Photometric and Radiometric Measurement of Light Sources
Radiometric Quantities
Radiometry measures the physical quantity of light emitted by a source, regardless of its perception by the human eye. Key radiometric quantities include:
- Radiant Flux (Φ): The total power emitted by a source, measured in watts (W).
- Radiant Intensity (I): The radiant flux emitted per unit solid angle, measured in watts per steradian (W/sr).
- Irradiance (E): The radiant flux received per unit area on a surface, measured in watts per square meter (W/m²).
Radiometric measurements are typically used for characterizing the total power output of a light-emitting diode (LED) and are often relevant in applications requiring precise power control or thermal management.
Photometric Quantities
Photometry measures light as perceived by the human eye, weighting different wavelengths according to the eye's sensitivity. Key photometric quantities include:
- Luminous Flux (Φv): The total perceived light emitted by a source, measured in lumens (lm).
- Luminous Intensity (Iv): The luminous flux emitted per unit solid angle, measured in candelas (cd).
- Illuminance (Ev): The luminous flux received per unit area on a surface, measured in lux (lx).
- Luminance (Lv): The luminous intensity per unit projected area of a surface, measured in candelas per square meter (cd/m²).
Photometric measurements are essential for assessing the perceived brightness and efficacy of an LED, critical for applications like lighting design and display technology.
Measurement Techniques
Various instruments are used for measuring radiometric and photometric quantities. These include:
- Spectrometers: Measure the spectral power distribution of the light source, providing data across the entire visible spectrum and beyond.
- Integrating spheres: Used to collect and measure the total light output from a source, minimizing directional effects.
- Photometers and lux meters: Measure illuminance (lux) and sometimes luminous intensity (candela).
- Goniophotometers: Measure the luminous intensity at various angles, characterizing the spatial distribution of light.
Factors Affecting Measurements
Several factors can influence the accuracy of measurements, including:
- LED characteristics: Variations in LED chip design, drive current, and temperature can affect output.
- Ambient conditions: Temperature and humidity can affect the light output and measurement accuracy.
- Calibration: Regular calibration of measurement instruments is crucial for reliable data.
Data Interpretation
Understanding the specific units and the context of the measurement is crucial for accurate interpretation. Consider whether radiometric or photometric data is relevant for a given application.