Assessment of spatially resolved temporal light modulation in extended scenes by fast imaging measurements

Modern LED-based light sources often use variants of pulse-width modulation (PWM) to control the operation current in the LEDs and by this the light output and colour of the lamp. The light emission of the LEDs follows this variable driving current which results in a temporal light modulation (TLM). These can lead to effects in the light’s perception by a human observer, e.g. stroboscopic effects, which can range from annoying to harmful. Therefore, the assessment of TLM is needed in the design phase of lighting installations like indoor and outdoor lighting or in automotive context. This work explores the possibility to use cameras for the spatially resolved measurement of TLM. To deal with the insufficient sampling rate and the need for low-pass filtering in the time domain, an approach for periodic signals to use equivalent-time sampling is chosen. Two variants of this method are explained in detail and exemplary measurements of TLM waveforms are presented. The measurement of waveforms with a cut-off frequency of about 20 kHz is demonstrated. Despite the used demonstrator setup did not implement a V(λ)-matching, it allowed to gain insights on the TLM of sources in a scene. The main limitation is the relatively small dynamic range that can be measured without manual adjustments to the device settings. This will require some future development to add some automatic attenuators.