Light Distribution – Pattern of Light Intensity in Space – Photometry

What is Light Distribution?

Light distribution is the spatial pattern describing how light intensity radiates from a luminaire or lighting fixture into its environment. It encompasses both the direction and magnitude of light output and is visualized using photometric techniques. The pattern is determined by the fixture’s optical system—reflectors, lenses, diffusers—and its mounting orientation and position.

Lighting engineers and designers analyze light distribution to ensure optimal performance for an application, whether illuminating a corridor, parking lot, or accentuating architectural features. Distribution is quantified with luminous intensity plots (in candelas), creating three-dimensional “light distribution curves” that show areas of high and low illumination.

Standards from organizations such as the Illuminating Engineering Society (IES) and International Commission on Illumination (CIE) guide appropriate distributions for different tasks. Photometric testing under controlled conditions produces data used in technical files (e.g., IES LM-63 format). Understanding light distribution is essential for choosing fixtures that maximize efficiency, meet safety codes, and provide visual comfort.

Why Light Distribution Patterns Matter

The pattern of light distribution is crucial for the efficacy, safety, and compliance of any lighting installation:

• Uniformity: Prevents hazardous dark spots and excessive glare in spaces like roadways, sports fields, and industrial sites.
• Efficiency: Directs light only where it’s needed, saving energy and reducing operational costs.
• Environmental Impact: Poor distribution causes light pollution (skyglow, trespass, glare), affecting residents, wildlife, and astronomy. Regulations like International Dark-Sky Association guidelines specify limits on uplight.
• Aesthetics & Experience: Distribution can highlight art, landscaping, or architecture, and create a sense of security and comfort.

Selecting the right light distribution type is a strategic decision—impacting visual performance, code compliance, environmental stewardship, and user experience.

Key Concepts in Photometry

Photometric Reports

A photometric report details how a luminaire emits light, serving as a foundation for lighting design, simulation, and code compliance. Generated with a goniophotometer, it measures luminous intensity at various angles, producing data in formats like IES LM-63 and EULUMDAT.

Key elements:

• Luminous flux (lumens)
• Luminous intensity distribution (candelas/angle)
• Color characteristics (CCT, CRI)
• Power consumption (watts)
• Polar luminous intensity graph and zonal lumen summary

Reports may specify mounting height, spacing criteria, and illuminance grids. They are essential for design software and required for regulatory approval and green certifications.

Luminous Intensity

Luminous intensity (candela, cd) is the measure of visible light emitted in a particular direction, calculated as luminous flux per solid angle (lumen/steradian). Unlike total light output (lumens), intensity describes directionality.

It is visualized in polar plots, showing the spatial distribution of light from a fixture. High-intensity zones correspond to the main beam; lower-intensity regions show peripheral or spill light. Standards often limit intensity at certain angles to control glare and light trespass.

Beam Angle

Beam angle is the angular width (in degrees) where a fixture emits at least 50% of maximum intensity. It determines how narrow or wide the light beam is:

• Narrow (10°–25°): Accent, spotlighting, highlighting
• Wide (60°–120°): General area lighting

Beam angle affects fixture placement, mounting height, and uniformity. Manufacturers specify both beam and field angles in datasheets, allowing designers to match the fixture to the task.

Mounting Height Ratio

The mounting height ratio compares the vertical distance from fixture to ground with the width of the area illuminated. For example, a 10-meter mounting height and a 2.75 ratio illuminates a 27.5-meter width. Lower ratios indicate narrow distributions (e.g., IES Type I), while higher ratios indicate wider patterns (Type IV/V).

Proper mounting height ratios ensure code compliance, efficiency, and visual safety.

Polar Luminous Intensity Graph

A polar luminous intensity graph represents the distribution of intensity from a luminaire at various angles. The fixture is at the center; radial lines indicate intensity (cd) at different angles. These graphs help identify symmetrical/asymmetrical patterns and are vital for controlling glare and spill in sensitive applications.

Zonal Lumen Summary

A zonal lumen summary quantifies what percentage of a fixture’s total output is emitted in defined angular zones (e.g., 0°–30°, 30°–60°, 60°–90°, etc). This is essential for compliance with codes that restrict uplight and light pollution.

Lighting Distribution Types

IESNA Distribution Types (Type I–V)

The Illuminating Engineering Society of North America (IESNA) defines five principal outdoor light distribution types:

• Type I: Linear; ideal for narrow paths/walkways.
• Type II: Wider; for entrance roads, alleys.
• Type III: Wide, forward; parking lots, perimeters.
• Type IV: Semi-circular, forward; site boundaries, security.
• Type V: Symmetrical; large open areas, plazas.

NEMA Beam Spread Types

The National Electrical Manufacturers Association (NEMA) classifies fixtures by beam angle where intensity drops to 10% of maximum:

NEMA types are key for spotlights and floodlights—where precise directionality is needed.

Comparing IESNA and NEMA

IESNA is used for large-scale area lighting; NEMA for focused, directional tasks.

How to Read a Lighting Distribution Diagram

A lighting distribution diagram (photometric curve) visually represents light output in different directions:

• Polar plot: Center is the fixture; radii show intensity at angles.
• C0–C180 and C90–C270 planes: Show longitudinal and lateral symmetry.
• Beam width: Identified by curve shape and length.

Reading these diagrams helps designers ensure uniformity, avoid dark spots, and meet lighting standards.

Light Distribution in Practice

• Roadways: Require Type II, III, or IV for uniform, safe coverage.
• Parking Lots/Plazas: Type V for 360° area lighting.
• Pathways: Type I for narrow, linear coverage.
• Art/Accent: Narrow NEMA types for focused beams.

Modern software (DIALux, AGi32) uses photometric data to model installations and ensure compliance.

Environmental and Regulatory Considerations

• Light pollution: Controlled by managing uplight (zonal lumen above 90°) and glare (max intensity at certain angles).
• Dark-Sky Compliance: Fixtures with minimal uplight and precise distribution reduce skyglow.
• Energy Codes: Many require photometric documentation for approval.

Summary

• Light distribution describes how fixtures emit light into space, affecting coverage, uniformity, and visual comfort.
• Key metrics: beam angle, mounting ratio, polar graph, zonal lumen summary.
• IESNA and NEMA classifications guide fixture selection for specific applications.
• Proper distribution ensures safety, energy efficiency, code compliance, and environmental protection.

Further Reading

• IES Lighting Handbook
• International Dark-Sky Association
• CIE Publications

If you need guidance on photometric analysis, fixture selection, or compliance, contact our lighting experts or schedule a demo .