WO2022144320A1

WO2022144320A1 - An optical wireless communication system

Info

Publication number: WO2022144320A1
Application number: PCT/EP2021/087630
Authority: WO
Inventors: Matthias Wendt; Andreas Felix Alfred BLUSCHKE
Original assignee: Signify Holding B.V.
Priority date: 2021-01-04
Filing date: 2021-12-24
Publication date: 2022-07-07

Abstract

A luminaire comprises a carrier board (40) with a first set of LEDs providing a main visible light source and a first, mains powered, driver for driving the first set of LEDs(42). A second set of LEDs (46) is for an emergency light source and has a second driver. There is separate wiring (45) between the first set of LEDs and the first driver and the second set of LEDs and the second driver. The second driver comprises a transmitter with a 5modulator for modulating the current to the second set of LEDs to provide optical wireless transmission. Thus the emergency lighting part of a luminaire is used in addition for the optical communication.

Description

An optical wireless communication system

FIELD OF THE INVENTION

This invention relates to optical wireless communication systems.

BACKGROUND OF THE INVENTION

LiFi (Light Fidelity) is a new type of Optical Wireless Communication (OWC), which also includes Visible Light Communication (VLC). OWC (and hence LiFi and VLC) use light as a media of communication, for replacing cable wire (wireline) communication. In this context visible light may be light that has a wavelength in the range 380nm to 740nm; and infrared (IR) light may be light that has a wavelength in the range 740nm to 1.5mm. There may be some overlap between these ranges.

Light based communication offers the ability for high data rate communication, for example even exceeding 10 Gbit/s, for devices having a line of sight between them. This for example applies to a set of communicating devices within an office environment.

Known LiFi products rely on a grid of optical access points mounted in the ceiling. The beams of these access points are wide enough (and thereby have a large field of view and/or coverage area) to create an overlap with the neighboring access points at the level of the desks beneath. The receiving devices in such a system are typically located at the desks or are being handheld at a height close thereto.

For ease of installation, the grid of access points is for example aligned with the luminaire grid in the ceiling. Each access point in such an installation must reach (illuminate, in the case of visible light) several square meters and hence illuminates a significant conical area. Such installations may utilize illumination light for the downlink (towards the dongles and/or mobile devices) and may use infrared light for the uplink (towards the access point) so as not to disturb mobile device users. Alternatively, both downlink and uplink may utilize infrared light thereby at least partially disentangling the lighting and communication infrastructure.

To communicate with the access points, currently often a dongle is connected to a user device such as a laptop or tablet. Alternatively end devices may be integrated in mobile user devices directly. These dongles, like access points, may emit a similar broad beam to be sure that at least one access point will receive the signal from the dongle. The beams of the access points and the dongles are generally fixed in direction, so no adjustment of the beam direction is required.

Each access point comprises a modem connected to one or multiple transceivers. The user devices connect to the access point via an optical link and they also comprise a modem connected to one or multiple transceivers.

United States patent application US2019/376653 Al discloses a lighting device, such as a light bulb, having a first fixed light source dedicated to lighting and a second light source dedicated to data transmission by light modulation. The light source includes two power supply modules, one for powering the first light source providing illumination light and one for powering the modulator that varies the light output of the second light source to provide both illumination and data transmission.

The function of the modem is to handle the protocols (modulate and demodulate) for transmitting and receiving data over the visible or invisible light connection. The modem transmitter includes an optical frontend which transforms an electrical signal of the transmit data to an optical signal (for example using an LED) and the modem receiver transforms the optical signal to an electrical receive data signal (using a photodiode).

SUMMARY OF THE INVENTION

A problem with optical wireless communications is that high bandwidth modulation is difficult to achieve using the LEDs of a typical LED module as normally used for general lighting applications. The LEDs and board on which they are carried are known as the level two (L2) module or board. A driver connects to the L2 board and converts a mains grid voltage into an appropriate LED current. In most cases, the LEDs on the L2 board are connected in series between the driver output terminals. There are many different types of LED L2 board.

The board is for example a printed circuit board (PCB) made from typical PCB materials like FR4, flex-on-rigid or a metal clad PCB (MCPCB) carrier for enhanced cooling. L2 boards are used in a modular manner in manufacturing solid state lighting luminaires.

The LEDs are typically distributed over the area of the board of such modules. Due to internal inductances and stray capacitances, high frequency components in the drive current do not reach the LED junction. Thus, the LED layout, for example with one or more sets of series LED strings (with the strings in parallel), is not optimized for high communication bandwidth.

The invention as defined by the claims aims to alleviate this problem.

According to examples in accordance with an aspect of the invention, there is provided a luminaire, comprising: a printed circuit board, PCB; a first set of LEDs mounted on the PCB, the first set together defining a main visible light source for the luminaire; a first, mains powered, driver for driving the first set of LEDs; a second set of one or more LEDs mounted on the PCB, the second set of LED having fewer LEDs than the first set and defining an emergency light source for the luminaire; a second driver for driving the second set of LEDs; and a first wiring between the first set of LEDs and the first driver and a second wiring between the second set of LEDs and the second driver, the first wiring and second wiring being separate, wherein the second driver comprises a transmitter with a modulator for modulating the current to the second set of LEDs to provide optical wireless data transmission, the luminaire characterized in that: the second set defines an emergency light source for the luminaire; the second driver is configured for driving the emergency lighting and comprises at least one of: a built-in-power source for powering the second set of LEDs when the main power supply has failed or a port for connection to an uninterruptable power supply delivered through a Power over Ethernet network.

This luminaire implements optical wireless data transmission using light sources which are normally allocated to emergency lighting. The advantage of this is that there are fewer LEDs than for the main lighting (because the emergency lighting is a low power mode for use e.g. when there is a mains power failure) and this means there are lower impedances and hence better suitability for high bandwidth/high frequency transmission.

By providing separate wiring to the two sets of LEDs, the impedance of the components in the circuit of the second set of LEDs can be controlled, taking into account the desired optical data transmission characteristics. For general lighting, a homogeneous light beam as well as good cooling are the important optimization criteria for the L2 board, and this results in a large areas distribution of LEDs. This is not a requirement for emergency lighting. For example, the emergency lighting LEDs are deliberately not driven to high output brightness, in order to keeping failure modes low. Hence cooling by distribution over a large area is not required, and the subsequent increased wiring length is not an issue. This gives additional freedom in the LED placement when aiming to optimize the wiring for high frequency operation when using the emergency lighting LEDs.

The frequency response of second set of LEDs, i.e. the emergency lighting LED(s), is better as a result of a lower amount of parasitic capacitance and inductance in the wiring of the second set as compared to the first set. There is additional freedom to control the wiring in the second set because of the smaller number of LEDs, which may even be a single LED.

The frequency response of the second set of LEDs will in practice depend on multiple factors, including the wiring and parasitic inductances and capacitances, the type of LEDs, the number of LEDs, and other circuity components.

High bandwidth OWC systems typically utilize a modulation bandwidth above 20 MHz in order to allow for the communication bandwidth in excess of 1 Gbit/second. Bandwidth requirements for the optical channel as e.g. discussed in the P802.11bb standardization group are 2 MHz - 186 MHz which is far beyond the bandwidth as realizable with an unchanged L2 layout.

The optical wireless communication may for example have a data rate greater than 100 Mbit/s, and even as high as or greater than as 1 Gbit/second as mentioned above. The use of a small number of LEDs of the second set and the associated low impedances enables this higher bandwidth operation.

The second driver in embodiments comprises a built-in power source for powering the second set of LEDs to provide emergency lighting. The built-in power source for example comprises a battery. Alternatively, or additionally, the second driver comprises a connection for receiving an uninterruptable power supply delivered through a Power over Ethernet (PoE) network. This is an alternative way to provide a power supply in an emergency situation, with centralized power backup rather than distributed power backup. The availability of PoE also provides the luminaire with access to a communication backbone.

The second driver may comprise a receiver with a demodulator to enable optical wireless reception. The luminaire can thus function as a transmitter and receiver of optical wireless data communications. The second driver is for example integrated on the PCB. In this way, the second driver can be as close as possible to the second set of LEDs, thereby reducing impedances and enabling higher frequency operation of the LEDs.

The LEDs of the second set may each comprise a cold white phosphor converted LED. This is particularly suitable for optical communication.

The LEDs of the second second set may instead each comprise a RGB LED. Only one color channel may be used for modulation, making reception easier. It also enables simple driver circuitry to be used for the other channels.

The LEDs of the second second set may instead each comprise a white LED for visible light output with an IR LED in series for the optical wireless transmission. In this way, the detection may be simpler using an IR filter.

In all examples, the LEDs of the first set may be arranged in series. This reduces the current demand but it results in large series impedances, and hence makes the first set not suitable for high frequency, high bandwidth communication.

The second set of LEDs may comprise a single LED. The second set may have only a few, or even just one, LED.

The invention also provides a method of operating the luminaire defined above, comprising: in a normal lighting mode, operating the first driver to control the first set of LEDs to emit visible light; in an emergency lighting mode, operating the second driver to control the second set of LEDs to emit visible light; and in an optical communications mode and at the same time as the normal lighting mode, operating the second driver to control the second set of LEDs with a modulated current to provide optical wireless data transmission.

The invention also provides a method of upgrading a luminaire, the luminaire comprising: a housing; and a driver and a set of LEDs mounted in the housing, wherein the method comprises replacing the driver and the set of LEDs with: a printed circuit board, PCB; a first set of LEDs mounted on the carrier board, the first set together defining a main visible light source for the luminaire; a first, mains powered, driver for driving the first set of LEDs; a second set of one or more LEDs mounted on the carrier board, the second set of LED having fewer LEDs than the first set and defining an emergency light source for the luminaire; a second driver for driving the second set of LEDs; and a first wiring between the first set of LEDs and the first driver and a second wiring between the second set of LEDs and the second driver, the first wiring and second wiring being separate, wherein the second driver comprises a transmitter with a modulator for modulating the current to the second set of LEDs to provide optical wireless transmission and wherein the second driver comprises at least one of: a built-in-power source for powering the second set of LEDs to provide emergency lighting when the main power supply has failed or a port (74) for connection to an uninterruptable power supply delivered through a Power over Ethernet network.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Figure 1 shows a typical configuration of a LiFi system;

Figure 2 shows an example of a downlight L2 board;

Figure 3 shows a modified L2 board;

Figure 4 shows an arrangement of drivers for use with the L2 board of Figure 3; and

Figure 5 shows another example of L2 board.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described with reference to the Figures.

It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

The invention provides a luminaire comprising a carrier board with a first set of LEDs providing a main visible light source and a first, mains powered, driver for driving the first set of LEDs. A second set of LEDs is for an emergency light source and has a second driver. There is separate wiring between the first set of LEDs and the first driver and the second set of LEDs and the second driver. The second driver comprises a transmitter with a modulator for modulating the current to the second set of LEDs to provide optical wireless transmission. Thus emergency lighting part of a luminaire is in this way used for the optical communication.

Figure 1 shows a typical LiFi system with a set of transmitting units 10 forming a ceiling mounted infrastructure and a LiFi receiving unit 12. The transmitting units are known as access points (APs) and are preferably linked to a backbone, e.g. by means of a wired link such as an Ethernet link using a twisted pair cable or an Optical Fiber network allowing the APs and/or a global system controller to align, e.g. on handover. The receiving units are known as end devices (EDs).

Each AP contains a modem connected to one or multiple LiFi transceivers. The end devices can connect to an AP via an optical link. Each ED also contains a modem connected to one or multiple LiFi transceivers. The function of the LiFi-modem is to handle the physical layer (PHY) and media access control layer (MAC) protocols for transmitting and receiving data over the visible or invisible light connection.

The LiFi transceiver comprises a transmitter to transform an electrical signal of the modem’s transmit data to an optical signal (e.g. via an LED, a VCSEL or laser diode) and to provide a receiver to transform an optical signal to an electrical of the modem’s receive data (e.g. via a photodiode). The end device is for example implemented by a dongle 14 attached to a mobile device such as a laptop. Instead of retro-fitting, it is envisaged that the receiver functionality is eventually integrated in the mobile devices themselves, in this manner laptops, tablets, mobile phones and/or other devices may use optical communication without the need for a dongle.

Typical known OWC systems use dedicated emitters for the modulated light. The originally promised use of general lighting installations for OWC has not yet been widely adopted. Typical luminaires are manufactured using so-called L2 boards (or L2 modules). However, problems arise when driving L2 modules with modulated currents at typical OWC modulation bandwidths.

Figure 2 shows an example of a downlight L2 board. 20. LED devices 22 are typically distributed over the board 20 of such modules in order to generate a homogeneous light beam as well as distribute the heat over the area of the board. Typically the LEDs are driven in a series connection to keep driving currents low. A drive conductor 24 winds over the board connecting all LEDs. A connection point such as connector 26 and associated hole 28 for a cable serve to connect to the LED driver electronics located remote from the L2 board 20. Mounting holes 30 are provided for mounting the board 20 to a luminaire housing.

The whole board needs good cooling to keep the LED temperature low. For that reason, often all of the free space of the board is metalized, on the frontside as well as the backside. Furthermore, the module is typically screwed using the mounting holes 30 to a metallic luminaire housing or a dedicated cooling aid.

This arrangement has resulting internal inductances of wiring and stray capacitances that effectively filter away a substantial part or even all the high frequency components in the drive current when trying to use these modules directly for OWC. The high frequency modulation is partly (for the higher frequencies) not reaching the junction of the LEDs and cannot be reflected in the light flux generated. The whole module acts like a low pass filter and is not able to effectively function as a broadband OWC transmitter, for example when the modulation frequency band is 50MHz and above.

The invention is based on the recognition that the cabling and driving of a single emergency LED (or small set of emergency light LEDs) is easier to tune towards a high communication bandwidth than the series and parallel connected LEDs of the main lighting on the L2 board. The invention is thus based on connecting one or a subset of LEDs of an L2 module with high bandwidth in mind.

Figure 3 shows a carrier board 40 in accordance with an example of the invention.

The carrier board has a first set of LEDs 42 mounted on the carrier board, the first set together defining a main visible light source for the luminaire in which the board is to be mounted. A first, mains powered, driver (not shown) is provided for driving the first set of LEDs 42 and it connects to a first connector 44 and to a conductor track 45 which extends between the LEDs 42. The LEDs are for example all in series, but there could be multiple parallel branches each comprising a series connection of LEDs. A second set of LEDs 46 is also mounted on the carrier board, the second set of LEDs having fewer LEDs than the first set. In the example shown, there is a single LED 46 in the second set. The second set of LEDs define an emergency light source for the luminaire in which the board 40 is to be mounted.

A second driver (not shown) is provided for driving the second set of LEDs and it connects to a second connector 48.

There is separate wiring between the first set of LEDs and the first driver (i.e. a cable which connects to the first connector 44 through opening 50) and the second set of LEDs and the second driver (i.e. a cable which connects to the second connector 48 through opening 52).

The second driver implements the OWC and hence comprises a transmitter with a modulator for modulating the current to the second set of LEDs to provide optical wireless transmission.

The second set of LEDs may comprise a single LED or small number of LEDs used for emergency lighting, and this set of LEDs is designed to provide the required bandwidth for broadband OWC.

The current loop for the emergency lighting is much smaller than for the main lighting as (in this example) only a single LED is connected directly to the additional connector. A dedicated layout of the conductors may be used to increase the potential bandwidth as delivered to the emergency lighting LED. Also, stray capacitance to any grounded conducting material may be minimized by keeping increased distances of any conductors from e.g. cooling planes on the PCB as compared to those for the illumination LEDs. So-called microstrips with controlled impedance may be used in the layout to optimize the high frequency performance in constructions where a metallic back side is present.

Figure 4 shows an arrangement of drivers for use with the module 40. A main LED driver 60 is connected to the L2 board 40 via the (conventional) driving wire 62 carrying the drive current. This wire 62 connects to the first connector 44 of the L2 board 40.

The emergency lighting set of LEDs is separate from the other LEDs and is connected to an OWC enabled emergency driver 70 by means of cable 72. The driver 70 comprises an emergency driver which eventually drives LED current whenever mains power is missing. In addition, it comprises the required modulator for driving high frequency modulated current into the emergency lighting circuit.

The cable 72 is chosen taking into account the desired high frequency capability. For example, twisted pair cabling may be used without ajacket, or a coaxial cable may be used. The emergency modulator driver 70 is connected to a data network by means of a connector 74 which may also serve for the power supply e.g. by using a Power over Ethernet (PoE) connection.

The emergency set of LEDs has to be operated whenever the OWC feature is required.

The double use of a single LED (or small set of LEDs) to be used for emergency lighting and for OWC offers a number of advantages. It provides simple integration with typical solid state lighting luminaires by using the same L2 board mounting configurations and form factors.

This means that the arrangement described above may be a retrofit modification to an existing luminaire. The retrofit modification involves upgrading a luminaire which comprises a housing and a driver and a set of LEDs mounted in the housing.

The driver and the set of LEDs may then be replaced with the L2 board explained above, together with the first driver for driving the first set of LEDs and the second driver for driving the second set of LEDs, wherein separate wiring is between the first set of LEDs and the first driver and the second set of LEDs and the second driver.

Instead of using the visible emergency lighting for OWC, an additional IR emitter may be used for the OWC. If the visible emergency lighting is of the “always on” type, then the additional IR emitter may always benefit from the availability of power. If the visible emergency lighting is of the “only on in emergency” type, e.g. when it switches on when the power to the main illumination is lost, then a further adaptation is required to ensure proper operation of the additional IR emitter. In the latter case, the second driver (emergency light driver) may always power the emergency lighting circuit using the signal carrying the modulation, but the visible emergency lighting may be shunted or otherwise switched off in the absence of an emergency, leaving power for the additional IR transmitter.

The second driver, for emergency lighting and for OWC, preferably has energy storage integrated into the driver allowing emergency light to be generated even when the power supply (for example through PoE) has failed.

The emergency function may instead be based on an uninterruptible power supply, UPS, again delivered through the PoE network. This has the benefit that distributed energy storage means is no longer needed, because the energy storage is centralized. The energy storage for an emergency power supply requires regular function checks and maintenance service. UPS supply means are already often connected to a network to allow for monitoring of the battery state of health as well as capacity. Providing the emergency lighting using power delivery over a PoE connection allows a second power source to be present at the luminaire. When the first, mains, power source fails, the emergency circuit as powered through PoE can automatically be activated.

In the communication context, the PoE can perform two tasks. Firstly, the Ethernet connection is used to transport data between the AP and the network infrastructure. Secondly, the power for the OWC function may come from PoE making it independent of any light switching circuit.

When upgrading an existing luminaire as discussed above, existing systems which are not connected (i.e. not networked) and hence with directly switched mains can be upgraded to light communication, as the OWC function may also be needed when the main lighting is off.

In an example, the PoE connection may also be used to connect any luminaire controller to the network in order to allow light controls through the network. The PoE connection may be a typical PoE with twisted Ethernet pair cabling utilizing 4 pairs of conductors. Alternatively, as single pair Ethernet systems with power delivery have been standardized with data rates up to 10Gbit and power delivery up to 45W, these may be used in the context of this invention. Single pair wiring allows for cheap cabling to enable emergency and OWC functionality in an upgrade scenario where the mains cable is already connected to the luminaire.

Alternatively, the data connectivity may be accomplished by means of power line communication (PLC) utilizing the mains wiring. This might be especially useful in cases where the emergency function is powered by means of local energy storage, here data may be provided using the same power lines without changes to the infrastructure.

In the example above, the first and second drivers are remote from the L2 board. However, the L2 board may also have an integrated receiver for OWC. This may furthermore be combined with the transimpedance amplifier of the driver circuitry of the OWC detector, thereby enabling a high signal to noise ratio (SNR).

The L2 board may even have the second driver integrated on the board, as this may further enable a high OWC bandwidth, by having the power stage next to the OWC emitter.

Figure 5 shows an example of L2 board 80 in which the second set of LEDs 46 the second driver 70 are integrated onto the L2 board. This allows easy upgrading of conventional luminaires to emergency lights including OWC functionality. The only required additional connection would then be a PoE connection for power and data. The cabling 86 to the second driver may be a flexible single twisted pair network connection with power delivery, and it connects to a connector 88.

Network and modulation interfaces 90 are also integrated. The connector 88 may be a 10 Mb/s single twisted pair connector or similar.

In order to enable easy filtering of the high frequency components used for optical communication using visible light, the LED or LEDs of the second set may for example comprise cold white (e.g. 6000k-6500k) phosphor-converted LEDs (using a yellow phosphor). A cold white LED has a more pronounced blue spectral peak, which results from the LED light shining through the phosphor as leakage. This is helpful as it means this component is not influenced by the slow light conversion process of the phosphor. This means the required bandwidth is possible even when using phosphor-converted LEDs. In summary, conversion phosphors do not transport OWC signals very well, whereas the blue light spectrum of the cold white LED chip can however transport the desired high bandwidth modulation.

There are also warm white phosphor-converted LEDs which can provide sufficient blue light leakage for use in OWC.

The emergency and OWC LED may instead be a RGB LED. This may beneficially only be modulated on a single color-channel making receiving OWC easier. In addition the circuitry of the other channels is then simplified allowing for increased availability as required for emergency lighting. For example, in many OWC LED drive stages, the high frequency signal is superposed on the LEDs DC drive current by means of an inductive coupling (e.g. a series-connected low impedance transformer) or a capacitive coupling. The circuit then has to be designed to make sure both can enter into a failure mode in order to avoid cutting the LED current which would lead to failure as an emergency light.

In another implementation, the emergency LED may be a white LED having an IR LED in series. This will allow an easy detection of the communications signal by using an IR filter to suppress all visible light for the OWC reception. In this implementation the illumination light is indicative of the availability of the OWC feature. Alternatively, if a user for power savings reasons, or otherwise only wants illumination light the be switched on when necessary, i.e. when ambient light is low and/or when there is an emergency, separate controls may be provided for the illumination light, which switches on the illumination on an on-demand basis, but continuously powers the IR LED.

When driving the emergency illumination LED and the IR LED using the modulated current, both the illumination output and the IR output would be modulated, however in case of a phosphor converted LED the modulation may be somewhat filtered, depending on the type of phosphor. When this is not desirable, an alternative implementation would be to place the IR LED in a position where it is powered directly by the driver (e.g. the first LED in the string) and to have the illumination LED placed down-stream thereof with a capacitor across the illumination LED to remove the high-frequency modulation component from its output.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

If the term "adapted to" is used in the claims or description, it is noted the term "adapted to" is intended to be equivalent to the term "configured to". Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A luminaire, comprising: a printed circuit board, PCB, (40); a first set of LEDs (42) mounted on the PCB, the first set together defining a main visible light source for the luminaire; a first, mains powered, driver (60) for driving the first set of LEDs; a second set comprising one or more LEDs (46) mounted on the PCB, the second set of LED having fewer LEDs than the first set; a second driver (70) for driving the second set of LEDs; and a first wiring wiring (62) between the first set of LEDs and the first driver and a second wiring (72) between the second set of LEDs and the second driver, the first wiring and second wiring being separate, wherein the second driver (70) comprises a transmitter with a modulator for modulating the current to the second set of LEDs to provide optical wireless data transmission, the luminaire characterized in that: the second set defines an emergency light source for the luminaire; the second driver is configured for driving the emergency lighting and comprises at least one of: a built-in-power source for powering the second set of LEDs when the main power supply has failed or a port (74) for connection to an uninterruptable power supply delivered through a Power over Ethernet network.

2. The luminaire of claim 1, wherein the data rate of the modulator used for optical wireless data communication is greater than 100 Mbit/s.

3. The luminaire of claim 1 or 2, wherein the PCB is one of an FR4 PCB, a flex- on-rigid PCB, or a metal clad PCB.

4. The luminaire of any one of claims 1 to 3, wherein the second driver (70) comprises a receiver with a demodulator to enable optical wireless data reception.

5. The luminaire of any one of claims 1 to 4, wherein the second driver (70) is integrated on the PCB.

6. The luminaire of any one of claims 1 to 5, wherein the LEDs (46) of the second set each comprise a phosphor-converted LED, for example with a color temperature in the range 6000K to 65000K.

7. The luminaire of any one of claims 1 to 6, wherein the LEDs (46) of the second set each comprise a RGB LED.

8. The luminaire of any one of claims 1 to 7, wherein the LEDs (46) of the second set each comprise a white LED for visible light output with an IR LED in series for the optical wireless data transmission.

9. The luminaire of any one of claims 1 to 8, wherein the LEDs (42) of the first set are arranged in series.

10. The luminaire of any one of claims 1 to 9, wherein the second set of LEDs (46) comprises a single LED.

11. A method of operating the luminaire of any one of claims 1 to 10, comprising: in a normal lighting mode, operating the first driver to control the first set of

LEDs to emit visible light; in an emergency lighting mode, operating the second driver to control the second set of LEDs to emit visible light; and in an optical communications mode, and at the same time as the normal lighting mode, operating the second driver to control the second set of LEDs with a modulated current to provide optical wireless data transmission.

12. A method of upgrading a luminaire, the luminaire comprising: 16 a housing; and a driver and a set of LEDs mounted in the housing, wherein the method comprises replacing the driver and the set of LEDs with: a printed circuit board, PCB; a first set of LEDs mounted on the carrier board, the first set together defining a main visible light source for the luminaire; a first, mains powered, driver for driving the first set of LEDs; a second set of one or more LEDs mounted on the PCB, the second set of LED having fewer LEDs than the first set and defining an emergency light source for the luminaire; a second driver for driving the second set of LEDs; and a first wiring (62) between the first set of LEDs and the first driver and a second wiring (72) between the second set of LEDs and the second driver, the first wiring and second wiring being separate, wherein the second driver comprises a transmitter with a modulator for modulating the current to the second set of LEDs to provide optical wireless data transmission, and wherein the second driver comprises at least one of: a built-in-power source for powering the second set of LEDs to provide emergency lighting when the main power supply has failed or a port (74) for connection to an uninterruptable power supply delivered through a Power over Ethernet network.