Comprehensive Guide to LED Lighting Terminology and Industry Standards

In the rapidly evolving field of illumination engineering, precision is paramount. Whether you are specifying a lighting system for a commercial facility or designing an architectural feature, understanding the fundamental physics of light and the rigorous standards that govern LED performance is essential.

This guide provides a definitive glossary of key lighting metrics and an in-depth analysis of critical IESNA standards, specifically LM-79 and LM-80, to help professionals navigate the technical landscape of modern LED technology.

Part 1: Fundamental Lighting Metrics

To accurately evaluate lighting performance, one must distinguish between the source of power, the output of light, and how that light is perceived by the human eye.

1. Luminous Flux (Ф)

• Unit: Lumen (lm)

• Definition: Luminous flux measures the total radiated power of a light source as perceived by the human eye. Unlike radiant flux, which measures total power across all electromagnetic wavelengths, luminous flux is weighted to the spectral sensitivity of human vision.

  
  

2. Luminous Intensity (I)

• Unit: Candela (cd)

• Formula: $I = \Phi / \omega$

• Definition: This metric describes the amount of light power emanating from a point source in a specific direction within a solid angle of one steradian. It helps define how "intense" a focused beam of light appears.

3. Illuminance (E)

• Unit: Lux (lx)

• Formula: $E = \Phi / A$

• Definition: Illuminance quantifies the total luminous flux incident on a specific surface area. It answers the practical question: "How much light is hitting this desk or floor?"

4. Luminance (L)

• Unit: Candela per square meter (cd/m²)

• Formula: $L = I / A$

• Definition: Often confused with illuminance, luminance is the intensity of light emitted from a surface per unit area in a given direction. It correlates closely with what we perceive as "brightness."

5. Luminous Efficacy (η)

• Unit: Lumen per Watt (lm/W)

• Formula: $\eta = \Phi / P$

• Definition: A critical measure of efficiency, luminous efficacy represents the ratio of luminous flux (light output) to power consumed. Higher efficacy indicates a more energy-efficient light source.

Part 2: Color Science and Quality

Beyond brightness, the quality of light—its color and ability to render objects naturally—is a defining characteristic of LED products.

• Correlated Colour Temperature (CCT): Measured in Kelvin (K), this defines the color appearance of a light source relative to an ideal black body radiator. Common classifications include:

• Warm White: Typically <3000K

• Natural White: ~4000K

• Cool White: >5000K

• Colour Rendering Index (CRI/Ra): A quantitative measure (0–100) of a light source's ability to reproduce object colors faithfully compared to a natural light source. An Ra of 100 indicates performance equal to an ideal black body radiator.

Part 3: Industry Standards – LM-79 and LM-80

As Light Emitting Diodes (LEDs) became a dominant source for outdoor and architectural lighting, the industry faced a challenge: LEDs behave differently than traditional filament or gas-discharge lamps. They are highly directional and critically dependent on thermal management.

In response, the Illuminating Engineering Society of North America (IESNA) developed specific testing protocols to ensure consistency and reliability.

Understanding LM-80-08

Published by the IESNA Solid State Lighting Subcommittee, LM-80-08 (Approved Method: Measuring Lumen Maintenance of LED Light Sources) is the industry standard for testing the longevity of LED packages, arrays, and modules.

Why LM-80 is Necessary:

There is a significant discrepancy between how component manufacturers and fixture manufacturers measure performance:

• Component Manufacturers: Typically test LEDs in Pulse Mode (short bursts of 10–20ms). This prevents the LED from heating up, allowing the junction temperature ($T_j$) to remain at ambient temperature ($25^\circ C$). While fast, this does not reflect real-world operating conditions.

• Fixture Manufacturers: Operate LEDs in Constant DC Mode inside a fixture. The proximity of multiple LEDs and continuous operation elevate the junction temperature well above $25^\circ C$, which degrades performance and color stability over time.

The LM-80 Testing Protocol:

To bridge this gap, LM-80-08 mandates strict testing conditions:

1. Thermal Control: LEDs must be tested at three specific case temperatures: $55^\circ C$, $85^\circ C$, and a third temperature selected by the manufacturer.

2. Duration: Lumen maintenance data must be recorded for at least 6,000 hours of constant DC mode operation.

3. Consistency: It controls forward voltage and current to ensure uniform data.