Monochromators are optical subassemblies used to isolate narrow portions of a light spectrum. They accept polychromatic input from a lamp or laser, and outputs monochromatic light. With monochromators, polychromatic light enters via a fixed port, such as a slit or optical fiber. Inside the monochromator, a dispersive element, grating, crystal prism, or mirror diffracts the light into its spectrum. If the monochromator can be rotated, the dispersive element is manual or motor-driven. The angle at which the element is rotated determines the wavelength of the output monochromatic light output, as well as the specific color of the light.
When selecting monochromators for an application, it is important to consider certain parameters and how they will affect the desired output. These parameters include bandpass, dispersion, resolution, acceptance angle, and blaze wavelength. Bandpass is the wavelength range in which the monochromator transmits. Dispersion, resolution, acceptance angle, and blaze wavelength are important parameters to consider when choosing monochromators. Dispersion in monochromators is the wavelength dispersing power, which is usually expressed as spectral range or slit width (nm/mm). Dispersion depends on the focal length, grating resolving power, and the grating order. Resolution is the minimum bandpass of the spectrometer, which is usually determined by aberrations in the optical system. With monochromators, the acceptance angle (f/#) is a measurement of the light-collecting ability and focal length / mirror diameter. Blaze wavelength is the wavelength of maximum intensity in the first order of monochromators.
Additional features to consider when choosing monochromators are nitrogen-purging abilities, vacuum capabilities, and fiber optic ready connection. Monochromators with a nitrogen purge feature have a port for nitrogen purging. Nitrogen purging is important because it allows monochromators to operate more deeply into ultraviolet (UV) light. Similarly, monochromators that operate with internal vacuum conditions are also able to extend their wavelength further into the ultraviolet range. Monochromators that have a fiber optic ready connection can be coupled with waveguides for the easier delivery of light output and data retrieval.
A range of accessories are available for use with monochromators, including cooled photo multiplier detectors, silicon detectors, light guides, arc lamp sources, and integrating sphere. Motorized drive monochromators are also available which can include electronic drivers to interface with software for automation.