What is Intensity Distribution?
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
| Characteristic | Symmetrical | Asymmetrical |
|---|---|---|
| Pattern | Even all around axis | Skewed to target areas |
| Typical Application | High bays, open offices, gymnasiums | Roadways, wall-washing, parking lots |
| Advantages | Uniform, simple installation | Focused, energy efficient, reduced spill |
| Disadvantages | May waste light, less efficient | Needs precise aiming, not always versatile |
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 .
Frequently Asked Questions
- What is intensity distribution in photometry?
- Intensity distribution refers to the way light intensity emitted from a source varies with direction in space. It is graphically represented using polar diagrams or photometric curves and is critical for ensuring light is applied where needed, minimizing glare and light pollution.
- How is intensity distribution measured?
- Intensity distribution is measured using instruments like goniophotometers, which record luminous intensity at various angles. The data is compiled into photometric files (IES or LDT) and used to generate curves and diagrams for lighting design.
- What is the difference between symmetrical and asymmetrical light distributions?
- Symmetrical light distributions emit light evenly in all directions around a central axis, ideal for general area lighting. Asymmetrical distributions focus light in specific directions, maximizing efficiency and minimizing spill, which is essential for applications like roadway or wall lighting.
- Why are intensity distribution curves important in lighting design?
- These curves help designers select and position luminaires to achieve the desired illumination pattern, meet regulatory requirements, reduce glare, and ensure safety and visual comfort.
- What standards govern intensity distribution in lighting?
- Standards from organizations such as IES, CIE, and ICAO define the required intensity distribution for different lighting applications, ensuring compliance for safety, efficiency, and visual comfort in diverse environments.
