| dc.description.abstract | Light-emitting diodes (LEDs) occupy an important portion in the optoelectronics industry due to their good characteristics, such as long life time and low energy consumption, meeting the worldwide demands for energy saving. The range of LED usage has gradually moved from indoor lighting to outdoor lighting. The applications include parking lot lighting and street lighting at night. Human eyes in these lighting environments are under mesopic vision, which is between scotopic and photopic visions and is most sensitive to green light with wavelengths between 507 and 555 nm. For this reason, adding green content to white light LEDs shall have the opportunity to provide illumination with less energy but the same brightness to users.
In this study, five different proportions of green to white LED chips are packaged, including 5W4G (5 white and 4 green chips), 4W4G, 4W3G, 4W2G, and 5W. These LEDs are measured by an integrating sphere. The spectra and energy consumptions are used to compute the energy saving ratios compared to the case of 5W in terms of mesopic luminance and luminous efficacy. Besides, psychophysical experiments are conducted to assess the visual acceptability and clarity of these LEDs by questionnaires.
The measurement results show that for different proportions of green to white LEDs at low driving currents (10-20 mA), the energy saving ratio is better when the LED has a higher S/P-ratio (scotopic and photopic ratio). This is true both in terms of mesopic luminance and luminous efficacy. However, at a higher driving current of 350 mA, when the S/P-ratio is lower than 1.952 (the case of 4W3G), the LED no longer has the energy saving property in luminous efficacy compared to the case of 5W. In psychophysical experiments, the cases 5W4G, 4W2G and 5W have approximate visual acceptability and clarity, and are better choices than the cases 4W4G and 4W3G, whose chromaticity coordinates are closer to the saturated green color. This study validates that raising the S/P-ratio of LEDs is beneficial for energy saving. Adding green chips into white light LEDs has the potential for energy efficient lighting with high visual acceptability and clarity. | en_US |