Evaluating Performance of LED-Based Luminaires — LED professional

Today, the preparation of lighting projects or tender specifications is more difficult than ever. Manufacturers of LED based luminaires provide different performance data for their products, often not based on standardized metrics, making it impossible to allow for an “apple-to-apple” comparison.

The intention of this article is therefore to give users, like specifiers, lighting designers, technical engineers and policy makers a guideline on how to define performance requirements, in particular related to the term “useful life or lifetime”. This article recommends a fixed set of performance data for LED based luminaires. This data set is focused on the information which is necessary for lighting application design.

Evaluating Performance of LED Based Luminaires

While the quality of LED technology has rapidly improved and the application considerations have not changed, the product data have remained unnecessarily complex. The main challenge for the professional market is to improve the way users of LED based luminaires evaluate the performance data of different manufacturers when preparing lighting projects or tender specifications.

Both “initial” and “useful lifetime” performance data should 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, evaluating LED based luminaires is complex because of two main reasons:
• The use of different technical definitions and related parameters to
  describe the performance of products, thus making them difficult to
  compare (for example the incorrect use of LED module or light source
  data instead of luminaire data)
• The technical design choices made for a product can make a tremendous
  difference in terms of performance over the useful lifetime

The establishment of simplified performance metrics that support the needs of good lighting design and allow easy product to product comparisons to be made can add value to the professional market.

Confusion due to the use of different sets of definitions can be eased by following the latest IEC/EN standards on performance of LED based luminaires. These standards give guidance on “what” (phenomena and metric) to publish and “how” (measurement and/or calculation method) to arrive at a set of comparable product specifications.

Lighting Requirements

Good lighting design calculations require different technical product parameters based on standardized and therefore comparable data. IEC 62722-2-1 Performance requirements for LED based luminaires, gives the following overview of the relevant “initial” and “useful lifetime” product information parameters that should be used for the planning of lighting designs.

Parameters to being considered for planning of lighting design:
• Rated input power (P in W)
• Rated luminous flux (Φ in lm)
• Rated luminous efficacy (η in lm/W)
• Rated luminous intensity distribution (in cd or cd/klm)
• Rated correlated color temperature (Tcp in K)
• Rated color rendering index (CRI)
• Ambient temperature related to performance of the luminaire (tq in ˚C)
• Rated median useful life (Lx in hours with x for the associated rated
  luminous flux maintenance factor in %)
• Rated abrupt failure value (in %)

In this context “rated” means the value of the parameter for the LED based luminaire declared by the manufacturer when operated under specified conditions. It is reminded that the tq value for which the performance data is declared shall always be reported even if this is 25˚C. Where applications require tq temperatures other than 25˚C all performance data is required to reflect the actual performance for these specific tq temperatures.

In this section, the initial luminaire performance parameters (1-7) are described that can be used as input for lighting design calculations. The useful lifetime luminaire performance parameters (8-9) are described in the section “Lifetime considerations”.

Common examples of misrepresentation of performance data are:
• Luminous flux output for LED module being stated instead of the luminous
  flux output for the complete luminaire
• Data based on 25 °C operation temperature of the LED module or
  light source instead of data based on the actual operating temperature
  of the source inside the luminaire
• Operating power being based on just that of the LED module or light
  source instead of that consumed by the complete luminaire
• Incorrect comparison of power / efficiency between luminaires containing
  built in control gear and those using remote control gear
• A combination of incorrect input power and luminous flux output values
  resulting in inflated efficacy

Lighting Application Requirements Following CEN

When considering if a product is the best solution for an application we need to understand what should be calculated to ensure that the correct lit environment is created.

When requirements are specific to the given lighting solution within the application space, a lighting design needs to be performed. In that case the data requirements when considering a particular lighting product should be application driven and consider what information is required to ensure the lighting solution is correct for the application space. Any data that is not driven by the application requirements should be considered of secondary importance.

Table 1 shows, according to European standards, which product requirements are relevant for each application and which of these requirements can be fulfilled wholly by the product data and therefore can be specified on a product datasheet.

Table 1: Product data directly linked to lighting application standards

Key to the standards:
• IEC 62722-2-1:2016 – Luminaire performance: Particular requirements for LED luminaires
• EN 12464-1:2011 – Light and lighting: Lighting of work places Part 1: Indoor work places
• EN 12464-2:2014 – Light and lighting: Lighting of work places Part 2: Outdoor work places
• EN 15193:2007 – Energy performance of buildings: Energy requirements for lighting
• EN 13201-2:2015 – Road lighting Part 2: Performance requirements
• EN 13201-5:2016 – Road lighting Part 5: Energy performance indicators
• EN 12193:2007 – Light and lighting: Sports lighting

Lifetime Considerations

There are two relevant useful lifetime performance values to be considered related to “gradual” and “abrupt” luminous flux output degradation of a LED based luminaire.

Gradual luminous flux output degradation relates to the lumen maintenance of the light source in a luminaire. It describes how much of the initial luminous flux output of the light sources in the luminaire is available after a certain period of time. Luminous flux output depreciation can be a combination of individual LEDs giving less light and individual LEDs giving no light at all.

Figure 2: IEC lifetime metric

Abrupt luminous flux 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.

Both “gradual” and “abrupt” luminous flux output degradations have been described in the IEC lifetime metric for LED based luminaires. IEC suggests applying a standard set of quantities for communication to the market: “Median Useful Life” and the associated “Abrupt Failure Value”.

As the Median Useful Life of LED based luminaires can be very long, it is important to understand that useful lifetime performance values are predictions rather than measurements. For manufacturers, it is not possible to measure the useful lifetime values with, for example, 50.000 h before launching new products. Instead, the manufacturers use shorter assessment periods and extrapolate the results to arrive at predictions.

The IEC performance standards currently describe lifetime metrics for LED based products but not how to measure/calculate the parameter of the lifetime metrics. As a consequence, the quality of the lifetime predictions varies wildly and there is a significant risk of apple-to-pear comparison.

Reputable manufacturers will calculate Median Useful Life and associated Abrupt Failure Value based on historical design data and knowledge, component level testing and thermal design.

Lifetime data are normally specified together with a specific ambient temperature (tq), the number of burning hours and the associated switching cycles.

Gradual Luminous Flux Degradation: Useful Life and Median Useful Life

The gradual light output degradation of a population of LED based luminaires at a certain point in time is called Useful Life and expressed in general as LxBy. The population includes operating LED based luminaires only; non-operative products are excluded.

Figure 3: Gradual light output degradation

Useful Life expresses the age at which a given percentile of LED based luminaires (y) cannot meet the lumen maintenance factor x. Light output lower than the required luminous flux maintenance factor x called flux degraded because they produce less light but still operate.

To unambiguously compare manufacturers’ lifetime data, IEC introduced Median Useful Life (Lx). Median Useful Life is the time at which 50% (B50) of a population of LED based luminaires are flux degraded. Median Useful Life is generally expressed as Lx so without the B50 notification.

Example:
Median Useful Life L90 is understood as the length of time during which 50% (B50) of a population of operating LED based luminaires of the same type have flux degraded to less than 90% (L90) of their initial luminous flux but are still operating.

Figure 4: Example of normal distribution for a L90 rated product

Besides the median value (B50), in the market an apparent demand for B10 or even B0 rated products exists. Although By is a defined performance characteristic, the standard IEC 62722-2-1 does not include any technical explanation for how this parameter should be verified or applied.

Figure 5: Product data analysis of an example of a LED based luminaire

Also lighting application design standards give no guidance for how a By factor should be accounted for. Closer technical evaluation as to what this really means is required.

It can be expected that around a distribution of products there will be a proportion above and a proportion below the rated performance value. The graph below shows an example of the normal distribution for a L90 rated product, illustrating the difference of a B10 or B50 value.

Detailed analysis of product data from LED based luminaires from various manufacturers shows that when projecting installation life up to 100.000 hours, the difference in flux degradation between B10 and B50 is about 1%.

For the L90 example at 100.000 hours this means that an initial luminous flux of 10.000 lumen will be 9.000 lumen in the case of B50. If the same luminaire is rated as B10, the corresponding value would be 8910 lumen. Bearing in mind that the rated light output data of both LED and traditional light sources are subject to a typical 10% tolerance this practical differential can be regarded as negligible.

As B10 and B50 are so close together, the spread due to depreciation is low and the median (B50) value represents with a sufficient degree of accuracy the lumen deprecation behavior of a number of products at the projected lifetime (in this example 100.000 hours). The measurement process for B50 is standardized and more widely accepted than any other By. Therefore, for reasons of accuracy and consistency between manufacturers the use of any other By cannot be recommended over the use of B50.

This indicates that for the commonly used L70, L80 or L90 values the By factor is not as significant as may be thought (or promoted) by some manufacturers and users. Consequently, there is little benefit in the continued promotion of By as a significant factor for making product to product performance comparisons. Therefore only the Median Useful Life, generally expressed as Lx without the unnecessary B50 notification, should be promoted.

Abrupt Luminous Flux Degradation: Time to Abrupt Failure and Abrupt Failure Value

An important parameter that should be considered with expected long life is system reliability. A LED based luminaire will last as long as the component used with the shortest life. There are several critical components of a LED based luminaire that influence the system reliability.

Figure 6: Abrupt luminous flux degradation

Degradation of optical material may cause a reduction of luminous flux rather than an abrupt degradation. Failure of one of the remaining principal components generally leads to complete failure of the LED based luminaire. This is not taken into account when indicating the rated Median Useful Life. For that reason, abrupt failures have to be considered separately so it can be taken into account at time of lighting engineering and planning. This is why the IEC lifetime metric also specifies time to abrupt failure, which takes into account failure modes of principal components in the LED based luminaire design.

The abrupt light output degradation of a population of LED luminaires at a certain point in time is called Time to Abrupt Failure and expressed in general as Cy. Time to Abrupt Failure expresses the age at which a given percentage (y) of LED based luminaires have failed abruptly.

To facilitate easy evaluation of manufacturers’ performance data, IEC introduced the Abrupt Failure Value (AFV) of a population of LED based luminaires. Abrupt Failure Value is the percentage of LED based luminaires failing to operate at Median Useful Life (Lx).

Example:
Abrupt Failure Value of 10% represents 10% of the population of initially operating LED based luminaires fail to produce any luminous flux at Median Useful Life.

The current IEC standards do not describe completely what failure modes of principal components to include in the Abrupt Failure Value (AFV) calculations. Since most of the abrupt failures in practice occur in relation to the LED control gear, LightingEurope recommends specifying the expected control gear failure rate of the device as the AFV indicated for the Median Useful Life of the LED-based luminaire.

Figure 7: Principal components of a LED based luminaire

Why Lifetime Is Not Always a Critical Factor

Looking at common practice, lifetime data for LED based luminaires seems to be a race for the highest number of hours belonging to the Median Useful Life L80B50. We have to be aware that in the professional market, requirements are specific to the lighting solution within the application and a lighting design needs to be performed.

As input to the lighting design the average installation life is often given, so one could argue the highest number of hours is not a relevant discriminator when selecting a LED based luminaire.

Consequently, this justifies the question what is the best recommended value for comparing the useful life of LED
based luminaires?
• Fixing of the “x” (lumen depreciation) from Median Useful Life Lx as a
  comparison value for different luminaires? In this case the “time” is not
  fixed and can have a variation from luminaire to luminaire
• Fixing of the “time” value from Median Useful Life Lx as a comparison
  value for different luminaires? In this case the “x” from Lx (lumen
  depreciation) is not fixed and can have a variation from luminaire
  to luminaire

To investigate the importance, the average installation life for different indoor- and outdoor applications have been calculated, based on the annual operating hours and the average time to refurbishment for a product in a specific application.

We also need to be aware that these values may not be realistic in all situations (e.g. in case of the use of automatic lighting controls or application requiring 24/7 illumination).

It can be concluded that for products used in the majority of indoor applications the average installation life will not exceed 50.000 hours. For products used in the majority of outdoor applications the average installation life will not exceed 100.000 hours.

Table 2: Possible examples of average installation life for different indoor applications

Table 3: Possible examples of average installation life for different outdoor applications

LightingEurope believes that “number of hours” should not be a dominant discriminator when selecting LED based luminaires for professional applications. For the lighting design, the maintained luminous flux at the average installation life for a specific application is much more relevant and may support energy saving through the reduction in over-design to account for losses through life.

LightingEurope therefore recommends:
• Lifetime claims should not exceed 100.000 hours, unless it is clearly
  required by specific lighting applications and verified by an appropriate
  life test period
• To enable apple-to-apple comparison it would be best practice to fix the
  “time” value for Median Useful Life to 35k, 50k, 75k and/or 100k and
  express the “x” from Lx (lumen depreciation) for time value(s) related to
  the applications where the product may be used

Table 4: Lumen maintenance factor groups [1]

Maintenance Factor Consideration in Different Applications

With LEDs rapidly becoming the new standard in (functional) lighting design for both indoor and outdoor installations, the need has arisen to provide more clarity on how the existing CIE maintenance factor (MF) determination methods can be applied to this technology.

Clarification is needed to prevent unsafe and uncomfortable situations during the lifetime of the installation. Current CIE technical reports describing the MF determination methodology contain detailed explanations with respect to conventional luminaires and light sources, but lack detail to accommodate LED-based lighting designs. However, the core of the CIE methodology – which is based on the same principles for both indoor and outdoor- is still accurate.

ISO/TC 274 is currently developing a Technical Specification that will provide a standardized way of working for determining the maintenance factor for both indoor and outdoor installations using the methodology as described in CIE 154:2003 & CIE 97:2005. Insights from recently published product performance standards such as IEC 62722-2-1 will be combined with the existing determination methodology from CIE technical reports.

By using the overall MF determination methodology and the content on the impact of the environment on luminaires in combination with the product performance metrics, a robust way of working can be established. This will allow for the determination of the maintenance factor of installations including the latest light source technologies. This will create a level playing field with respect to comparison of lighting designs in the market, provide clarity to all involved parties (from end-users to policy makers), and ensure safety and comfort over the lifetime of the installation.

Further LightingEurope Recommendations

Manufacturers of LED based luminaires should publish apple-to-apple comparable product information following the parameters and as described in IEC 62722-2-1.

Recommended initial performance values to be provided:
• Input power (P in W)
• Luminous flux (Φ in lm)
• Luminous efficacy (η in lm/W)
• Luminous intensity distribution (in cd or cd/klm)
• Correlated color temperature (Tcp in K)
• Color rendering index (CRI)
• Ambient temperature (tq) related to performance of the luminaire (in ˚C)

Recommended over time performance values to be provided:
• Lumen maintenance factor “x” (in %) at the associated median useful
  life Lx (in hours)
• Abrupt failure value (in %) at the same associated median useful life
  Lx (in hours). The expected “control gear failure rate” should be
  provided for this data

Lumen maintenance factor groups (buckets) should be introduced to enable initial product comparison. Separate product specific lumen maintenance factor values for input to lighting designs may also be published.

References:
[1] Details on terms, their definitions and references can be found on
     the LightingEurope website. Also, application requirements from
     EN standards, divided into stated and implied requirements can be
     found there. –
     Please, visit https://www.lightingeurope.org/images/publications/general/

Disclaimer:
This information is for general guidance on matters of interest only. While we have made every attempt to ensure that the information has been obtained from reliable sources, LightingEurope is not responsible for any errors or omissions, or for the results obtained from the use of this information.
The content of this document is a recommendation only and is not binding to any party. LightingEurope members are not bound to adhere to this document.
All information is provided with no guarantee of completeness, accuracy, timeliness or of the results obtained from the use of this information, and without warranty of any kind, express or implied, including, but not limited to warranties of performance, merchantability and fitness for a particular purpose.
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The data and information presented in this guide should not be taken as forming a basis of warranty conditions which are the responsibility of individual manufacturers.

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