WO2010055339A2

WO2010055339A2 - Lamp unit, light fitting and method of making an optical lens for a lamp unit

Info

Publication number: WO2010055339A2
Application number: PCT/GB2009/051519
Authority: WO
Inventors: Gary William Morris
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-11-17
Filing date: 2009-11-12
Publication date: 2010-05-20
Anticipated expiration: 2011-05-17
Also published as: GB0820901D0; WO2010055339A3

Abstract

A lamp unit (10) comprises a base (12), at least one LED (14), and an optical lens (16) mounted adjacent to the LED. The base includes a heat sink (20) for the LED. The optical lens is configured to provide a substantially spherically- uniform distribution of light around the optical lens, whereby the lamp unit can replace an incandescent lamp unit. The light fitting (42) comprises a lamp unit and a power supply for the lamp unit, the power supply being separate from the lamp unit. The method of making the optical lens for the lamp unit includes the step of printing a number of reflective elements onto a sheet of prismatic film whereby the optical lens will provide a substantially spherically uniform distribution of light from the LED.

Description

LAMP UNIT, LIGHT FITTING AND METHOD OF MAKING AN OPTICAL LENS FOR A LAMP UNIT

FIELD OF THE INVENTION

This invention relates to a lamp unit, to a light fitting incorporating the lamp unit, and to a method of making an optical lens for the lamp unit.

BACKGROUND TO THE INVENTION

Light fittings incorporating lamp units are in widespread use to provide illumination in private, commercial and industrial buildings. Traditionally lamp units have been incandescent and used a heated metal filament, but such lamp units are increasingly being replaced by more energy efficient alternatives.

The most-widely used alternative to an incandescent lamp unit is a fluorescent tube. Linear fluorescent tubes are significantly more efficient than incandescent lamps and have been in widespread use for many years. More recently, manufacturers have developed fluorescent tubes wound into a coil, and many of these fluorescent lamp units have a bayonet or screw fitting so that they can be used in place of a traditional incandescent lamp unit (or "bulb").

It has been recognised, however, that whilst fluorescent tubes are more energy efficient than incandescent lamp units, yet more efficient lamp units are available in the form of light emitting diodes (LEDs).

LEDs are particularly energy efficient because they can be manufactured to emit light only in the visible spectrum, i.e. little or none of the electrically energy is wasted in creating non-visible radiation.

One method of lighting a room is to use lamp units recessed into the ceiling, such lights commonly being called "downlights". Whilst it is possible to use an incandescent lamp unit in a downlight manufacturers have appreciated that it is possible to use an alternative lamp unit, and many downlights utilise halogen lamp units. Certain types of halogen lamp unit can be connected directly to the mains electricity supply, but others require a transformer to ensure that the lamp unit receives an appropriate voltage and current. In a downlight the transformer is typically located above the ceiling and therefore out of sight.

An advantage of halogen lamp units is their small size, it being a requirement of many downlights that they occupy a very small volume (and in particular a very shallow depth) within the ceiling.

Halogen lamp units are more energy efficient than incandescent lamp units, but are not as efficient as LEDs, and so it is known to replace a halogen lamp unit in a downlight with an LED. An LED lamp unit typically requires a dedicated power supply or driver to ensure that the lamp unit receives the appropriate voltage and current, and since the power supply differs from the transformer (if fitted) for the halogen lamp unit it is necessary to install the power supply for the LED lamp unit, typically above the ceiling.

Halogen lamp units provide a very concentrated light source which can appear harsh when viewed directly. A halogen downlight will typically utilise a reflector to concentrate the light beam in one direction and a diffuser so as to reduce the harshness or glare of the lamp unit, the reflector and the diffuser together acting to direct the light in a chosen angular direction. . LED lamp units also provide a very concentrated light source and therefore also appear harsh when viewed directly. Typically, an LED downlight will use a lens and/or a reflector to concentrate the light beam in one direction.

The presence of a reflector in a downlight makes this form of lighting highly directional. This is acceptable in a downlight since it is appropriate to direct the light downwardly onto the floor or other surface, but it is not appropriate for all lighting applications. Accordingly, few buildings are illuminated solely by downlights, and additional background or ambient lighting is utilised which is less directional. Many private and commercial buildings (such as public houses for example) therefore utilise both downlights and ambient lights in the form of wall light fittings and/or hanging light fittings, the ambient lights being fitted with incandescent lamp units or fluorescent coils which are better suited to ambient lighting. Often the lamp units will be illuminated almost continuously, i.e. some public houses are open more than twenty hours each day and the owners require that the premises be well lit during all of those hours.

It is a requirement upon the owner or operator of many commercial and industrial buildings (such as the licensee of a public house for example) that the lamp units be inspected regularly, for example once each day, and the cost of that inspection in terms of time, and the cost of replacing faulty lamp units, is considerable.

SUMMARY OF THE INVENTION

The inventor has set out to provide a new lamp unit which is suitable for use in a wall light fitting or hanging light fitting, for use as ambient lighting. The inventor also seeks to provide a light fitting providing long-term and sustainable energy savings.

According to the invention there is provided a lamp unit comprising a base, at least one LED, and an optical lens mounted adjacent to the LED, the base including a heat sink for the LED, the optical lens being configured to divert light emitted from the LED by way of reflection and/or refraction, and/or to absorb some of the light emitted from the LED, whereby to provide a substantially spherically uniform distribution of light around the optical lens.

By "spherically uniform distribution of light" is meant a substantially uniform distribution of light in all directions from the optical lens. The lamp unit therefore differs from the lamp unit used in a downlight in which the distribution of light is highly directional. The invented lamp unit therefore emits light with a distribution similar to that of an incandescent lamp unit or a fluorescent coil lamp unit. The inventor has realised that it is this uniform distribution of light that makes an incandescent lamp unit and a fluorescent coil lamp unit suitable for ambient or background lighting, and that by providing an optical lens which can distribute the light from an LED substantially uniformly the LED can be used for ambient lighting.

Alternatively stated, an optical lens which provides a spherically uniform distribution of light will appear uniformly bright across its surface, i.e. it will not appear brighter in one region than in another; accordingly, the light output from the lamp unit can be maximised whilst keeping the glare to a minimum.

Clearly, it is impossible in practice for a lamp unit to provide a totally spherically uniform distribution of light, and it is recognised that the lamp unit will not illuminate in certain directions, for example in the direction of the (opaque) base. Also, it can be arranged that in certain applications the distribution of light is deliberately not spherically uniform, for example the light may be directed solely or predominantly in a chosen direction. In applications in which the lamp unit is used to illuminate a picture for example it can be arranged that light is emitted only or predominantly in the direction of the picture. For the avoidance of doubt the term "substantially spherically uniform" will still be used herein to describe applications in which the distribution is skewed or deliberately made non-uniform to suit a particular requirement. Thus, the inventor has appreciated that the technically most difficult step is providing a substantially uniform distribution of light; once it becomes possible to provide a substantially uniform distribution of light it is easy to modify the optical lens to produce a desired non-uniform distribution.

The optical lens can divert light by way of refraction and/or reflection. In some embodiments, part of the optical lens is adapted to absorb some of the incident light; in such embodiments it is preferred that the part is only a small proportion of the optical lens so that the reduction in energy efficiency caused by the light absorption is minimised. It is recognised that the life of an LED lamp unit is dependent upon its working temperature, with a lower working temperature giving a longer life. The heat sink is provided to enable the LED to operate within the manufacturer's recommended working temperature range, and the performance of the heat sink can vary according to the working temperature.

It is recognised that the working temperature will depend upon the LED being used, with different LEDs, from different manufacturers, having different working temperatures. Also, the working temperatures of LEDs are generally increasing with the availability of new diodes. One presently-available LED operating at a working temperature of between 4O⁰C and 5O⁰C will have a rated working life in excess of 50,000 hours (equivalent to around 7 years at 20 hours per day).

Accordingly, the use of LEDs for ambient lighting offers not only a saving in terms of energy usage, but also a saving in terms of the regular inspection required in some buildings, and a saving in terms of the stockholding of replacement lamp units or bulbs. It is expected that in some commercial buildings such as public houses, installing the invented light fittings in place of the existing light fittings would avoid the requirement for regular inspection of the lamp units, and would also avoid the need to store replacement lamp units. The cost benefit of that

(added to the cost benefit of the energy saving) would be measurable, and the owner or operator could readily calculate the savings likely to be made and could offset the savings against the cost of installing the invented light fittings. In calculations undertaken by the inventor the cost of installing the invented light fitting could be recovered in savings within around 1 year in a commercial building such as a public house.

Desirably the LED produces white light, such LEDs being available from many manufacturers. The particular colour produced (often referred to as the "colour temperature") can vary depending upon the LED chosen. It is also possible to produce white light by using complementary LEDs, for example a separate red, blue and green LED in a single lamp unit. A suitable optical lens can be made from optical lighting film (OLF) manufactured by the Minnesota Mining and Manufacturing company (3M). The inventor modifies this film by applying a pattern of reflective elements. The reflective elements are preferably regularly, and in some embodiments substantially uniformly distributed over the optical lens and create a substantially uniform pattern of reflective and transmissive areas, depending upon the presence or absence of a reflective element. The reflective elements reflect incident light and their density and distribution is chosen to make the light distribution from the optical lens substantially spherically uniform.

Preferably the reflective elements are substantially circular dots. Preferably also the reflective elements occupy around 50% of the area of the optical lens. Preferably the reflective elements are white in colour, but alternatively can be silver or mirrored. Desirably the reflective elements are applied by a screen printing process.

The optical lens preferably has a substantially flat surface (to which the reflective elements are applied) and a non-flat surface, the non-flat surface comprising a series of substantially linear prisms. Ideally the axis of the prisms is arranged substantially parallel to the longitudinal axis of the lens.

The optical lens is preferably in the form of a tube having a closed end. In many embodiments the tube is less than 10 cm in length, and ideally around 6 cm in length. With such a lens the lamp unit can be of similar size to a conventional incandescent lamp unit, and can therefore fit within a conventional lamp shade if required.

The closed end of the tubular optical lens can be mirrored so that substantially no light passes out of the closed end. The lamp unit will therefore emit light only from the sides of the tubular optical lens. Alternatively, the closed end is part-mirrored or opalised, so that a chosen proportion of incident light passes out of the closed end of the lens. In another embodiment the optical lens is a moulded prismatic tube, with the prisms running around the circumference of the tube, inside the tube. The angle of the prisms may vary along the length of the lens so as to make the emission of light substantially uniform over the length of the lens.

Preferably, there are at least two LEDs in the lamp unit, and ideally there are six LEDs. The number of LEDs which are used depends upon the illumination required and the illumination provided by each LED. In one embodiment the LEDs each use 1.2 watts of power and six LEDs are required to provide approximately the same light output as a 40 watt incandescent lamp unit.

In lamp units utilising more than one LED the base preferably includes a printed circuit board to deliver the electrical voltage and current to the LEDs. Desirably the LEDs and mounted in series upon the printed circuit board.

There is also provided a light fitting including a lamp unit as herein defined. The light fitting preferably includes a power supply for the lamp unit.

Whilst it would be possible to make the lamp unit replaceable within the light fitting this is not preferable, and the inventor wishes to avoid the temptation to provide his lamp unit with a bayonet or screw fitting so as to be interchangeable with an incandescent or fluorescent coil lamp unit, notwithstanding that this would be possible. Thus, the lamp unit will be far more energy efficient than an incandescent lamp unit or a fluorescent coil lamp unit, but is expected to be considerably more expensive even when manufactured in large volumes. The inventor wishes to avoid the possibility that a user might wish to use an incandescent lamp unit or a fluorescent coil lamp unit with the light fitting and thereby avoid the long term energy savings of the LED lamp unit so as to save cost in the short term. By making it necessary to use an energy efficient lamp unit with the light fitting the light fitting can deliver a sustainable energy saving.

The inventor does, however, seek to make the lamp unit as compatible as possible with existing light fittings, by making the base of dimensions compatible with other light fittings so as to avoid the manufacturer of a light fitting having to retool to manufacture a new size or shape of light fitting. In this way the light fitting manufacturer can install the power supply and the lamp unit in an existing light fitting in place of the componentry which would otherwise be installed for a screw-fit or bayonet-fit lamp unit.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:

Fig.1 shows an exploded view of the components of a lamp unit according to the present invention;

Fig.2 shows a side view of a light fitting according to the invention;

Fig.3 shows a detailed view of the structure of the optical lens; and

Fig.4 shows an exploded view of the components of the lamp unit for use in a downlight.

DETAILED DESCRIPTION

The lamp unit 10 comprises a base 12, a number of LEDs 14, and an optical lens 16. The base 12 is metallic, and includes a heat sink 20.

In this embodiment there are four LEDs, mounted in series upon a printed circuit board 22. In other embodiments there are more or fewer than four LEDs depending upon the light output required, and the light output of each LED; in one particularly preferred embodiment there are six LEDs. The LEDs emit white light having a chosen colour temperature. Fig.1 does not show the electrical wires through which electrical current passes to and from the printed circuit board 22, but it will be understood that the wires are connected to the printed circuit board 22 and pass through the base 12, which has a longitudinal hole (not seen) provided for this purpose. In order to cooperate with a standard light fitting and avoid the light fitting manufacturer having to retool to utilise the lamp unit 10, the longitudinal hole is threaded to allow the lamp unit to be secured to the light fitting in typical fashion.

The base 12 comprises a tubular collar 24, a floor 26 and the heat sink 20. The printed circuit board 22 can fit into the collar 24, and in use will lie upon the floor 26 within the collar.

The optical lens 16 is tubular and is sized to fit into the collar 24, so that it too in use will lie within the collar 24. In this embodiment the optical lens lies upon the periphery of the printed circuit board 22 but in alternative embodiments the optical lens lies upon the floor 26 surrounding the printed circuit board.

The collar 24 in this embodiment is around 1 cm deep, so that around 1 cm of the optical lens lies within the collar 24. The optical lens 16 is 7cm in length so that in the assembled lamp unit around 6cm of the optical lens is visible. This is a similar dimension to many incandescent lamp units and the lamp unit 10 can therefore be used with the lamp shade of a conventional light fitting, if required.

The base 12 is preferably aluminium, and preferably also an integral unit made by casting or sintering, but it could be machined from solid or fabricated from separate parts if desired. Aluminium is a good thermal conductor and the structure of the base 12 ensures that heat generated by the LEDs 14 during use is absorbed into the base and dissipated through the radiating fins 30 of the heat sink 20.

The form of the heat sink 20, and the material from which the base 12 is made, can vary to suit the application, but it is desired that the base 12 be configured to enable the LEDs to operate continuously at the manufacturer's recommended working temperature (for example 4O⁰C to 5O⁰C).

It will be understood from Fig.2 that in use the base 12 is located within a part 40 of a light fitting 42, ideally a decorative part. The base 12 (including the heat sink

20) is configured to fit the part 40. If the light fitting 42 is newly-designed the part

40 and base 12 can be designed together, but the inventor wishes a light fitting manufacturer to be able to fit the lamp unit 10 without redesigning the light fitting

42, and so the base 12 has been designed to fit within the part 40 which is of a size and shape to accommodate a (standard) screw or bayonet fitting for a conventional lamp unit.

It will be understood that a standard screw or bayonet fitting allows the user to replace a faulty lamp unit with a new lamp unit, but such interchangeability is not provided with the invented lamp unit, i.e. the base 20 (and therefore the whole of the lamp unit 10) is designed to be substantially permanently mounted within the part 40.

In common with a conventional light fitting, the collar 24 is threaded and mounts a correspondingly-threaded shade ring 34, whereby a lamp shade (not shown) may be secured to the light fitting in known fashion.

The light fitting 42 of Fig.2 is a wall light fitting, having a wall-mounting part 44 which can be secured directly to the wall (not shown) in known fashion. The electrical wires will enter the light fitting through the wall-mounting part 44, the wall-mounting part 44 including electrical connectors for this purpose.

In order to make the wall light fitting 42 interchangeable with a light fitting for an incandescent or fluorescent coil lamp unit, it is necessary that the light fitting 42 include a power supply or driver for the lamp unit 10. In this embodiment the power supply is located in the wall-mounting part 44, although in other embodiments it may be located elsewhere in the light fitting. The power supply could, for example, be mounted upon the printed circuit board in a "driver on board" configuration.

The detailed structure of a practical optical lens 16 is shown in Fig.3. The optical lens is a tube with a substantially circular cross-section. One end 50 of the tube is open to accommodate the LEDs, the other end 52 is closed. In this embodiment the closed end 52 is opalised (for example cut from a sheet of opalescent material such as is widely available). Whilst it will be understood that some of the light energy is absorbed into the opalised material at the closed end, and therefore reduces the overall efficiency of the light, the inventor has found that it is easy to balance the light output from the closed end with the light output from the sides of the tube 16 so as to provide substantially uniform spherical distribution of emitted light.

In another embodiment the closed end 52 is mirrored so that no (or substantially no) light passes through the end 52.

The optical lens 16 is manufactured from a sheet of prismatic material having one substantially flat side 54 and one side 56 carrying a series of substantially linear prisms 60. In one suitable material the prisms 60 are substantially triangular in cross-section with a base width of between approximately 0.25 mm and 0.41 mm, and a height of between approximately 0.13 mm and 0.25 mm. The material is ideally approximately 0.38 - 0.64 mm thick.

Accordingly, in this embodiment the sheet of prismatic material forms the entire curved sidewall of the optical lens 16. In another embodiment (which is particularly simple to manufacture), a sheet of prismatic material is curved around the inside surface of a transparent tube, the transparent tube playing no part in light distribution but serving to support the prismatic material.

The prismatic material is initially substantially transparent, so that little or no light is absorbed into the material and wasted as heat. When wound into a tube with the prisms 60 lying parallel to the longitudinal axis of the optical lens as in the embodiment of Fig.3, the prisms 60 will cause light to be directed substantially uniformly around the lens 16, i.e. when viewed in horizontal cross section the distribution of light is substantially uniform around the lens. However, when viewed in vertical cross-section the distribution of light from such a lens would not be uniform, with some regions (for example close to the collar 24) being much brighter than others.

To distribute the light more evenly along the length of the optical lens 16, the flat surface 54 carries a pattern of reflective dots 62 (only some of the dots being shown in Fig.3). In this embodiment the dots 62 are white, and it has been found that white dots will reflect all or almost all of the incident light from the white-light

LEDs 14, so that little of the light energy is wasted. In another embodiment the dots are silver or mirrored, which may increase the proportion of incident light which is reflected.

The dots 62 reflect the incident light and therefore act to prevent the passage of light out of the lens 16 at the location of the dots. Thus, light can only leave the lens 16 if it is incident upon the areas of the lens between the dots 62. It has been found that with dots 62 occupying around 50% of the area of the circular wall of the lens 16, the light distribution along the length of the lens will be substantially uniform. With six LEDs emitting light equivalent to a 40 watt incandescent bulb, and with an optical lens around 6cm in length, the lamp unit 10 is suitable for ambient lighting, even without a lamp shade should this be desired.

Accordingly, in this embodiment the reflective dots 62 act to retain light within the optical lens 16, the light rays repeatedly reflecting within the lens until they are incident upon a transparent area of the lens 16. As the light rays pass through the transparent parts they are diverted by the prisms 60 so as to further distribute the light.

It has been found that with a relatively short optical lens such as that described (6 cm exposed above the collar 24) the pattern of substantially circular dots 62 can be substantially regular and uniform along the length of the optical lens, though a variable density along the lens may be utilised in some applications. In particular, it may be desirable to have the part of the optical lens which lies inside the collar 24 made totally reflective to avoid the transmission of light in that area. Also, in longer optical lenses it may be necessary or desirable to have a lesser density of dots adjacent to the LEDs than adjacent to the closed end, so as to make the distribution of emitted light more uniform along the length of the lens. In one practical embodiment of a large optical lens for the invention the dots cover approximately 3% of the surface area adjacent to the collar 24, increasing steadily to approximately 33% coverage adjacent to the closed end 52.

Also, in the embodiment shown the density of the dots 62 is circumferentially uniform around the optical lens 16, but in other embodiments the density is nonuniform so as to direct a larger proportion of light in one radial direction than another. If, for example, the lamp unit is used as a picture light, the reflective dots may be merged into a continuous reflector around a part of the optical lens, which part will face away from the picture in use. Also, for use in a wall-mounted light fitting, the reflective dots may be denser in the part of the optical lens which faces the wall than in the part of the lens which faces the room.

It will be understood that the light is not distributed only radially from the optical lens 16, and that some of the light rays emitted will be directed upwardly and others downwardly relative to the optical lens. Thus, just as an incandescent lamp unit distributes light in a substantially uniform spherical pattern, so can the present lamp unit.

In the embodiment of Fig.3 the reflective dots 62 are arranged on the inside surface of the optical lens 16, whilst the prisms are located on the outside surface of the lens. This arrangement can be reversed if desired, but the distribution of light has been found to be less uniform in the reversed arrangement. Also, the reflective dots can be printed onto the prisms, but this is a more complex printing operation and the distribution of light is expected to be less uniform. It will therefore be understood that in the embodiment of Fig.3 the optical lens 16 creates the chosen substantially uniform distribution of light by way of refraction (and some reflection) at the surfaces of the prism 60, and reflection at the reflective dots 62).

In other embodiments the distribution is created solely by prismatic surfaces upon the optical lens. The optical lens could in one embodiment be manufactured as a moulded tube with the prisms running around the inside surface of the tube, i.e. the axis of the prisms being substantially circumferential. The vertical and horizontal distribution of light can be determined by choosing the form of the prisms, and in particular the density and size of the prisms can vary along the length of the lens, and the angles of the prism faces can vary along the length of the lens. It is expected that a suitable optical lens could be created which would provide the required distribution without needing any reflective elements.

Fig.4 shows an alternative use of some of the components of the lamp unit 10. Thus, the inventor has sought to make the components of the lamp unit 10 suitable for use in a downlight also, so as to maximise economies of manufacturing.

As above indicated, the major difference of a downlight is that it is directional, and does not therefore utilise a tubular optical lens such as 16. Instead, it uses a flat lens 70 which is common to downlight applications. The lens 70 carries a number of substantially pyramidal prisms which act to distribute and soften the light somewhat.

The lens 70 is secured to the base 12 by an inwardly-directed lip (not seen) of the shade ring 34. It will be observed that the shade ring 34 has been inverted, so that the outwardly-directed lip 72 is uppermost as drawn (and will be lowermost in use as a downlight). The outwardly-directed lip 72 is sized to match the housing of a downlight, so that the lamp unit can fit the housing and bezel etc. of an existing downlight.

Claims

1. A lamp unit comprising a base, at least one LED, and an optical lens mounted adjacent to the LED, the base including a heat sink for the LED, the optical lens being configured to provide a substantially spherically uniform distribution of light around the optical lens.

2. A lamp unit according to claim 1 in which the optical lens comprises an optical lighting film.

3. A lamp unit according to claim 2 in which the optical lighting film carries a pattern of reflective elements.

4. A lamp unit according to claim 3 in which the reflective elements are regularly distributed over the optical lens.

5. A lamp unit according to claim 3 in which the reflective elements are substantially circular dots.

6. A lamp unit according to claim 3 in which the reflective elements are white in colour.

7. A lamp unit according to claim 1 in which the optical lens is in the form of a tube having a closed end.

8. A lamp unit according to claim 7 in which the optical lens has an inner surface and an outer surface, the outer surface comprising a series of substantially linear prisms.

9. A lamp unit according to claim 8 in which the axis of the prisms is arranged substantially parallel to the longitudinal axis of the tube.

10. A lamp unit according to claim 7 in which the closed end is mirrored.

11. A lamp unit according to claim 1 in which the optical lens is a moulded prismatic tube, with the prisms running around the circumference of the tube.

12. A lamp unit according to claim 10 in which the prisms are located inside the tube.

13. A lamp unit according to claim 11 in which the angle of the prisms varies along the tube.

14. A lamp unit according to claim 1 in which the base includes a printed circuit board carrying the LED(s).

15. A lamp unit according to claim 14 in which there is a plurality of LEDs mounted in series upon the printed circuit board.

16. A lamp unit according to claim 14 in which the printed circuit board carries a driver for the LED(s).

17. A light fitting comprising a lamp unit and a power supply for the lamp unit, the power supply being separate from the lamp unit, the lamp unit comprising a base, at least one LED, and an optical lens mounted adjacent to the LED, the base including a heat sink for the LED, the optical lens being configured to provide a substantially spherically uniform distribution of light around the optical lens.

18. A method of making an optical lens for a lamp unit comprising the steps of: providing a sheet of optical lighting film having a series of closely-spaced prisms, printing a number of reflective elements onto the sheet, bending the sheet into a tube, and fitting an end cap to an end of the tube.

19. The method according to claim 18 in which the sheet of optical lighting film has a first surface and a second surface, the series of closely-spaced prisms being arranged on the first surface, and the number of reflective elements being printed onto the second surface.