(1) Performance Degradation of Phosphors at Higher Temperatures While there is limited information on the direct relationship between phosphor excitation efficiency and temperature in LEDs, there is ample evidence showing that temperature significantly affects both the performance and lifespan of phosphors. Manufacturers have conducted tests indicating that at 60°C, the excitation efficiency of phosphors decreases by approximately 2%, but it recovers after cooling. This test was conducted over a short period, yet it clearly shows that rising temperatures lead to reduced phosphor performance. The irreversible degradation, however, is a cumulative process that occurs gradually over time. In practice, users often observe that white LEDs become brighter after being used for some time, as shown in Figure 1. This phenomenon is commonly seen in low-power LEDs within the first 1,000 hours. Some products from mid-2008 may already be near or have reached this point. For low-power packaged LEDs, this effect can last up to 2,000 hours. Possible explanations include: A: The interaction between the phosphor and the encapsulating gel temporarily reduces performance, which then recovers when the temperature stabilizes. B: The phosphor and gel work together to enhance performance. C: The blue LED chip initially performs better during the early stages of operation. Experiments have shown that during the initial phase, the luminous flux of white LEDs increases before eventually decreasing. This behavior is also observed in red LEDs from different manufacturers, making it difficult to determine whether the issue lies with the phosphor, packaging materials, or the manufacturing process in the short term. However, in life tests of low-power blue LEDs, an increase in luminous flux during the initial period has been observed. As shown in Figure 2, most LEDs experience a rise in luminous flux within about 200 hours. Plug-in white LEDs typically show this effect within 100 hours. It can be inferred that the phosphor's performance degrades first in white LEDs. After the initial brightness boost in low-power white LEDs, the situation becomes less favorable. Imagine a plane without wings—results are obvious. High-power white LEDs usually see a luminous flux increase of about 100 hours, followed by instability until 6,000 hours. Over time, some products exhibit significant fluctuations in light output. After 6,000 hours, the luminous flux begins a steady decline. Currently, high-power white LEDs typically reach the end of their life (50% light decay) within 1.5 to 20,000 hours. (2) Rapid Decline of Blue LEDs Themselves Compared to other LED types, blue LEDs have the shortest lifespan. A low-power plug-in blue LED lasts around 7,000 to 10,000 hours under 20mA current. Meanwhile, a low-power red LED shows no noticeable light decay even after operating for 8,000 hours at 50mA. Red LEDs consume 1.8 times more power than blue LEDs but do not suffer from performance degradation. Yellow and green LEDs generally have lifespans exceeding 10,000 hours. Therefore, the inherent weakness of blue chips leads to shorter lifespans for white LEDs made from them. From a material perspective, blue and green LEDs use different epitaxial layers, but they share similar sapphire substrates with comparable thermal conductivity. Hence, the structure of the epitaxial layer determines how well the chip withstands heat. Improving this structure should be a key focus for future advancements. If the epitaxial structure cannot be improved significantly, replacing the substrate material with one that conducts heat better could be a viable alternative. (3) Poor Thermal Conductivity of LED Package Materials In low-power plug-in LEDs, the chip mounting bracket is typically made of iron, which has poor heat dissipation properties. Additionally, the portion of the bracket that extends out has a small cross-sectional area, increasing thermal resistance. Even piranha-style brackets face similar issues due to their limited cross-section. These material and structural choices contribute to the low thermal conductivity of low-power packages. The main cause of light degradation in white LEDs is heat. Materials involved in the thermal path of LEDs must be carefully considered. The base material issue has already been discussed, and the remaining factors involve the solid glue, powder glue, and protective glue (lens). It has been found that using silver paste for solid crystal mounting results in a longer lifespan compared to epoxy resin, although the initial luminous flux is nearly one-third lower. On the other hand, epoxy resin-based powder blends offer higher initial brightness—about 25% more—than silica gel, but with a shorter lifespan. (4) Effects of Ultraviolet Radiation on LEDs UV radiation primarily affects the chip materials, phosphors, and encapsulants. Among these, the encapsulant is most vulnerable. LEDs are generally not exposed to direct sunlight, so any UV light entering the device is mostly diffuse. As a result, the impact of UV on the chip and phosphor is relatively minor. Small Diaphragm Vacuum Pump,Small Vacuum Electric Pump,Small Electric Vacuum Pump,DC Small Vacuum Pump,DC Electric Vacuum Pump Shenzhen DYX Technology Co.,Limited , https://www.dyxpump.com