Introduction

Spectrometers divide the light centered at wavelength λ into narrow spectral ranges, Δλ. If the resolution R = λ/Δλ > 10, the goals of the observation are generally different from those in photometry, including both measuring spectral lines and characterizing broad features.

There are three basic ways of measuring light spectroscopically:

1. Differential-refraction-based, in which the variation of refractive index with wavelength of an optical material is used to separate the wavelengths, as in a prism spectrometer.

2. Interference-based, in which the light is divided so a phase-delay can be imposed on a portion of it. When the light is re-combined, interference among components is at different phases depending on the wavelength, allowing extraction of spectral information. The most widely used examples are diffraction grating, Fabry–Perot, and Fourier spectrometers. Heterodyne spectroscopy also falls into this category, but we will delay discussing it until we reach the submillimeter and radio regimes in Chapter 8.

3. Bolometrically, in which the signal is based on the energy of the absorbed photon. This method is applied in the X-ray, for example, using CCDs or bolometers, and will be discussed in Chapter 10.