A. A. Gaertner

*National Research Council of Canada, Ottawa Canada* 

### **1. Introduction**

Optical radiation bathes the world in which we live. The sun, moon and stars have provided us with our primary sources of light since before the beginning of life upon the earth. Very early in our history we observed that we could produce light by other means than depending upon the daily rotation of the earth. As our societies developed, in addition to creating intricate visual experiences with light, we discovered new ways of using this light to effect changes upon many of the materials we found in the world around us. As we sought to control these effects, we realised that the light itself had many properties that affected the outcome of our investigations and that we needed to untangle the characteristics of the light and the properties of the materials in response to this light. We discovered that there were effects caused by 'light' that we could not observe visually, which led to the distinction between light and radiation. To evaluate these effects, to discover new effects, and to produce (or reduce) them efficiently, we have come to depend critically upon the accurate measurement of the optical radiation required in our endeavors. In this chapter we will discuss some of the procedures and equipment necessary to obtain accurate measurements of the amount of optical radiation that we have available for our activities.

The measurement of optical radiation presents the practitioner with many opportunities and challenges. These stem from various sources: the properties of the radiation itself (wavelength, polarization), the geometrical configuration of the radiation field (direction, solid angle), the interaction of the radiation field with the detectors, devices and materials under test (responsivity, reflectance, transmittance), and, not in the least, to the vocabulary and definitions in use, locally and internationally, to describe the measurement process and the results of the measurements.

There are two international organizations whose work is particularly important for the fundamentals of measurement and of optical radiation measurement in particular:

