Introduction

In the introduction we saw that explaining the concept of light may require more than just one theory. However, before we begin our journey through the disciplines of optics, we must identify the physical parameters needed to quantify the phenomena that are associated with light. But even before that, we should recognize that the phenomenon we call light is only a part of the broader phenomenon of radiation. If we consider radiation to be

then light may be defined (American National Standard 1986) as

(This definition is sometimes extended to include ultraviolet and infrared radiation.) Note that, by the present definition, radiation that is spectrally detectable to the eye will be called light even if it is too faint to be seen. Although these definitions include terms (such as electromagnetic waves and spectra) that are yet to be explained, it is evident that light represents a subcategory of radiation. Thus, once the physical quantities that specify radiation are defined they may also be used for the quantification of light. Furthermore, the measurement of radiation – or radiometry – does not depend on how we define what is detectable by the eye. Therefore, radiometry is more objective than photometry, which is the process of measuring light.