Optics in Instruments

Chapter 3

A Revision of Photometry and Radiometry ¹

3.1. Introduction: the role of photometry and radiometry

For a long time, the purpose of photometry and radiometry was to characterize visible radiations – that is, those of wavelengths between 0.4 and 0.7 µm – given that the eye was the only optical sensor at mankind’s disposal. In response to the diversification of optical instruments, the abilities of photometry and radiometry on both sides of the visible spectrum have increased, and they now cover the complete range of optics, and the domain of the electromagnetic spectrum between X-rays and radar, which ranges from ultraviolet to infrared, i.e. from wavelengths of 0.01 µm to 1 mm.

The role of the optical instruments is to study the optical radiations in all their aspects, theoretical and experimental, to define their parameters and the laws which govern them, from the emission of light and its propagation to its reception. All the elements of an optical chain are involved: sources, media of propagation, optical or natural surfaces, optical components and instruments, detectors, etc., with their associated metrology.

The laws of geometrical optics and radiometry form the basis of optical design. The performance of an instrument relies to a large extent on the quality of its radiometry, be it at the level of its design, its manufacture or its control: at the design stage, radiometry evaluates the radiative and energetic limitations of the future system, which it must optimize ...