| 摘要: | 現今白光LED的產生方式有透過藍光LED晶片激發螢光粉產生白光，螢光粉組成類型則有（1）單劑型：使用產生黃光的Silicate螢光粉或YAG螢光粉，（2）混合型：使用產生綠光的β-SiAlON螢光粉加產生紅光的KSiF螢光粉。
一般白光LED只考慮到白光色點是否可以更為精準，而其應用在顯示器是否可以得到更大的色域涵蓋範圍，並不會將混光後的白光分光再使用，因為不同螢光粉的激發高峰和能量消散有時間差，到達激發高峰前和消散過程中有連續的顏色變化，本研究利用調控時間差以觀察到白光LED的多色變化。
本研究利用脈衝寬度調變(PWM)方式，固定輸入電源的頻率，以改變佔空比(Duty cycle)的方式來進行實驗，用藍光LED晶片激發混合型螢光粉；產生綠光的β-SiAlON(波長530nm)螢光粉和產生紅光的KSiF(波長700nm)螢光粉；以10Hz的發光頻率，分別以佔空比40%、50%、60%、70%、100%等五個條件值，相對的開關時間間隔分別為0.04秒、0.05秒、0.06秒、0.07秒、常開，使用高速攝影機以240fps的拍攝條件觀察兩種類螢光粉激發時間差，再以1Hz和佔空比50%作為參考背景，比較各個條件下的光色數量和能量變化。
實驗結果顯示：
1. 藍光單獨照射β-SiAlON螢光粉時，激發到能量高峰需時約0.0042秒，β-SiAlON螢光粉能量消散需時約0.0042秒，上升與下降的時間比例約1:1，藍光單獨照射KSiF螢光粉時，激發到能量高峰需時約0.0168秒，KSiF能量消散需時約0.063秒，上升與下降的時間時間比例約1:4。
2. 藍光照射β-SiAlON加KSiF混和型螢光粉時，因β-SiAlON螢光粉，激發到能量高峰產生綠色的時間為0.0042秒比KSiF到能量高峰產生紅色的時間0.0168秒快4倍，所以會先看到青色光再看到白色光。
3. 藍光照射β-SiAlON加KSiF混和型螢光粉時，因β-SiAlON螢光粉，綠光能量消散的時間為0.0042秒比KSiF螢光粉紅光能量消散的時間0.063秒快15倍，所以當斷電時，青色光只有0.0042秒，之後0.063秒的過程只看到紅色光斑逐漸轉暗到完全熄滅，混和型螢光粉白光LED亮暗全程時間，到達白光能量高峰時間為0.0168，消散時間為0.063，與兩種螢光粉分開獨立照射所得結果完全一致。
4. 實驗中混和型螢光粉白光LED佔空比40%、50%、60%、70%中ON的時間，分別為0.4、0.5、0.6、0.7，都大於到達能量高峰的0.0168秒，所以無法避開合成白光的時間，所以在形成白光前的顏色變化都一樣。
5.在能量高峰前，ON時間低於時0.0168秒，能調制出R:G:B比例的光，在OFF後，能調制出R:G:B、R:G、R比例的光。
6.人眼視覺暫留約0.0625秒，當PWM的佔空比Ton的時間小於0.0625秒時就可以看到較長時間的紅光。
在現階段LED或是LCD產業中，會面臨白光LED混光時間無法被清楚定義，而造成產品產生無法預期的雜訊光；本研究方法可以解決所有LED應用之產品在檢驗和設計過程中，改善電源的開關時序，以避免產生雜訊光，另一方面可以提升單色光LED透過簡單的電源控制方式而產生多色光，增加單一LED可應用的場景，如光通訊、顯示、照明等，以減少資源的浪費。
;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. |
