LM-80: Measuring Lumen Maintenance Life

As a new type of light source, LED in recent years have been developing rapidly, according to the data of National Bureau of Statistics, the amount of the street lamps is about 90 million in the whole nation. If every street lamp costs 800 $, the costs of the whole market will be closer to 80 billion $. Although the LED illuminance has the advantages of energy conservation, environmental protection, high efficiency and long life, and many other advantages, but its reliability research is still lacking. As the light source, LED should be received an overall assessment not only in the photosynthetic efficiency, but also in color temperature ( Color Temperature Meter ) and color rendering index. It has no practical significance of paying attention to the luminous efficacy only, and neglecting the color temperature and color rendering index. It also has no practical significance of paying attention to the luminous efficacy only, and neglecting the light output lumens maintenance life. The theoretical service life of LED is 100000 hours, if still using a conventional LED life test under the normal rated stress; it is difficult to make an objective evaluation of the life and reliability of the product. How to scientifically forecast the LED flux maintenance life to ensure the reliability of the LED use not only needs to establish forecasting model, but also needs to make sure the certain test time for the scientific forecast. Below analyses and interprets the LM-80 report and LED flux maintenance life prediction figure. It reveals the prediction of LED light output if should be based on the mathematical statistics inspection for at least 6000 h, which has certain practical significance for the LED reliability research.

Pulbished by IES (Illuminating Engineering Society of North America), IES LM-80-08 “Methods of LED Light Source " has the requirement of the test temperature and humidity: when doing photometric measurement, LED light source should work in the minimum temperature among the three base temperature Ts, and use the same drive current. The three base temperature Ts should be: 55 ℃, 85 ℃ and the temperature chosen by the manufacturer. The base temperature and the drive current chosen by the manufacturer shall represent the customer expected application places, and should be in the recommended operating temperature range. In the life test, the base temperature should be controlled in the range of 2 ℃. In the test environment, the air temperature should be kept in the base temperature-5 ℃ scope. During the whole life test, the relative humidity should be less than 65%. The requirement of this publication to maintain the test time and interval of the flux ( Flux Meter ) is that in the provision of environmental temperature, it should work at least 6000 h, at least collect a data every 1000 h. In order to improve the expected model, it is better to work for 10000 h. And according to the U.S. Department of Energy (DOE) in terms of life threshold requirement of the solid-state lighting products (SSL), the energy star. If the minimum lumen maintenance rate of 6000 h is 91.8%, the maximum life is L70 = 25000 h. if the minimum lumen maintenance rate of 6000 h is 94.1%, the maximum life is L70 = 35000 h. The data published by the energy star is also suitable for other power LED. The method of estimating graphics push LM-80 LED flux maintain life prediction figure. 

Predicted figure of LM-80 measuring lumen maintenance of LED light source extrapolated by the graphics estimation methods.

Under the conditions of 85 ℃ environment temperature and 0.35 A current, according to the light output data collection of 1000 h, 1500 h, 2000 h, 3000 h, 4000 h, 5000 h and 6000 h, use the index function model to extrapolate, and get the calculation result L70 = 69857. But use the graphic estimation method to extrapolate Ф/Ф 0 = 0.7, the service life is about 70000 hours, both the data are not significant. Please note that, when conducting the regression for the , the necessary correction is needed o reduce the error. For example, in the initial state of the low stress test, Ф/Ф 0 May be greater than 1, ASSIST suggests to take 1000 h as Ф 0; And in the initial state of high stress test, Ф/Ф 0 will fall away from the index function, when doing the regression, the weighted processing is needed.

Life test in a short time can predict the life characteristics of LED ( LED Test Instruments ) products under the conditions of normal stress; it is the effective way of the reliability evaluation of LED lighting products for long-term use. This paper introduces the LED light output life forecasting model, with introducing the basic principle of mathematical statistics and the linear regression formula it analyses and interprets the LM-80 report and LED flux output maintenance life prediction figure. The results reveal that the LED light output life forecasting shall establish on the base of mathematical statistics inspection for at least 6000 h, and has certain practical significance of the reliability research for LED ( LED Tester ).

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