GB2449987A - Dimmable LED light fitting having occupancy sensor - Google Patents

Abstract

A light fitting 10 is powered from live L and neutral N mains supply lines including an override switch 32. The fitting comprises an array of LEDs 34, the output of which can be varied by a control circuit 36 including a timer 40, an occupancy sensor e.g. PIR 38, a photometer 42 monitoring ambient light level, and a trimpot potentiometer 44. The light fitting is turned on to maximum output only when a person is detected in the space to be illuminated and the ambient light level is below that. If no person is detected for a period determined by the timer, the output of the light fitting is reduced to a lower level, which can be adjusted by means of the potentiometer. If the ambient light level is above the lower level, the light fitting is turned off.

Description

LIGHT FITTING

This invention relates to light fithngs, and more particularly to the automatic control of light fittings.

It is commonplace for light fittings to switched on automatically, for instance in response to the detection of a person in the vicinity by a passive infra-red detector (PIR) or the like. Typically such light fittings are switched off automatically some time after the person ceases to be detected. However there are very many situations where it is undesirable for the light to be extinguished altogether. One example is a stairwell in a multi-occupancy building, where it is advisable to provide not less than a specified minimum level of illumination at all times, for safety reasons. Other reasons for providing not less than a minimum level of illumination are to enable persons entering a space to see that it is clear in advance, rather than having to enter in darkness, and to enable monitoring by closed circuit television (CCIV) and to ensure that at least some illumination will be provided even if the lighting control malfunctions. In such situations the light could be kept on full power all the time, but this would be both costly and environmentally unacceptable.

Alternatively the light could be kept at a low level all the time, but this would be generally uncomfortable for people and unsatisfactory for those needing to work in the illuminated space.

It is known from United States Patent US 6 275 163 for the output of an automatically switched light to be adjusted between a minimum and a maximum brightness level. In the arrangement of US 6 275 163, when a sensor detects a person in a prescribed area, the light is automatically turned on at the minimum level, and a dimmer then automatically brings the light up to its maximum level. The light remains at its maximum level until the sensor ceases to detect a person in the area, when the dimmer takes the light back down to its minimum level, and thereafter the light is turned off. The level of brightness may be adjusted, by means of a preset control or manually during use by means of a key, but there is no provision in US 6 275 163 for the light to stay at not less than its minimum level.

Fluorescent lighting units are very commonly used and are generally robust and able to provide an acceptable level of illumination. However, particularly in large-scale industrial and commercial applications the replacement of tubes and starters and other maintenance tasks becomes an expensive overhead.

An example of this is in relation to illuminated signs of the kind (commonly known as flexi-front' or flexi-face') having a vinyl or similar printed sheet stretched across the front of a light box to form a screen which in use is backlit. Conventional flexi-front signs have heretofore used fluorescent tubes as a light source, typically switched on automatically when darkness falls. To maintain these fluorescent light fittings is troublesome and time-consuming. A lift or hoist is usually needed to access the sign, the screen has to be removed before any replacement of tubes, starters or control circuits etc can be carried out, and then the screen to be relocated and stretched across the front of the box again. A large sign, say 4m long by im high, may require as many as 15 fluorescent tubes, usually arranged in banks, so to keep the sign in good order requires a regular maintenance programme, failing which individual tubes or even banks of them will cease to operate, and the sign will not be properly illuminated. For commercial premises, especially, this may result in loss of business or, at least, damaged public perception of the enterprise.

It is an object of the present invention to provide a light fitting (such as but not limited to a flexi-front sign) with reduced maintenance requirements.

Thus according to the invention there is provided a light fitting for illuminating a space, which light fitting comprises a plurality of LEDs and a control circuit operative to control output of the LEDs, characterjsed in that the control circuit is sensitive to an independent variable and automatically controls the output of the LEDs in relation thereto.

The use of LEDs with automatic control provides a synergy with significant advantages over the use of fluorescent tubes as was conventional heretofore, for two reasons. First, LEDs have a generally longer life than fluorescent tubes. And second, the life of a fluorescent tube decreases with "burn time" -that is, with the length of time it is on for each start -so a fluorescent tube may last for 34,000 hours if it is left on continuously, but if it is left on for only three hours its life may fall to 20,000 hours. By contrast, LEDs may have a life of 50,000 hours and this is not affected by burn time. It follows that in automatically controlled applications, where burn time may be short and/or unpredictable, the use of LEDs as in the present invention offers substantial advantages.

The independent variable to which the control circuit is sensible may be the presence or absence of a person in said space, the control circuit being operative to set the output of the LEDs to a high value when a said person is present in said space. In addition the control circuit may be operative to set the output of the LEDs to a low but non-zero value when no person is present in said space. Those skilled in the art will appreciate that the high output, which will generally be the maximum output of the LEDs, will be determined by the needs of persons using the illuminated space. For a typical work area this may be 500 lux. The lower output may be determined from health and safety considerations. Illumination as low as say 20 ux may be sufficient to show up trip hazards, but persons entering the space may feel much more comfortable with rather higher levels of illumination, and preferably the lower output is not less than 10% of said high output.

The control circuit may be operative to light most or all of the LEDs when a said person is present and to light fewer of the LEDs when a said person is not present. Alternatively the control circuit may operative to set most or all of the LEDs to a high output when a said person is present and to set most or all of the LEDs to a low output when a said person is not present.

The control circuit may include a PIR, preferably responsive around 3600 within said space, for detecting a said person in said space. And particularly for use in cold stores and other refrigerated works spaces the PIR is preferably operative below -18 C.

The control circuit may be adjustable, eg by means of a trim-pot, to set the low value of the light output.

In another application such as illuminated signs, the independent variable to which the control circuit is sensitive may be ambient illumination, the control circuit being operative to set the output of the LEDs to a high value when the ambient illumination is below a threshold level.

In such application, the control circuit may reduce output of the LEDs to zero if the ambient illumination is above said threshold level, and the control circuit may include a timer whereby the output of the LEDs is reduced from its high value after a predetermined period.

A light fitting according to the invention may comprise a reflector carrying the LEDs in an array, preferably covered by a cover such as a diffuser secured to the reflector. There may be a seal between the reflector and the cover configured and arranged so that the light fitting conforms to the 1P65 standard of the Comité Européen de Normalisation Electrotechnique (CENELEC), which specifies that the fitting must be dust-tight and protected against water jets. The control circuit may be carried by the reflector, on a face thereof opposite from the LEDs.

The light fitting may be generally circular, or generally rectangular, to conform to the configuration of regular fluorescent fillings.

The invention extends to the use of a light fitting as aforesaid in an illuminated sign (such as a flexi-front sign) comprising a casing having a translucent front panel illuminated from behind by the light fitting. There may be a plurality of said light fittings within said casing each configured and arranged to illuminate said front panel from behind, and a single control circuit may control the plurality of fittings.

The invention will now be described by way of example only with reference to the accompanying schematic drawings, in which -Figure 1 shows a lengthwise cross section of a light fitting embodying the invention and including a cover; Figure 2 is a plan view from below of the light fitting of Figure 1 with it cover removed to show its reflector; Figure 3 is an enlarged transverse cross section of the reflector at at A-A of Figure 2; Figure 4 shows a schematic block diagram of the light filling of Figures 1 to 3; Figure 5 is a flow diagram illustrating the operation of the light filling of Figures 1 to 4; Figure 6 shows in front elevation an illuminated sign to which the invention is applied, which sign has a flexi-front screen backlit in use; and Figure 7 is a view corresponding to Figure 6 but with the flexi-front screen of the sign removed; Referring first to Figures 1 to 3, the light fitting 10 shown therein comprises a box 12 which may be secured to a ceiling by means of fixings 12, to illuminate a space such as a work area. Secured to the box 12 is a reflector 14 and a light-diffusing cover 16. The reflector 14 carries an array 18 of LEDs facing downwards in use to illuminate the space below the light fitting 10. As can be seen from Figure 3, the reflector 14 comprises a flat base carrying the LEDs 18 with inclined wings each side to direct light from the LEDs outwardly.

A gasket 20 provides sealing engagement between the box 12 and the cover 16, inconformity with CENELEC 1P65 standard. This is an important feature in cold stores and the like which it is necessary to clean by disinfectant sprays and/or hosing at prescribed intervals.

Within the box 12 and on the opposite face of the reflector 14 relative to the LED array 18 is a driver 20 for the LEDs 18, being part of a control circuit (not detailed, but which may be of any appropriate form). The control circuit also includes a sensor 22 responsive to the presence or absence of a person. The sensor 22 is carried on the diffuser 16.

The control circuit is so arranged that when a person is detected by the sensor 22, all the LEDs 18 are switched on by the driver 20, to provide full illumination from the light fitting. When nobody is detected by the sensor 22, only some of the LEDs 18 are switched on, to provide a low level of illumination in the work space. A timer, not shown, in the control circuit determines when these remaining LEDs are switched off.

It will be understood that the light fitting 10 may include other numbers of LEDs than those shown, and it may be arranged to switch different numbers of LEDs on and off according to need. Alternatively the control circuit may be arranged to vary the output of all the LEDs 18.

Referring now to Figure 4, the light fitting is indicated in broken lines at 10. The light fitting 10 is powered from live L and neutral N mains supply lines, the live L line including a switch 32 providing override control. The light fitting 10 comprises an array 34 of LEDs the light outputs of which are varied by means of a control circuit 36 of any convenient form. The control circuit 36 provides three stages of control as follows.

The first control stage comprises a first sensor in the form of a PIR 38 arranged on the surface of the light fitting 10 to provide 3600 coverage of the space below the fitting 10 that is to be illuminated thereby. The PIR 38 is operatively associated with a timer 40. The second control stage comprises a second sensor in the form of a photometer 42 arranged (away from the LED array 34) to monitor ambient light level. The third control stage comprises a trimpot indicated at 44.

The operation of the light fitting 10 will now be described (assuming throughout that the override switch 12 is closed).

When the PIR 38 detects a person in the space below the light fitting it causes the LED array 34 to be turned on to full power. When the PIR 38 ceases to detect a person, the timer 40 maintains full power for a predetermined period (say 5 mm) and then the control circuit 36 causes the output of the LED array 34 to be reduced. This lower output is adjustable by means of the trimpot 44 and (unless and until another person is detected by the PIR 38) it is delivered as long as the photometer 42 determines that the ambient light level is below a predetermined threshold such that illumination is required from the light fitting 10. However, if the photometer 42 determines that the ambient light level is above the predetermined threshold, being a level adequate for health and safety purposes, then the light fitting 10 is turned off automatically. If a person is then detected by the PIR 38, the light fitting 10 is turned back on, to its maximum output, unless of course the photometer 42 determines that the ambient light level is above that value, in which case it is not necessary for the light fitting 10 to be turned on.

The flow diagram of Figure 5 illustrates the operation of the lighting control in a typical installation. The light fitting 10 is turned on to a maximum output, at which it provides at least 500 lux illumination, only when (a) a person is detected in the space to be illuminated and (b) the ambient light level is below 500 lux. If no person is detected, the output of the light fitting is reduced to a lower output. This lower output is adjustable by means of the trimpot 44 (Figure 4): it may for instance be set at 50 lux, which is enough for health and safety purposes and also perceived as comfortable for persons about to enter the space. If but only if the ambient light level in the space is above that provided by the light fitting at its lower output, the light fitting can be turned off. The result is to maintain adequate lighting at all times whilst substantially reducing energy consumption. It is to be understood that the lower light output can be adjusted.

Whilst not detailed in the drawings, those skilled in the science will appreciate that the invention may be embodied as a luminaire light fitting for use in staircases and corridors of hotels, sheltered accommodation etc, where at least a minimum level of lighting is required all the time. For stock rooms, plant rooms etc, the invention may be embodied as a baton-style fitting, individually controlled so that only those in the vicinity of a worker are turned on to full output. For freezer rooms, outdoor use or other applications in potentially wet environments, a light fitting according to the invention may be made to conform to lP65 standard.

It should also be noted that the invention provides for a self-contained light fitting with the lamp(s) and all controls in a single unit, and tests indicate that the PIR can be located behind the light diffuser or similar covering.

Those skilled in the art will now appreciate that the invention may readily be applied to an illuminated sign such as a flexi-front sign, as will now be described with reference to Figures 6 and 7. Referring therefore to Figures 6 and 7, these show an illuminated sign comprising a casing 50 which is open at the front and a printed screen 52 disposed across the front of the casing to be backlit by light fittings 10 within the casing 50. The casing is formed of galvanised steel, but may be of another material such as aluminium alloy or synthetic plastics material. The screen 52 is a flexi-front screen, which is to say a vinyl or similar sheet stretched across the front of the casing 50, but it is to be understood that the invention is not necessarily limited to the use of flexi-front screens. For illustrative purposes, the screen 52 is shown to be printed with the name of a company, but it is to be understood that it may carry other material such as directions, warnings or advertising.

Conventionally, signs such as that shown in Figures 6 and 7 have heretofore used fluorescent lights to illuminate the screen 12. The present invention, however, uses LED light fittings instead. Thus, referring to Figure 7, the casing 50 contains a plurality of light fittings 10 each including an array 18 of LEDs which in use project light generally towards the front of the casing 50, thereby to illuminate the screen 52.

It will be noted that the combination of casing 50 and screen 52 provide an enclosure for the light fittings 10 such that the light fittings 10 do not need individual covers over the LED arrays 18.

It is also to be noted that the plurality of light fittings 10 are controlled by a single control circuit 54. This avoids the possibility of individual light fittings switching on and off at different times, which would adversely affect the appearance of the sign. The control circuit 54 includes a photometer 56.

When darkness falls or the ambient light level otherwise falls below a set threshold, the photometer 56 detects this and the control circuit 54 turns on the LED arrays 18 of each light fitting, whereby the sign becomes illuminated. A timer 58, which is optional, causes the control circuit 54 to turn the LED arrays 18 off again after a set period of time, to save power during the night.

Those skilled in the science will now appreciate that the illuminated sign shown in the drawing is cheaper to maintain than a conventional fluorescent-powered sign, because the LEDs used have a longer life than fluorescent tubes, particularly with relative short burn times of say a few hours each.

Claims (26)

1 A light fitting for illuminating a space, which light fitting comprises a plurality of LEDs and a control circuit operative to control output of the LEDs, characterised in that the control circuit is sensitive to an independent variable and automatically controls the output of the LEDs in relation thereto.

2 A light fitting as claimed in Claim 1 characterised in that said independent variable is the presence or absence of a person in said space and the control circuit is operative to set the output of the LEDs to a high value when a said person is present in said space.

3 A light fitting as claimed in Claim 2 characterised in that the control circuit is operative to set the output of the LEDs to a low but non-zero value when no person is present in said space.

4 A light fitting as claimed in Claim 3 characterised in that the control circuit is operative to light most or all of the LEDs when a said person is present and to light fewer of the LEDs when a said person is not present.

A light fitting as claimed in Claim 3 characterised in that the control circuit is operative to set most or all of the LEDs to a high output when a said person is present and to set most or all of the LEDs to a low output when a said person is not present.

6 A light fitting as claimed in any preceding claim characterised in that the control circuit includes a PIR for detecting a said person in said space.

7 A light fitting as claimed in Claim 6 characterised in that the PIR is responsive around 3600 within said space.

8 A light fitting as claimed in Claim 6 or Claim 7 characterised in that the PIR is operative below -18 C.

9 A light fitting as claimed in any of Claims 3 to 8 characterised in that said low value is not less than 10% of said high value.

A light fitting as claimed in any of Claims 3 to 9 characterised in that the control circuit is adjustable to set said low value.

11 A light fitting as claimed in Claim 10 characterised in that the control circuit includes a potentiometer manually adjustable to set said low value.

12 A light fitting as claimed in any preceding claim characterised in that said independent variable is ambient illumination and the control circuit is operative to set the output of the LEDs to a high value when the ambient illumination is below a threshold level.

13 A light fitting as claimed in Claim 12 characterised in that the control circuit reduces output of the LEDs to zero if the ambient illumination is above said threshold level.

14 A light fitting as claimed in any preceding claim characterised in that the control circuit includes a timer whereby the output of the LEDs is reduced from its high value after a predetermined period.

A light fitting as claimed in any preceding claim characterised in that said fitting comprises a reflector carrying said LEDs in an array.

16 A light fitting as claimed in Claim 15 characterised in that the LEDs are covered by a cover secured to the reflector.

17 A light fitting as claimed in Claim 16 characterised in that there is a seal between the reflector and the cover configured and arranged so that the light fitting conforms to CENELEC lP65 standard.

18 A light fitting as claimed in Claim 16 or Claim 17 characterised in that the cover comprises a diffuser.

19 A light fitting as claimed in any of Claims 15 to 18 characterised in that the control circuit is carried by the reflector, on a face thereof opposite from the LEDs.

A light fitting as claimed in any of Claims 15 to 19 characterised in that the fitting is generally circular.

21 A light fitting as claimed in any of Claims 15 to 19 characterised in that the fitting is generally rectangular.

22 A light fitting substantially as hereinbefore described with reference to and as shown in Figures 1 to 5 of the accompanying drawings.

23 An illuminated sign comprising a casing having a translucent front panel and, within the casing, a light fitting as claimed in any preceding claim configured and arranged to illuminate said front panel from behind.

24 An illuminated sign as claimed in Claim 23 characterised in that there is a plurality of said light fittings within said casing each configured and arranged to illuminate said front panel from behind.

An illuminated sign as claimed in Claim 24 characterised in that a single said control circuit controls said plurality of light fittings.

26 An illuminated sign substantially as hereinbefore described with reference to and as shown in Figures 6 and 7 of the accompanying drawings.