Lifetime and Reliability by Günther Sejkora
Although questions regarding lifetime and reliability of Solid-State-Lighting (SSL) products are essential for customers, knowledge about it is still fragmentary. It was easy to give answers to these questions in the times of conventional lamps (incandescent, fluorescent, high pressure): light output degraded over time and end of life was reached when light output fell below a certain level or when the lamp “burned out”.
Lifetime was restricted by these events from several hundred up to 15,000 or 20,000 hours. This time was far below the useful lifetime of any other components in the luminaire. Only lifetime of the lamp was discussed at that time. When SSL entered the picture, the same concept was adopted and lifetime of the LED was promoted instead of talking about lifetime of a luminaire or a module.
Maybe we should first take a look at failure behaviour of components and products. A parametric failure occurs, when a performance parameter (e.g. light output or chromaticity for an LED) comes out of range and therefore the device can’t be used any more. A catastrophic failure means that due to a spontaneous effect the component or product becomes inoperable and can’t be repaired (e.g. burn out of a lamp filament). Both failures finish the lifetime of a product or component.
Typically, at the beginning of the product’s life the failure rate starts at a certain level and drops, due to early failures, to a certain constant level where it stays until the beginning of the wear-out phase. The failure rate rises again due to catastrophic and parametric failures until the end of the product’s life. For LED nearly all catastrophic failures occur during processing and installation of the LED product and after that lifetime is limited only by parametric failures (depreciation of light output, color shift). This applies only for the LED and not for the power supply, driver and other components in the luminaire.
Does a high lifetime and high reliability of the LED automatically mean the same for an SSL product? Surely not! There are many components (power supply, drivers, optical elements, gaskets) with comparable or even lower lifetimes than the LED in a luminaire or module. Also, environmental conditions of the LED may differ according to the SSL product or the application where it is used. Lifetime of the product will be lower than the lifetime of each component integrated in the luminaire, reliability will be worse than reliability of each element in the product.
What customers really want to know are confinable figures for lifetime and reliability for the luminaire or the module. Up until now we don’t have standardized methods to specify the lifetime of LEDs but at least we are on the way. Similarly, we could standardize specifications for lifetime of power supplies and drivers. Using mathematical algorithms we could at least calculate lifetime expectancies for the electronic parts of an SSL luminaire.
Maybe these figures for electronic components in luminaires will not tell the full truth about lifetime and reliability of LED luminaires or modules. Maybe we could do better integrating lifetime figures for other components in the product. But reliable figures for LED and electronics will be much better than anything we use today: estimations of LED lifetimes, determined without any standardized methods, without knowing the environmental conditions of the LED, without taking into account any other components.
Günther Sejkora
He received his PhD from the University of Innsbruck after studying physics, IT and mathematics. He spent more than 20 years in the Research & Development department at Zumtobel Lighting and then went on to start his own company, “items” where, together with industrial partners, he has carried out more than 50 R&D and technology projects in the fields of LED lighting and lighting controls. He was Managing Director of the Kompetenzzentrum Licht GmbH from 2010 to 2015 and is currently the Research and Innovation Manager at Luger Research.
