Light Beam – Directional Projection of Light in Photometry

What Is a Light Beam?

A light beam is a concentrated, directional emission of electromagnetic energy within the visible spectrum. Unlike omnidirectional light, a beam is confined along a principal axis, occupying a finite cross-sectional area. In photometry and lighting design, a light beam is characterized by measurable properties such as luminous intensity (candelas), beam angle, divergence, and spatial coherence.

In practice, light beams are created by lamps, LEDs, or lasers using reflectors and lenses to control spread and direction. In scientific contexts, beams are described using both geometrical optics (as bundles of rays) and physical optics (as electromagnetic waves), depending on the required level of detail.

Standards from organizations like the International Commission on Illumination (CIE) and the International Civil Aviation Organization (ICAO) define requirements for beam geometry, intensity, and visibility in applications such as runway lighting, signaling, and architectural illumination. Correct engineering of beams is critical to safety, compliance, and visual comfort.

Light Beam vs. Light Ray vs. Light Wave

• Light Ray: An abstract, infinitesimal path used in geometrical optics; represents direction but has no width or physical extent.
• Light Beam: A real, physical bundle of light with measurable width, intensity, and divergence; can be shaped or focused.
• Light Wave: A full physical description as an electromagnetic field, incorporating wavelength, phase, and coherence; vital for explaining interference, diffraction, and advanced optical behavior.

Summary: Rays are for tracing direction, beams for practical and photometric design, and waves for detailed scientific modeling.

Beam Spread and Beam Angle

• Beam Spread: Qualitative description (e.g., spot, flood) of how widely a light source projects.
• Beam Angle: Quantitative metric (in degrees), defined as the angle between directions where intensity drops to 50% of the peak value (per CIE/IES standards).

Table: Typical Beam Angles and Use Cases

Always rely on actual beam angle specifications and photometric test data to achieve the required lighting effect.

Beam Divergence and Focus

Beam divergence is the angular spread of a beam as it propagates. Low divergence beams (collimated) retain brightness over distance—critical for searchlights, runway approach lights, and lasers. High divergence beams quickly spread their energy over a wider area.

Focus refers to the point (beam waist) where the beam is narrowest and most intense. Laser beams are notable for their sharp focus and minimal divergence, described by the Gaussian beam model.

In photometry and safety-critical fields like aviation, standards (e.g., ICAO Annex 14) specify maximum divergence for ground lights to ensure visibility at required distances.

Cutoff Angle and Visual Comfort

The cutoff angle is the angle from the vertical at which the light source becomes shielded by the fixture, preventing direct view and reducing glare. Deep cutoff fixtures (smaller cutoff angle) enhance visual comfort and minimize distracting brightness, especially important in architectural, healthcare, and aviation lighting.

Measurement of cutoff angle is performed photometrically, and compliance is essential for both user comfort and regulatory approval.

Luminous Intensity and Photometric Metrics

Luminous intensity (candelas, cd) is the amount of visible light emitted in a given direction. It is measured using photometric instruments and detailed in polar diagrams. Intensity is distinct from total luminous flux (lumens) and from illuminance (lux).

In aviation and signaling, minimum and maximum intensities are regulated to ensure beams are visible for safety. In architecture, intensity ensures adequate task and accent lighting with minimal energy use.

Beam Quality and Spatial Coherence

Beam quality describes how closely a beam approximates an ideal (typically Gaussian) profile, with low divergence and high focusability. Spatial coherence—high in lasers, low in conventional bulbs—enables tight focusing, interference, and precise optical control.

High beam quality is critical in scientific, medical, and industrial applications for accuracy and efficiency.

Lumen Output (Brightness)

Lumen output is the total visible light emitted, integrated over all directions. In general lighting, lumens measure overall brightness. In directional lighting, consider both lumens and beam angle to assess how much light reaches the target.

Manufacturers use standardized tests (like IES LM-79 for LEDs) to report lumen output, ensuring product comparability.

Candela (Luminous Intensity)

Candela is the SI unit measuring the intensity of light in a specific direction. It is used to specify spotlights, floodlights, and aviation ground lights. High candela values denote beams that are visible at greater distances or through adverse conditions.

Candela ratings are verified in laboratory settings and summarized in photometric data files.

Lux and Foot-Candles (Illuminance)

Illuminance measures how much luminous flux (lumens) falls on a surface area—expressed in lux (lm/m²) or foot-candles (lm/ft²). It is the key metric for assessing if lighting is adequate for tasks, safety, or compliance.

Illuminance is calculated using the inverse square law and measured with lux meters. Aviation, architectural, and workplace standards set minimum lux levels for various environments.

Center Beam Candlepower (CBCP)

CBCP is the peak luminous intensity (in candelas) at the center of a directional beam. It is crucial for spotlight, accent, and aviation lighting where the brightest possible spot is needed at a specific distance or location.

CBCP should be considered alongside beam angle and lumen output for optimal performance.

Beam Angle Classification Table

Note: Always refer to manufacturer data for precise definitions, as subjective terms may vary.

Lasers and Optical Elements

Lasers produce highly collimated, coherent beams with minimal divergence—ideal for signaling, measurement, surgery, and communications. Optical elements (lenses, reflectors, diffusers) are used to shape, focus, or spread light beams for specialized applications.

Standards and Compliance

International standards (CIE, ICAO, IES, EN) dictate the photometric requirements for beam angles, intensity, cutoff, and illuminance in diverse fields:

• ICAO Annex 14: Aviation ground lighting, runway/approach beam geometry, minimum intensity
• CIE/IES: Beam angle definitions, photometric testing, glare control, energy efficiency
• EN 12464: Interior workspace illuminance requirements

Practical Applications

• Aviation: Runway and taxiway lights, approach signals, obstruction markers
• Architecture: Accent and display lighting, glare control, energy efficiency
• Science/Industry: Laser alignment, measurement, material processing
• Healthcare: Focused illumination in surgical and examination areas

Conclusion

A light beam is more than just visible light—it is a rigorously defined, engineered phenomenon. Understanding beam properties (angle, divergence, cutoff, intensity) is vital for safety, comfort, and regulatory compliance in professional lighting, aviation, and scientific applications.

For expert guidance in light beam specification or compliance, contact our team or schedule a demo .

References

• CIE 13.3-1995: Method of Measuring and Specifying Colour Rendering Properties of Light Sources
• ICAO Annex 14: Aerodromes, Volume I, 8th Edition, 2018
• IES TM-30-18: IES Method for Evaluating Light Source Color Rendition
• EN 12464-1: Light and lighting — Lighting of work places — Part 1: Indoor work places
• IES LM-79-19: Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products

For further reading, consult the latest publications from CIE, ICAO, IES, and national standards bodies.