Light Spectrum – Distribution of Light Energy by Wavelength – Photometry

Introduction

The light spectrum is a foundational concept in photometry, color science, and the engineering of lighting systems for human environments, including critical sectors such as aviation, transportation, and healthcare. Understanding how light energy is distributed by wavelength enables us to measure, reproduce, and optimize visual experiences, safety, and energy efficiency.

This glossary entry provides an in-depth overview of the light spectrum, its measurement, and its significance—especially regarding the visible range, spectral power distributions, photometric versus radiometric units, and the human visual response. Applications in aviation and industry highlight why spectral engineering is vital for safety, compliance, and performance.

The Electromagnetic Spectrum and Visible Light

All light is electromagnetic radiation, but only a narrow band—from about 380 to 780 nanometers (nm)—is visible to the average human eye. The electromagnetic spectrum itself ranges from high-energy gamma and X-rays (less than 1 nm) to radio waves (meters or longer).

• Ultraviolet (UV): 10–380 nm (invisible to humans, but can cause sunburn)
• Visible Light: 380–780 nm (perceived as colors from violet to red)
• Infrared (IR): 780 nm–1 mm (felt as heat, used in night vision)

Within the visible band:

• Violet: 380–450 nm
• Blue: 450–495 nm
• Green: 495–570 nm
• Yellow: 570–590 nm
• Orange: 590–620 nm
• Red: 620–780 nm

The human eye’s sensitivity does not drop off abruptly at the boundaries; instead, perception fades smoothly, and individual variation exists.

Applications:

• Aviation: Cockpit displays and runway lights are engineered within the visible spectrum for optimal detection by pilots.
• Lighting Design: Spectral knowledge is crucial for displays, workspaces, and safety signaling.

Distribution of Light Energy by Wavelength

A light source rarely emits all wavelengths equally. Its spectral power distribution (SPD) describes the intensity of emitted light at each wavelength. This distribution is unique for every source and governs its color, color rendering performance, and energy efficiency.

• Continuous SPD: All visible wavelengths present, as in sunlight or incandescent lamps. Yields excellent color rendering.
• Line SPD: Sharp peaks at specific wavelengths, typical of lasers and low-pressure gas discharge lamps. Often poor color rendering.
• Band SPD: Groups of closely spaced wavelengths, seen in high-pressure discharge lamps or phosphor-based LEDs.

SPD determines:

• Color appearance: Which wavelengths dominate.
• Color rendering: How natural object colors appear under the light.
• Efficacy: How much of the energy output is visible (aligned with eye sensitivity).
• Specialized suitability: For tasks like plant growth or signaling.

Measurement:
Spectroradiometers and spectrophotometers are used to analyze SPD, supporting certification and quality control in industries like aviation (to meet ICAO or FAA standards).

Types of Spectra

1. Continuous Spectra

Produced by solids, liquids, or high-pressure gases heated to incandescence (e.g., the sun, incandescent bulbs). All visible wavelengths are present, yielding white light and high color fidelity.

2. Line Spectra

Produced by low-pressure gases (e.g., sodium, mercury vapor lamps) or lasers. Only specific wavelengths are emitted, giving sharply defined lines. Each chemical element produces a unique line spectrum.

3. Band Spectra

Produced by molecules or high-pressure gases, where groups of lines merge into bands. Seen in some gas-discharge lamps and atmospheric emissions (like auroras).

Significance:
The type of spectrum affects how objects appear and how well humans can distinguish colors—critical for cockpit, cabin, and airfield lighting.

Radiometry: Physical Measurement of Light

Radiometry quantifies electromagnetic radiation in absolute physical units, regardless of human perception.

Key Quantities

• Radiant Energy (Qₑ): Joules (J)
• Radiant Flux (Φₑ): Watts (W) — energy per second
• Radiant Intensity (Iₑ): Watts per steradian (W/sr)
• Radiance (Lₑ): W/(m²·sr) — brightness in a direction
• Irradiance (Eₑ): W/m² — power per area

Each can be resolved by wavelength (e.g., spectral radiance in W/(m²·sr·nm)). Radiometric data are the starting point for engineering lighting and sensor systems, including those used in aviation navigation and safety.

Photometry: Measurement Weighted by Human Vision

Photometry weights measurements to match the human eye’s sensitivity, producing values meaningful for human-centric design.

Key Quantities

• Luminous Flux (Φᵥ): Lumens (lm)
• Luminous Intensity (Iᵥ): Candelas (cd)
• Luminance (Lᵥ): cd/m² (nits)
• Illuminance (Eᵥ): Lux (lx), where 1 lx = 1 lm/m²

The Luminosity Function (V(λ))

Represents average eye sensitivity under photopic (daylight) conditions, peaking at 555 nm. For any spectral distribution, photometric quantities are calculated by weighting the radiometric values by V(λ) and integrating across the visible range.

Importance:
Photometry bridges objective energy and subjective human experience, guiding lighting design, safety standards, and regulatory compliance.

Radiometric vs. Photometric Units

Conversion:
Photometric = Radiometric × V(λ) × 683 lm/W (at 555 nm).

Human Visual Response: Photopic and Scotopic Sensitivity

• Photopic Vision: Daylight, cone cells, peaks at 555 nm (green-yellow), covers 380–780 nm.
• Scotopic Vision: Night, rod cells, peaks at 507 nm (blue-green), less sensitive to red.
• Mesopic Vision: Intermediate light, both rods and cones contribute.

Implications:

• Lighting for night operations (e.g., red cockpit lights) is designed to preserve scotopic (night) vision.
• Airfield lighting considers both photopic and scotopic responses to maximize visibility for pilots under all conditions.

Spectral Power Distribution (SPD) and Color Quality

The SPD is the “spectral fingerprint” of a light source, dictating:

• Color Rendering Index (CRI): How naturally colors appear under the source.
• Correlated Color Temperature (CCT): The “warmth” or “coolness” of white light.
• Visual Comfort and Performance: SPD impacts glare, eye strain, and visibility.

Example:
Incandescent lamps have a smooth SPD, high CRI, and warm CCT. Fluorescent lamps or “peaky” LEDs may have poor CRI, making some colors look unnatural—a key concern in cockpit and airfield lighting.

Applications in Aviation and Industry

• Aviation:
  • Runway and taxiway lights must have precise colors and intensities, certified by SPD and photometric measurements.
  • Cockpit lighting is engineered for optimal readability and to support night adaptation.
  • Lighting standards (ICAO, FAA) specify spectral and photometric criteria.
• Display Technology:
  • Spectral tuning improves color accuracy and reduces visual fatigue.
• Safety and Compliance:
  • Correct SPD ensures signals are distinguishable and meet regulatory requirements.

Conclusion

The light spectrum—and its distribution by wavelength—lies at the heart of color science, photometry, and practical lighting design. Whether for aviation, manufacturing, or everyday environments, understanding and controlling SPD, radiometric and photometric quantities, and the human visual response is essential for safety, efficiency, and user satisfaction.

For manufacturers, designers, and regulators, mastery of the light spectrum enables the development of lighting and signaling systems that deliver optimal performance, compliance, and comfort in every context.