GB2324598A

GB2324598A - LED display with shades to shield LEDs from external light

Info

Publication number: GB2324598A
Application number: GB9707992A
Authority: GB
Inventors: Michael Alexander Graham Clark; Mark Edward Pleydell
Original assignee: Microsense Systems Ltd
Current assignee: Microsense Systems Ltd
Priority date: 1997-04-21
Filing date: 1997-04-21
Publication date: 1998-10-28
Anticipated expiration: 2017-04-21
Also published as: GB2324598B; GB9707992D0

Abstract

A LED display 7 is provided, preferably for a traffic signal or a traffic light. The display 7 has a plurality of louvres 10, preferably inclined at an angle above the horizontal, which shield the LEDs 8 from phantom illumination by sunlight. At least the lower surface of the louvres 10 is reflective so that any light rays which are emitted from a LED 8 in an upward direction are deflected down below the horizontal to e.g. traffic at ground level. In an alternative arrangement a plurality of honeycombed shaped shading elements (11, Fig.6) may be used.

Description

A DISPLAY The present invention relates to a display, preferably for a traffic signal or a traffic light.

Known traffic lights comprise three separate traffic signals (red, yellow and green) and use a filament lamp as a light source. However, filament lamps suffer from a relatively short lifetime and are relatively inefficient.

Recently, light emitting diodes (LEDs) have begun to find favour as an alternative light source as they offer higher efficiencies and far longer service lives.

In addition to efficiency and service life, other important considerations in the design of traffic signals are the availability of different colours, the uniformity of the output across the emitting surface, the intensity distribution and the degree of immunity from phantom illumination.

LEDs are readily available in the main colours used for traffic signals i.e. red, yellow and green, and provided sufficient LEDs are used, an array of LEDs can provide a uniform output.

However, using LEDs as a light source for a display, such as a traffic signal, is problematic because light is emitted from the LEDs in all directions whereas the signal only needs to be directed to traffic which is usually at ground level. Therefore any light emitted in a direction above the horizontal is wasted.

Another problem suffered by traffic signals in general, and with LEDs in particular, is phantom illumination, namely where external light, e.g.

sunlight, is incident upon the traffic signal and is internally reflected and then emerges from the traffic signal such that the traffic signal appears to be illuminated when in fact no illumination is intended.

This presents obvious dangers if it is not clear whether a traffic signal is in fact illuminated. This problem is of particular concern with LED displays since LEDs are strongly reflective.

A known solution to the problem of phantom illumination in a filament lamp traffic signal is disclosed in GB-2194321. Louvres are provided in front of an array of asymmetric lens elements. The louvres act to block or re-direct external light so as to shade the traffic signal from phantom illumination by sunlight. The louvres, however, serve only to block external light from being internally reflected by the traffic signal and they have no effect upon light emerging from the traffic signal.

It is therefore desired to provide a LED display which overcomes the above-mentioned problems.

According to the present invention, there is provided a display comprising: an array of light emitting diodes; and a plurality of shades for shading each said light emitting diode from external light, each said shade having in use an upper and a lower surface, wherein at least said lower surface is substantially reflective.

The present invention provides a solution to both the problem of light being wasted by being emitted above the horizontal and of phantom illumination by providing shades which downwardly deflect any light which is initially emitted upwardly and which at the same time shade the LEDs from external light thereby substantially preventing phantom illumination.

The present invention also has the advantage of avoiding using a complex convex-concave lens array which is required in known traffic signals with a filament lamp.

Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 shows a conventional filament lamp traffic signal; Fig. 2 shows a front view of a LED traffic signal according to an embodiment of the invention; Fig. 3a shows a side view of a LED traffic signal according to an embodiment of the invention; Fig. 3b shows a side view of a preferred embodiment of the invention with louvres inclined to the horizontal; Fig. 4 shows a side view of a less preferred embodiment which employs a honeycombed shaped shade to shield each individual LED from phantom illumination; Fig. 5a shows a side view of the less preferred embodiment with the honeycombed shade removed; Fig. 5b shows a side view of the less preferred embodiment with the honeycombed shade present; and Fig. 6 shows a perspective view of the less preferred embodiment.

Fig. 1 shows a conventional traffic signal 1 with a filament lamp light source 2, such as a tungsten halogen lamp. The filament lamp light source 2 is fixed at or near the focus of a parabolic mirror 3 so that a substantially parallel light beam is formed which is then incident on a plurality of lens elements formed into a lens array 4. Louvres 5 are placed the other side of the lens array 4 to prevent external light from reaching the lens array 4 thereby preventing phantom illumination. The louvres 5 may be reflective to redirect unwanted sunlight incident upon the traffic signal 1, but the louvres 5 do not reflect or otherwise deflect light emitted from the light source 2. An outer protective screen 6 is provided at the front of the traffic signal to protect the traffic signal 1.

Figs. 2 and 3a show a LED traffic signal 7 according to an embodiment of the invention. A plurality of LEDs 8 are mounted on a board 9 which is preferably blackened, with the LEDs 8 being arranged in an array or in an otherwise generally ordered fashion.

The array may preferably be arranged to be generally circularly shaped. A plurality of louvres 10 or shading elements are arranged, preferably in generally horizontal rows. The individual LEDs 8 are preferably arranged so that they are closer to the lower surface of a louvre 10 which is arranged above the LED 8 than the upper surface of a louvre 10 which is arranged below the LED 8.

The louvres 10 are reflective at least on the bottom or lower surface and preferably blackened on their upper surface. However, in a less preferred embodiment the louvres 10 may be reflective on both their upper and lower surfaces. The relative extent of the louvre 10 dictates the degree of protection against phantom illumination. Increasing the extent of the louvre 10 gives better phantom protection but reduces the angular spread of the light output.

As shown in Fig. 3b, LEDs 8 may be thought of as comprising a generally cylindrical shaped base portion 16 capped with a domed shaped emitting portion 17. A small (- 1-2%) fraction of the light emitted from the LED 8 may "leak" from the cylindrical portion 16.

However, this is not of a general concern and the effect may either be ignored or the louvres 10 or some other blocking means may optionally extend so as to block any light leaking from the cylindrical surface 17 of the LED 8. Any reference to the emitting region of a LED 8 is taken to mean the front domed shaped portion 17 of an LED 8 and not the cylindrical portion 16.

For convenience of assembly the louvres 10 may extend down to the board 9 onto or into which the LEDs 8 are mounted, although this is not necessary.

Preferably the louvres 10 have a vertical spacing of approximately 10 mm, the emitting region 17 of the LED 8 is approximately 5mm in diameter and the louvres 10 are mounted approximately 6 mm from the LED mounting board 9.

The louvres 10 preferably extend between 15-35 mm forward of the base of the emitting area or region 17 of the LEDs 8. Further preferably, the louvres 10 extend 20-30 mm. An extension of 25 mm is particularly preferred. The above mentioned extensions of the louvres 10 are given with respect to the preferred vertical spacing of approximately lOmm and with respect to the preferred LED diameter of approximately 5mm.

The ratio of the louvre height or extension to that of the LED diameter is preferably between 3:1 to 7:1.

Further preferably, the ratio is between 4:1 to 6:1 and a ratio of 5:1 is particularly preferred.

The ratio of the louvre height or extension to that of the louvre spacing is preferably between 1.5:1 to 3.5:1. Further preferably, the ratio is between 2:1 and 3:1 and a ratio of 2.5:1 is particularly preferred.

If the louvres 10 are too short then they afford reduced phantom protection at low angles of incident light, and the reflective side of the louvre 10 may not extend far enough so as to be able to reflect downwardly a sufficient proportion of the upwardly emitted light.

If the louvres 10 extend too far then the downwardly emitted light from a LED 8 may be cut off by the next louvre 10 down.

Fig. 3b shows a preferred embodiment of the invention in which the louvres 10 are inclined at an angle a measured above the horizontal. The angle of inclination a may be varied so as to vary the amount of light reflected from the underside of the louvre 10.

The angle a is preferably between +10 to -5 (i.e.

between 10" above the horizontal and 5 below the horizontal) . Further preferably, the angle a is between 0 to +5 , and an angle of +2.5 is particularly preferred.

Instead of (or in addition to) using louvres 10 to shade the LEDs 8, in a less preferred embodiment, a plurality of different shaped shades may be used. The shades may, for example, have a cellular or partially cellular structure. For example, each individual LED 8 may be shielded by a shade having a generally circular, a generally semi-circular, a generally rectangular, a generally semi-rectangular, a generally hexagonal or a generally semi-hexagonal cross section.

Figs. 4-6 show an individual LED 8 mounted on a board 9 with a generally hexagonal or honeycombed shade 11. At least the uppermost surface 12 of the shade 11 (measured with respect to the rays emitted from LED 8) is generally or substantially reflective and preferably at least the lowermost surface 13 is generally or substantially non-reflective.

Preferably, at least part of the two surfaces 14 adjoining the uppermost surface 12 are also generally or substantially reflective and further preferably, at least part of the two surfaces 15 adjoining the lowermost surface 13 are also generally or substantially non-reflective.

The reflective cell wall(s) 12,14 reflect any upwardly emitted light rays from the LED 8 back into a downward path as shown in Fig. 5b. Since the LED 8 is preferably displaced so as to be closer to the uppermost reflecting surface 12 of the shade 11 than the lowermost surface 13 (with respect to the LED 8), any downwardly emitted light rays are preferably not deflected or otherwise substantially hindered by the shade 11.

As in the preferred embodiment shown in Fig. 3b which uses louvres 10 inclined to the horizontal, the cells may also be inclined to the horizontal within the same ranges and at the same preferred angles as previously mentioned.

The arrangement of LEDs 8 and louvres/shades 10,11 may preferably be protected from the ambient environment by a window which may optionally have some optical power and which serves as a barrier to the ingress and contamination of the louvres/shades 10,11 and LEDs 8.

The window or protective covering may be slightly domed outwards to assist in the control of stray light reflections.

A traffic light comprising, for example three individual displays or traffic signals e.g. red, yellow and green may be provided. A single display may also be used in a traffic signal for directing traffic, e.g. a green arrow for filtering traffic.

Although reference has been made throughout to using the display in traffic signals and traffic lights, it is understood that the display may also be used in conjunction with other means of transport such as railways, airports and shipping and that the display may be used for displaying information in any form, e.g.

departure times, general information and advertising.

Claims

1. A display comprising: an array of light emitting diodes; and a plurality of shades for shading each said light emitting diode from external light, each said shade having in use an upper and a lower surface, wherein at least said lower surface is substantially reflective.

2. A display as claimed in claim 1, wherein said upper surface is substantially reflective.

3. A display as claimed in claim 1, wherein said upper surface is substantially non-reflective.

4. A display as claimed in any preceding claim, wherein at least some of said plurality of shades are arranged in rows.

5. A display as claimed in claim 4, wherein at least some of said plurality of shades arranged in rows are inclined above or below the horizontal.

6. A display as claimed in claim 5, wherein said shades are inclined at an angle within the range 10 above the horizontal to 5 below the horizontal.

7. A display as claimed in claim 5 or 6, wherein said shades are inclined at an angle within the range 0-5" above the horizontal.

8. A display as claimed in claim 5, 6 or 7, wherein said shades are inclined at an angle of 2.5 above the horizontal.

9. A display as claimed in any preceding claim, wherein at least some of said plurality of shades have a cellular or partially cellular structure.

10. A display as claimed in any preceding claim, wherein at least some of said plurality of shades have a generally circular or semi-circular cross section.

11. A display as claimed in any of claims 1-8, wherein at least some of said plurality of shades have a generally rectangular or semi-rectangular cross section.

12. A display as claimed in any of claims 1-8, wherein at least some of said plurality of shades have a generally hexagonal or semi-hexagonal cross section.

13. A display as claimed in any preceding claim, wherein in use each said light emitting diode is disposed above a mid-way point between two adjacent shades.

14. A display as claimed in any preceding claim, having a ratio y/x in the range of 1.5:1 to 3.5:1, wherein y is the extension of a shade for shading said light emitting diode measured with respect to an emitting region of said light emitting diode to the distal end of said shade and x is the spacing between adjacent shades.

15. A display as claimed in claim 14, wherein said ratio y/x is in the range of 2:1 to 3:1.

16. A display as claimed in claim 14 or 15, wherein said ratio y/x is 2.5:1.

17. A display as claimed in any of claims 1-13, having a ratio y/z in the range of 3:1 to 7:1, wherein y is the extension of a shade for shading said light emitting diode measured with respect to an emitting region of said light emitting diode to the distal end of said shade and z is the diameter of a light emitting diode.

18. A display as claimed in claim 17, wherein said ratio y/z is in the range 4:1 to 6:1.

19. A display as claimed in claim 17 or 18, wherein said ratio y/z is 5:1.

20. A display as claimed in any preceding claim, wherein said display is used in a traffic light or signal.

21. A traffic light or signal comprising at least one display as claimed in any preceding claim.

22. A display, substantially as hereinbefore described with reference to Figs. 2-6 of the accompanying drawings.

23. A traffic light or signal, substantially as hereinbefore described with reference to Figs. 2-6 of the accompanying drawings.