Monochromators

This presentation is about the monochromators and specifically their use in spectroscopy. It includes definition, principle, origin of term, principle, types, prism monochromator, diffraction grating monochromator, difference in both of them, their uses and working as well, optical filters and their uses and application.
A Monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light chosen from a wider range of wavelengths available at the input. And the unwanted radiations are blocked by the slit allowing only the desired ray to pass (monochromatic).
A dispersive element disperse the polychromatic light into several bands of single wavelength and then a slit is used which stops the unwanted bands of light, allowing only the desired monochromatic light to pass through its exit point.
By fixing the slit and rotating the dispersive element, the direction of the dispersed light is turned so that the colour of the resulting monochromatic light changes.
When electromagnetic radiation passes through a prism, it is refracted because the index of refraction of the prism material is different from that of air.
Shorter wavelengths are refracted more than longer wavelengths.
By rotation of the prism, different wavelengths of the spectrum can be made to pass through an exit slit and through the sample.
A prism works satisfactorily in the ultraviolet and visible regions and can also be used in the infrared region.
Because of its nonlinear dispersion, it works more effectively for the shorter wavelengths.
Glass prisms and lenses can be used in the visible region.
Quartz or fused silica must be used in the ultraviolet region.
The entire monochromatic compartment must be kept dry.

1. Monochromators 10/29/2020 Analytical II Chemistry Presentation 1

2. Group Members Asad Aziz ~ 7204 Majid Ashiq ~ 7205 Sufian Raza ~ 7206 Arooj Mansha ~ 7234 Ain- ul - Batool ~ 7213 10/29/2020 Analytical II Chemistry Presentation 2

3. Outline 1-Definition 2-Terminology 3 -Principle 4 -Types Prism Monochromator. Diffraction grating Monochromator. Optical Filters 5 -Applications. 10/29/2020 Analytical II Chemistry Presentation 3

4. Definition A Monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light chosen from a wider range of wavelengths available at the input. And the unwanted radiations are blocked by the slit allowing only the desired ray to pass (monochromatic). 10/29/2020 Analytical II Chemistry Presentation 4

5. Terminology Mono Single Chroma Colour Ator Donating Agent 10/29/2020 Analytical II Chemistry Presentation 5

6. Principle A dispersive element disperse the polychromatic light into several bands of single wavelength and then a slit is used which stops the unwanted bands of light, allowing only the desired monochromatic light to pass through its exit point. By fixing the slit and rotating the dispersive element, the direction of the dispersed light is turned so that the colour of the resulting monochromatic light changes. 10/29/2020 Analytical II Chemistry Presentation 6

7. Types of Monochromators Monochromators are of following three types. 10/29/2020 Analytical II Chemistry Presentation 7

8. 1- Prism Monochromator When electromagnetic radiation passes through a prism, it is refracted because the index of refraction of the prism material is different from that of air. Shorter wavelengths are refracted more than longer wavelengths . By rotation of the prism, different wavelengths of the spectrum can be made to pass through an exit slit and through the sample. 10/29/2020 Analytical II Chemistry Presentation 8

9. Features of Prism Monochromators. A prism works satisfactorily in the ultraviolet and visible regions and can also be used in the infrared region. B ecause of its nonlinear dispersion, it works more effectively for the shorter wavelengths . Glass prisms and lenses can be used in the visible region. Q uartz or fused silica must be used in the ultraviolet region. The entire monochromatic compartment must be kept dry. 10/29/2020 Analytical II Chemistry Presentation 9

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11. Diffraction Grating Monochromator The dispersive element in grating monochromator is a reflecting diffraction grating. It provides a constant dispersion for all wavelengths and a low dependence on temperature. However, they produce relatively large amounts of scattered light and require the use of filters to block higher order light.  Diffraction gratings are often used in modern instruments due to their superior dispersion properties . The most popular design for grating is the Czerny-Turner monochromator 10/29/2020 Analytical II Chemistry Presentation 11

12. Czerny-Turner Monochromator The input light is focused onto the input slit and therefore divergent after the slit. It is  collimated  by a curved mirror and hits a diffraction grating , which deflects different wavelength components in slightly different directions. A second curved mirror translates different beam directions into different positions on the exit slit, so that only light in a narrow wavelength region can get through that slit. 10/29/2020 Analytical II Chemistry Presentation 12

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14. Prism vs Diffraction Grating Monochromator Prism monochromator Exploits differences in the material refractive index according to wavelength Wavelength dependency of dispersion is Variable, high for UV and low for visible High temperature dependency for dispersion Low Polarization Low stray light Diffraction grating monochromator Exploits diffraction from a reflective surface with a regular grating structure Wavelength dependency of dispersion is High and approximately constant . Low temperature dependency for dispersion High polarization High stray light 10/29/2020 Analytical II Chemistry Presentation 14

15. Optical filters Optical Filters are used to selectively transmit wavelength or range of wavelengths while rejecting the remainders. They are of following two categories Absorptive optical filters Dichroic optical filters 10/29/2020 Analytical II Chemistry Presentation 15

16. Absorptive and Dichroic Optical filters Absorptive optical filter Absorptive filters have a coating of different organic and inorganic materials that absorb certain wavelengths of light, thus allowing the desired wavelengths to pass through. Since they absorb light energy, the temperature of these filters increases during operation. Dichroic optical filter Dichroic filters are more complicated in their operation. They consist of a series of optical coatings with precise thicknesses that are designed to reflect unwanted wavelengths and transmit the desired wavelength range. 10/29/2020 Analytical II Chemistry Presentation 16

17. Types of optical filters: Long-Pass Filter : A long-pass configuration transmits longer wavelengths above a specified range while attenuating shorter wavelengths. These filters are commonly used with dichroic mirrors and emission filters. Short-Pass Filter:  A short-pass configuration transmits shorter wavelengths over an active range while attenuating longer wavelengths. Bandpass Filter:  Short-pass and long-pass filters can be combined to form a b andpass filter, which features lower transmittance values and rejects any wavelengths outside a predetermined interval. 10/29/2020 Analytical II Chemistry Presentation 17

18. Applications of Monochromators. Monochromators are often used in  spectroscopy . Monochromators are commonly used in measurement devices such as spectrometers or microplate readers. They are a popular device for wavelength selection in a range of detection technologies, such as absorption and fluorescence intensity In  spectrophotometers for measuring the wavelength- dependent  absorbance or reflectance of a sample A common application is in combination with a  photodetector with such a setup one can record the optical spectrum of a light source 10/29/2020 Analytical II Chemistry Presentation 18