In the world of LED technology, understanding the difference between hot-state and cold-state lumens is crucial for ensuring optimal performance and longevity. Recent tests have consistently shown that hot-state lumens, measured at an elevated temperature of 85°C, are significantly lower than their cold-state counterparts, which are typically tested at 25°C.
The disparity stems from the fundamental properties of LED devices, which are highly sensitive to temperature. As the temperature rises, various material parameters within the LED chip undergo changes, affecting its light output. Specifically, the junction temperature—the hottest point within the semiconductor chip—plays a pivotal role in determining the LED's luminous flux, or lumens.
Cold-state lumens, measured at 25°C, represent the idealized performance of an LED under minimal thermal stress. This test is often conducted using instantaneous pulse currents, minimizing heat generation and providing a snapshot of the LED's maximum potential light output. However, in real-world applications, LEDs are subjected to much higher temperatures, particularly in automotive headlights and other demanding environments.
Under these conditions, hot-state testing becomes essential. By simulating the harsh operating temperatures of 85°C, hot-state lumens provide a more accurate assessment of an LED's performance in its intended use case. The lower lumens recorded during hot-state testing reflect the degradation of light output due to increased thermal stress, which can lead to reduced efficiency and accelerated aging.
Moreover, the temperature-induced changes within the LED material can create non-radiative recombination centers, reducing the amount of light emitted. Additionally, the encapsulants used to protect the LED can degrade at high temperatures, further diminishing light output.
The lower hot-state lumens compared to cold-state lumens underscore the importance of considering temperature in LED testing and design. By understanding and mitigating the effects of thermal stress, manufacturers can develop more reliable and efficient LED products that meet the demands of diverse applications.