Characterization of LED Temperature Dependence and Power-Efficient LED Modulation Methods for Use in Fluorescence Absorption Spectroscopy

Temperature characteristics of LEDs impact the performance of optical instruments. We have characterized these effects by powering various colors of high-powered LEDs in constant current mode and analyzing the light output with a spectrometer. Light intensity, center wavelength, and spectral full width at half maximum (FWHM) was recorded as temperature changed, allowing for a better understanding of the relationship between temperature and the characteristics of light. Results show that an increase in temperature (range of 40 °C) decreases the efficiency (average of 15%; peak of 46%), increases the FWHM (average of 6.7%), and causes the center wavelength to shift to the red. Furthermore, we present the impact of sinusoidal current modulation upon the wavelength spread of LEDs. Circuit approaches for power efficient modulation are proposed that minimize changes in LED characteristics and produce pure sine-waves. Temperature characterization measurements and modulation methods are critical for low-cost and portable medical instruments.