Evaluating performance of LED-based luminaires
This article sheds light on the evaluation of manufacturers’ performance claims by explaining the different “initial” and “over time” performance criteria for LED-based luminaires.
There has been a significant increase in the use of LED-based luminaires in recent years. Initially, there were no universal standards available to measure or compare the performance of LED-based lighting products. This situation is compounded by new and unproven entrants flooding into the market, some making dubious claims about their products’ performance. There is a lot of confusion among customers about which LED systems to choose.
In this regard, the main challenge for the professional market is to improve the way users of LED-based luminaires (such as specifiers, lighting designers, technical engineers and policy makers) evaluate the performance claims of different LED luminaire manufacturers when preparing lighting projects or tender specifications. Today, they often compare – unwittingly – apples with pears.
Standardised quality criteria
As things stand at present, evaluating LED systems is complex. There are two main reasons for this:
- Different manufacturers use different technical definitions to describe the performance of their products, making them difficult to compare.
- The technical design of a product can make a difference in terms of performance. Even if two luminaires are based on exactly the same LEDs, their performance can be vastly different because of the design choices made.
Fig. 1: “Over time” performance.
If we look at efficacy for example, we can see that the design of the product can make a huge difference to the luminaire’s system performance. The effectiveness of the heat management, the driver and the optics can all make or break the efficacy of the total LED-based luminaire.
When evaluating performance claims from different manufacturers,
- Apply a standardised set of quality criteria for comparison.
- Only evaluate products that have been measured in compliance with appropriate IEC standards.
This will allow you to judge comparison claims on an equal, like-for-like basis – apples with apples, so to speak.
Fig. 2: IEC lifetime metric.
IEC performance criteria
Both “initial” and “over time” performance must be evaluated to have confidence in how LED-based luminaires will perform and how long they will sustain their rated characteristics over their years of operation. At present, it can be difficult to know whom to trust or what to believe.
Standardisation of performance requirements is an important first step towards full transparency regarding the performance of LED-based luminaires in the professional market. Therefore, the IEC recently developed and published specific performance standards for LED-based luminaires.
These standards describe how to measure “initial” performance and to provide a lifetime metric for “over time” performance.
The initial product specifications will typically be measured, whereas performance over time will be calculated using the IEC lifetime metric for LED-based lighting products.
Fig. 3: Light output decreases over time.
IEC “over time” performance criteria
There are two relevant “over time” performance values to be considered, related to gradual and abrupt light output degradation of an LED-based luminaire at rated life (see Fig. 1).
Gradual light output degradation relates to the lumen maintenance of a luminaire over time. It tells you how much of the initial lumen output of the luminaire is maintained after a certain period of time. The lumen depreciation can be a combination of degradation of optical elements used, individual LEDs giving less light and individual LEDs giving no light at all.
Abrupt light output degradation describes the situation where the LED-based luminaire no longer gives any light at all because the system, or a critical component therein, has failed.
The IEC lifetime metric for LED-based luminaires specifies useful life and time to abrupt failure (see Fig. 2).
Fig. 4: Measure a batch of LED-based luminaires to obtain a range of values.
Gradual light output degradation (useful life)
The gradual light output degradation of a population of LED-based lighting products at a certain point in time is called useful life and is generally expressed as LxBy. Useful life describes the lumen maintenance of an LED-based luminaire over time.
Useful life is expressed as LxBy and means length of time during which y% of a population of operating LED-based luminaires of the same type fails to provide at least x% of the initial luminous flux. Lx describes the lumen maintenance: L80 means that the luminaires of this specific type still give 80% of their initial light output.
Fig. 5: The data rearranged.
By describes what percentage of the population is true. The example L80B50 reflects the age (in hours) at which 50% of the population have failed parametrically. Parametrically, in this case, means an LED-based luminaire producing less light than 80% of its initial flux but still operating.
If we take a closer look at a typical curve showing how light output decreases with time, we can read off the point in time where the light output decreases to a certain value (see Fig. 3).
However, to understand the useful life of the respective LED-based luminaires, we must investigate what is actually happening at that point.
We measure a whole batch of products and obtain a range of values (see Fig. 4).
Fig. 6: The median is 80 and there are as many points above the median as below it.
When we average all the products we have measured, we create a point on our depreciation curve. Some of the products will be above that average and some below. They will not all be of the same value. We then rearrange the data in a more meaningful way (see Fig. 5).
We can now see how many of the products are below and how many are above the average. It is useful to look at the median, which is the point on the graph where we have the same number of points above the median as below it.
With LEDs, the distribution is logarithmic for LED lumen decay. With such a distribution, the median is close to the average. This is significant as we will use the average for our design calculations and the median for predicting life beyond measurements.
In this example, the median is 80 and we have as many points above the median as below it. We say that the life is the time to which the average is 80% of the initial average value (see Fig. 6).
Light output lower than the target value is called a “parametric failure” because the product produces less light but still operates.
If we place this data back on the graph, we see that the average is at 80% and we have 50% parametric failures and 50% still operating above our rated value of 80% (see Fig. 7).
Fig. 7: 50% of the lamps have failed parametrically while 50% operate above 80%.
In the IEC, this is defined as the useful life, and we use the Lx term to quantify the average value and the By to tell us how many are above the average and how many below. In this example, we would say that the useful life (L80B50) = 50 000 hours. Useful life therefore has to contain both the L and the B components.
We may decide that having 50% of our sample below the average is too much and we would want to consider a smaller percentage. If we want, say, 10% of our batch below the average, the time to achieve this will be less.
In this example, we would say that the useful life (L80B10) = 45 000 hours (see Fig. 8).
Fig. 8: Useful life (L80B10) = 45 000 hours.
However, for general applications, we would normally take y to be 50, as shown in Fig. 9.
Fig. 9: For general applications, take y to be 50.
In this specific case, the average and the median values will be the same, so we define the rated median useful life Lx. In this case, we don’t need to use the B term as median y is always 50.
Rated median useful life, or rated lifetime in the new IEC general definition, is the value marked on product datasheets, leaflets or websites.
Abrupt light output degradation
Besides lumen maintenance (useful life), there are other factors to consider when evaluating performance over life. LED-based luminaires and modules are sophisticated products consisting of many components. System reliability is an important parameter that should be considered with expected long life. An LED-based luminaire will last as long as the component with the shortest life. There are several critical components in an LED-based luminaire that influence the system reliability.
The IEC lifetime metric therefore also specifies time to abrupt failure, which takes into account failure modes of critical components in the LED-based luminaire design.
The abrupt light output degradation of a population of LED-based lighting products at a certain point in time is called time to abrupt failure and is expressed as L0Cy. Time to abrupt failure describes the situation where the LED-based luminaire no longer gives any light at all.
Lx describes the lumen maintenance: L0 means that the LED-based luminaires of this certain type give 0% of their initial light output. Cy describes for what percentage of the population that is true.
The example L0C10 reflects the age (in hours) at which 10% of the population fails abruptly. If we take a closer look at a failure curve, we see what percentage of failures we have at a given time (see Fig. 10).
Fig. 10: The age at which 10% of the population fails abruptly.
However, to understand the time to abrupt failure of the respective LED-based luminaires, we must investigate what is actually happening at that point.
Taking a closer look at our data will show more detail (see Fig. 11).
Fig. 11: Some LEDs have failed completely.
Within the sample batch of LED products, we can see that some have failed completely or abruptly. This may be due to mechanical or driver failure, among others.
If we order this data in a similar way as before, we see that the failures are a percentage of the total (see Fig. 12).
Fig. 12: The failures are a percentage of the total.
So, at any given point on our failure curve, we simply plot the percentage of failures compared with the original sample and note the time at which this occurs (see Fig. 13).
Fig. 13: Percentage failures compared with the original sample.
Note that we are only concerned with abrupt failures and not failure due to loss of light.
We call this the “time to abrupt failure” or Cy. In this example the time to abrupt failure (C10) = 53 000 hours.
We can now look at a specific failure that occurs at the rated median useful life (see Fig. 14). The rated abrupt failure value (AFV) is the percentage of LED-based lighting products that fail to operate at the rated median useful life Lx. In this example, the rated median useful life is 50 000 hours and the rated abrupt failure value is 8%.
Fig. 14: Failure at the rated median useful life.
Unfortunately, the industry has not yet reached consensus on what critical components have to be taken into account when calculating time to abrupt failure. Philips Lighting has therefore decided not to publish this value.
Conclusion
Luminaire life is always a combination of gradual and abrupt light degradation. Note that luminaire life claims must always be specified together with a specific ambient temperature, number of burning hours and associated switching cycles. As mentioned here, the design of the LED-based luminaire can have a significant impact on the luminaire performance, including its lifetime.
It is therefore important to realise that data provided by LED or LED board suppliers cannot simply be translated one-to-one as LED-based luminaire performance data. We should be wary of claims such as “these luminaires use the same LEDs so therefore their (over-time) performance is the same”.
It is also important to remember that over-time performance values are predictions rather than measurements. As the useful life and time to abrupt failure of LED-based luminaires are so long, it is not possible for manufacturers to measure these before launching new products. Instead, they use shorter measurements and extrapolate those to arrive at predictions.
The quality of these predictions varies wildly as there is no standard in place yet that describes how these predictions or extrapolations should be done. The IEC only describes a lifetime metric for LED-based products at this point – which parameters should be stated in terms of useful life and time to abrupt failure, but not how to calculate these.
Philips has developed a tool to calculate useful life and time to abrupt failure for LED-based luminaires. Calculations are based on real-life endurance test data of LED boards, accelerated testing of critical components and an understanding of which design parameters are critical to extend luminaire lifetime.
Contact Philips Lighting SA, Tel 011 471-5000, lighting.za@philips.com
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