1. Preface The significant percentage respect to the initial luminous flux emission The continuous growth of LED lighting is significantly...
The significant percentage respect to the initial luminous flux emission
The continuous growth of LED lighting is significantly transforming the lighting industry, providing a range of millions of colours and dynamic effects that conventional lighting cannot match in terms of design. Furthermore, the miniaturization of the dimensions and the low thermal radiation allow LEDs to be incorporated almost everywhere. In addition, ensuring a long life together with energy and management savings, they offer potentially efficient lighting solutions.
However, there is a problem: in recent years the market has been invaded by a large number of new unknown operators, with questionable information about the performance of their products, too much optimistic to be true and even technically unsupported.
The need for accurate information leads designers and end users to demand the characteristics of such new lighting systems in order to be able to adequately assess how long a LED device keeps a significant percentage of the initial emission of light during the years of operation and depending on the so-called "child mortality" or sudden failure.
In the standardization process, there are three elements that can be standardized, such as:
The performance prescriptions described in the IEC standards provide a definition of qualitative criteria and how to measure them.
The lifetime of LEDs is, in most cases, longer than practical test times. Consequently, verification of manufacturer's claims on life expectancy should be considered as indicative, because based on extrapolations not yet validated by durability tests as other components concur in the supply circuitry.
In order to substantiate a life-long statement, extrapolation of test data is required. In this respect, a general method is being studied to allow the projection of measured data over the limited time of the test.
The IEC Regulations propose the following list of qualitative criteria to be considered when evaluating manufacturer's declarations:
Currently, several LED lighting manufacturers produce test results according to the LM-80 as the basis for Lx, By and Cz statements as maintenance thresholds for LED lighting fixtures.
The LM-80 requires to test LEDs for 6.000 hours and recommends testing for 10.000 hours. It requires tests at three surface temperatures (55°C, 85°C and a third temperature determined by the manufacturer) to see the effects of the temperature on the light output and specify the additional test conditions to ensure consistent and comparable results.
In fact, the main LED manufacturers try their products at the minimum of 6.000 or 10.000 hours provided by the LM-80, and then apply extrapolation methods as described in TM-21 (see Reference Table below) to get the values L90, L70 and L50. The device manufacturers translate these curves into specific curves of the LED lighting fixture.
For a better understanding, see below an extrapolation diagram, (source CREE) and the reference regulations.
IES LM-79-08: Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products – Illuminating Engineering Society of North America, 2008
LM-79: Prescrive i metodi di prova uniformi in condizioni controllate per le prestazioni fotometriche e colorimetriche, nonché le misure di potenza elettrica degli apparecchi di illuminazione a LED. Questa può essere usata per misurare le specifiche iniziali elettriche e fotometriche di un apparecchio LED.
IES LM-80-08: Approved Method: Measuring Lumen Maintenance of LED Light Sources – Illuminating Engineering Society of North America, 2008
LM-80: Regarding the measurement of the luminance maintenance of LED light sources (single LEDs or multi chips). It consists of a real size for the first 6.000 hours, combined with an extrapolation until the end of life. Many lighting fixture manufacturers translate the LED light source curve in the illumination LED device maintenance curve using the TM-21 recommendations.
There are two constraints:
IES TM-21-11: Projecting Long Term Lumen Maintenance of LED Packages – Illuminating Engineering Society of North America, 2011
TM-21: Provides recommendations for the long-term projection of LED luminous flux maintenance using the data obtained during tests in accordance with IES LM-80-08
The life of the LED module and power supply unit should be stated separately. If the lifetime of the power supply is shorter than LED, replacement of the power supply will be necessary before the lifetime of the unit is completed. This means that there is no unicity in the statement of the lifespan of the lighting equipment. A useful metric for "median useful life" has been introduced in IEC 62717. This is the time elapsed until the 50% of the LED lighting fixtures in use reach the declared light flow, for example L80.
Most manufacturers state in the catalogue the parameters, referring to 100% of the initial light flux:
L70 B10 C10, where:
L70 indicates the percentage value of maintenance under operating conditions with respect to the initial light flux and operating temperature.
Eg.1: If at 25°C a LED declares 100.000h of life, L70 indicates that it will be able to guarantee 70.000h (70% of the nominal value)
Eg.2: If at 45°C a LED declares 90.000h of life, L70 indicates that it will be able to guarantee 63.000h (70% of the nominal value)
Eg.3: If at 60°C a LED declares 50.000h of life, L70 indicates that it will be able to guarantee 35.000h (70% of the nominal value)
B 10 indicates the percentage value of the lighting fixtures that are supposed to be extinguished under operating conditions with respect to the initial light flux and operating temperature.
E.g.: In case of malfunction, B10 indicates that the luminous flux, in its overall circuitry, may decrease depending on the remaining number of lighting equipment on (possible 10% decrease compared to the initial volume).
C10 indicates the percentage value of the lighting fixtures that may break down and not to re-start in operating conditions with respect to the initial quantity.
E.g.: In the event of a definite failure, C10 indicates that the luminous flux, in its overall circuitry, may decrease as a function of the remaining number of lighting equipment (possible 10% decrease compared to the initial quantity).
Cortem Group, according to the type of lighting fixtures and the built-in LED model installed, has designed and manufactured specific housings suitable for the dissipation features required by the manufacturer of LEDs, in order to dissipate LEDs heat generated during operation and thus optimizing, during the engineering phase, the ability to last in time with the least power and luminous flux loss.
In addition, always paying close attention to the functional guarantee and the end-of-life aspect, Cortem Group analysed all the constructive and product quality variables, in order to minimize possible defects resulting from so-called "child mortality" and possible implications for a proper dimensioning of the drivers, for uses in environments with positive or negative temperatures.
All of the above considerations are of an exemplary nature and they are not intended for a specific product of Cortem. For each specific product is necessary to refer to the technical data sheets.