Service life of LED light sources

The luminous flux of LED light sources also decreases with increasing operating duration. This phenomenon is referred to as luminous flux degradation. However, total failures of LED light sources only occur after a very long period of time when the degradation is far advanced. Therefore, total failure plays only a minor role when considering the service life of this type of LED products. Total failure is only recognizable for individual LEDs, e.g. as a defective pixel on an LED display.

Following the failure rate of incandescent lamps, the time of degradation to 50% of initial luminous flux (rated luminous flux) is a common definition for household LED retrofit lamp service life.

Rated service life

For LED luminaires, there is no conventional definition of service life, be it “nominal service life” or “economic service life”.

Instead, it is common to relate the service life specified by the manufacturer to the respective specified level of luminous flux degradation. This procedure is suggested by the standards regarding luminaire performance (DIN EN 62722-1; Luminaire performance – Part 1: General Requirements [106], DIN EN 62722-2-1; Part 2-1: Particular requirements for LED luminaires [107] and LED modules [DIN IEC/PAS 62717; LED modules for general lighting – Performance requirements [16]).

A more general definition of rated service life based on these standards leads to an expression in the form of L_xB_y (e.g. L₈₀B₁₀ = 50,000 h). The index x describes the percentage of residual luminous flux of a luminaire due to degradation. The index y describes the percentage of a large number of luminaires which statistically undercut this luminous flux, meaning the portion of luminaires with increased drop in luminous flux (“gradual failure fraction”).

The definition of “average rated service life” L_x is common on the market, without specification of B_y. In this case, it is assumed that the index y of the general definition is 50. Therefore, L_x refers to the statistic average of the residual luminous flux remaining at the end of service life for a large number of luminaires.

• L_x, the average rated service life

• Φ(L_x), the luminaire luminous flux at L_x,

• Φ_B, the rated luminous flux (initial luminous flux), and

• x, the statistically averaged residual luminous flux of a luminaire at the end of service life L_x, in %.

The service life specification

L₈₀, 50,000 h for a given luminaire,

e.g. means that a large number of these luminaires in total after 50,000 operating hours still generate at least 80% of their rated luminous flux (available initially and in total). Therefore, this is an average value.

Until the rated service life is reached, the progress of the drop in luminous flux (degradation) can be regarded as linear in simplification (see figure).

This means:

(see symbols above)

In practice, this approximation can often be used beyond the scope of the rated service life specified by the manufacturer, for a period of up to 1.5 L_x (see also tables in chapter “The lamp maintenance factor of an LED luminaire”).

Common average rated service life specifications refer to different degrees of degradation: L_90-, L_85-, L_80-, L_70- and L₅₀. These values can be converted into each other to a certain extent.

The choice of rated service life index has a significant impact on the maintenance factor to be specified in the lighting planning stage. Further information can be found in the tables in chapter “The lamp maintenance factor of an LED luminaire”.

For LED luminaires, a calculation tool (TRILUX LIFETIME CALCULATOR) for determining LLMF and LSF is available to users of the TRILUX portal.

The LED as a semiconductor in which electrical energy is converted is temperature-sensitive, similar to a power transistor inside an amplifier or a processor inside a computer. The extent of the luminous flux degradation – and therefore the rated service life – depends particularly on the operating temperature of the LED inside the luminaire. Correct service life specifications therefore require reliable thermal management of the LED luminaire (see also chapter “Thermal management”) and usually relate to an ambient temperature (rated temperature) of 25°C, unless otherwise stated by the manufacturer. Based on the aforementioned standards, it is possible to specify an ambient temperature t_q other than 25°C in the data sheet at which specified technical quality criteria are achieved (see also chapter “Operating conditions” and chapter “Performance labelling of LED luminaires”).

Failure rate

Total failures of LED lamps or luminaires are expressed by the C_z value (catastrophic failure) with the numeric value of z indicating the expected failure rate in per cent at a given time.1

Luminaire classification specification

C₅ = 100,000 h at t_q = 35° C

would therefore state e.g. that the LED luminaires in question feature a total failure rate of 5% at an ambient temperature of 35°C and after 100,000 operating hours.

The value of a luminaire’s total failure rate at the end of the average rated service life Lx (B50, see above) is referred to as AFV (“abrupt failure value”). In practice, significant failure rates in LED products only occur with very advanced degradation. At average rated service life specifications x ≥ 80 the AFV is therefore negligible.

When determining the maintenance factor, the total failure rate expressed by LSF (“lamp survival factor”) must be considered (see also chapter “The lamp maintenance factor of an LED luminaire”).

Therefore, the lamp survival factor [Symbol] only matters for the determination of the maintenance factor after the end of the rated service life. In the tables in chapter “The lamp maintenance factor of an LED luminaire”, this factor is already considered.

Rated service life with constant light output (CLO)

With constant light output (CLO), the luminous flux of an LED product is constantly regulated to the level corresponding to the residual luminous flux which is statistically expected at the end of the rated service life. If luminous flux is used as the basis for lighting design, energy can be saved until the end of the rated service life which would otherwise only lead to unnecessary excess lighting.

For a luminaire with rated service life L_x the following applies:

and

with

• Φ(t), the luminaire luminous flux at the time of t,

• L_x, the average rated service life,

• Φ'_B, the rated luminous flux (initial luminous flux) of a luminaire with the same rated service life without CLO, and

• x, the residual luminous flux percentage of Φ'_B at the end of the rated service life.

Upon expiration of the rated service life, degradation can no longer be compensated and the luminaire luminous flux decreases at a rate corresponding to a luminaire without constant light output (see figure above).

Generally, the power consumption of the luminaire in initial state P_new required to provide constant luminous flux Φ_new is specified in the data sheet of such luminaires. In addition, the required power consumption value P_x at the end of the rated service life should be specified as well.

The result for a given average rated service life L_x is derived as follows:

The power consumption is constant after the end of the rated service life, when the luminous flux Φ_new can no longer be maintained. It has reached its maximum value:

Over the course of the rated service life, the power consumption increases continuously (see figure):

The resulting time-dependent factor at which power consumption increases is defined as power lifetime factor PLF:

The tables in chapter “The lamp maintenance factor of an LED luminaire” list the resulting PLF values.

The energy requirement W_new(t) of a newly installed luminaire up to a point within the rated service life is derived as a statistic average of: