When Do LEDs Become Less Efficient in Terms of Lumen Depreciation?

LEDs, like all light sources, experience a gradual decrease in light output over time due to a process called lumen depreciation. This phenomenon is influenced by various factors, including temperature, current, and product design. Understanding the mechanisms behind lumen depreciation and the industry standards used to measure it is crucial for assessing the long-term performance and efficiency of LED fixtures.

Lumen Depreciation and Lumen Maintenance

Lumen depreciation refers to the reduction in the luminous flux (light output) of an LED over time. This decrease occurs gradually and is a natural consequence of the aging process of the LED. The rate of lumen depreciation is influenced by several factors, including:

1. Temperature: Elevated temperatures can accelerate the degradation of the LED’s internal components, leading to faster lumen depreciation.
2. Drive Current: Operating an LED at higher currents than recommended can generate more heat, which can contribute to faster lumen depreciation.
3. Product Design: The quality of the LED’s packaging, heat sink, and other components can impact the rate of lumen depreciation.

Lumen maintenance, on the other hand, is the complement of lumen depreciation. It represents the percentage of the initial luminous flux that remains at a given time. For example, if a luminaire produced 1,000 lumens when it was new and now produces 700 lumens after 30,000 hours of operation, it would have a lumen maintenance of 70% at that point.

L70 Rating and Lumen Maintenance Life

To quantify the lumen depreciation of LEDs, the industry uses the L70 rating. This rating indicates the number of hours it takes for an LED to reach 70% of its initial light output. The L70 rating is a crucial metric for assessing an LED’s lifespan and long-term performance.

The L70 rating is determined through a standardized testing procedure known as LM-80, which is approved by the Department of Energy. The LM-80 standard specifies a minimum testing period of 6,000 hours, with 10,000 hours being the preferred testing period. During the LM-80 test, the lumen maintenance of LED arrays and modules is measured at different temperatures.

Building upon the LM-80 data, the TM-21 standard is used to project the lumen degradation of an LED package, array, or module beyond the LM-80 test period. TM-21 allows for the determination of a depreciation curve for the luminous flux at each tested temperature, which is then used to estimate the L70, L80, or L90 “Lumen Maintenance Life” of the LED product.

Factors Affecting Lumen Depreciation

Several factors can contribute to the rate of lumen depreciation in LEDs, including:

1. Temperature: As mentioned earlier, elevated temperatures can accelerate the degradation of the LED’s internal components, leading to faster lumen depreciation. The LM-80 standard specifically measures lumen maintenance at different temperatures to understand this relationship.

2. Drive Current: Operating an LED at higher currents than recommended can generate more heat, which can contribute to faster lumen depreciation. Manufacturers typically provide guidelines on the maximum recommended drive current for their LED products.

3. LED Chip and Package Design: The quality and design of the LED chip and package can significantly impact the rate of lumen depreciation. Factors such as the materials used, the manufacturing process, and the thermal management of the LED can all influence its long-term performance.

4. Thermal Management: Effective thermal management is crucial for maintaining the LED’s efficiency and longevity. Proper heat sinking and airflow around the LED can help mitigate the effects of elevated temperatures and slow down lumen depreciation.

5. Operating Environment: The ambient conditions in which the LED is used, such as temperature, humidity, and exposure to contaminants, can also affect the rate of lumen depreciation.

6. LED Binning and Sorting: The LED manufacturing process involves binning and sorting LEDs based on their performance characteristics, including initial luminous flux and color temperature. Proper binning and sorting can help ensure consistent lumen maintenance across LED fixtures.

Measuring Lumen Depreciation: LM-80 and TM-21

As mentioned earlier, the industry uses two primary standards to measure and project the lumen depreciation of LEDs:

1. LM-80: The LM-80 standard, approved by the Department of Energy, is used for measuring the lumen depreciation of solid-state (LED) arrays and modules. It specifies a minimum testing period of 6,000 hours, with 10,000 hours being the preferred testing period. LM-80 measures the lumen maintenance of light sources at different temperatures and color maintenance at different temperatures.

2. TM-21: TM-21 is an IES-recommended method for projecting the lumen degradation of an LED package, array, or module beyond the LM-80 test period. It uses the data collected during the LM-80 test to determine a depreciation curve for the luminous flux at each tested temperature. This data is then used to estimate the L70, L80, or L90 “Lumen Maintenance Life” of the LED product.

The combination of LM-80 and TM-21 provides a comprehensive and standardized approach to understanding the lumen depreciation of LEDs over time, which is crucial for assessing their long-term efficiency and performance.

Lumen Depreciation and Energy Efficiency

Lumen depreciation is an important factor to consider when evaluating the energy efficiency of LED lighting systems. As LEDs lose light output over time, the overall energy efficiency of the system decreases. This is because the same amount of electrical power is being used to produce a lower amount of light.

To maintain the desired light output, users may be tempted to increase the drive current to the LEDs, which can further accelerate lumen depreciation and reduce the overall energy efficiency of the system. Proper thermal management and adherence to the manufacturer’s recommendations for drive current are essential to mitigate this issue.

Additionally, the L70 rating can be used to estimate the point at which an LED fixture will need to be replaced or retrofitted to maintain the desired light output and energy efficiency. By understanding the lumen depreciation characteristics of their LED lighting systems, users can make informed decisions about maintenance, replacement, and upgrades to ensure optimal energy efficiency over the long term.

Conclusion

Lumen depreciation is a natural phenomenon that affects all LED light sources over time. Understanding the factors that influence this process, as well as the industry standards used to measure and project lumen depreciation, is crucial for assessing the long-term performance and efficiency of LED lighting systems.

By considering the L70 rating, lumen maintenance, and the testing methods outlined in LM-80 and TM-21, users and professionals can make informed decisions about the selection, installation, and maintenance of LED fixtures to ensure optimal light output and energy efficiency throughout the product’s lifespan.

References:
– Lifetime of White LEDs
– LED Lifetime and L70
– Lumen Maintenance
– LED Lifespan and Lumen Depreciation