Glossary of White Light and Photometry

White Light

White light is electromagnetic radiation encompassing all wavelengths within the visible spectrum, typically from 380 to 780 nanometers (nm). Unlike monochromatic light, which consists of a single wavelength and appears colored, white light is a composite of multiple spectral components, each correlating to different colors. The most familiar source is sunlight, appearing white due to its continuous and balanced spectrum. Artificial sources such as incandescent bulbs, some LEDs, and fluorescent lamps are engineered to mimic this composition so that humans perceive them as white.

The sensation of white arises when the human eye’s three cone cell types—sensitive to short (S), medium (M), and long (L) wavelengths—are stimulated in proportions that the brain interprets as white. This can result from a continuous spread of wavelengths (natural or incandescent sources) or from a mixture of a few discrete wavelengths (as in RGB digital displays).

White light is vital in all fields involving vision: from aviation (where lighting clarity and color accuracy are critical for safety and navigation), to architecture, industry, and remote sensing. Its spectral content can be split into the familiar rainbow of colors using a prism, revealing its composite nature—a phenomenon first studied by Isaac Newton.

Electromagnetic Spectrum

The electromagnetic spectrum is the full range of wavelengths or frequencies of electromagnetic radiation, from gamma rays (with wavelengths less than a nanometer) to radio waves (with wavelengths of kilometers). The visible spectrum—relevant for white light—covers only a tiny portion, from about 380 to 780 nm.

Adjacent to the visible range are ultraviolet (100–380 nm) and infrared (780 nm–1 mm). These regions are invisible to the human eye but are important in technology and science. For example, aviation lighting systems are designed to maximize visibility by emitting wavelengths least affected by atmospheric conditions like fog.

Visible Spectrum

The visible spectrum is the portion of the electromagnetic spectrum that the average human eye can perceive, typically from 380 to 780 nm. Within this band, different wavelengths correspond to different colors:

Boundaries are not sharply defined due to overlapping cone sensitivities and individual differences in vision. The visible spectrum’s limits may be adjusted in technical standards (e.g., 400–700 nm for photometry).

Spectral Distribution

Spectral distribution describes how a light source emits energy across the visible spectrum. It is usually shown as a graph of intensity versus wavelength.

Types of spectral distribution:

• Continuous spectrum: Emitted by sources like the Sun or incandescent bulbs; includes all visible wavelengths.
• Line spectrum: Emitted by gas discharge lamps; consists of sharp peaks at discrete wavelengths (e.g., sodium or mercury vapor lamps).
• Band spectrum: Contains groups of closely spaced lines, found in high-pressure lamps or certain natural phenomena.

Spectral distribution determines color appearance, color temperature, and color rendering. In aviation, lighting must comply with standards for spectral distribution to ensure safety and performance.

Color Perception

Color perception is the subjective experience resulting from the stimulation of the retina by different wavelengths. The human eye has three types of cones:

The brain interprets the combined responses to create the sensation of color. When all three are stimulated equally, the result is “white.” The eye’s sensitivity peaks at about 555 nm under daylight conditions (photopic vision) and shifts to 507 nm (scotopic vision) in low light—a phenomenon called the Purkinje effect.

Color perception depends on context, adaptation, and the spectral composition of the light. Metamerism occurs when different spectra are perceived as the same color. Aviation standards specify chromaticity boundaries for white and colored lights to ensure safety.

Additive Color Mixing

Additive color mixing is how different colored lights combine to produce new colors—including white—by stimulating the retinal cones in specific proportions. Mixing red, green, and blue (RGB) light in the right intensities yields white. This principle is used in digital displays, cockpit instruments, and airport signage.

Additive mixing is distinct from subtractive mixing (used in pigments and filters). International standards define chromaticity coordinates for aviation “white” and other operational colors, ensuring reliable color recognition.

Color Temperature

Color temperature describes the hue of white light by comparing it to the spectrum of an ideal blackbody radiator at a given temperature (in kelvins, K). Lower color temperatures (2700–3500 K) appear warm (yellow/red), while higher (5000–6500 K) appear cool (blue-white).

Color temperature is crucial for aviation and industrial lighting, supporting visual comfort, safety, and accurate color discrimination.

Luminous Flux

Luminous flux is the total amount of visible light emitted by a source in all directions, weighted by the human eye’s sensitivity. Measured in lumens (lm), it differs from radiant power (watts) by considering only visible wavelengths.

Luminous flux (Φ_v) is calculated by integrating the spectral power distribution with the eye’s luminous efficacy function (V(λ)), peaking at 555 nm. It’s a key parameter for specifying lamp and luminaire performance in aviation and industrial settings.

Luminous Intensity

Luminous intensity measures how much luminous flux is emitted in a specific direction, within a given solid angle. It’s measured in candelas (cd), where one candela equals one lumen per steradian.

Directional lights (e.g., runway lights, beacons) are specified by their luminous intensity. ICAO standards define required values and angular distributions for aviation safety.

Illuminance

Illuminance quantifies the amount of luminous flux incident on a surface per unit area, measured in lux (lx), where one lux is one lumen per square meter. It determines how brightly a surface is lit and is crucial for visibility in airports, cockpits, and workspaces.

ICAO and other bodies specify required illuminance levels for safe navigation and operations. Lux meters measure illuminance and help ensure lighting systems meet standards.

Luminance

Luminance is the brightness of a light-emitting or reflecting surface as seen by an observer, considering both intensity and projected area. Measured in candelas per square meter (cd/m²), luminance is key for display readability, signage, and lighting ergonomics.

It’s calculated as: L = I / (A · cosθ)

where L is luminance, I is intensity, A is area, and θ is the angle between surface normal and viewing direction. Aviation standards specify luminance values for visibility and safety.

Photometry

Photometry is the science of measuring visible light as perceived by the human eye. It involves quantities such as luminous flux, intensity, illuminance, and luminance, all weighted by the eye’s spectral sensitivity. Photometry is essential in lighting design, safety, and regulatory compliance—especially in aviation, architecture, and industry.

Photometric measurements use sensors and filters calibrated to the human visual response, ensuring consistency and relevance to real-world perception. International standards (ICAO, CIE) define procedures and thresholds for photometric parameters in critical environments.

By understanding white light, its spectral properties, and photometric quantities, engineers and operators can design lighting systems that optimize visibility, safety, and comfort—especially in demanding applications like aviation and transportation.rtation.