Luminous flux and degradation: causes and prevention

Through detailed analysis, statistical data, case studies, and technical tables, we will explore the mechanisms, causes of performance decay, and best practices for maintaining high light intensity levels over the long term, with particular attention to commercial, industrial, and infrastructure applications.

Luminous flux: what is it?       

Before addressing degradation, it is essential to establish fundamental photometric concepts. We will therefore review all relevant quantities, their mathematical relationships, and their practical significance in the design and evaluation of LED systems.

What is luminous flux? Fundamental definition      

Luminous flux (Φ) is a photometric quantity that quantifies the luminous power perceived by the human eye, emitted by a source in all spatial directions. It does not measure total radiant energy, but only the visible component, weighted according to the spectral sensitivity curve of the average human eye (photopic V(λ) curve).

While radiant power (watts) measures all emitted electromagnetic energy, luminous flux (lumens) measures only the portion visible to our eyes, applying a spectral weighting factor. This is fundamental for comparing sources with different spectral distributions.

Symbol and unit of measurement of luminous flux

Luminous flux formula

The mathematical definition of luminous flux begins with its relationship to luminous intensity. For a non-isotropic source, the total flux is obtained by integrating the luminous intensity I(θ,φ) over the entire solid angle (Ω):

Φ = ∫ I(θ,φ) dΩ (integral extended over 4π steradians)

Where:

           
• Φ = Luminous flux (lumens)           
• I = Luminous intensity (candela, cd)           
• dΩ = Infinitesimal element of solid angle (steradian, sr)           
• θ, φ = Angular coordinates (e.g., zenith and azimuth)
           

For sources with approximately symmetric distribution, the formula can be simplified. For example, for uniform emission within a solid angle cone Ω, the formula becomes: Φ = I * Ω.

Related photometric quantities

Luminous flux does not exist in isolation. For a complete evaluation of a lighting system, it must be considered together with three other fundamental quantities: luminous intensity, illuminance, and luminous efficacy.

Luminous intensity: purpose and meaning

Luminous intensity (I) is the photometric quantity that describes the luminous power emitted by a source in a particular direction. Its unit of measurement is the candela (cd). Unlike flux (total), intensity is directional.

Purpose of luminous intensity:

Luminous intensity serves to:       

           
• characterize the spatial distribution of light (photometric curves);           
• design installations that direct light only where needed, minimizing light pollution and waste;           
• define requirements for signage, headlights, projectors, and all applications where beam control is critical;           
• calculate illuminance on a surface, given the source-surface geometry.
          

What does luminous flux express in relation to intensity? It expresses the "sum" of all luminous intensities emitted in every direction. If intensity is the "directional detail", flux is the "global total".

Illuminance: luminous flux reaching a surface

Illuminance (E) measures the incident luminous flux per unit area. Its unit is the lux (lx), equivalent to one lumen per square meter (lm/m²).

Fundamental difference between lumens and lux: Lumens (flux) describe the source's output. Lux (illuminance) describe how much of that light actually reaches a specific plane (a desk, a road, a workbench). 10,000 lumens emitted toward the sky will produce no useful lux on the ground. The relationship is given by: E = Φ / A (for uniform incident flux over area A), but generally depends on distance and angle of incidence (Lambert's cosine law).

Luminous efficacy: source efficiency

Luminous efficacy (η) is the ratio between the total emitted luminous flux (Φ in lm) and the absorbed electrical power (P in W). It is measured in lumens per watt (lm/W). This parameter is crucial for evaluating the operating economy of an installation. Modern LEDs for professional applications regularly exceed 150 lm/W, with the most advanced models approaching 200 lm/W in laboratory conditions.   

Luminous flux degradation in LEDs – Phenomenology and primary causes

Luminous flux degradation, known as Lumen Depreciation (L), is the irreversible process of reduction in a LED's light output over time. Understanding its physico-chemical causes is the first step toward developing effective mitigation strategies. This phenomenon is not a simple "failure" but a progressive degradation influenced by multiple factors.

Internal degradation mechanisms of the LED chip

Within the semiconductor, several microscopic mechanisms lead to reduced efficiency of radiative recombination (which produces photons).

Diffusion and migration of crystal defects

High current densities and elevated junction temperatures accelerate the diffusion of impurity atoms and the migration of lattice defects (such as vacancies and dislocations) into the LED's active regions. These defects act as non-radiative recombination centers, where electron energy is dissipated as heat instead of being converted into light. Studies on InGaN LEDs show that increasing junction temperature from 85°C to 135°C can accelerate the flux degradation rate by up to 5 times.

Phosphor layer degradation (for white LEDs)

White light LEDs typically use a blue-emitting chip (InGaN) coated with a phosphor converter (e.g., YAG:Ce). This phosphor layer is subject to:                 

• thermal degradation: high temperatures (above 150°C) cause oxidation and aggregation of phosphor particles, reducing their conversion efficiency;           
• degradation from high-energy photons: UV radiation from the chip itself or high-energy blue photons can cause ionization and defect creation in the phosphor lattice (a "bleaching" phenomenon);           
• moisture degradation: without a perfectly hermetic encapsulant, moisture can react with the phosphor, altering its optical properties.       

Phosphor degradation leads not only to flux reduction but often also to a shift in correlated color temperature (CCT) toward cooler (or warmer, depending on phosphor chemistry) tones.

External and system-level causes accelerating degradation       

Often, the rate of luminous flux degradation is determined more by operating conditions and overall system quality than by the LED chip itself.

Junction temperature (Tj): the dominant factor       

Junction temperature (Tj) is the single most influential parameter on LED lifetime. The relationship is exponential. Arrhenius' empirical rule, often applied, suggests that for every 10°C reduction in Tj, LED useful life approximately doubles.

Statistical data

A study by the U.S. Department of Energy (DOE) on LED modules for general lighting showed that maintaining Tj at 105°C instead of 135°C reduces lumen loss after 36,000 hours from 30% to less than 15% for medium-quality products.

*Average indicative values for commercial/professional quality LEDs. **L70 = Hours to 30% reduction of initial flux.

Drive current: overcurrent and ripple, enemies of luminous flux       

LEDs are driven by constant current. Operating above the nominal current (overdriving) drastically increases thermal and electrical stress, accelerating all degradation mechanisms. A 10% current increase can lead to a 40% increase in power dissipation and a 15-20°C rise in Tj in a system with non-optimal thermal management.

Current quality is also crucial. High current ripple (e.g., > 30% of DC current) causes thermomechanical fluctuations in the chip and interface materials, leading to microcracks and delamination. High-quality drivers for professional installations typically maintain ripple below 10%.

Environmental factors: humidity, corrosive gases, radiation       

In industrial or outdoor environments, LEDs may be exposed to:       

           
• humidity and condensation: leading to contact corrosion, dendrite formation, and degradation of optical materials and silicone encapsulants;           
• corrosive gases (H2S, SOx, NOx, Cl2): common in chemical plants, wastewater treatment facilities, and ports. They can corrode metal contacts, reflectors, and optics, reducing light extraction;           
• solar UV radiation (for outdoor): damages plastics, dyes, and encapsulation materials, causing yellowing and reduced optical transmittance.
       

Useful life, standards and degradation projections for luminous flux

Accurately predicting long-term LED behavior is essential for evaluating lifecycle costs and planning maintenance. This chapter explores international standards and mathematical models that enable estimation of luminous flux degradation, transforming laboratory data into reliable real-world projections.

Fundamental standards: IESNA LM-80 and IES TM-21

The LED lighting industry relies on two complementary standards developed by the Illuminating Engineering Society of North America (IESNA), which have become the global reference for characterizing lumen maintenance.

IES LM-80: measurement method for lumen maintenance

The IES LM-80-20 standard ("Approved Method: Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays and Modules") defines uniform procedures for measuring variations in luminous flux and chromaticity of LED sources (packages, arrays, modules) over time under controlled environmental conditions.

Critical aspects of LM-80

• Minimum test duration: requires at least 6,000 hours of measurements, with recommendations for 10,000 hours. Reputable manufacturers extend tests to 12,000 or 15,000 hours for greater data reliability.
• Measurement temperature: LEDs must be tested at at least three different case temperatures (Ts): 55°C, 85°C, and a third chosen by the manufacturer (often 105°C or 25°C for controlled environments). This allows studying degradation kinetics as a function of heat.
• Test current: devices are tested at their nominal current, providing data relevant to real applications.
• Sampling and statistics: testing is performed on a statistically significant sample (minimum 20 units per test condition) to account for production variability.

The LM-80 report provides raw measurement data, typically in the form of a graph or table showing lumen maintenance (expressed as a percentage of initial value) versus time for each test temperature.

IES TM-21: method for projecting useful life from LM-80 data

The IES TM-21-11 (and subsequent revisions) standard ("Projecting Long Term Lumen Maintenance of LED Light Sources") provides the method for extrapolating LM-80 data (typically 6,000–10,000 hours) to project LED behavior up to 36,000 hours (approximately 10 years at 10 hours/day) or, in some cases, beyond.

The core of TM-21: exponential degradation equation

TM-21 assumes that luminous flux degradation follows an exponential trend after an initial stabilization period. The mathematical model used is:

Φ(t) = B * e^(-αt)

Where:

• Φ(t) = Luminous flux at time t (as % of initial value)
• B = Intercept factor (often near 1 or slightly above for positive "burn-in" phenomena)
• α = Degradation rate (positive constant)
• t = Operating time (hours)

TM-21 rigorously defines how to calculate parameter α from the latter portion of LM-80 data (typically from 5,000 hours onward) using a least-squares method. Fundamental limitation: The standard prohibits projecting beyond 6 times the duration of collected data. Therefore, from 10,000-hour LM-80 data, the maximum allowed projection is to 60,000 hours.

Interpretation of Lxx curves and useful life parameters

LED useful life is not defined by "failure" but by reaching a certain percentage reduction of initial luminous flux.

Definitions of L70, L80, L90 and L50

Lxx represents the time (in hours) after which luminous flux has reduced to xx% of its initial value. Examples:

• L70: time to reach 70% of initial flux (30% loss). Most common standard for general lighting, acceptable for many applications.
• L80: time to reach 80% of initial flux (20% loss). Increasingly required for commercial applications and high-end offices.
• L90: time to reach 90% of initial flux (10% loss). Standard for applications where precise illuminance levels are critical (hospitals, museums, laboratories) or where maintenance costs are extremely high (tunnels, highway lighting on viaducts).
• L50: time to reach 50% of initial flux. Sometimes used to define "end of life" more drastically.

Industry statistical data: An aggregated analysis of 200 LM-80/TM-21 reports from global manufacturers (2020–2023) shows that for professional-quality LEDs operated at Ts=85°C, the average projected L90 value at 60,000 hours is approximately 45,000 hours, with a range from 30,000 hours (entry-level products) to over 70,000 hours (high-end products).

Lamp Lumen Maintenance Factor (LLMF) in lighting design calculations

In professional lighting design according to standards such as EN 12464 series, the LLMF (Lamp Lumen Maintenance Factor) is a multiplier ≤1 applied to initial flux to account for expected degradation at the end of the planned maintenance period. A designer specifying a product with L90=50,000 hours for an installation with a 5-year maintenance cycle (approximately 22,000 hours at 12h/day) can use an LLMF of 0.92–0.95, reducing initial oversizing and saving energy. Ignoring LLMF leads to oversized installations at commissioning that degrade toward acceptable levels but waste energy for years.

Prevention and mitigation strategies for degradation

Understanding degradation causes and models enables implementation of proactive strategies to control them. This chapter provides a detailed framework of concrete actions, from design phase to operational management, to maximize luminous flux stability over time.

Thermal management: the undisputed pillar of longevity

The primary objective is to minimize junction temperature (Tj). This is achieved by managing the entire thermal chain from chip to environment.

Heat sink design: materials, geometry and surface

Materials: extruded aluminum (with thermal conductivity ~200 W/mK) is the standard. For very high thermal loads or limited spaces, copper alloys (~400 W/mK) or, in cutting-edge applications, carbon-matrix composites (diamond composites, up to 1500 W/mK) are used.

Geometry (fins): efficiency depends on exchange surface area. The relationship is not linear: doubling fin height does not double heat exchange due to decreasing thermal gradient along the fin. CFD (Computational Fluid Dynamics) simulation software is essential for optimizing fin thickness, spacing, and profile based on natural or forced airflow.

Surface finish: a black paint with high thermal emissivity (ε > 0.9) can increase radiative exchange by 20–30% compared to natural anodized aluminum (ε ~ 0.7–0.8), especially without forced ventilation.

Thermal interface (TIM - Thermal Interface Material)

The layer between the LED module and heat sink is a critical point of thermal resistance. Silicone-based thermal greases (Rth ~ 0.2–0.5 K/W for a typical area) are common. For superior performance:

• graphite pads: high anisotropic conductivity, ideal for spreading heat over large surfaces;
• metal-based pastes (indium/gallium): exceptional conductivity but expensive and delicate to apply;
• bicomponent thermally conductive adhesives: provide both conduction and mechanical bonding, excellent for vibration resistance.

Correct TIM application is fundamental: excessive thickness or air bubbles dramatically increase resistance. A University of Padua study showed that non-uniform thermal grease application can cause Tj differences of up to 15°C between identical LEDs on the same heat sink, leading to differential degradation and visually non-uniform aging of the luminaire.

Critical component selection: beyond the LED chip

LED drivers: stability, ripple and protections

A quality driver must guarantee:

• precise and stable current regulation (±2–3%) across the entire temperature and input voltage range;
• residual current ripple < 10% (ideal < 5%) at switching frequency. 30% ripple can reduce estimated LED life by 20–40%;
• integrated protections: OVP (Over Voltage), OCP (Over Current), OTP (Over Temperature) at driver level and, in advanced models, with remote sensor on the LED board;
• high efficiency (>90%): each percentage point of additional efficiency reduces thermal losses in the driver itself and luminaire, indirectly contributing to lower Tj;
• dimming compatibility: if required, must be flicker-free and without compatibility issues that could cause current instability.

Primary and secondary optics: resistant materials

Lenses and reflectors must maintain their optical properties. PMMA (Acrylic) is economical but yellows under UV and high temperatures (above 80°C). PC (Polycarbonate) is more heat-resistant but can yellow. For professional applications, high-transmittance optical silicone and borosilicate glass are superior choices, with operating temperatures up to 150°C and excellent UV stability. Optics degradation can absorb or scatter 10–30% of light, mimicking LED degradation itself.

Operational strategies: dimming and load management

Operating LEDs below nominal power is one of the most effective strategies for dramatically extending their life.

Thermal derating and dimming

Derating: design the installation to use LEDs at 70–80% of their maximum nominal current. For example, use a 100W module in a thermally limited luminaire rated for 75W. This immediately lowers Tj by 10–20°C, exponentially increasing L70/L80 life.

Dimming: operation at reduced levels (e.g., 70% in offices at night) not only saves energy but proportionally reduces generated heat. The relationship between current and flux is nearly linear, while the relationship between current and heat (dissipated power) is super-linear due to increased series resistance with temperature. Dimming to 50% can increase L70 life by 3–4 times.

Active temperature management (Thermal Foldback)

Advanced systems for outdoor or hot environments integrate a temperature sensor on the heat sink. When a critical threshold is exceeded (e.g., 80°C on the body), the driver progressively reduces current (and therefore flux) to keep Tj within safe limits, preventing accelerated degradation. This is an intelligent compromise between immediate performance and long-term durability.

Case studies – Analysis and solutions for specific environments

Theory meets practice. This chapter analyzes real degradation scenarios, their root causes, and implemented solutions, providing a repertoire of immediately applicable knowledge.

Case study 1: Open-plan office lighting – Differential degradation

Scenario

In a 1,000 m² open-plan office with linear suspended LED luminaires (3000K, 4000 lm each), after 3 years (approximately 8,000 hours) visible non-uniformity in illuminance is observed. Some rows appear "cooler" and less bright.

Investigation and causes

• Measurements with lux meter and thermal camera reveal that weaker rows are above PC workstations, where monitor and computer heat (rising warm air) raises plenum ambient temperature by 8–10°C compared to central rows.
• Spectral analysis shows that "hot" luminaires have a CCT increase of 150K (from 3000K to 3150K) and 18% flux loss versus 9% for luminaires in cooler zones. This indicates accelerated phosphor degradation due to higher operating temperature.
• Further analysis reveals insufficient thermal separation between LED module and housing, and blocked airflow in the plenum due to cables.

Solutions implemented

1. Environmental modification: installation of deflectors to direct workstation heat toward walkways, reducing thermal load on the plenum.
2. Luminaire upgrade: replacement with models featuring more efficient heat sinks and drivers with thermal foldback, automatically limiting current during high temperatures.
3. Maintenance planning: introduction of a periodic rotation program (every 2 years) of luminaires between hot and cool zones to uniform aging, extending overall fleet life.

Result

After intervention, non-uniformity was corrected. Projections based on the new thermal profile indicate L80 life exceeding 60,000 hours for all luminaires.

Case study 2: Coastal street LED lighting

Scenario

On a seaside promenade, after 4 years, 30% of architectural LED floodlights on poles show drastic flux reduction (>40%) and visible corrosion.

Investigation and causes

• Inspection reveals luminaires were not rated for marine environments (inadequate corrosion class, e.g., only IP66 without anti-corrosion certification such as ISO 12944-2 C5-M).
• Salty air, rich in chlorides, corroded PCB traces, connector contacts, and the aluminum heat sink surface, increasing its thermal resistance.
• Saline moisture ingress through joints (due to thermal cycling and gasket wear) caused ionic migration and short circuits on drivers, leading to failures or irregular current operation.

Solutions implemented

1. Replacement with suitable products: installation of luminaires with IP68/IP69K rating, PCBs with high-quality conformal coating (e.g., acrylic or polyurethane), heat sinks in marine-grade aluminum (anodized thickness >15μm) or AISI 316L stainless steel, and hermetic brass nickel-plated connectors.
2. Preventive maintenance: establishment of a biennial cleaning cycle for luminaires with fresh water and gasket inspection, with programmed gasket replacement every 5 years.

Predictive maintenance and installation condition monitoring

Transitioning from corrective or time-based scheduled maintenance to condition-based predictive maintenance is the evolutionary step to maximize operational efficiency and prevent costly plant shutdowns. This chapter describes technologies and methodologies for real-time monitoring of luminous flux health status.

Integrated monitoring systems (IoT for Lighting)

Controlled lighting networks (DALI-2, Zigbee, Bluetooth Mesh, LoRaWAN) serve not only for on/off control but can become distributed sensor networks.

Monitorable parameters

• Operating hours: the simplest parameter, but fundamental for comparison with Lxx curves.
• LED module or heat sink temperature: measured with integrated NTC thermistor. A rising temperature trend under identical ambient conditions indicates thermal interface degradation or ventilation obstruction.
• Module supply current and voltage: increased voltage at constant current (for constant-voltage LEDs) or anomalous current variations may indicate driver or module issues.
• Relative luminous flux: some high-end systems integrate a reference photodiode measuring a small fraction of emitted light, providing real-time decay estimation.

Data analysis and advanced predictive models

Raw data must be transformed into actionable information.

Dashboard and alerting

Software platforms aggregate data from thousands of light points, presenting:

• thermal maps of the installation;
• graphs of estimated relative flux vs. expected degradation curve (customized TM-21);
• automatic alerts when a luminaire or zone deviates from expected parameters (e.g., temperature > threshold, estimated flux < 85% of reference value for that zone).

Integration with maintenance plans (CMMS)

Alerts automatically generate work orders in the CMMS (Computerized Maintenance Management System), directing technicians to the correct luminaire with the right spare part and preliminary diagnosis, reducing intervention times by 60–70%.

Benefit statistics: a study conducted on a portfolio of 50,000 light points managed with predictive maintenance showed a 40% reduction in total maintenance costs compared to scheduled maintenance, a 15% increase in average system efficiency (because degraded luminaires are repaired before consuming excess energy), and zero catastrophic failures causing dark zones.

Best practices summary

We summarize in an operational decalogue the most critical actions for preserving luminous flux in professional installations, integrating concepts discussed in previous chapters.

Decalogue of design and management for maximum longevity

1. Request and analyze complete manufacturer reports, favoring data at 20,000+ hours.
2. Invest in thermal management: slightly oversize heat sinks, use quality TIMs, and design for an operating Tj not exceeding 85–95°C for critical applications.
3. Choose quality drivers with ripple <10% and integrated protections, preferably with thermal foldback functions for difficult environments.
4. Adopt a derating strategy: use LEDs at 70–80% of their maximum nominal power when possible.
5. Select environment-appropriate materials: verify IP, IK, and corrosion resistance classes (e.g., ISO 12944) for outdoor or industrial applications.
6. Implement dimming and occupancy sensors not only to save energy but to reduce cumulative thermal stress.
7. In lighting design, use the correct LLMF based on the chosen product's Lxx curves to avoid oversizing.
8. Plan easy maintenance access (optics cleaning, thermal inspection) in luminaires.
9. Consider IoT control and monitoring systems even for medium-sized installations to enable predictive maintenance.
10. Document everything: create an installation register with models, operating hours, interventions, and periodic measurements (lux, temperature).

Luminous flux: a more performant future

Research aims to mitigate degradation causes at their root: LEDs on gallium nitride (GaN) substrates on silicon or larger-diameter sapphire reduce crystal defects; quantum dot (QD) phosphors offer greater thermal and spectral stability; COB (Chip on Board) with ceramic substrates dramatically improve heat extraction. The synergy between advanced materials, intelligent control electronics, and data management promises to bring future LED installations toward L90 useful lives exceeding 100,000 hours, making luminous flux degradation an increasingly marginal phenomenon—though its understanding will remain fundamental for anyone designing, installing, or managing quality lighting.

Understanding luminous flux and the complex degradation phenomena affecting it is a non-negotiable requirement for designing and managing efficient, durable, and economically advantageous professional LED installations. The key lies in rigorous junction temperature control, selection of high-quality components with reliable data, design of an effective thermal system, and adoption of a data-driven maintenance approach.

By implementing the strategies illustrated in this guide, designers, installers, and facility managers can ensure that LED installations deliver optimal and consistent luminous performance, maintaining high luminous flux for tens of thousands of hours, thereby maximizing return on investment and sustainability of lighting interventions.