Beam Pattern – Distribution Pattern of Light Intensity

A beam pattern is the foundational descriptor of how a light source distributes its intensity in space. In photometry and lighting engineering, understanding beam patterns is essential for designing lighting systems that are safe, efficient, and compliant with regulatory standards. This glossary entry explains the concept of beam pattern, types of light distributions, photometric measurement, and practical application in fields such as architecture, industry, and aviation.

What is a Beam Pattern?

A beam pattern specifies the spatial arrangement of light intensity (measured in candela, cd) emitted by a luminaire or lighting device. It mathematically describes how brightly a source shines in each direction, not just the visible spot or shape of the light on a surface.

The beam pattern is typically visualized using photometric diagrams, most often polar plots, where the length of a line at a given angle from the center represents intensity in that direction. The result is a unique “fingerprint” for each type of luminaire, revealing whether its light is narrowly focused, broadly spread, symmetric, or asymmetric.

Key characteristics include:

• Beam angle: The angular width where intensity is at least 50% of the peak value.
• Field angle: The angular width where intensity drops to 10% of the peak.
• Symmetry: Whether the light is distributed equally in all directions or targeted in a specific direction.

Importance in Lighting Design

The beam pattern determines how well a luminaire meets requirements for visual comfort, safety, energy efficiency, and compliance with standards such as those from the Illuminating Engineering Society (IES), International Commission on Illumination (CIE), and the International Civil Aviation Organization (ICAO). In modern practice, accurate beam pattern data—often in standardized digital formats—is vital for lighting simulation, design, and certification.

Light Distribution Patterns: IESNA and NEMA Types

Light distribution patterns help lighting professionals match the right fixture to the right application, ensuring optimal performance and regulatory compliance.

IESNA Distribution Types (I–V)

The Illuminating Engineering Society of North America (IESNA) classifies luminaires by their horizontal light spread, relative to mounting height, for street and area lighting. Each type is best suited to specific tasks:

Technical Note:
Type III and IV patterns minimize dark spots and maximize useful coverage, while Type V provides uniform illumination in all directions, ideal for central installations in open spaces.

Aviation Context

In aviation, analogous distribution types are specified for runway, taxiway, and approach lighting. Beam patterns are tightly controlled to ensure visibility for pilots without causing glare or light pollution, following ICAO Annex 14 and national standards.

NEMA Beam Spread Types (1–7)

The National Electrical Manufacturers Association (NEMA) classifies beam patterns based on the angular spread (in degrees) at which light intensity falls to 10% of its peak—useful for spotlights, floodlights, and directional fixtures.

Selection Guidance:
Choose NEMA types for applications needing targeted beams or accent lighting (e.g., sports, architectural features), and IESNA types for general area coverage.

Luminous Intensity and Photometric Distribution

Luminous intensity (candela, cd) quantifies light output in a particular direction. Photometric distribution is the complete mapping of intensity versus angle, forming the backbone of lighting calculations.

Photometric Data Files

Manufacturers provide digital photometric files—such as IES (.ies) or EULUMDAT (.ldt)—containing measured intensity values at defined angles. These files are essential for:

• Simulating lighting layouts in software (e.g., DIALux, AGi32)
• Verifying regulatory compliance (e.g., ICAO for aviation, local codes)
• Calculating illuminance, uniformity, glare, and spill light

Aviation Example:
ICAO Annex 14 sets minimum and maximum intensity requirements for airfield lights in specific angular sectors. Certified photometric data ensures compliance and safe navigation.

Advanced Beam Shaping

Optics such as reflectors, lens arrays, and diffusers can sculpt the photometric distribution, enabling custom beam patterns for architectural or technical demands.

Symmetric vs. Asymmetric Light Distribution

Symmetric Distribution

A symmetric beam pattern emits light equally in all directions around its axis.

• Photometric curves for the main planes (e.g., C0–C180 and C90–C270) are nearly identical.
• Applications: Open areas, high-bay warehouses, plazas, some apron lighting.
• Advantages: Uniform lighting, simple installation, minimal aiming required.
• Drawbacks: Potentially wasted light outside the target area.

Asymmetric Distribution

An asymmetric pattern purposefully directs more light in a preferred direction.

• Photometric curves differ between main planes, with a pronounced “throw.”
• Applications: Roadways (projecting light onto a carriageway), wall washing, perimeter lighting, approach/runway lights in aviation.
• Advantages: Greater efficiency, reduced glare and spill, compliance with dark-sky requirements.
• Drawbacks: Requires careful aiming and design.

Determining Symmetry

• Compare photometric curves (C0–C180 vs. C90–C270): identical = symmetric; different = asymmetric.
• Check the application: Directional tasks need asymmetry.

ICAO Regulation:
Aviation fixtures often require asymmetry (e.g., forward throw for approach lighting), with beam shapes precisely defined in standards.

Reading Photometric Curves and Diagrams

Photometric curves graphically show how light is distributed around a luminaire.

C-Planes and Reference Axes

• C0–C180: Along the main axis of the fixture.
• C90–C270: Perpendicular to the main axis.
• Intensity is measured at regular angular increments in each plane.

Polar Diagrams

A polar plot displays intensity radii at each angle from the center:

• Circular plot: Symmetric, uniform distribution.
• Elongated lobe: Focused, narrow beam.
• Off-center bulge: Asymmetric, directional throw.

Calculating Beam and Field Angles

• Beam angle: Angular width above 50% of maximum intensity.
• Field angle: Angular width above 10% of maximum.

Example:

If the peak is 1200 cd, find the angles where intensity drops to 600 cd (50%); the angle between these points is the beam angle.

Aviation Example

Approach lights must have tightly controlled vertical and horizontal beam angles, per ICAO Annex 14, to maximize pilot visibility and minimize ground-level glare.

Applications and Use Cases

Outdoor Environments

• Type I/II: Linear walkways, narrow roads.
• Type III: Wider streets, parking lots.
• Type IV: Site perimeters, building edges.
• Type V: Open plazas, intersections.
• Asymmetric: Roadways, facades, sports fields, dark-sky compliance.

Aviation

• Runway/taxiway edge lights: Linear or asymmetric patterns for guidance.
• Approach/threshold lights: Asymmetric, forward-throw beams for maximum pilot visibility.
• Apron lighting: Symmetric or semi-symmetric for uniform ground illumination.

Indoor Environments

• Symmetric: Offices, warehouses, classrooms, industrial bays.
• Asymmetric: Wall washing, displays, accent lighting, galleries/museums.

Comparative Summary Table

Best Practices Checklist for Beam Pattern Selection

1. Match distribution to application: Choose the beam pattern (IESNA/NEMA type, symmetry) that fits the task and environment.
2. Review photometric files: Inspect polar plots and digital data (IES/LDT) for compliance and suitability.
3. Assess mounting and area size: Use isolux diagrams to verify adequate coverage at intended mounting heights.
4. Control glare and pollution: Select asymmetric patterns and/or shielding for roads, perimeters, and aviation to limit spill and skyglow.
5. Optimize energy use: Targeted beams improve efficiency and reduce waste.
6. Simulate before installation: Use lighting software to check illuminance, uniformity, and glare indices.
7. Reference standards: Ensure compliance with IES, CIE, ICAO, and local codes.
8. Verify symmetry: Confirm the luminaire’s distribution matches the application (use manufacturer data or lab reports).

Further Resources

• IES Lighting Handbook
• ICAO Annex 14, Vol. 1 – Aerodrome Design and Operations
• CIE Technical Reports on Photometry
• Manufacturer photometric data sheets and simulation files (IES/LDT)
• DIALux, AGi32 lighting simulation software

In summary:
Beam pattern is the defining characteristic of a luminaire’s performance, crucial for delivering the right light, in the right place, at the right intensity. Whether specifying airfield lighting for aviation safety, designing roadway or architectural lighting, or optimizing industrial and indoor environments, understanding and applying beam pattern data ensures success in every lighting project.