Light Under the Microscope: Introduction to Photometric Measurements
Measuring Light as It Is Perceived: Where Physics Meets Human Vision
Photometric measurement, based on radiometry, is an optical characterization method that aims to quantify light according to how it is perceived by the human eye. Photometry was primarily developed in France at the beginning of the 19th century by Pierre Bouguer to better compare the luminous intensity of light sources, with the goal of improving lighthouse visibility at sea and facilitating navigation.
This article invites you to explore how these measurements work and why they have become essential in many technological applications.
How can we measure light as we perceive it? At the intersection of optics and human perception, photometry makes it possible to quantify visible light while taking into account the sensitivity of the human eye. Used in the development of lighting systems, displays, sensors, and scientific instruments, this discipline—rooted in optical sciences—transforms a natural phenomenon into precise data that are essential for many technological applications.
Several standardized quantities in the International System of Units (SI) are used to characterize light in photometry. Luminous flux, expressed in lumens, corresponds to the total power emitted by a light source in all directions. Luminous intensity, measured in candelas, refers to the flux emitted in a specific direction per unit solid angle. Illuminance, measured in lux, represents the density of luminous flux received by a surface. Finally, luminance measures the light emitted or reflected by a surface in a given direction per unit area and solid angle, making it possible to evaluate perceived brightness—an essential parameter in applications related to visual comfort and signaling.
These quantities are measured using specialized instruments: integrating spheres for luminous flux, lux meters for illuminance, and luminance meters for the apparent brightness of surfaces. For more complex analyses—particularly the angular distribution of light—goniophotometers are used to map the spatial distribution of luminous intensity.
Photometric measurements are governed by rigorous international standards, such as those established by the CIE (International Commission on Illumination), the IES (Illuminating Engineering Society), and the ISO (e.g., ISO/CIE 23539:2023, ANSI/IES LS-4-20(R2023)). These standards ensure traceability, comparability of results, and compliance with regulatory requirements.
Among the advantages of these measurements are their ability to quantify light according to human visual perception, their relatively straightforward implementation, and their non-destructive nature. They provide actionable data to optimize lighting systems, monitor the quality of optical devices, and ensure visual comfort, while offering a strong balance between accuracy, speed, and reproducibility.
In summary, photometric measurements play a crucial role in translating light into clear, usable data aligned with human perception. Through strict standards and specialized instruments, they help ensure quality, safety, and visual comfort across many fields—from public lighting to advanced technologies. Mastering these measurements ultimately means mastering light itself, enabling us to better illuminate both our everyday environments and our technological innovations.
Optech supports companies in the implementation of precise photometric measurements, whether for research, quality control, or the development of innovative products.
Discover how our experts can support your project today.