Glossary of Light Pattern, Spatial Distribution of Light, and Photometry

Lighting is both a science and an art, crucial to visibility, safety, and aesthetics in environments ranging from airports and roads to architecture and industrial spaces. The following glossary unpacks essential terms and concepts—light pattern, spatial distribution, photometry, and related topics—providing foundational understanding for professionals and enthusiasts alike.

Light Pattern

A light pattern is the distinct geometrical footprint or distribution of light as it falls onto a target plane or surface, cast by a lighting fixture or luminaire. The pattern’s shape, coverage area, intensity gradients, and directionality are determined by a combination of the light source characteristics, the optics (lenses, reflectors, diffusers), and the mounting position.

Light patterns are fundamental to lighting engineering because they directly influence visibility, safety, energy efficiency, and compliance with regulatory standards. For example, in aviation lighting, precise light patterns ensure optimal visibility on runways and taxiways. In architectural lighting, tailored patterns highlight features or prevent light trespass.

Manufacturers design fixtures to produce specific patterns—from narrow spots to wide floods—using advanced optics and photometric testing. The resulting patterns are documented in photometric reports (IES or LDT files) and used in simulation software for accurate lighting design.

Spatial Distribution

Spatial distribution refers to how light is emitted and propagates in three-dimensional space from a light source. It quantifies the luminous intensity (in candelas) at every angle around a fixture, forming a comprehensive map of light delivery.

This distribution is crucial in applications like airport apron lighting (requiring broad, uniform coverage) or runway edge lighting (requiring tightly controlled beams). Spatial distribution is measured with goniophotometers and represented as polar or Cartesian intensity diagrams. These datasets underpin lighting simulations and ensure compliance with standards from organizations like ICAO and IES.

Photometry

Photometry is the science of measuring visible light in terms of human brightness perception. It quantifies luminous intensity, luminous flux, illuminance, and luminance, using units like candelas (cd), lumens (lm), lux (lx), and nits (cd/m²). Photometry is distinct from radiometry, as it weights light power by the eye’s sensitivity.

Photometric measurements enable objective fixture comparison, regulatory compliance, and efficient lighting design. In aviation, standards from ICAO ensure airfield lighting delivers required brightness, uniformity, and color.

Photometric properties are determined in labs using integrating spheres (for total flux) and goniophotometers (for angular intensity). The resulting data populates photometric files and guides design simulations.

Luminous Intensity

Luminous intensity describes the amount of visible light emitted by a source in a specific direction, measured in candelas (cd). It characterizes directional output, informing how a fixture illuminates its surroundings.

Luminous intensity distribution is key in selecting fixtures for applications like runway centerline lights, where visibility from specific approach paths is critical. Measured with goniophotometers, intensity is visualized on polar plots, ensuring compliance with standards for safety and glare control.

Light Distribution Patterns (Types)

Light distribution patterns categorize luminaires by the shape and area of their emitted light, most commonly using IES Types I–V:

• Type I: Narrow, linear pattern for walkways and corridors.
• Type II: Slightly wider, for alleys and narrow roads.
• Type III: Wide, forward-throw arc for roadway perimeters and parking rows.
• Type IV: Forward-throw for building perimeters and large lateral areas.
• Type V: Symmetrical, circular/square for central pole areas and open spaces.

Selecting the correct pattern ensures optimal illumination, efficiency, and regulatory compliance.

Beam Angle

The beam angle is the angular width of a light beam where intensity drops to 50% of its peak (full width at half maximum, FWHM). Narrow beams (10°–30°) are for accent or focused tasks, while wide beams (60°–120°) serve general area lighting.

Choosing the right beam angle prevents over-illumination, glare, and spill. It’s especially crucial in environments like airfields, where precision dictates safety.

Intensity Distribution

Intensity distribution maps luminous intensity as a function of direction, typically shown on a polar plot. It reveals beam shape and fall-off, guiding fixture choice for targeted applications like airfield approach lights or street lighting.

Data is gathered with goniophotometers and used in lighting simulations for compliance with standards and to minimize unwanted glare and trespass.

Photometric Distribution

Photometric distribution is the complete spatial arrangement of light from a luminaire. Documented in IES/LDT files, it combines optical design and geometry, allowing precise simulation and ensuring standards for brightness, uniformity, and glare are met.

Designers use photometric distributions to optimize fixture layout and minimize energy use and light pollution.

Light Distribution Curve (Polar Curve)

A light distribution curve (polar curve) graphically represents luminous intensity at different angles. Essential for comparing fixtures and interpreting test data, polar curves help designers select and position lights for required coverage and compliance.

Symmetrical fixtures show circular/elliptical curves; asymmetrical ones display directional peaks.

Goniophotometer

A goniophotometer is a laboratory instrument measuring the angular distribution of light from a fixture. By rotating the source and/or detector, it captures intensity data for photometric files critical to design and compliance.

Types A, B, and C goniophotometers enable various measurement geometries. Their data verifies that products meet standards for distribution and glare.

Symmetric and Asymmetric Distribution

Symmetric Distribution

Symmetric distribution emits light equally in all directions around a central axis—ideal for uniform area lighting, such as parking lots or warehouses. It ensures spatial uniformity but may waste light in unnecessary directions.

Asymmetric Distribution

Asymmetric distribution focuses light in specific directions, using specialized optics to avoid spill and maximize useful illumination. It’s key for roadways, perimeters, and runways, improving efficiency and reducing light pollution.

Solid Angle

A solid angle measures the extent of a 3D angle over which a source emits light, expressed in steradians (sr). It underpins definitions like the candela (one lumen per steradian) and is vital in calculating intensity and coverage for beams.

Solid angles help ensure fixtures meet regulatory beam spread and intensity requirements.

Lighting Design and Application Use Cases

Lighting design integrates photometric data, spatial characteristics, and patterns to achieve visual, functional, and compliance goals. In aviation, lighting must meet ICAO standards for intensity, color, and glare. In urban design, correct selection of distribution types prevents waste and enhances safety.

Application Examples:

Conclusion

Understanding light patterns, spatial distribution, photometry, and related concepts is essential for effective lighting design—ensuring safety, efficiency, and compliance in diverse environments from airfields to urban spaces.

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