Introduction

In lighting design, ensuring that a site consistently meets illuminance standards throughout its entire operational lifespan is of greater importance than merely calculating the initial brightness of a newly installed system. However, over time, the illuminance provided by luminaires inevitably declines due to factors such as the lumen depreciation of light sources, dust accumulation, and environmental soiling. To quantify this phenomenon, we introduce the Light loss factor (LLF), a concept derived from IESNA standards.

The Maintenance factor (MF) is, in essence, the same concept as the LLF, differing only in the specific regulatory framework to which it belongs—the MF corresponds to CIE standards. The core definition shared by both is identical: the ratio of the maintained illuminance on a lit surface after a period of use to the initial illuminance (a ratio that is typically less than 1). By incorporating the LLF or MF, designers can—at the initial design stage—allocate a sufficient margin of luminous flux (What’s luminous flux of LED luminaire?) to compensate for future losses caused by light depreciation and contamination, thereby ensuring that the lighting system consistently meets minimum illuminance requirements throughout its designated maintenance cycle. Let us now delve into the main text to learn more about the LLF, including its calculation methods and its relationship to the MF.

What’s light loss factor?

Simply put, LLF (Light loss factor) refers to the reduction in luminous flux emitted by a lighting fixture after a period of use—typically prior to the conclusion of a maintenance cycle—due to various causes. Specifically, LLF is defined as the ratio of the average illuminance on a surface after a period of use to its initial illuminance. It is calculated as the product of several independent loss coefficients, including the Lamp lumen depreciation (LLD), the Luminaire dirt depreciation (LDD), and equipment-related factors (including the Lamp burnout factor). Unlike the Maintenance factor, LLF does not account for factors such as changes in the reflectance of room surfaces. Among these components, LLD and LDD align with the corresponding parameters defined in CIE standards, while the Lamp burnout factor corresponds to the Lamp survival factor found in CIE standards. Furthermore, IESNA standards for calculating LLF incorporate additional parameters, such as the Luminaire ambient temperature factor, the Luminaire voltage factor, and the Ballast factor. Once the specific values ​​for each of these parameters have been determined, the final LLF value can be calculated. The ultimate purpose of introducing LLF is to enable lighting designers to proactively increase the initial luminous flux—thereby offsetting future light decay (What’s light decay?) during operation—and to ensure that the lighting system maintains the required illuminance levels throughout its entire operational lifecycle. In the next chapter, we will discuss the various factors that influence the magnitude of LLF, as well as the methods used to calculate them.

Determinants of the light loss factor

The Light loss factor (LLF) is not a single fixed value, but rather the product of multiple independent loss coefficients that vary over time. These influencing factors can typically be consolidated into a single multiplier for use in lighting design (ZGSM lighting design solutions) calculations. An exception to this is the Equipment factor (EF), which depends primarily on the initial selection of equipment rather than on changes over time. In terms of control mechanisms, the various components of the LLF can be categorized into three groups: those controlled through equipment selection—such as the inherent performance characteristics of the luminaires and light sources themselves (which fall under the scope of the Equipment factor); those controlled through maintenance schedules—such as operations involving periodic cleaning and lamp replacement; and those beyond the owner’s control—such as external factors like power grid voltage fluctuations and changes in environmental emissions. The task of the lighting designer is to reasonably determine and apply a realistic overall LLF value. The overall Light loss factor is calculated by multiplying all the independent influencing coefficients together. If a specific loss is negligible—meaning its corresponding coefficient is close to 1.0—it may be disregarded; otherwise, it should be estimated based on empirical data derived from similar installations, equipment, and maintenance procedures. In all instances, the Lamp lumen depreciation (LLD) and the Luminaire dirt depreciation (LDD) are the minimum factors that must be taken into consideration. If the calculated overall LLF is excessively low (indicating an unacceptably high level of light loss), it becomes necessary to re-select the luminaires or light sources, or to adjust the cleaning and maintenance protocols. For example: LLF = LLD × LDD × BF (where all coefficients are typically less than 1.0).

What’s lamp lumen depreciation (LLD)?

Lamp lumen depreciation (LLD) refers to the phenomenon wherein the light output (lumens) of most light fixtures gradually diminishes over the course of their operational lifespan as operating time accumulates. LLD is typically represented by a lumen maintenance curve, which illustrates the percentage of the fixture’s current light output relative to its initial output at a specific point in its operating life. For HID lamps, lifespan testing is conducted based on burn cycles exceeding 10 hours per start-up and under specific ballast conditions; this is because operating duration is a key factor influencing luminous flux levels, and variations in ballast performance can also lead to significant discrepancies in the luminous flux of HID lamps. As traditional lighting technologies currently account for a diminishing share of the market, we will not elaborate further on them here. For LED light sources, the lumen maintenance rate is influenced by a multitude of factors, including operating temperature, drive current, manufacturing processes, and material composition. If an LED is operated at temperatures (About working temperature of LEDs and LED driver) or current levels exceeding the manufacturer’s recommendations, it will significantly accelerate lumen depreciation and shorten the rated lifespan—a consequence that becomes clearly evident when projecting a fixture’s LLD using the TM-21 methodology. The lumen maintenance lifespan of an LED is typically denoted by the notation Lxx; for instance, L70 represents the cumulative operating hours at which the light output has declined to 70% (or a specified percentage) of its initial value. LLD characteristics may vary significantly across different manufacturers, and even among different LED models produced by the same manufacturer. During the design phase, it is essential to select an appropriate LLD value by consulting the tables and curves provided by the manufacturer, while also taking into account specific operational parameters—such as the duration of each burn cycle—and prevailing environmental conditions. The table below presents the LLD curve for Osram LED components; since the actual test duration was 9,000 hours, the reported L70 value is capped at 54,000 hours (as the projected L70 value is limited to a maximum of six times the actual test duration).

What’s luminaire dirt depreciation (LDD)?

LDD (Luminaire dirt depreciation) refers to the phenomenon in which dust gradually accumulates on various components of a luminaire—including the inner and outer surfaces of the optics (lens cover), the inner surface of the luminaire’s reflector, and the lamp (light source) itself—resulting in an additional reduction in the luminaire’s total light output. This reduction occurs independently of the natural lumen depreciation of the lamp itself over time; together, these two factors determine the long-term performance of a lighting system. The magnitude of LDD depends primarily on the cleanliness of the luminaire’s installation environment (classified into five levels, ranging from very clean to very dirty) and the established cleaning and maintenance schedule. For traditional, non-LED light sources, the corresponding LDD factor can be determined by consulting standard reference curves; however, the RP-8-21 standard (More about RP-8-21 standard) provides limited guidance regarding LED luminaires. Here, we elaborate on this subject by referencing the relevant CIE standards. These standards recognize that the structural design of LED luminaires differs from that of traditional light sources—for instance, LEDs typically lack upward-facing surfaces and often feature enclosed designs—resulting in a smaller surface area available for dirt accumulation; consequently, the impact of LDD is less significant for LEDs compared to traditional light sources. According to the CIE 154:2003 (Outdoor) standard, the Luminaire maintenance factor (LMF) for LED luminaires must be determined by comprehensively considering three primary factors: the IP rating, environmental conditions (pollution category), and the cleaning interval. For example, for an LED street light (ZGSM street lights) with an IP66 rating, situated in a highly polluted environment, and subject to a three-year cleaning cycle, consulting the relevant tables yields a Luminaire maintenance factor of 0.83. In contrast, if one were to consult the corresponding tables provided by the IESNA, the coefficient for this scenario would fall between 0.55 and 0.75—a significant discrepancy. If you require further assistance in determining LDD values, please contact ZGSM for additional information.

What’s luminaire burnout factor (LBF)?

The IESNA standard includes a “Lamp burnout factor” (LBF); however, detailed descriptions regarding this parameter are relatively scarce. Based on our company’s experience, we recognize that this factor is a critical consideration when calculating the Maintenance factor (MF). We frequently observe streetlights where certain fixtures have ceased to function—or where individual LED chips (bulbs) have failed—which inevitably impacts the brightness and illuminance levels on the roadway. In such instances, it becomes necessary to replace these failed light sources. The IESNA standard merely stipulates that one should consult the manufacturer’s statistical data regarding the burnout rates for specific lamp types in order to determine the appropriate LBF. Furthermore, it advises that for critical, continuous roadway lighting applications, lamp performance should be monitored regularly—for instance, through nighttime inspections or automated fixture monitoring systems (e.g., smart controls – Smart street lighting system)—to facilitate the development of a replacement schedule that mitigates the adverse effects of LBF as early as possible. Conversely, the CIE offers a more detailed treatment of this subject, albeit utilizing a different terminology: the “Lamp survival factor” (LSF). The LSF represents the probability that a light source and/or fixture will continue to operate for a specified duration; this factor can be determined through a lifespan analysis of the fixture’s various components. For example, within an LED fixture, the probability of an individual LED chip failing is typically relatively low, whereas the probability of the LED driver failing is comparatively higher; consequently, the LSF is often extrapolated based on the projected lifespan of the LED driver. Taking Inventronics EUM DG series power supply as an illustration: it boasts a Mean time between failures (MTBF) of 473,000 hours. Through relevant calculations, we can determine that fixtures equipped with this power supply exhibit a failure rate of approximately 4.6% after five years of operation (assuming 12 hours of daily usage); based on this figure, we establish an LSF of 95.4%. For further information regarding MTBF and its significance, please refer to our blog post: “Mean time between failure of LED Drivers and the Importance of MTBF.”

What’s equipment factors (EF)?

Non-time-variant light loss factors are primarily determined by the inherent equipment characteristics of the luminaire itself. While some of these losses cannot be corrected, they can significantly impact the luminaire’s luminous flux and, consequently, reduce actual illuminance levels. The RP-8-21 standard categorizes non-time-variant light loss factors into three types: the Luminaire ambient temperature factor (LATF), the Luminaire voltage factor, and the Ballast factor. ZGSM considers the Luminaire ambient temperature factor to be the most critical of the three, as ambient temperature exerts a profound influence on the light source’s output (LED street light with NTC thermistor and over temperature protection). The Luminaire voltage factor ranks second in importance; however, given that the vast majority of mainstream luminaires today are LED-based, the Ballast factor typically requires no additional consideration.

Regarding the Luminaire ambient temperature factor, professionals in the LED industry are well-acquainted with the LM-82 report, a standard used to evaluate a luminaire’s luminous flux performance across various ambient temperatures (Ta). Citing relevant test reports from ZGSM as an example, a luminaire’s luminous flux was observed to decrease by 3% and 5% under operating conditions of Ta +25°C and Ta +40°C, respectively. Therefore, during the lighting design phase, it is essential to clearly define the luminaire’s operating temperature range and select the appropriate LATF parameter by referencing the corresponding LM-82 report. As for the Luminaire voltage factor, minor voltage fluctuations within a ±5% range of the power grid nominal voltage have a negligible impact on the luminous flux of LED luminaires; however, they can significantly affect traditional luminaires—such as high-pressure sodium lamps—that utilize ballasts. Typically, if an LED driver (More about LED driver) is designed to accommodate a wide input voltage range, the resulting fluctuation in luminous flux will be minimal. Nevertheless, a specific exception exists: some luminaires are rated for a nominal voltage range of 100–277 VAC but are designed to automatically reduce their power output when operating within the actual range of 100–180 VAC. Such a design is not entirely without merit; when the grid voltage drops significantly below the rated voltage, the driver circuit actively reduces power—a measure that effectively serves as an electrical protection mechanism—though opinions regarding the merits of this specific design vary across the industry.

How to calculate the LSF?

IESNA standards state that if the impact of individual coefficients is negligible, they may be omitted from calculations. Generally, to calculate the Light loss factor (LLF), three key components must be multiplied together: the Lamp lumen depreciation (LLD), the Luminaire dirt depreciation (LDD), and the Luminaire burnout factor (BF). For instance, if a project employs a spot replacement strategy (More about street light maintenance and its benefits) by which it replace individual failed lamps as they occur then the BF may be disregarded. In many applications, clients are unfamiliar with the LDD parameter—and sometimes overlook it entirely—leading many to consider only the LLD. However, in practice, implementing spot replacement can be quite costly; furthermore, luminaire maintenance frequencies often follow predictable patterns. Therefore, ZGSM recommends that all three of these key factors be taken into account.

First, the LLD is determined based on the LED luminaire’s L70 rating and its projected operating lifespan (What’s lifespan of LED lights?). For example, if a luminaire has an L70 rating of 100,000 hours, and assuming a linear depreciation model, it would retain 85% of its initial luminous flux after 50,000 hours; in this scenario, the LLD would be 0.85. If the projected lifespan is 5 years (operating 12 hours per day), the total operating time amounts to 21,900 hours, yielding a calculated LLD of 0.93. Naturally, one can also derive the LLD directly by cross-referencing LM-80 and TM-21 data tables—a method recommended by IESNA—which primarily takes into account the operating temperature and drive current of the LEDs. Next, we must estimate the LDD based on environmental conditions, referencing curves such as “Average Dirt Depreciation Rate vs. LED Optics.” However, these guidelines do not offer extensive detail on the specific methodology for determining the LDD. ZGSM suggests that for luminaires utilizing PMMA lenses and glass surfaces, the LDD after 5 years of operation can be estimated as approximately 0.96 × 0.885 = 0.85. Regarding the BF, we have already calculated the Burnout factor for our current mainstream power supplies after 5 years of operation in a previous section; consequently, we assign a value of 0.95. Finally, by multiplying these values ​​together—i.e., LLF = 0.93 × 0.85 × 0.95—we arrive at a result of 0.75. This signifies that, at the designated target point in time, the lighting system will retain 75% of its initial luminous flux.

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Summary

The Light loss factor (LLF) and the Maintenance factor (MF) are fundamentally identical concepts, derived respectively from IESNA and CIE standards; they are defined as the ratio of the maintained illuminance to the initial illuminance after a specific period of use. The LLF is derived by multiplying several independent loss coefficients, which primarily fall into three categories: Lamp lumen depreciation (LLD)—the natural decline in the light source’s luminous flux over time—which corresponds to the Lamp lumen maintenance factor (LLMF) in CIE standards; Luminaire dirt depreciation (LDD)—the loss of light output caused by the accumulation of dust on optical surfaces, the magnitude of which is directly influenced by environmental cleanliness and the luminaire cleaning schedule—which corresponds to the Luminaire maintenance factor (LMF) in CIE standards; and Lamp burnout factor (BF)—representing the probability that the light source or driver will continue to operate normally throughout a specified period—which corresponds to the Lamp survival factor (LSF) in CIE standards. Additionally, IESNA introduces Equipment factors (EF) that do not vary over time—such as ambient temperature factors, voltage factors, and ballast factors—among which, for LED luminaires, particular attention must be paid to the impact of ambient temperature on luminous flux (in accordance with LM-82 reports). When calculating the LLF, the LLD, LDD, and BF values ​​are typically multiplied together. For instance, if an LED luminaire has been in operation for five years, and the LLD is 0.93, the LDD is 0.85, and the LBF is 0.95, then the LLF = 0.93 × 0.85 × 0.95 ≈ 0.75; this signifies that the system is still capable of maintaining 75% of its initial luminous flux. If you require further information regarding LLF and MF, please contact us for additional details. Alternatively, if your project references CIE standards, please check the blog – Maintenance Factor.

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