A control signal for controlling a light emitting device is PWM modulated in time, the PWM modulation comprising PWM pulses and PWM periods. The PWM pulse instantaneous frequency of a PWM pulse is the reciprocal of the instantaneous PWM period of the PWM pulse. The PWM pulse instantaneous frequency depends on the PWM duty cycle of the PWM pulses of the control signal. The PWM pulse instantaneous frequency of the PWM pulses is a first PWM pulse instantaneous frequency at a first PWM duty cycle of the control signal, and is a second PWM pulse instantaneous frequency at a second PWM duty cycle of the control signal. In an operating condition, the first PWM duty cycle is less than the second PWM duty cycle and the first PWM pulse instantaneous frequency is less than the second PWM pulse instantaneous frequency.