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 color) 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.
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
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.
Henry A. Rowland (1848–1901) was an American
physicist and the first president of the American Physical Society. He was also the first chairman of the Physics
Department at Johns Hopkins University. Although he
did outstanding work in several areas of electricity and
magnetism, he is best known for developing methods to
produce high-quality diffraction gratings.
A diamond point then travels over the grating, which is ruled on a concave mirror surface. Machines of this type were the models for many of the ruling engines constructed after Rowland’s time. Ruling engines are among the most sensitive and precise macroscopic mechanical devices ever made. The resulting gratings have played an integral role in many of the most important advances in science over the past century
Photon Detectors Widely used types of photon detectors include phototubes, photomultiplier tubes, silicon photodiodes, photodiode arrays, and charge-transfer devices such as charge coupled and charge-injection devices.
The photomultiplier tube (PMT) is similar in construction to the phototube
but is significantly more sensitive. The photocathode is similar to that of t
he phototube with electrons being emitted on exposure to radiation.
Albert Abraham Michelson (1852–1 931) was one of the most gifted and inventive experimentalists of all time. He was a graduate of the United States Naval Academy and eventually became professor of physics at The University of Chicago. He studied the properties of light and performed several experiments that laid the foundation for our modern view of the universe. He invented the interferometer described in Feature 25-7 to determine the effect of the Earth’s motion on the velocity of light. For his many inventions and their application to the study of light, Michelson won the 1907 Nobel Prize in Physics. At the time of his death, Michelson and his collaborators were attempting to measure the speed of light in a mile long vacuum tube located in what is now Irvine, California.
Michelson interferometer
It consists of a collimated light source, shown on the left of the diagram, a stationary mirror at the top, a moveable mirror at the right, a beam-splitter, and a detector. The light source may be a continuum source as in FTIR spectroscopy, or it may be a monochromatic source such as a laser or a sodium arc lamp for other uses such as, for example, measuring distances. The mirrors are precision-polished ultraflat glass with a reflective coating vapor deposited on their front surfaces. The moveable mirror is usually mounted on a very precise linear bearing that allows it to move along the direction of the light beam while remaining perpendicular to it as shown in the diagram. The key to the operation of the interferometer is the beamsplitter, which is usually a partially silvered mirror similar to the “two-way” mirrors often seen in retail stores and police interrogation rooms. The beam-splitter allows a fraction of the light falling on it to pass through the mirror, and another fraction is reflected. This device works in both directions, so that light falling on either side of the beam-splitter is partially reflected and partially transmitted
(Skoog, 2013)
(https://www.chemicool.com/definition/monochromators.html, 2017)
Fundamentals of Analytical Chemistry 