Photometry is the science and technique of measuring the intensity of light as perceived by the human eye or detected by instruments. It focuses on quantifying visible light, often in terms of brightness, illuminance, or luminous flux.
Photometry plays a vital role in fields like astronomy, lighting design, photography, and vision science.
🔍 Key Concepts in Photometry
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Luminous Flux (Φ)
Total amount of visible light emitted by a source, measured in lumens (lm). -
Luminous Intensity (I)
Light power emitted in a particular direction, measured in candelas (cd). -
Illuminance (E)
Amount of luminous flux hitting a surface per unit area, measured in lux (lx) = lumens per square meter. -
Luminance (L)
Brightness of a surface perceived by the eye, measured in cd/m².
📐 Photometric Quantities vs Radiometric Quantities
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Photometric quantities consider human eye sensitivity (visible spectrum weighted by the luminosity function).
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Radiometric quantities measure total electromagnetic energy, regardless of human vision.
⚙️ Instruments for Photometry
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Photometer: Measures light intensity or illuminance.
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Lux meter: Measures illuminance (lux).
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Integrating sphere: Measures total luminous flux from sources.
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CCD cameras and sensors: Used for imaging and quantitative photometry.
🌟 Applications of Photometry
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Astronomy: Measuring star brightness, light curves of variable stars.
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Lighting design: Evaluating and designing indoor and outdoor lighting.
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Photography and cinematography: Exposure measurement.
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Vision science: Understanding human perception of brightness.
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Material analysis: Reflectance and transmittance measurements.
🔬 Photometry vs Spectrophotometry
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Photometry measures light intensity over a broad wavelength range (weighted by human eye response).
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Spectrophotometry measures intensity as a function of wavelength (spectral distribution).