Light Source (Object Emitting Light) – Photometry

A light source is any physical entity or engineered device that emits electromagnetic radiation within the visible spectrum (approximately 380 to 780 nanometers). Light sources are foundational to both natural and artificial illumination, acting as the origin points for photons that interact with their environment and human observers. In photometry, these sources are characterized by how their emission is perceived in terms of brightness and color by the human eye, rather than solely by physical energy output.

Light sources can be:

• Natural (e.g., the sun, stars, fire)
• Artificial (e.g., incandescent bulbs, LEDs, fluorescent lamps, lasers)
• Biological/Chemical (e.g., fireflies, bioluminescent algae)

The emission mechanism varies:

• Thermal emission (incandescent, sunlight)
• Electroluminescence (LEDs, OLEDs)
• Gas discharge (fluorescent, neon)
• Chemical reaction (bioluminescence)

Each source type exhibits a unique spectral power distribution (SPD), impacting perceived color, luminous efficacy, and suitability for specific applications.

Figure: CIE 1931 Photopic Luminosity Function (V(λ)). The human eye’s peak sensitivity is at 555 nm.

Photometry: Measuring Light as Perceived by Humans

Photometry is the science of quantifying visible light according to human vision. Unlike radiometry, which measures absolute energy (watts) across all wavelengths, photometry applies a weighting function (the luminosity function) to account for the eye’s varying sensitivity to different wavelengths.

Core Photometric Quantities

Why Photometry is Essential

• Lighting engineering: Ensures spaces are lit for comfort, function, and safety.
• Aviation: Guarantees runways, approach lights, and beacons are visible and unambiguous in all conditions.
• Regulatory compliance: Meets standards like ICAO Annex 14, FAA, and CIE for color, intensity, and distribution.

Human Eye Sensitivity: The Luminosity Function

The human eye is most sensitive to green-yellow light (555 nm) under photopic (well-lit) conditions. This sensitivity is modeled by the luminosity function (V(λ)), standardized by the CIE. Under low light (scotopic) conditions, sensitivity shifts toward blue (507 nm).

This function allows translation of physical radiant energy into perceptual quantities:

• Green light at 555 nm is perceived as much brighter than red or blue light of the same radiant power.
• Design of signals and displays must consider this for maximum visibility and safety.

Spectral Power Distribution (SPD)

SPD describes the amount of light a source emits at each wavelength. It determines:

• Color (appearance of the light)
• Luminous efficacy (how much visible light is produced per watt of power)
• Color rendering (how naturally colors appear under the source)

SPDs vary:

• Broad, continuous (sunlight, incandescent)
• Spiked, narrow (low-pressure sodium, lasers)
• Multi-peaked (white LEDs with phosphor)

SPD is critical for:

• Ensuring regulatory compliance (e.g., specific chromaticity for aviation lighting)
• Achieving desired visual effects and efficiency

Key Photometric Quantities Explained

Luminous Flux (Lumen, lm)

Measures total visible light output. Weighted by the eye’s sensitivity and used to compare overall output of different sources.

Luminous Intensity (Candela, cd)

Measures light emitted in a specific direction per unit solid angle. Critical for signal lamps, beacons, and focused lighting.

Luminance (Candela/m², nit)

Describes the perceived brightness of a surface in a given direction. Important for displays, signage, and cockpit indicators.

Illuminance (Lux, lx)

Amount of light falling on a surface. Used in lighting design to ensure sufficient visibility for tasks and safety.

Luminous Exitance

Amount of light leaving a surface per unit area. Assesses visibility of illuminated or self-luminous surfaces.

Luminous Efficacy (lm/W)

Efficiency of converting input power into visible light. Higher values mean more efficient lighting. LEDs outperform incandescents by a large margin.

Radiometry vs. Photometry

• Radiometry: Measures all electromagnetic radiation (watts, W/sr, W/m²). Used for energy-based analysis, sensor calibration, non-visual applications.
• Photometry: Measures only visible light, weighted by human sensitivity (lumens, candelas, lux). Used where human perception is the priority.

Measurement Principles and Instruments

• Integrating Sphere: Captures all light from a source for total luminous flux measurements.
• Goniophotometer: Maps intensity distribution at different angles, essential for directional lighting.
• Photometer: Measures illuminance, luminous intensity, and other quantities with photopic filters matching eye sensitivity.
• Luminance Meter: Measures surface brightness, critical for displays and signage.

All instruments must be calibrated using standards traceable to the SI candela to ensure measurement reliability.

Application in Aviation and Regulated Environments

Aviation lighting must meet stringent requirements:

• Reliability: Continuous operation under harsh conditions.
• Luminous Intensity and Distribution: To ensure detection by pilots at required distances and angles.
• Color and Chromaticity: For unambiguous signaling (e.g., red for obstruction, green for taxiway, white for runway edge).
• Compliance: ICAO and FAA standards define minimum and maximum values for photometric parameters.

Example: Calculating Luminous Flux from Spectral Data

Luminous flux is calculated by integrating the spectral power distribution weighted by the luminosity function:

[ \Phi_v = 683 \cdot \int_{380,nm}^{780,nm} V(\lambda) \cdot \Phi_{e,\lambda}(\lambda) d\lambda ]

Where:

• (\Phi_{e,\lambda}(\lambda)): Spectral radiant flux (W/nm)
• (V(\lambda)): Standard luminosity function
• 683 lm/W: Maximum photopic efficacy at 555 nm

Summary Table: Photometric vs. Radiometric Quantities

Conclusion

Light sources are the origin of all visible illumination, and understanding their photometric properties is essential for effective, efficient, and compliant lighting in technical, commercial, and regulatory environments—especially in high-stakes fields like aviation. Photometry bridges the gap between raw physical emission and human visual experience, ensuring that lighting systems meet both objective and perceptual requirements.

For optimal results in lighting design, always consider:

• The nature and SPD of the light source
• Human visual sensitivity
• Application-specific standards and safety needs

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