Beam Angle – Angular Width of Light Output from Fixture – Photometry

Beam Angle

Beam angle is a foundational concept in lighting design and photometry. It refers to the angular spread of light emitted from a fixture, measured between two directions from the beam’s centerline where the luminous intensity drops to 50% of its maximum value (Full Width at Half Maximum, FWHM). Expressed in degrees (°), the beam angle determines how concentrated or diffuse the light appears. Narrow angles (<30°) produce focused beams for accent or display purposes, while wide angles (>60°) provide general, uniform illumination.

Beam angle is determined through photometric testing with a goniophotometer. The results are visualized in polar diagrams and documented in IES files, forming the basis for lighting calculations in architecture, industry, and aviation. For example, a fixture with a 20° beam angle will project a small, intense light spot, ideal for highlighting artwork or signage, while a 120° beam will gently illuminate an entire room or open area.

In regulated environments like airfields, beam angle selection is critical: international standards (e.g., ICAO Annex 14) specify exact angular distributions for runway, taxiway, and approach lighting to provide optimal guidance for pilots and ensure safety. The correct beam angle ensures compliance, prevents glare, and minimizes light pollution.

Field Angle

The field angle describes the total visible spread of a light beam, defined as the angle between directions where luminous intensity falls to 10% of peak value. The field angle includes the peripheral “spill” light outside the main beam, which is crucial for applications where both the core and transition zones of illumination matter.

For example, in stage lighting, the field angle ensures an actor is fully lit even during movement. In architectural and aviation lighting, the field angle helps predict how much light will reach adjacent surfaces or runways, ensuring uniformity and compliance. Field angle is always equal to or greater than the beam angle, and both are presented in photometric plots and product datasheets.

CBCP (Center Beam Candlepower)

Center Beam Candlepower (CBCP) is the peak luminous intensity (in candelas, cd) emitted along the centerline (0°) of a beam. CBCP quantifies the maximum brightness delivered to a target area—critical for applications requiring focused, high-impact illumination.

CBCP is measured during photometric testing, and is especially important for spotlights, searchlights, and aviation approach lights, where high-intensity beams must penetrate weather or cover long distances. A narrow-beam fixture with high CBCP concentrates light onto a small spot, while a wide-beam fixture with the same lumen output will have lower CBCP. CBCP is a primary selection criterion for precise, task-oriented lighting.

Luminous Intensity

Luminous intensity quantifies the amount of visible light emitted in a particular direction, measured in candelas (cd). It describes how much light is sent along a given angle, forming the basis for all directional lighting calculations. Unlike luminous flux (lumens), which sums output in all directions, intensity focuses on a specific direction.

Luminous intensity data is measured with a goniophotometer and presented in polar diagrams and IES files. It is essential for calculating illuminance (lux) at a surface, modeling glare, and verifying compliance with aviation and architectural standards.

Illuminance

Illuminance measures the amount of luminous flux (lumens) arriving at a surface per unit area, expressed in lux (lx) or footcandles (fc). It answers: “How brightly is this surface lit?” The basic formula:

E (lux) = I (cd) / d²

where I is luminous intensity and d is distance from the source. Illuminance calculations, often performed with simulation software, ensure that lighting meets required brightness for safety, comfort, and regulatory standards—such as those found in airports, workplaces, and public spaces.

Light Distribution

Light distribution characterizes how light is emitted spatially from a luminaire. It is visualized in polar diagrams or as “cones of light,” showing intensity at various angles. Light distribution is determined by the fixture’s optics—reflectors, lenses, and diffusers.

Manufacturers classify distribution as narrow, medium, wide, or asymmetric, depending on the application. In aviation, precise distribution patterns are mandated by ICAO/FAA to direct light along runways and avoid glare. In architecture, it informs fixture placement and aiming for uniform coverage or dramatic effects.

Goniophotometer

A goniophotometer is the instrument used for photometric testing—measuring how a light source distributes intensity at various angles. By rotating the fixture or detector, it records intensity data, which is used to calculate beam angle, field angle, CBCP, and create IES files.

Modern goniophotometers are automated, ensuring precise and repeatable results. In regulated industries, goniophotometric testing is required to certify that fixtures meet performance and safety standards.

IES File

An IES file is a standardized digital format (.ies) used to describe the photometric properties of a luminaire. Created from goniophotometer data, it includes light distribution, intensity at various angles, total lumens, and other key metrics.

Lighting designers import IES files into simulation software (like DIALux or AGi32) to model real-world performance, verify compliance, and compare fixtures. For airfield, industrial, and architectural projects, IES files are essential for regulatory approval and effective lighting design.

Accent Lighting

Accent lighting uses focused beams (narrow beam angles, 10°–30°) to highlight specific features, objects, or architectural details. It creates visual interest and contrast, drawing attention to art, merchandise, or signage. The effectiveness of accent lighting depends on careful beam selection and aiming to avoid unwanted spill light.

Accent lighting is also used in aviation for signage or special markers. Precise photometric planning ensures the intended visual impact.

Ambient Lighting

Ambient lighting provides general illumination for overall visibility and comfort. It uses wide beam angles (typically 60° or more) and diffuse distribution to minimize shadows and create a uniform environment. Ambient lighting design considers room size, ceiling height, and surface reflectance to achieve the desired mood and meet regulatory requirements for brightness.

Task Lighting

Task lighting delivers dedicated illumination for specific activities—reading, working, machine operation—using medium beam angles (30°–60°) to balance intensity and coverage. In aviation, task lighting is used in control towers and maintenance areas, while in homes and offices it appears as desk lamps or under-cabinet fixtures. Calculating required illuminance and using correct beam angles ensures visual comfort and performance.

Mounting Height

Mounting height is the vertical distance from a fixture to the illuminated surface. It determines the diameter of the beam and the resulting illuminance. The beam spread formula:

Beam Diameter = 2 × Mounting Height × tan(Beam Angle ÷ 2)

Mounting height, combined with beam angle, is critical for achieving the correct light distribution, especially in large spaces or where regulatory standards apply (e.g., runways).

Landscape Lighting

Landscape lighting illuminates outdoor features—gardens, pathways, facades—for safety, security, and aesthetics. It uses a mix of narrow beams for accenting and wide beams for general coverage, with careful fixture placement to avoid glare and light pollution. Compliance with local regulations (e.g., dark sky ordinances) and photometric planning are key considerations.

Asymmetric Optics

Asymmetric optics are designed to distribute light unevenly, favoring one direction. This is achieved with specialized reflectors or lenses and is used in wall-washing, pathway, or roadway lighting, where uniform coverage from one side is desired. In aviation, asymmetric optics direct light along runways without causing glare for pilots.

Conclusion

Understanding beam angle and related photometric concepts—field angle, CBCP, luminous intensity, illuminance, light distribution, and more—is essential for effective lighting design in any context. These parameters guide fixture selection, placement, and aiming, ensuring visual comfort, task performance, compliance, and energy efficiency in architectural, industrial, and aviation settings.

For lighting professionals, mastery of these terms and tools—such as IES files and goniophotometers—enables precise specification and successful project outcomes.

See Also

• Photometry
• Glare
• Runway Lighting
• IES File
• Illuminance
• Aviation Lighting Standards
• Lighting Simulation Software