Maintenance factors in lighting design
Lighting design is an important part of creating functional, safe and aesthetically pleasing environments. A key consideration in effective lighting design is the concept of maintenance factors, which help ensure that lighting systems perform as intended over time. In this article, we explore maintenance factors used in lighting calculations, their definitions and importance for lighting design.
Lamp Lumen Maintenance Factor (LLMF)
LLMF takes into account the reduction in light output of lamps due to aging and operating conditions (without accounting for external factors). It quantifies the expected reduction in luminous flux over time for a specific lamp type. LLMF is essential to ensure that the lighting design meets the required illuminance levels over the lifetime of the lighting system. For example, if a lamp initially produces 1,000 lumens and the LLMF is 0.93, the effective light output will be 930 lumens.
Lamp Survival Factor (LSF)
LSF reflects the proportion of lamps that are expected to remain functional at a given time. It takes into account the failure rates of lamps during their lifetime. LSF is important for determining the number of lamps needed to achieve the desired light output over time. Due to modern LED technology, the probability of LED damage is considered low in most projects, so this factor can often be set to 1.
Luminaire Maintenance Factor (LMF)
LMF takes into account the reduction in light output due to factors affecting the luminaires themselves, including dirt build-up and decline in performance over time. By including LMF, designers can better estimate the effective lighting level over the lifetime of the lighting system. Since LEDs usually do not have upturned surfaces and are often sealed, the area for dirt accumulation is reduced. This enables the use of a factor of at least 0.9 in most cases.
Room Surface Maintenance Factor (RSMF)
RSMF takes into account the impact of room surface conditions, such as dirt and wear that can reduce the reflectance of surfaces over time. RSMF is critical to ensure that lighting systems are designed with an understanding of the room's environment. For example, a room with low reflectance due to dirty walls may require a higher initial light output to achieve the desired lighting level.
Calculation of Maintenance Factor (MF)
The maintenance factor is calculated by the following formula:
MF = LLMF × LSF × LMF × RSMF
Let's look at an example of a step-by-step calculation for a 50 m² office with the following values:
LLMF: 0.93 (93% of original lumens retained)
LSF: 1.0 (100% of lamps are expected to work)
LMF: 0.91 (91% of luminaire output is retained)
RSMF: 0.95 (95% of room reflectance is retained)
Insert the values in the formula:
MF = 0.93 × 1.0 × 0.91 × 0.95 ≈ 0.80
If the design specifies a required lighting level of 500 lux and the total installed light output of the lighting system is 36,000 lumens (8 luminaires x 4,500 lumens), the effective lighting level is calculated as follows:
This means that the effective lighting level meets the required level of 500 lux. Yes, this formula does not take into account the installation height of lamps, reflection levels from room surfaces, etc. In this case, we only show how MF works.
Conclusion
Maintenance factors play a crucial role in lighting design, given the realities of equipment aging, lamp failure and the accumulation of dirt and dust. By incorporating maintenance factors into lighting calculations, designers can create systems that maintain desired lighting levels over time, ensuring both functionality and efficiency.
For more detailed information on the definition of each factor, please refer to the CIE 97:2005 standard.