Indian Society Of Lighting Engineers, Karnataka State Centre

Summary report on two lectures on LED
(1) By Vikrant Mahajan of Labsphere, US, &
(2) By Hemant Prasad of Sapa, Sweeden

The program was chaired by the Chairman & HOD of the Electrical & Electronics Department of the University Vishveshvaraya College of Engineering.

Comments from a participant - Shri Sandeep Shrikhande:

The program conducted was very good, We learnt a lot about the LED and heat sink units and the revolution going to happen. I am also having the same openion that still in market there are so many vendors are in LED business (they approach us) but when we ask them about the complete specification/data-sheet they will fail to produce it.

Also we would be more interested if we can get more workshops on –

1.  Retrofitting or equivalent item(like lamp & ballast) for existing light fixtures.
2. Common chart for different vendors where it is clearly mention about re-fitment.
3. To get more knowledge about color rending.

For your information and thanks for giving us the opportunity to attend such a interesting program.
Regards,
Sandeep Shrikhande

Dr.Gail Devoid & Mr.Vikrant Mahajan of Labsphere presented on the present state of standards applicable to LEDs and LED Luminaires.

While there are no National Standards available, only IESNA / IES standards are available nd they are being used fr testing LEDs and Luminaires based on LEDs.

• The key standards are LM-79-08 and LM-80-08.
• Key observations about LM-80:
  
• Key observations about LM-79:
  
  

Mr.Hemant Prasad, representative for SAPA, Sweeden, gave the presentation on the recent developments in head sink designs for LEDs. While SAPA has been in the field of cooling for various applications from automobiles to generating sets, it has also a special design software for catering to the requirements of the LEDs and LED assemblies or arrays.

The presentations (both slides (ppt) and audio are available on request. Please contact secretary@islek.in if you want them.

The lectures were followed by discussions. The discussions brought a number of new issues and some of them are covered in the following note about the need for national standards, testing, certification and marking.

LED Lighting Life & Performance Prediction
(Rounding off the summary of the discussions following the lectures)
By Bhavani Prasad.
(Note: We will try to bring out the details of the highly interesting discussions that took place as an FAQ on the subject, as early as possible. We request participants to mail the questions so that we do not miss anything.)

LED's have moved up from simple indication application to lighting applications consequent to the development of White LEDs with high brightness. They are also very attractive alternatives to the existing sources of lighting inview of their advantages, such as (1) small size, (2) absence of delicate compnents within, (3) long life, (3) high efficiency, (4) less heat production, (5) better utilization of the emitted light.. The disadvantages are the high cost, lack of standards for performance, non availability of established data about life, performance over the later part of its claimed long life. Another disadvantage is thesource brightness, which in many applications causes discomfort glare. (LED being a point source the entire light flux comes out from a very small area and as such the source is extremely bright.)

The manufacturers of LED lighting products often claim the long life of LEDs as one of the major benefits. Besides energy savings, long life of LED lighting products can also result in noticeable maintenance cost reductions. The term “long life” for general lighting applications refers to 25,000 to 50,000 hours of operation time. In fact some known information about the colour LEDs which have been in use now for over 25 to 30 years in indication applications suggest that the life could be even higher. As a result, products properly installed in a given application may last as long as five to 10 years without needing replacement. How can the LED lighting manufactures claim such long life product without conducting years of reliability and durability tests? Without the claim on the long life LEDs are not a viable alternative to the existing sources. Purchasers are for right reasons quite sceptical about the life, particularly after the bad experience with CFL lamps. The bad experience has been compounded or strengthened by the CFL manufacturers coming with a replacement guarantee of one year initialy, which came down to 6 months and is now totally withdrawn. Consumers, who have to make purchase decisions, do not have the support of any independent authority for testing and certification. Public, as a common man consumer is at great risk in the absence of standards. Manufacturer's claims are undependable as premature failures are often covered up by claims of power supply fluctuations or improper use etc., Public, in the context of the organizations, have problems of normal requirements of public procurement, due to lack of standards for performance in terms of output, stability of output over the life period and the life.

While National Standards have not yet appeared, some LED lighting standards committees in the industry have struggled to establish an acceptable standard method to be used by the LED integrators to project final product life. One such series of standards is the series being brought out by IESNA (Illumination Engineering Society of North America). Such standards are generally perceived to be biased towards the manufacturer, since the manufacturer takes active interest in the formulation of the standard and the consumer has very little oppertunity to contribute his requirements. In view of the high cost of LEDs, the long life is an extremely critical contributer to a purchase decision.

Regarding the life of LEDs some ideas have already crystallized. Unlike an incandescent lamp, which has a very nominal degradation in light output till it fails and stops working due to filament breakage (and as such a very defined end of life point), LEDs face multiple problems of output detrioration such as

1. reduction of output of luminous flux due to changes in p-n junction ageing,
2. changes in the absorption characteristics of the transparent encapsulation medium,
3. changes in the spectral distribution of the light output causing a change in the colour of the output light,
4. changes in output brought about by change in the operating temperature due to ageing of the contact between the diode strata and the heat sink.

The three major challenges to developing a standard prediction method are:

1. variation of LED chip and packaging technologies that lead to significantly different lumen degradation behaviors especially within the first few thousands hours;
2. short testing duration based on IESNA LM-80; and
3. data collection errors due to repeatability and uncertainty.

One approach used by some LED manufacturers is to first identify what causes the lumen output degradation for a given LED package. An LED package is defined, per ANSI/IESNA RP-16 as,

• “An assembly of one or more LED dies that includes wire bond or other type of electrical connection, possibly with an optical element and thermal, mechanical, and electrical interfaces.” In general, LED package degradation over time occurs for the die (or chip), phosphor, encapsulation material and lens used in the package. For simplicity it can be grouped into two parts –
  • A) chip degradation and
  • B) package degradation.
• Each part of the degradation or aging behaviors can be tested and analyzed separately. Based on years of semiconductor product development and testing experiences, experts in the industry have a well established approach to shorten the test duration, which is called accelerated life tests.
• In the accelerated life tests, much more severe operational and environmental conditions are applied to accelerate the degradations. This may include higher operating current, higher temperature and humidity.
• By conducting the accelerated life tests, the data collected can be used to compare with the degradation data under the normal conditions. Then the correlation between these two conditions can be found, and the behavior can be modeled mathematically.
• For the chip part, depending on the technologies used by each LED manufacturer, the degradation behavior may be close to one of the mathematical models, which could be an exponential equation.
• For the package part, the degradation behavior may be found differently, in which a different equation can be used with the data collected.
• Once each part of the degradation behavior is identified, one can superimpose these fitting curves and then a special mathematical model is developed to describe the combined degradation behavior for the LED package.
• Because of the variations in the LED packages, the models or the equations and the parameters used in the equations vary.
• The validity and accuracy of these models should also be confirmed by lumen maintenance test data under the non-accelerated life test condition.

Once the models for different LED packages are established, the LED manufacturers can also apply them to the family or group of products where the technologies or designs are similar. Using the prediction model associated with the LED package, the estimated lumen maintenance information over a long period of time, (25,000 to 50,000 hours or longer), can be provided.

The prime mover for LED Lighting systems appears to be the Energy Star program in the US. But without a national standard to refer, there are problems for all classes of purchasers

• individual small consumer, who cannot afford to specify or test,t as well as
• the large corporate or Government/Public Services/ Public sector consumers.

The US Energy Star Program requirement was stated to be that: “The LED package(s) / module(s) / array(s) used in the fixture has/have been tested according to LM-80, and the package(s) / module(s) / array(s) demonstrated at least 91.8 percent lumen maintenance at 6,000 hours (residential indoor) or 94.1 percent lumen maintenance at 6,000 (residential outdoor and all nonresidential).” The thresholds of 91.8 percent and 94.1 percent are based on an exponential model that assumes the LED packages’ lumen decay over the time follows an exponential curve. For reasons stated previously, an exponential curve and 6,000 hour threshold can’t simply and reliably, in practice, describe the wide range of LED packages and their lumen maintenance. Also for the same reasons stated previously, the standards committees continue to struggle to establish standard lumen prediction models. Some alternative approaches for setting up a 6,000 hour threshold have been discussed in the standards committees. In the meantime, it is very important for the LED users to know the LED package lumen maintenance information before they commit to a luminaire design, production and installation.

Essential steps are:

1. Existance of a standard with test procedures and bench marks
2. Research Data on the extent to which the test data on life can be expected to be true in reality
3. Possibility of the manufacturer (here we have to consider these at different levels) to have the test facilities
   1. manufacturer of the LED device
   2. the integrator who produces the basic module of LED+current management+thermal management and also
   3. the assembler who produces the end device as sold in the market), to have his in house facility for the production testing his products before labelling and issuing data sheets and also
4. independent test houses who will have to test and certify the products.

First, the LED users including luminaire manufacturers and lighting designers, need to extend the efforts to understand and qualify the LED package suppliers and their products. Two pieces of information may be requested from the LED package manufactures. One is the testing data such as IESNA LM-80, and another equally important part is how the lumen prediction curves associated with the LED package being tested are derived by the LED manufacturer. This effort may help the LED users to distinguish reliable and reputable LED package manufacturers.

Second, the LED users need to understand that LED package lumen maintenance prediction information should not be used to directly predict the luminaire product rated life or lumen maintenance. All LED luminaires contain many other components that impact the performance and decay over time.

There are two other major components or subsystems that have been identified, based on past experiences, which contribute to luminaire level lumen output decay over time. One contributor is the LED driver. Over the long period of time (25,000 hours or longer), without accounting for catastrophic failure, the driver efficiency reduces. In turn its output characteristics such as current may also change. (In the case of the CFL lamps there have been extensive failures in the control gear pcb-in particular the capacitor.) LED is a current device, its lumen output is sensitively relating to the operation current. When the LED driver’s output current changes over a long period of time, even when the LED package maintains unchanged lumen output, the luminaire level light output may be reduced due to the drive degradation. Or the current output may increase over time and cause deterioration of life. The second contributor is the optical elements used in the luminaire. This includes secondary optics (lens, reflector, prism, diffuser and others) and the cover lens. If these optical elements are made of plastics or having metalized reflective surfaces, the optical characteristics will change over time. Transmittance will be reduced due to material hazing and coloring. The reflectance will be reduced due to surface blemish, cracking and out-gassing. The optical elements transmittance and reflectance losses over time significantly affect the overall luminaire lumen maintenance. For instance, a 30 percent transmission loss for plastic material over 10,000 hours of operational time is not uncommon. There are other environmental variables which may also contribute to premature loss of transparency such as exposure to UV (both internal & external from Sun), atmospheric gases such as sulfur-di-oxide etc., The encapsulation of the diode may also deteriorate.

Besides LED package lumen depreciation, other subsystems’ performance degradation must also be accounted for in long term luminaire lumen maintenance. Based on the knowledge from other lighting industries such as automotive and transportation lighting, the environmental impact of temperature, humidity, salt and other chemical corrosion to the luminaire light output reduction over time have been identified and corresponding testing and standards were established and are in use today. The general illumination industry standard committees are currently discussing the standardization for luminaire level reliability and durability. At the same time, it is encouraged that luminaire manufacturers make more efforts to identify other subsystems degradation behaviors beyond LED packages lumen maintenance.

The bottom line is that in order to get the full view of lumen maintenance degradation over time, one needs to look at not only the LED but also the luminaire and its components as a whole.

END