A first lamp for use with one or more other lamps (12b, 12c, 12d) in a multi-lamp luminaire, each lamp being operable to emit respective illumination embedded with a predetermined coded light message (S1....Sn); wherein the first lamp comprises: one or more light-emitting elements for emitting the respective illumination; a local controller; a communication interface (Ml) configured to enable the local controller to communicate with a corresponding controller on each of the one or more other lamps in the multi-lamp luminaire, the communication comprising sending and/or receiving one or more signals; and a coded light transmitter operable to modulate the coded light message into the respective illumination of the first lamp; characterised in that the local controller is configured to coordinate with the corresponding controllers of the one or more other lamps, based on the communication via said communication interface, to prevent unsynchronized instances of said coded light message being transmitted from different ones of the lamps in the multi-lamp luminaire, the local controller being configured such that said coordination comprises either: a) coordinating such that each of the first lamp and the one or more other lamps transmit a respective instance of the same coded light message, whereby said first lamp transmits the same coded light message whilst synchronizing the instance of said coded light message to be transmitted starting at the same time as the instance of the same coded light message for each of the one or more other lamps, or b) coordinating such that only one of the lamps in the multi-lamp luminaire transmits the message, and none other of the lamps in the multi-lamp luminaire transmit any coded light, such that when the first lamp is to transmit said message then the local controller selects to operate the first lamp in a coded-light-transmission mode in which the coded light transmitter transmits said message, whereas when one of the other lamps is to transmit the message the local controller selects to operate the first lamp in a no-coded-light-transmission mode in which the first lamp does not transmit said message. one or more light-emitting elements for emitting the respective illumination; a local controller; a communication interface (Ml) configured to enable the local controller to communicate with a corresponding controller on each of the one or more other lamps in the multi-lamp luminaire, the communication comprising sending and/or receiving one or more signals; and a coded light transmitter operable to modulate the coded light message into the respective illumination of the first lamp; characterised in that the local controller is configured to coordinate with the corresponding controllers of the one or more other lamps, based on the communication via said communication interface, to prevent unsynchronized instances of said coded light message being transmitted from different ones of the lamps in the multi-lamp luminaire, the local controller being configured such that said coordination comprises either: a) coordinating such that each of the first lamp and the one or more other lamps transmit a respective instance of the same coded light message, whereby said first lamp transmits the same coded light message whilst synchronizing the instance of said coded light message to be transmitted starting at the same time as the instance of the same coded light message for each of the one or more other lamps, or b) coordinating such that only one of the lamps in the multi-lamp luminaire transmits the message, and none other of the lamps in the multi-lamp luminaire transmit any coded light, such that when the first lamp is to transmit said message then the local controller selects to operate the first lamp in a coded-light-transmission mode in which the coded light transmitter transmits said message, whereas when one of the other lamps is to transmit the message the local controller selects to operate the first lamp in a no-coded-light-transmission mode in which the first lamp does not transmit said message. a) coordinating such that each of the first lamp and the one or more other lamps transmit a respective instance of the same coded light message, whereby said first lamp transmits the same coded light message whilst synchronizing the instance of said coded light message to be transmitted starting at the same time as the instance of the same coded light message for each of the one or more other lamps, or b) coordinating such that only one of the lamps in the multi-lamp luminaire transmits the message, and none other of the lamps in the multi-lamp luminaire transmit any coded light, such that when the first lamp is to transmit said message then the local controller selects to operate the first lamp in a coded-light-transmission mode in which the coded light transmitter transmits said message, whereas when one of the other lamps is to transmit the message the local controller selects to operate the first lamp in a no-coded-light-transmission mode in which the first lamp does not transmit said message.