What is a Monochromator?
A monochromator is an optical device with a dispersive or diffractive element that separates or spreads the wavelengths from a broadband source and produces a very narrow band output. The term monochromator is derived from the Greek words, mono & chroma which mean single & color respectively, and hence it refers to a device that makes the output a single color/wavelength. The dispersive element in a monochromator can be a prism or a diffraction grating.
In a prism monochromator, a prism is used to obtain monochromatic light. There are two interfaces in a prism where refraction occurs. The variation in refractive index for different wavelengths in the material of the prism causes each wavelength to have a different refracted angle. This results in angular dispersion, i.e., each wavelength in the incident parallel beam is deflected at varying angles. The prism in the monochromator is oriented in such a way that the selected wavelength reaches the output slit and exits the same to form the required monochromatic output.
In a grating monochromator, a diffraction grating is used as the monochromator to obtain the narrow-band output. The grating has a large number of parallel and closely separated ridges in its reflective-coated grooved surface. Their period is comparable to the incident wavelengths.
When light encounters obstacles or apertures comparable to its wavelength, the phenomenon of diffraction takes place and the incident wave spreads out. Since the diffraction grating has a large number of ridges placed close to each other periodically, the diffracted waves produced from each of them will spread out. These waves at each wavelength in the incident polychromatic light will have a unique angle/direction that favors constructive interference. And hence, the monochromatic light of each wavelength is available in a different direction. So, the required monochromatic light is obtained at the narrow output slit by changing the angular orientation of the diffraction grating.
Monochromators are ideal for spectrophotometry, spectroscopy, holography, and tunable lasers applications. In absorbance, transmittance, reflectance, and fluorescence spectrophotometer, monochromators are used for illumination and analysis purposes.