Optical Lock-in Spectrometry Reveals Useful Spectral Features of Temporal Light Modulation in Several Light Source Technologies

This paper presents a study of the spectral characteristics of temporal light modulation in several technologies of lighting products. An optical lock-in spectrometer was designed for this purpose and integrated in a spectral radiant flux measurement facility. It was applied to incandescent and fluorescent lamps, as well as lamps based on white phosphor-converted LEDs and tunable RGB LEDs. The results are well correlated with the light emission processes of each technology. For incandescent lamps, the spectral modulation follows a 1/lambda relationship in agreement with the blackbody radiation laws. Measurements performed on halophosphate and tri-phosphor tubes agree well with published data. The modulation and phase spectra of fluorescent lamps reveal a variable modulation rate across the visible range, directly related to the fluorescence lifetimes of the different luminophores, which were estimated from our data using a model of single exponential decay.The spectral modulation of white phosphor-converted LED lamps is nearly constant across the visible spectrum, demonstrating that their color parameters can be assessed from the lock-in modulation amplitude spectrum. In the case of tunable RGB LED lamps using PWM, the spectral modulation widely differs from the steady-state spectral distribution and changes with the user settings, confirming the possible occurrence of temporal color artifacts. Optical lock-in spectrometry can be used to improve spectral and color measurements of solid-state lighting, opening new opportunities for laboratory and remote sensing applications. Other foreseeable applications of optical lock-in spectrometry are also presented.