chapter 8, Resolution and Noise

Excerpt

8.1 Introduction

There are several issues that communicate how “good” a spectrum is. In this chapter, we will focus on two of them: resolution and noise. Both are factors that can affect the quality of a spectrum. Keep in mind that not all of the issues discussed here are applicable to all types of spectroscopy. A good spectroscopist should recognize which issues will impact the particular spectrum being measured.

8.2 Resolution in Dispersive Spectrometers

A spectrum, recall, is usually some graphical representation of electromagnetic radiation absorbed or emitted versus the energy of that radiation. One concern of anyone measuring a spectrum should be, how close can two different spectral transitions be and still be differentiated as different signals? This question relates to the idea of resolution.

The American Society for Testing and Materials (ASTM) defines spectral resolution as the ratio λ/Δλ, where λ is the wavelength of radiant energy being examined and Δλ is the spectral bandwidth expressed in wavelength units; or, alternatively, the ratio ν/Δν, where ν is the wavenumber of the radiant energy being examined and Δν is the spectral bandwidth expressed in wavenumber units.

That is, the resolution is some unitless numerical value that depends on the wavelength or frequency (i.e., wavenumber) of the light and the spectral bandwidth of the spectrometer's monochromator. For dispersive spectrometers, the spectral bandwidth is the wavelength (or frequency) interval that is coming out of the exit slit or its equivalent. Spectroscopists recognize that no monochromator will pass a single frequency at a time; rather, a range of frequencies always comes out together.

This definition of resolution depends on the spectral bandwidth, which is itself determined by the dispersive ability of the monochromator and the size of the instrument's exit slit. Since the exit slit's width can be controlled by the experimenter or the spectrometer (i.e. automatically), the dispersive ability of the monochromator ultimately determines the resolution.