Basics of Light Measurement

Light, which is the visible part of the electromagnetic radiation spectrum, is the medium through which human beings receive the major part of environmental information. Evolution has optimized the human eye into a highly sophisticated sensor for electromagnetic radiation. Joint performance between the human eye and the visual cortex, which makes up a large part of the human brain, outshines even the latest technical and scientific developments in image processing and pattern recognition. As a matter of fact, most of the information flow from external stimuli to our brain is transferred visually. Photometry deals with the measurement of this visible light energy.

However, optical radiant energy does not only encompass visible “light”, but also radiation that is invisible to the human eye. The term optical is used because this radiation follows the laws of geometrical optics.

Radiometry deals with the measurement of all optical radiation, including the visible portion of this radiant energy.

This tutorial is an introduction to the radiometric, photometric, colorimetric, reflection and transmission principles as well as quantities, symbols, units and the basic nature of light and color. Sections covering a sampling of current applications, detectors, electronics and calibration are included. A list of reference sources is provided for future study.

SI (Système International) units are used throughout this tutorial. Many international organizations, including the CIE (Commission Internationale de l'Eclairage), have exclusively adopted this system of units. The terminology used follows that of the CIE International Lighting Vocabulary.