Monochromator

Monochromator

Introduction

The term monochrome and its variants come to us from the Greek words mono “single” and chroma “colour”.

No light source is truly monochromatic; no light source emits light of a single wavelength, all sources containing contributions from a finite range of wavelengths, termed its spectrum.
It is often of interest to decompose a source into its component wavelengths, for the purpose of determining the spectral distribution (UV, visible, infrared etc), or to employ that source as a means of testing samples under “monochromatic” stimulation.In either case, a means of wavelength dispersion is required.
▲Figure 1-4:- Solar spectrum- from ultra violet to infrared

Light dispersion
The image of the dispersion of light can be little better conjured up than that of a rainbow.
Though not an entire description of the processes in play, the apparently “white” light from the sun, on travelling through the droplet is refracted, or bent from its path. The amount light is refracted depends upon its wavelength; blue light on one extreme of the visible spectrum is refracted more than the rest of the visible spectrum, through to red light on the other extreme, resulting in a rainbow.
In effect the sun contains wavelength components from the UV to the infra-red (heat radiation), not visible to the human eye, but nevertheless part of the solar spectrum.

Light Dispersion Mechanisms

In terms of scientific instrumentation for use in the laboratory or field, the following are the principal manners of either determining the spectrum of a source.

Mechanism Dispersion Process Pros Cons
Prism Refraction
- Simple
- Inexpensive 
- Prism material absorbs light
- Non-linear dispersion 
Reflection diffraction grating Diffraction Can be optimised
- Linear dispersion 
- Complex process
- Delicate optics 
- Expensive
- Delicate optics 
Transmission diffraction grating Diffraction Relatively simple
- Linear dispersion 
Expensive
- Complex process 
- Delicate optics 
- Grating material absorbs light 
Band Pass Filter Band pass filters
- Simple 
- Relatively inexpensive (in visible-NIR only)
- Low spectral resolution 
- Low throughput 

Of the above techniques, only the reflection diffraction grating can be used over wide spectral range in a practical application, providing potentially very high spectral resolution.

Wave Interference

Light can be considered as having a wavelike nature. When two such waves are brought into proximity, they interact, the resultant wave depending on the amplitudes, frequencies and relative phases of the two waves. In the context of diffraction gratings, it is sufficient to consider the case of the superposition of two waves of equal frequency (and therefore wavelength). The resultant wave is simply the sum of the two.
With a phase difference of zero or a whole number of wavelengths, constructive interference obtains, ie mλ where m is an integer.
With a phase difference of half a wavelength, they interfere destructively, ie mλ/2 where m is a whole number
Between these two conditions varying degrees of interference result.

Figure 1-5:-Wave interference