| dc.description.abstract | Currently, white LEDs are generated by exciting fluorescent powders through blue LED chips to produce white light. Types of fluorescent powder compositions include: (1) single-agent type: using Silicate fluorescent powder or YAG fluorescent powder to generate yellow light, and (2) mixed type: using β-SiAlON fluorescent powder to produce green light combined with KSiF fluorescent powder to produce red light.
In general, white LEDs only consider whether the white light chromaticity can be more precise. Their application in displays does not utilize the split light after mixing white light, because different fluorescent powders have time differences in peak excitation and energy dissipation. Continuous color changes occur before reaching the excitation peak and during the dissipation process. This study observes multi-color changes in white LEDs by adjusting these time differences.
This study uses Pulse Width Modulation (PWM) to experiment by fixing the input power frequency and varying the duty cycle. It employs blue LED chips to excite mixed-type fluorescent powders: β-SiAlON (wavelength 530nm) and KSiF (wavelength 700nm). At an emission frequency of 10Hz, five conditions of duty cycle—40%, 50%, 60%, 70%, and 100%—correspond to switch time intervals of 0.04 seconds, 0.05 seconds, 0.06 seconds, 0.07 seconds, and constant on. High-speed cameras record at 240 frames per second (fps) to observe the time differences in excitation between the two types of fluorescent powders. A reference background of 1Hz and 50% duty cycle is used to compare changes in light color quantity and energy under various conditions.
The experimental results show:
1. When blue light alone irradiates β-SiAlON phosphor, it takes about 0.0042 seconds to reach the energy peak, and the energy dissipation of β-SiAlON phosphor takes about 0.0042 seconds. The rise and fall time ratio is approximately 1:1. When blue light alone irradiates KSiF phosphor, it takes about 0.0168 seconds to reach the energy peak, and the energy dissipation of KSiF phosphor takes about 0.063 seconds. The rise and fall time ratio is approximately 1:4.
2. When blue light irradiates a mixture of β-SiAlON and KSiF phosphors, the time for β-SiAlON phosphor to reach the energy peak and emit green light is 0.0042 seconds, which is 4 times faster than the 0.0168 seconds required for KSiF phosphor to reach the energy peak and emit red light. Therefore, cyan light is observed first before white light.
3. When blue light illuminates a mixture of β-SiAlON and KSiF phosphors, the time for β-SiAlON phosphor to dissipate its green light energy is 0.0042 seconds, which is 15 times faster than the 0.063 seconds required for KSiF phosphor to dissipate its red light energy. Therefore, when the power is cut off, cyan light is visible for only 0.0042 seconds, followed by a process of gradually dimming red light over 0.063 seconds until complete extinction. Throughout the cycle of brightness and darkness of the mixed phosphor white LED, the time to reach the peak white light energy is 0.0168 seconds with a dissipation time of 0.063 seconds, which aligns entirely with the results obtained from separately illuminating the two phosphors.
4. In the experiment, for the mixed phosphor white LEDs with duty cycles of 40%, 50%, 60%, and 70%, the ON times are 0.4, 0.5, 0.6, and 0.7 seconds, respectively. All these times exceed the 0.0168 seconds required to reach the energy peak for white light. Therefore, it is not possible to avoid the time required to synthesize white light, resulting in consistent color changes before the formation of white light.
5. When the ON time is less than 0.0168 seconds before reaching the energy peak, it can modulate light with an R:G:B ratio. After turning OFF, it can modulate light with R:G:B, R:G, and R ratios.
6. The human naked eyes retains visual impressions for about 0.0625 seconds. When the duty cycle is below 21% or the frequency exceeds 20Hz, continuous red light can be perceived.
In the current LED or LCD industry, the undefined mixing time of white light LEDs leads to unpredictable optical noise in products. This study proposes a method to improve the power switching timing during testing and design processes, thereby preventing the occurrence of optical noise across all LED applications. Additionally, it enables monochromatic LEDs to produce multicolored light through simple power control methods, thereby expanding their applicability in scenarios such as optical communication, displays, lighting, etc., and reducing resource wastage. | en_US |