation of the outer surface of the shell 5 of the
reflector 3 and the
3o apex of the prism being adjacent the envelope of the fluorescent tube 2.
The two prisms 6
have identical cross-sections in the form of an isosceles triangle and are
positioned and
oriented symmetrically relative to the tube 2. Light that passes through one
of the prisms 6
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as it leaves the cavity 4 will be deviated by the prism and, through an
appropriate
orientation of the prism and selection of the prism angle, it is possible to
control the
direction in which that light will leave the luminaire. It is further possible
to adjust the
amount of light that passes through the prisms 6 by altering the extent to
which the
reflector 3 wraps around the fluorescent tube 2.
In the case of the reflector illustrated in Fig 3, the extent of the reflector
3 (measured as the
distance between the apexes of the prisms 6) is such that the reflector wraps
around 55%
of the circumference of the tube 2. The prisms 6 have a prism angle a of
76°and each is
1o oriented so that the outer face of the prism is at an angle (3 of
68° to the plane containing
the prism apexes with the result that the prism apexes are directed into the
cavity 4. Fig 3A
illustrates the effect of the reflector 3 on the angular distribution of light
from a luminaire
of this construction, in the plane of Fig. 3 (i.e. transverse to the length of
the tube 2). Fig
3A shows that, in this plane, the light has an intensity peals in the forwards
direction (0° in
15 Fig. 3A) and declines to zero on each side of the forwards direction more
rapidly than
would the light from a Lambertian source. In the orthogonal plane (i.e. along
the length of
the tube 2), the light also has an intensity peals in the forwards direction
but the effect of
the reflector 3 is less apparent.
2o Fig. 4 is similar to Fig. 3 and illustrates a luminaire in which the prisms
6 are oriented so
that the outer face of each prism is at an angle (3 of 8° to the plane
containing the prism
apexes, with the result that the apex of each prism is directed out of the
cavity 4. Fig 4A is
similar to Fig. 3A and illustrates the effect of the reflector 3 on the
angular distribution, in
the plane of Fig. 4, of the light from a luminaire of this construction. It
will be seen that,
25 in this plane, the light again has an intensity peak in the forwards
direction (0° in Fig. 4).
In the orthogonal plane (i.e. along the length of the tube 2), the light also
has an intensity
peak in the forwards direction but the effect of the reflector 3 is less
apparent.
Luminaires of the type shown in Figs. 3 and 4, which provide a beam of light
with an
3o intensity peals in the forwards direction, are particularly suitable for
edge illumination of
light guides because they will enable a comparatively high level of light to
be injected into
the guide thereby enabling the efficiency of the system to be increased. The
light guides
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can have various uses, for example, as electronic displays or edge-lit signs
or may,
themselves, also function as luminaires. Luminaires of the type shown in Figs.
3 and 4 are
also suitable for use in "wall-washing" lighting systems for illuminating
surfaces (e.g.
sign faces) and for illuminating merchandise in retail locations. In addition,
due to their
compact construction, they are particularly suitable for installation above
the aisles
between storage racks in warehouses to illuminate the raclcs effectively
without hindering
the mobility of forldift trucks.
Fig. 5 is a similar view to those of Figs. 3 and 4 but illustrates a luminaire
that will provide
l0 a completely different light distribution. In this case, the extent of the
reflector 3
(measured as the distance between the apexes of the prisms 6) is such that the
reflector
wraps around 70% of the circumference of the fluorescent tube 2. In addition,
although
the prisms 6 still have an apex angle a of 76° as in Figs. 3 and 4,
they are oriented so that
the outer face of each prism is at an angle (3 of 38° to the plane
containing the prism
15 apexes. Fig 5A illustrates the effect of the reflector 3 on the angular
distribution, in the
plane of Fig. 5, of the light from a luminaire of this construction. The
distribution has a
so-called "batwing" form, in which the light intensity has two peaks, one on
each side of
the forwards direction (in this case, at an angle of about 40°) and
then declines to zero
following a Lambertian distribution as the angle widens. In the orthogonal
plane (i.e.
20 along the length of the tube 2), the effect of the reflector 3 on the
angular distribution of
the light is less apparent.
Fig. 6 illustrates a modification of the construction shown in Fig. 5. The
principal
modification comprises continuing the reflector 3 beyond the prisms 6 to form
similar
25 outwardly-inclined extensions 9 along each edge of the curved shell 5 of
the reflector. A
highly-efficient specularly-reflecting layer 10, similax to the layer 8, is
formed on the outer
surface of each extension 9. The extensions 9 function to intercept light that
would
otherwise leave the luminaire 1 at a comparatively wide angle (including, in
each case,
some light from the prism 6 on the other side of the reflector), and cause it
to be emitted in
3o a more forwards direction. Fig 6A illustrates the angular distribution, in
the plane of Fig.
6, of the light from a luminaire of this construction. The distribution still
has the
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"batwing" form, but the light intensity in the two peaks is increased and
declines to zero
very rapidly at about 60° from the forwards direction on both sides.
Luminaires of the type shown in Figs. 5, providing a wide-angle "batwing"
distribution,
are particularly suitable for general space lighting applications. It is
already known, when
a plurality of ceiling-mounted luminaires is used to illuminate a floor space,
that
luminaires providing a "batwing" distribution are most efficient in that they
can be spaced
more widely apart without compromising the uniformity of the illumination
provided.
Luminaires of the type shown in Fig. 6 are preferred for lighting spaces, such
as offices, in
l0 which computer display screens are used. In that case, because the light
emitted by each
luminaire is contained within an angle of about 60° around the downward
vertical, the
glare from the display screens will be much less troublesome to the users. It
will be
appreciated that, for ceiling mounting, the luminaires of Figs. 5A and 6A
would be
oriented so that the emitted light is directed downwards towards the floor
area of the space
to be illuminated.
In the luminaire constructions illustrated in Figs. 3 to 6, the orientation of
the prisms 6, the
prism angle and the extent of the reflector 3 can all be altered to modify the
distribution of
the emitted light. Of these three factors, it has been found that the
orientation of the
2o prisms 6 has the greatest effect on the light distribution. Increasing the
extent of the
reflector 3 (measured between the apexes of the prisms 6), and hence the
degree to which
the light source 2 is surrounded, will reduce the efficiency of the luminaire
since less light
will be emitted but will also reduce the amount of light that is emitted in an
uncontrolled
manner. For a luminaire with a "batwing" light distribution as in Figs. 5A and
6A, for
example, the reflector 3 (measured between the apexes of the prisms 6)
preferably
surrounds about 75% of the circumference of the light source 2 although
anything between
55% and 85% is satisfactory. For a luminaire with a narrow light distribution
as in Figs.
3A and 4A, on the other hand, a smaller proportion of the light source 2 would
normally
be surrounded by the reflector 3. In all cases, if the extent to which the
reflector surrounds
3o the light source is changed, consideration may need to be given to the
mechanism used for
maintaining the position of the reflector relative to the light source.
9
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One preferred construction for providing a batwing light distribution, which
is of the type
shown in Fig. 6, uses a fluorescent tube 2 having a diameter of 25mm and the
distance
between the apexes of the two prisms 6 of the reflector 3 is sufficient to
surround about
75% of the circumference of the tube. The length of the prism sides, between
the base and
the apex, is l Omm; the prism apex angle a is 74°; and the prisms are
oriented so that the
outer face of each prism is at an angle (3 of 40° to the plane
containing the prism apexes.
The extensions 9 have a width of 20mm and are inclined outwards at an angle of
100° to
the plane containing the apexes of the prisms 6.
to As already indicated above, the reflectors 3 of the luminaires of Figs. 3
to 6 have a
controlling effect on the distribution of light in planes transverse to the
length of the
fluorescent tubes 2. If additional control of the light output of any one of
those luminaires
is required in the orthogonal plane, this can be achieved by, for example,
providing
louvres on the forward side of the tube 2 arranged to run across the tube.
The luminaires of Figs. 3 to 6 can be provided with any appropriate additional
features
known to be suitable for use with fluorescent tubes. For example, when the
reflecting
layer 8 is provided by a polymeric film, a loaded polymer material may be
provided as
described in EP-A-0 811 305 behind the polymeric reflecting film to assist in
starting and
2o regulating the fluorescent tube.
From the above description of Figs. 3 to 6, it will be understood that the rib
7 at the rear of
the fluorescent tube 2 serves only to maintain the space between the tube and
the reflector
shell 5. It does not contribute to the distribution of the light from the
luminaire, and could
be omitted if some alternative mechanism were provided for maintaining the
space
between the light source and the reflector.
A particular practical advantage of the luminaire constructions illustrated in
Figs. 3 to 6 is
that the space between the fluorescent tube 2 and the baclc reflector 3 is
closed, along the
length of the tube, by the prisms 6 and will consequently remain much cleaner
than in a
conventional arrangement. When the luminaire is used for space lighting, only
the outer
surfaces of the tube 2 and the prisms 6 will normally require cleaning.
to
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Although the prismatic ribs 6 of the reflectors 3 of Figs. 3 to 6 all have
cross-sections in
the form of isosceles triangles, other forms of prisms could be employed to
vary the
distribution of the light from the luminaire. The modifications that may be
made to the
prisms 6 include, for example, the provision of rounded sides, microstructured
surfaces,
and an asymmetric cross-section. The prisms 6 of any one reflector need not
have the
same shape and, depending on the light distribution required, one of the
prisms may be
omitted.
1o It is also possible to alter the relative positions of the fluorescent tube
2 and the reflector 3
so that they are no longer concentric. Fig. 7, for example, shows a luminaire
with a
reflector 3 similar to that of Fig. 5 except that the rib 7 is shortened so
that the space
between the reflector and the tube at the rear of the latter is decreased.
Generally,
however, a wider space between the tube 2 and the reflector 3 is preferred
because light
I5 will then undergo fewer reflections before emerging from the luminaire
cavity 4.
The shape of the back reflector 3 can also be modified from the generally semi-
circular
form shown in Figs. 3 to 6. The reflector 3 may, for example, have a parabolic
form or
comprise flat surfaces. Moreover, although it is preferable for the ribs 6, 7
to be formed in
20 one piece with the reflector shell 5, that is also not essential. The shell
5 could, for
example, be formed in metal (which may, in itself, be sufficiently
reflective), with the ribs
6, 7 being attached to it. In such a construction, the prismatic ribs 6 would,
of course, be
formed from an optically-transparent material.
25 Although the luminaries of Figs. 1 to 7 utilise fluorescent tubes as the
light sources, they
could use any alternative form of elongate light source provided that this
does not
completely absorb light that is returned to it by the reflector 3. Preferably,
the light source
absorbs none, or substantially none, of the light that is returned to it by
the reflector 3.
Suitable alternative light sources include large diameter optical fibres and
light guides.
Fig. 8 is a cross-sectional schematic view illustrating a baclclight system 20
that includes a
luminaire 11 in accordance with the invention, and a solid light guide 12. The
light guide
11
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12 is shown as having a rectangular cross-section, with the elongated
luminaire positioned
along one edge 12A. The use of a rectangular light guide is not essential,
however, and a
light guide of any other suitable shape could be used. The reflector 13 of the
luminaire is
selected so that the output of the luminaire is a narrow, forwardly-directed
beam as
illustrated in Figs 3A and 4A, thereby ensuring that as much of the light as
possible will
enter the light guide 12 through the adj acent edge 12A.
The light guide 12, which may be solid or hollow, has a front surface 14 and a
back
surface 15. When the backlight system is in use, a component such as a
polarizes, diffuser,
to liquid crystal display panel, graphics film or print may be placed above
the front surface
14. That component is not shown in Fig. 7 but is well lcnown and will not be
described in
greater detail here. The light guide 12 further includes some form of light
extraction
mechanism to direct light from within the guide out through the front surface
14.
Examples of lcnown extraction mechanisms include diffusing dots on, or
channels in, the
15 back surface I 5 of the guide.
A particularly advantageous feature of lurninaires constructed as illustrated
in Figs. 3 to 7
is that they can be very compact in comparison with conventional luminaires
employing
fluorescent tubes, but will nevertheless provide effective illumination in a
wide variety of
20 locations. Luminaires that require less space offer greater design freedom
in many areas
including, for example, building construction, interior design and electronic
displays.
12
