Photometry (optics)

Introduction

Photometry is a branch of optics that deals with the measurement of visible light as perceived by human vision. Unlike radiometry, which measures all electromagnetic radiation, photometry is concerned specifically with the human eye's response to light. This field is crucial in various applications including lighting design, photography, astronomy, and vision science.

Basic Concepts

Luminous Flux

Luminous flux, measured in lumens (lm), is the total amount of visible light emitted by a source. It is a photometric equivalent of radiant flux, but weighted by the luminosity function, which accounts for the varying sensitivity of the human eye to different wavelengths of light.

Luminous Intensity

Luminous intensity is the amount of luminous flux emitted per unit solid angle, measured in candelas (cd). It describes the concentration of light in a particular direction and is essential in designing lighting systems to ensure adequate illumination.

Illuminance

Illuminance, measured in lux (lx), is the luminous flux incident on a surface per unit area. It quantifies how much light is received by a surface and is a critical parameter in workplace lighting standards and architectural design.

Luminance

Luminance, measured in candelas per square meter (cd/m²), describes the amount of light emitted, passing through, or reflected from a surface in a given direction. It is a measure of the brightness perceived by the human eye and is used in display technology and visual ergonomics.

Measurement Techniques

Photometers

Photometers are instruments designed to measure various photometric quantities. They often use a photodetector with a filter that mimics the human eye's response to light. Common types include lux meters, luminance meters, and integrating spheres.

Spectroradiometers

Spectroradiometers measure the spectral power distribution of a light source. By applying the luminosity function to the spectral data, one can derive photometric quantities such as luminous flux and illuminance.

Integrating Spheres

Integrating spheres are used to measure the total luminous flux of a light source. The interior of the sphere is coated with a diffuse reflective material, ensuring that light is evenly distributed and measured accurately by a photodetector.

Applications

Lighting Design

Photometry is fundamental in lighting design, ensuring that spaces are adequately and efficiently illuminated. Standards such as those from the Illuminating Engineering Society (IES) provide guidelines for appropriate illuminance levels in different environments.

Photography

In photography, photometry is used to determine the correct exposure settings. Light meters measure the illuminance or luminance of a scene, helping photographers achieve the desired brightness and contrast in their images.

Astronomy

Astronomical photometry involves measuring the brightness of celestial objects. This data helps in understanding the properties of stars, planets, and other astronomical phenomena. Instruments like charge-coupled devices (CCDs) are commonly used for these measurements.

Vision Science

In vision science, photometry helps in understanding how the human eye perceives light and color. This knowledge is applied in developing visual displays, improving lighting conditions, and studying visual impairments.

Advanced Topics

Colorimetry

Colorimetry is closely related to photometry but focuses on measuring color. It involves quantifying the color of light sources and surfaces based on human vision. Instruments like colorimeters and spectrophotometers are used in this field.

Mesopic Photometry

Mesopic photometry deals with light levels that are intermediate between photopic (daylight) and scotopic (nighttime) vision. This area is important for understanding human vision in low-light conditions and has implications for street lighting and automotive lighting.

High Dynamic Range (HDR) Imaging

HDR imaging captures a wider range of luminance levels than standard imaging techniques. Photometry plays a role in calibrating HDR systems to ensure accurate representation of both very bright and very dark areas in a scene.

References

CIE (Commission Internationale de l'Éclairage). "International Lighting Vocabulary." CIE S 017/E:2011.
Illuminating Engineering Society. "The Lighting Handbook." 10th Edition, 2011.
Wyszecki, Günther, and Stiles, W.S. "Color Science: Concepts and Methods, Quantitative Data and Formulae." 2nd Edition, Wiley-Interscience, 2000.