Intensity Distribution: Pattern of Light Intensity Variation in Photometry

Introduction

Intensity distribution describes how the intensity of light, or luminous flux, from a light source is distributed in different directions throughout three-dimensional space. This foundational concept in photometry and lighting engineering underpins the effective design and deployment of lighting for roads, airports, workplaces, and public spaces.

Understanding and controlling intensity distribution ensures that light is delivered efficiently and safely—minimizing glare, light pollution, and energy waste, while maximizing visibility and comfort. International standards (from IES, CIE, ICAO, etc.) require precise measurement and documentation of intensity distribution for compliance, guiding everything from streetlights to runway approach systems.

Key Photometric Terms and Definitions

Luminous Flux

Luminous flux (lumens, lm) is the total perceived power of light emitted by a source per unit time, accounting for the sensitivity of the human eye to different wavelengths. It measures the overall “amount” of light, irrespective of direction.

Luminous Intensity

Luminous intensity (candela, cd) quantifies the luminous flux emitted in a specific direction per unit solid angle (steradian). It reflects how focused or spread out the light is in that direction—a crucial property for directional lighting.

Solid Angle

A solid angle (steradian, sr) describes a three-dimensional angular span, with 4π steradians comprising a full sphere. It is used to define the section of space over which intensity is measured.

Beam Angle

The beam angle is the angular width (in degrees) at which luminous intensity drops to 50% of its maximum along the main axis. It classifies lighting as spot, flood, or wide, and determines how broadly or narrowly light is distributed.

Luminance

Luminance (cd/m²) is the intensity of light emitted or reflected from a surface in a particular direction per unit area, representing the “brightness” perceived by the eye.

Illuminance

Illuminance (lux, lx) is the total luminous flux incident on a surface per unit area. It is the primary design metric for how much light reaches a workspace, roadway, or other area.

Photometric Distribution

Photometric distribution describes the spatial pattern of luminous intensity, usually visualized via polar diagrams, intensity tables, or 3D plots.

Luminous Intensity Distribution Curve

A distribution curve graphically shows how intensity varies with angle—essential for lighting design and selection.

Symmetrical vs. Asymmetrical Light Distribution

Symmetrical distributions emit evenly in all directions around a central axis; asymmetrical distributions are tailored to direct more light in specific directions, improving efficiency and minimizing unwanted spill.

Measurement Principles and Representation

Photometric Testing

Goniophotometer

A goniophotometer rotates a light source through different angles, measuring intensity at each, and is the principal tool for mapping intensity distribution. Data from goniophotometers forms the basis of photometric files and diagrams.

Integrating Sphere

An integrating sphere measures total luminous flux, not spatial distribution, by capturing all emitted light in a uniformly reflective sphere.

Photometric Distance

Photometric distance ensures the measurement setup approximates a point source, typically 5–15 times the size of the light source, to maintain accuracy.

Photometric Data Files

• IES Files (.ies): Used mainly in North America, encode measured intensity at different angles.
• EULUMDAT (.ldt): European standard, similar in function.

These files power lighting simulation software (DIALux, AGi32, Relux), allowing accurate virtual modeling and compliance checking.

Graphical Representation

Polar (Candela) Diagrams

Show intensity (cd) as the radius at each angle from the source in a 2D plane, often overlaid for different axes.

Cartesian Plots

Plot intensity against angle in a straight line—helpful for spotting side peaks or beam cutoff characteristics.

3D Plots

Provide a full spatial visualization of light output, especially useful for complex or non-symmetrical luminaires.

Zonal Lumen Summary

Tabulates the percentage of total light emitted within specified angular zones, helping designers ensure light is concentrated where needed.

Interpreting Intensity Distribution

Reading Photometric Curves

• Symmetrical luminaires: Identical curves across main planes; circular pattern.
• Asymmetrical luminaires: Different curves; skewed or elliptical pattern.
• Beam angle: Width at 50% of peak intensity.
• Peaks: Directions of maximum light output.

Calculating Beam Angle

• Find the maximum intensity (the axis).
• Identify angles where intensity falls to 50% of peak.
• Beam angle = angle between these two points.

Determining Symmetry

• Compare values at mirrored angles or overlay C0–C180 and C90–C270 curves.
• Symmetrical: curves match; asymmetrical: curves differ.

Assessing Suitability

• Direct/indirect output: Is most light going where needed?
• Glare: Intensity near the “glare zone” (60–90° from nadir).
• Uniformity: Evenness of coverage.

Symmetrical vs. Asymmetrical Light Distribution

Visual:
Symmetrical: circular polar curve.
Asymmetrical: elliptical or skewed curve.

Practical Use Cases

Roadway Lighting

Roadway luminaires use asymmetrical distributions to cast light along the pavement while minimizing glare and spill onto nearby properties. Modern LED streetlights use precise optics to achieve these patterns, ensuring compliance with IES and CIE standards for roadway visibility and safety.

Airport and Runway Lighting

ICAO specifies precise intensity distributions for approach, threshold, and runway lights to ensure visibility in all weather conditions. Here, photometric data ensures that lights are visible from required distances and angles, and that glare is minimized for pilots.

Office and Commercial Spaces

Symmetrical luminaires provide even light in open-plan offices and retail spaces, improving comfort and reducing shadows. Asymmetrical wall-washing luminaires highlight architectural features or create visual interest with controlled spill.

Industrial and Sports Facilities

High-bay and floodlighting often require careful management of intensity distribution to ensure uniform coverage and minimize glare—especially important for visual tasks and safe operation.

Standards and Compliance

International standards bodies define the requirements for intensity distribution in various contexts:

• IES (Illuminating Engineering Society): North American photometric standards (e.g., LM-79 for LED testing).
• CIE (International Commission on Illumination): Global guidelines for measurement and reporting.
• ICAO (International Civil Aviation Organization): Specifies intensity and distribution for airfield lighting.
• ISO, EN, ANSI: Additional regional or application-specific standards.

Photometric files and test reports must be produced by accredited laboratories in compliance with these standards.

Conclusion

Intensity distribution is a cornerstone of modern lighting design, enabling engineers and architects to deliver light exactly where it is needed—safely, efficiently, and in compliance with international guidelines. Understanding how to read, interpret, and apply photometric data ensures optimal illumination, reduces waste, and enhances the quality of built environments.

Whether you are illuminating a runway, designing a city street, or planning an office, mastery of intensity distribution principles will help you achieve superior lighting outcomes.

For expert guidance on optimizing intensity distribution in your next lighting project, contact our team or schedule a demo .