Glossary of Light Sensor Terms and Concepts

Light Sensor

A light sensor is a device designed to detect and respond to the presence or intensity of light. It converts the energy from incident light (photons) into an electrical signal (current, voltage, or resistance), serving as a cornerstone in photometric systems. Light sensors are used in applications ranging from industrial automation and environmental monitoring to aviation and consumer electronics. Also referred to as photoelectric sensors, ambient light sensors, or luminosity sensors, their role is to provide quantitative data on light levels, enabling automated control, safety systems, and scientific measurement.

Key characteristics of light sensors include spectral sensitivity (the wavelength range detected), response time (how quickly output changes with light), and linearity (accuracy of output correlation with light intensity). In aviation, for example, light sensors are critical for cockpit and runway lighting control, ensuring optimal visibility for pilots. In consumer electronics, they allow displays to adjust brightness for comfort and power efficiency. According to ICAO, light sensors in aerodrome lighting control systems automate switching and dimming of runway lights for safety and efficiency.

Photodiode

A photodiode is a semiconductor light sensor that converts light into an electrical current via the photoelectric effect. When photons strike the pn-junction, electron-hole pairs are generated, creating a current proportional to the light intensity. Photodiodes offer fast response times (microseconds or less), high sensitivity, and a linear output, making them ideal for precise photometric applications such as scientific instruments, cockpit displays, and weather monitoring systems.

Photodiodes come in configurations like PIN (for speed), avalanche (for low-light gain), and silicon (for visible/near-IR sensitivity). Their performance depends on quantum efficiency, dark current, and noise. Photodiodes can operate in photovoltaic mode (voltage output) or photoconductive mode (current output). ICAO specifies photodiodes in calibration devices for aerodrome lighting, ensuring regulatory compliance.

Phototransistor

A phototransistor is a light-sensitive transistor that amplifies the current generated by absorbed photons, providing higher output than photodiodes. The base region is exposed to light, and the resulting small photocurrent is amplified by the transistor. Phototransistors are sensitive to low light, but their response is slower and less linear than photodiodes.

Phototransistors are used in light meters, optical switches, and safety devices. In aviation, they can trigger lighting controls based on ambient light. Spectral response depends on material and filters. While phototransistors offer enhanced sensitivity, they are used where cost and simplicity are more important than speed or absolute accuracy.

Photoresistor (LDR – Light Dependent Resistor)

A photoresistor or Light Dependent Resistor (LDR) is a passive sensor whose resistance decreases with increased incident light. Made from photoconductive materials like cadmium sulfide (CdS), LDRs show large resistance changes from darkness to illumination. They are simple and inexpensive, widely used for automatic lighting and educational demonstrations.

LDRs are non-linear, slow to respond, and typically sensitive to visible light. Their use in aviation is limited to non-critical applications due to accuracy and environmental concerns (CdS is hazardous). Solid-state sensors like photodiodes are preferred for safety-critical systems.

Ambient Light Sensor

An ambient light sensor measures environmental light intensity, often matching the human eye’s spectral response. Modern sensors use silicon photodiodes with on-chip filtering and signal conditioning, outputting analog or digital signals. They are vital in smartphones, avionics, and airport lighting control, adjusting display and light levels for comfort, efficiency, and regulatory compliance.

Features include wide sensitivity range, fast response, and temperature stability. Advanced models offer calibrated lux output, programmable gain, and proximity sensing. Integration is usually via I2C/SPI for real-time control in embedded systems.

Photometry

Photometry is the measurement of visible light as perceived by the human eye, using units like lux, lumen, and candela weighted by the CIE photopic response. Photometry is fundamental for designing and regulating lighting, especially in aviation, where precise measurement ensures runway and taxiway safety.

Photometers use filtered photodiodes or standard illuminants, with calibration traceable to national standards. ICAO and ASTM standards detail protocols for accurate, reproducible light measurement.

Lux

Lux (lx) measures illuminance, or luminous flux per square meter. It quantifies how much visible light strikes a surface—crucial for specifying and verifying lighting in offices, airports, and public spaces. In aviation, lux is used to set minimum lighting for runways and aprons, as per ICAO Annex 14.

Lux meters use calibrated photodiodes and cosine correction for accuracy. Measurements help ensure safety, energy efficiency, and regulatory compliance.

Lumen

Lumen (lm) quantifies luminous flux—the total visible light emitted per second. One lumen equals the flux from one candela over one steradian. Lumen ratings are essential in comparing lighting efficiency and ensuring compliance in aviation and other fields.

Integrating spheres and calibrated photodetectors are used to measure total luminous flux, considering all emitted directions.

Candela

Candela (cd) is the SI base unit of luminous intensity—power emitted in a specific direction per steradian. One candela equals one lumen per steradian. Aviation lighting systems are specified in candelas to ensure visibility. Measurement uses goniophotometers for directional output, fundamental in lighting design and certification.

Spectral Response

Spectral response is a sensor’s sensitivity to different light wavelengths. For photometry, sensors should match the CIE photopic curve (peak at 555 nm), but silicon sensors may also detect UV and IR, potentially skewing results. Optical filters and calibration correct this.

Spectral response is critical in applications from regulatory lighting measurements to horticulture and UV monitoring. Sensor datasheets provide spectral curves for application matching.

Calibration

Calibration aligns a sensor’s output with reference standards for accurate, traceable measurements. This involves exposing the sensor to known light levels and recording the output, accounting for non-linearity, temperature, and spectral mismatch. ICAO specifies calibration intervals and traceability for aviation lighting inspections.

Calibration ensures compliance, safety, and scientific accuracy, with field calibration using certified meters or reference photometers.

Response Time

Response time is the interval between a change in light and the corresponding sensor output change, measured from 10% to 90% of final value. Fast response is vital in optical communications, automation, and pilot-controlled lighting. Photodiodes are fastest (nanoseconds); phototransistors are slower, and LDRs are slowest (tens to hundreds of milliseconds). In aviation, fast response supports real-time lighting control for safety.

Linearity

Linearity describes how closely a sensor’s output tracks light intensity. Photodiodes are highly linear; phototransistors and LDRs less so. Linearity is essential for accurate photometric measurements and regulatory compliance, with correction functions applied as needed in calibration.

Cosine Correction

Cosine correction ensures a sensor accurately measures light intensity regardless of the angle, following the cosine law. This is achieved with diffusers or shaped entrance windows and is required for regulatory compliance in photometric equipment, especially for outdoor or multi-directional lighting.

Illuminance Meter (Lux Meter)

An illuminance meter or lux meter is a calibrated, cosine-corrected photodiode instrument for measuring lux. Used in aviation, architecture, and horticulture, lux meters verify lighting compliance and optimize designs. Proper use requires careful positioning and regular calibration.

Luminous Flux

Luminous flux (in lumens) is the total perceived power of light emitted, weighted by the human eye’s sensitivity. It is used to specify lamp output and is measured using integrating spheres and calibrated detectors. Aviation standards require minimum luminous flux for various lighting fixtures.

Luminous Intensity

Luminous intensity measures the amount of luminous flux emitted in a specific direction, expressed in candela. It is fundamental in regulatory lighting design, ensuring directional visibility and compliance.

Further Reading and References

• ICAO Annex 14 – Aerodromes: Aerodrome Design and Operations
• CIE (Commission Internationale de l’Éclairage)
• ASTM International Lighting Standards
• National Institute of Standards and Technology (NIST) – Photometry
• EASA (European Union Aviation Safety Agency)

Light sensors are integral to safety, efficiency, and automation in modern systems. Their correct selection, calibration, and application ensure compliance, reliability, and optimal performance across diverse industries—including the critical field of aviation.