GB2412255A

GB2412255A - Testing light emitting elements in a light emitting device

Info

Publication number: GB2412255A
Application number: GB0405848A
Authority: GB
Inventors: Ian Harwood
Original assignee: Cooper Lighting and Security Ltd
Current assignee: Eaton Electrical Systems Ltd
Priority date: 2004-03-16
Filing date: 2004-03-16
Publication date: 2005-09-21
Anticipated expiration: 2024-03-16
Also published as: GB2412255B; GB0405848D0

Abstract

A light emitting device 12, incorporating a plurality of electronic light emitting elements 10a, 10b and at least one light sensor 14 wherein the light emitting device is operable to illuminate individual electronic light emitting elements separately, in order to determine the light output of the individual electronic light emitting elements. The light emitting device may be a sign.

Description

1 2412255 Light Emitting Device This invention relates to a light emitting

device and a monitoring system for a light emitting device, particularly, but not limited to, an exit sign monitoring system. The invention also relates to a method of monitoring a light emitting device.

There is a legal requirement to test emergency lighting including exit signs. Published standards describe acceptable test regimes, for example BS5266 ptl.

Traditionally the testing was carried out manually using a switch to disconnect the mains and visual inspection to confirm that the luminaires and signs were working correctly.

For many years automatic test systems have been used. The driver for such systems is generally to reduce the cost of manual testing but some responsible building owners select an automatic system to ensure that the testing is actually done.

Automatic test systems generally fall into two types. A first type is a self-contained type, where the whole testing system is contained in a luminaire and a local inspection is needed to ensure that the system indicators are displaying system healthy.

A second type is a central control type, where a control and indicating equipment is connected to monitoring interfaces such that the state of a whole building's emergency lighting system can be inspected by viewing a control panel. A connection from the panel to the interfaces can be via a dedicated communication cable, mains signalling, radio or shared communication network such as a EMS system or data communications network.

All automatic systems have a basic function of monitoring the battery charge and the emergency system operation together with simulating a failure of the primary mains supply. Other facilities are sometimes provided to confirm the presence of mains electricity, battery voltage monitoring, lamp presence monitoring etc. However the basic features allow the battery charge to be monitored regularly, the emergency luminaire to be tested for a short duration at regular intervals to check that it is functioning correctly and then for full duration (typically once per year) to check that the battery

capacity is in specification.

In the past the monitoring of the emergency function was typically either done optically - via a light sensor - or electrically by measuring the lamp current or the battery current. The latter is by far the most popular.

Electrical monitoring is valid for filament lamps because the lamp generally gives out a reasonably constant light over its life until the filament fails. Similarly fluorescent lamps tend to degrade slowly and then towards the end of their lives the lamp voltage rises until a lamp cathode fails and the lamp stops working. This can be monitored either by checking the lamp current or the battery current, which typically rises with the lamp voltage and hence power rises and then falls when the lamp fails.

Optical monitoring is achieved by mounting a light sensor such that it sees light from the lamp, but not from the environment.

In this way limits can be set which bear a reasonable relation to the correct operation of the system.

LED technology is advancing such that LEDs are now more efficient than tungsten lamps and under correct operating conditions can last longer than fluorescent lamps. They are easy to operate from low voltage and so are a very attractive alternative to the older light sources for emergency lighting.

LEDs are particularly suitable for the illumination of exit signs and it is this application to which this

specification has particular relevance.

It is an object of the invention to provide a monitoring system for a sign, in particular an LED-lit sign.

According to a first aspect of the present invention, a light emitting device incorporates a plurality of electronic light emitting elements and at least one light sensor, wherein the light emitting device is operable to illuminate individual electronic light emitting elements separately, in order to determine a light output of the individual electronic light emitting elements with the at least one light sensor.

Preferably, the electronic light emitting elements are LEDs.

The light emitting device may be a sign. Preferably, the sign is a sign adapted to display information, such as an exit sign or emergency sign, or an emergency exit sign.

The plurality of electronic light emitting elements is preferably adapted to illuminate information material of the sign. The light emitting device may be an emergency light.

Preferably, the light emitting device is operable to illuminate individual electronic light emitting elements sequentially, which sequence may be an overlapping sequence.

Preferably, the light emitting device is operable to illuminate individual electronic light emitting elements separately in a monitoring mode, preferably under the control of control means of the sign.

The light emitting device may be operable to vary a voltage or current supplied to one of the electronic light emitting elements, preferably based on an output of the light sensor, preferably to adjust a light output of the electronic light emitting element.

The light emitting device may be operable to take into account ambient light levels when determining a light output of individual electronic light emitting elements with the at least one sensor.

The light emitting device may be operable to determine said light output in synchronization with a mains electrical supply or a source of ambient light, such as a lighting unit, including a fluorescent lighting unit.

Preferably, the at least one light sensor is positioned to detect equal levels of light from the plurality of electronic light emitting elements, given equal light output. Compensation may be provided, preferably by control means, where the light output of one or more light emitting elements is less, due to its location, than others of the LEDs.

According to a second aspect of the invention a monitoring system for a light emitting device having a plurality of electronic light emitting elements comprises at least one light sensor and control means, wherein the monitoring system is operable to illuminate the plurality of electronic light emitting elements separately, to determine a light output of individual light emitting elements with the at least one light sensor and to control a light output of each of the plurality of electronic light emitting elements according to an output of the light sensor.

According to a third aspect of the invention a method of monitoring a light emitting device comprises checking the light output of a plurality of electronic light emitting elements separately with at least one light sensor.

The method may include varying the light output of one or more of the plurality of electronic light emitting elements, preferably based on the detected light output.

Preferably, the light emitting elements are illuminated sequentially, which sequence may be an overlapping sequence.

The invention extends to an emergency light in the form of a light emitting device according to the first aspect. The invention extends to an emergency lighting system comprising one or more light emitting elements of the first aspect.

All of the features described herein can be combined with any of the above aspects, in any combination.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which: Figures la and lb show front and side views respectively of an illuminated exit signs and Figures 2a to 2c show graphs of light output of LEDs of the exit sign in Figure la.

LEDs decrease in light output over their life but their electrical characteristics remain constant. Therefore if electrical means is used to verify emergency operation the test only validates that the control circuit is operating - not that the sign is performing its correct function.

If optical means are used then the monitoring can become complex and expensive if multiple LEDs are used to illuminate the sign. The reason being that it becomes very difficult to use a single light sensor to measure the light output from multiple LEDs and give a warning if one of them drops below its specified output, if a sensor is used per LED then the monitoring function can become as costly as the emergency function it is checking.

Using two high output side illuminating LEDs lOa and lOb and utilising the technology described in UK patent 2 253 301 an exit sign 12 has been designed that exceeds the light output requirements for internally illuminated signs in Europe. The sign 12 could be monitored using two light sensors, one for each LED. This application describes a method where monitoring can be accomplished using a single light sensor 14.

The sign 12 separately controls, with control circuitry 18, the current through each LED lOa, lOb, this is convenient from a current switching point of view and allows the light output to be controlled and compensated for degradation over the life of the product and during the discharge of a battery 16 that powers the sign 12. The compensation is for light output degradation and battery voltage changes.

The LEDs lOa, lOb can be switched on and off rapidly without degradation or even a user perceiving the switching frequency. If the light sensor 14 is located such that it sees approximately equal light from each LED lea, b (assuming equal light output from the LEDS) then by synchronizing the light monitoring with the LED switching it is possible to separately monitor the light output performance of each LED lea, lOb and even use the light output measurement to control the LED drive level via feedback, so that continuous adjustment of LED output can be made. The latter can be achieved by measuring the light level when both LEDs are off. By doing this it is possible to account for ambient light and effectively subtract it before applying the test criteria to establish whether the LEDs lOa, b are operating correctly.

A typical arrangement for this invention is shown in Figures la and lb, although alternative mounting arrangements could be used and yet still use the monitoring technique described herein.

Alternative numbers of LEDs could be used and more than one light sensor could be used, but the key issue is that each light sensor is arranged to see a multiple number of LEDs and that controls are incorporated to allow all but one LED to be turned off in sequence to allow the monitoring of each device, plus a background measurement.

Figures 2a to 2c show three graphs of light output on the y-axis against time on the x-axis. The first graph in Figure 2a shows the output for the first LED lea being switched on for a brief period. The second graph, in Figure 2b, shows the second LED lOb being switched on for a similar brief period that partially overlaps with the period in the first graph. The third graph in Figure 2c shows a sum of the light outputs detected at the sensor 14 and shows: an ambient light level before illumination of the first LED lea; then a first level for the first LED lOa; a second, combined, level of light for both the first and second LEDs lOa/b; a third level of light for the second LED lOb alone; returning to an ambient level. It can be appreciated from Figure 2c that the ambient light level can be registered first and re-checked for consistency at the end of a monitoring process. If either of the LED outputs is too low then changes can be made by the control system to increase light output, or if this is not possible, to flag an error requiring replacement of an LED lOa/b. As can be seen in Figure 2c the output of the first LED lOa is slightly lower than that of the second LED lOb; this would be catalogued and corrected for by control circuitry of the panel 12.

Use of the method of having no LED illuminated to get a level of ambient or external but direct light (even bright ambient light) level and then illuminating the LEDs so that they are illuminated singly at some point is particularly advantageous. The method is very accurate over a wide range of ambient light levels for providing a measurement of light output of one of the LEDs lOa/b.

As referred to above, if the ambient, or non-LED, light varies at a slow rate then measuring the LED lOa/b light output on a short timescale (for example milliseconds) will result in the ambient light variation having a negligible effect. In any event a change in ambient light over the period of checking is accounted for by averaging the change from the first and last sections of the graph in Figure 2c.

An alternative compensation for ambient light variation, for example in a situation where fluorescent light at a lOOHz flicker forms all or part of the ambient light, is to take a measurement with the light sensor 14 which is synchronized to the mains power supply i.e. taking a measurement at the same time point in the mains cycle.

Synchronising to the mains is done via simple mains detection or could be done by observation of the envelope of the detected light.

The monitoring method described herein has significant advantages in that it allows light output from multiple LEDs to be checked with a single light sensor. Although the description relates to LEDs used for signs, the above may be applied in relation to emergency lighting also.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features

disclosed in this specification (including any

accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims 1. A light emitting device incorporates a plurality of electronic

light emitting elements and at least one light sensor, wherein the light emitting device is operable to illuminate individual electronic light emitting elements separately, in order to determine a light output of the individual electronic light emitting elements with the at least one light sensor.
2. A light emitting device as claimed in claim 1, in which the electronic light emitting elements are LEDs.
3. A light emitting device as claimed in claim 1 or claim 2, in which the light emitting device is a sign.
4. A light emitting device as claimed in any preceding claim, which is operable to illuminate individual electronic light emitting elements sequentially, which sequence may be an overlapping sequence.
5. A light emitting device as claimed in any preceding claim, which is operable to illuminate individual electronic light emitting elements separately in a monitoring mode.
6. A light emitting device as claimed in any preceding claim, which is operable to vary a voltage or current supplied to one of the electronic light emitting elements, based on an output of the light sensor.
7. A light emitting device as claimed in any preceding claim, which is operable to take into account ambient light levels when determining a light output of individual electronic light emitting elements with the at least one sensor.
8. A light emitting device as claimed in any preceding claim, which is operable to determine said light output in synchronization with a mains electrical supply or a source of ambient light
9. A light emitting device as claimed in any preceding claim, in which the at least one light sensor is positioned to detect equal levels of light from the plurality of electronic light emitting elements, given equal light output.
10. A monitoring system for a light emitting device having a plurality of electronic light emitting elements comprises at least one light sensor and control means, wherein the monitoring system is operable to illuminate the plurality of electronic light emitting elements separately, to determine a light output of individual light emitting elements with the at least one light sensor and to control a light output of each of the plurality of electronic light emitting elements according to an output of the light sensor.
11. A method of monitoring a light emitting device comprises checking the light output of a plurality of electronic light emitting elements separately with at least one light sensor.
12. A method as claimed in claim 12, which includes varying the light output of one or more of the plurality of electronic light emitting elements, based on the detected light output.
13. A method as claimed in claim 11 or claim 12, in which the light emitting elements are illuminated sequentially.
14. An emergency light in the form of a light emitting device according to any one of claims 1 to 9.
15. An emergency lighting system comprising one or more light emitting elements according to any one of claims 1 to 9.
16. A light emitting device substantially as described herein with reference to the accompanying drawings.
17. A method of monitoring a light emitting device substantially as described herein with reference to the accompanying drawings.