Optical Imaging and Spectroscopy

SPECTROSCOPY

Parmi les appareils utilises en analyse spectrale, il est maintenant classique de distinguer, d'une part, les spectrometres, o le spectre est explore dans le temps, element par element et, d'autre part, les spectrographes qui permettent d'obtenir des informations simultanement sur tous les elements du spectre.

—A. Girard [93]

9.1 SPECTRAL MEASUREMENTS

A spectrometer analyzes the power spectral density S(ν) of an optical signal. Of course, optical signals are functions of space, time, and polarization as well as wavelength or frequency, but we limit our discussion in this chapter to instruments that measure the power spectral density in a single optical mode or the average spectral density over a range of modes, returning to the larger issue of spectral imaging [determination of S(r, ν)] in Section 10.6.

Spectral information is isolated from optical signals by three mechanisms:

• Spatial Dispersion. Dispersive elements, such as prisms and gratings, redirect refracting and diffracting waves as a function of wavelength. These elements are combined with spatial filters to produce spectrally informative spatial patterns. Sections 9.2 and 9.3 integrate the dispersive elements described in Chapter 4 in spectrometer designs. The most recent trend in dispersive spectrograph design focuses on the use of volume diffractive structures, such as photonic crystals, multiplex holograms, ...