A monochromator is an optical dispersing device that is used to select a narrow band of light (i.e., optical radiation) from a wider range of wavelengths available at the input. The Greek roots “mono-” and “chroma-” refer to “single” and “color” respectively. Ideally, a monochromator should produce a single wavelength of optical radiation at its output. Although lasers produce light that is much more monochromatic than the optical monochromators discussed here, only some lasers are easily tunable but are not as simple to use.
Monochromators are included in many optical measurement instruments and systems for applications where tunable monochromatic light is required. A monochromator combined with optical detectors can be used to obtain the spectral power distribution (SPD) of light sources, reflectance or transmittance of objects, etc.
Monochromators are important for color measurement because many color-related optical characteristics are dependent on wavelength. In color science, reflected or transmitted light from a sample is usually measured, where a monochromator can be used before or after the incident light illuminates a sample. If a monochromator is used before the incident light illuminates the sample, the incident light is monochromatic light. On the other hand, if a monochromator is used after the light illuminates the sample, the monochromator is used to convert the reflected or transmitted light to monochromatic signals for analysis. It is typically used in a spectrometer (or spectroradiometer) or a spectrophotometer.
There are different types of monochromator based on its color selection mechanisms and/or designs, e.g., prism, Czerny–Turner, holographic grating, double, etc. The principles of these monochromators are introduced in the “Overview” section below.
The main function of a monochromator is to separate the color components of a light. It can use either the optical dispersion phenomenon in a prism or that in a diffraction grating.
A prism monochromator uses a prism as the dispersing element (Fig. 3). It is not necessary for the collimating and focusing elements to be concave mirrors, but they can be optical lens. Usually, reflective mirrors are preferred over optical lens because mirrors conserve more optical radiation energy during the transmission process. However, because of the higher cost and small angular dispersion, prism monochromators are rarely made today .
A holographic grating monochromator uses holographically constructed concave gratings to simplify the optics. This eliminates some of the lenses and mirrors that would normally be required with conventional flat plane grating monochromators.
The criteria for the selection of a grating 
Specification of monochromators
Groove density (or groove frequency):
The number of grooves contained on a grating surface (lines/mm)
Groove density affects the mechanical scanning range and the dispersion properties of a system. It is an important factor in determining the resolution capabilities of a monochromator. A higher groove density results in greater dispersion and a higher resolution
Select a grating that delivers the required dispersion, such as that for a charge-coupled device (CCD), array detector, or a required resolution (with an appropriate slit width) when using a monochromator
Mechanical scanning range:
The wavelength region in which an instrument can operate
Refers to the mechanical rotation capability (not the operating or optimal range) of a grating drive system with a specific grating installed
Select a grating groove density that allows operation over the desired wavelength region
The angle in which the grooves are formed with regard to the normal grating, often called the blaze angle
Diffraction grating efficiency plays an important role in spectrograph throughput. Efficiency at a particular wavelength is largely a function of the blaze wavelength if the grating is ruled, or a modulation if the grating is holographic
Select a blaze wavelength that encompasses the total wavelength region of the applications. More attention should be paid to covering the short wavelength of the spectrum region
Quantum wavelength range:
The wavelength region of highest efficiency for a particular grating
Normally determined by the blaze wavelength
Select a grating with maximum efficiency over the required wavelength region for the applications
When using a monochromator, some parameters need to be set up properly, as they affect the measurement results. For example, smaller slit widths produce smaller spectral bandwidths, which corresponds to a narrower spectral band at the exit slit (i.e., closer to the ideal single-wavelength output). However, smaller slit widths mean that less optical radiation power is transmitted through the monochromator, which results in lower output signals and decreases the signal-to-noise ratio of the signals measured. The bandwidth of commonly used monochromators is from the order of 10−2 to 10 nm. Detailed information on monochromators can be found in manufacturers’ user manuals.
It is important to evaluate the performance of a monochromator to reduce measurement uncertainties (i.e., gain more confidence regarding the accuracy of the results measured). To evaluate the performance of a monochromator, the following characteristics should be considered: wavelength accuracy, spectral bandwidth, stray light, system reproducibility, and system linearity.
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