CIE x049-P05

Lock-in detection is a well-established technique to measure the amplitude and phase of a modulated signal in the presence of noise, background, and parasitic signals. This technique is commonly used in many applications to extract small, modulated signals buried in noise that could not directly be measured otherwise.
In most lighting products, temporal light modulation is usually a periodic fluctuation of the light output resulting from the power supply. Lock-in detection is inherently well-suited to the characterisation of these optical oscillations superimposed to a large luminous background.
Different methods of lock-in detection are applicable to photometric and spectroradiometric measurements in the presence of temporal light modulation: electrical lock-in detection with a lock-in amplifier, digital lock-in detection based on synchronous sampling, and purely optical lock-in detection.
The practical implementations of these methods are detailed, illustrated, and compared. Their respective advantages and drawbacks are presented in the case of photometric and in the case of more complex measurements based on array spectrometers and imaging photometric instruments. New applications enabled by temporal light modulation are given.