Introduction

Ray tracing, with either paraxial or real rays, is insufficient to assess the efficiency of optical systems or indicate the quality of the final image. Apart from the effect of aberrations, it is also necessary to understand basic photometric principles, photometric quantities such as source luminance and surface illuminance, and a number of other factors such as vignetting which affect the image plane illuminance in optical systems.

Before beginning a study of the photometry of optical systems, it is necessary to understand the four basic photometric quantities, namely luminous flux, luminous intensity, luminance and illuminance. For a long time, photometry was regarded as an independent field of study with its own fundamental units and international standards. For example, luminous intensity has been a fundamental basic physical quantity for many years with a physical standard using a sample of thorium oxide held at the melting point of platinum (2042 K) as the light source (Sanders and Jones 1962). This light source had a defined luminance of 60 x 104 cd/m2. More recently, however, there has been a trend to regard photometry as a branch of radiometry and derive all photometric quantities from radiometric quantities.

Radiometry may be defined as the measurement of the energy or power in an electromagnetic beam, measured over the entire spectrum. The division of the entire electromagnetic spectrum is shown in Figure 1.2, Chapter 1. Light is only a very small part of this spectrum.

The nature of light

Light is that part of the electromagnetic spectrum that elicits a visual response in the eye and is in the range of approximately 400–700 nm. The eye is not equally responsive to electromagnetic energy in this range.