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GENERAL INFORMATION

chapter 4, Spectrometer Specifications

from: Introduction to Imaging Spectrometers

Author(s): William L. Wolfe

Published: 11 April 1997

Chapter DOI: http://dx.doi.org/10.1117/3.263530.ch4

Chapter Page Count: 4 pages

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Front Matter

1. Introduction

2. Optics Overview

3. Radiometry Review

4. Spectrometer Specifications

5. Imaging Introduction

6. Detector Descriptions

7. System Sensitivity

8. Filter Phenomena

9. Prism Spectrometers

10. Grating Spectrometers

11. Michelson Interferometer Spectrometers

12. An Imaging Fourier Transform Spectrometer

13. Fabry-Perot Interferometer Spectrometers

14. A Challenging Application

15. A Satellite Spectrometer

16. A Mars Rover Experiment

17. Some Trade-Offs

18. Other Examples

A2. Appendix to Chapter 2 - Optics Operations

A6. Appendix to Chapter 6 - Detectors

A9. Appendix to Chapter 9 - Prisms

A10. Appendix to Chapter 10 - Gratings

A12. Appendix to Chapter 12 - FTS Foundations

Back Matter

Preview

Chapter Contents

4.1 Spectral Variables
4.2 Resolution
4.3 Resolving Power

Excerpt

In the very general sense of the word, every spectrometer is a filter and every filter is a spectrometer. Each is a device for isolating a relatively small portion of the entire spectrum. Thus, in this section discussing the descriptors, we use the terms filter and spectrometer interchangeably. The important concepts include wavelength, wave number, spectral resolution, line width, resolving power, and closely related concepts.

4.1 Spectral Variables

There exist many ways to represent the spectral variable of a radiometric quantity. Perhaps the most familiar is the wavelength. It is the distance between two points of equal phase on a sinusoidal wave. It is usually given the symbol λ, and is measured in micrometers, μm, nanometers, nm, and angstroms, Å. The wave number may be the next most frequently used spectral variable. It is designated by either σ or v; I will use σ. It is defined as the number of waves in a centimeter, sometimes called a kayser, and usually is expressed in cm⁻¹. Therefore the relationship between wavelength (λ in μm) and σ wave numbers (σ in reciprocal centimeters) is

The wave number in kaysers is simply the reciprocal of the wavelength in centimeters, but it is 10,000 times this if the wavelength is in micrometers. These are the two most frequently used variables, but k, v, f, and x are also used. The radian wave number k is 2 π/λ or 2 πσ; it is also the magnitude of the wave vector. When people use the term frequency, they often mean v, which has the units of Hertz or cycles per second. This is sometimes cited as f but I will save that symbol for focal length. The nondimensional frequency x is useful in radiometric work. It is defined as c₂/λT, where c₂ is the second radiation constant and T is absolute temperature.

4.2 Resolution

A spectral line usually has some predetermined shape, like Gauss, Lorentz, or Doppler. Lines are narrow maxima in the spectrum.

http://dx.doi.org/10.1117/3.263530.ch4

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