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Optical Spectrometers

Introduction

A spectrometer is an optical system that transmits a specific band of electromagnetic spectrum. Dispersion of different wavelengths is accomplished with the separating capability of refraction (prism) or diffraction (diffraction grating). Typical applications are isolation of a narrow band of radiation from a continuum light source for absorption measurements, or analysis of the emission from excited atoms or molecules.


Monochromator designs

A typical monochromator design is shown below. It consists of a diffraction grating (dispersing element), slits, and spherical mirrors. The light source emits a broad spectrum of radiation as represented by the multi-colored line from the lamp to the grating. (The yellow color of the light source represents all colors.) The diffraction grating disperses light by diffracting different wavelengths at different angles. The grating is positioned so that green light passes through the exit slit and all other colors are blocked. The particular wavelength that passes through the monochromator is selected by rotating the angle of the grating. The mirror and slit positions remain fixed. If this grating was rotated clockwise slightly, what color light would pass through the exit slit? Scanning a spectrum is accomplished by rotating the grating with a motor. The detector measures the power of the light that strikes it, converting the light power to an electrical signal.

Schematic of a Czerny-Turner monochromator

spectrometer


Monochromator parameters

Bandpass
The wavelength range that the monochromator transmits.
Dispersion
The wavelength dispersing power, usually given as spectral range / slit width (nm/mm). Dispersion depends on the focal length, grating resolving power, and the grating order.
Resolution
The minimum bandpass of the spectrometer, usually determined by the aberrations of the optical system.
Acceptance angle (f/#)
A measure of light collecting ability, focal length / mirror diameter
Blaze wavelength
The wavelength of maximum intensity in first order.

Related instruments

Spectrograph
A spectrometer that records a wide bandpass with a photographic plate or an array detector. The spectrometer requires a flat image field.
Polychromator
A spectrometer with multiple detectors for simultaneous detection of multiple analytes.

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