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

ABSTRACT

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/λ 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.

KEYWORDS:

• Lighting
• optical lock-in detection
• temporal light modulation
• spectral modulation
• photometry
• spectroradiometry
• temporal color modulation
• color flicker

Acknowledgments

The authors are grateful to Prof. Geneviève Chadeyron of the SIGMA Clermont Institute for fruitful discussions concerning luminophores used in lighting applications.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was entirely funded by the Centre Scientifique et Technique du Bâtiment (CSTB) through the internal research program “Indoor Environmental Quality: Measurements and Modeling.”