Interference Filters & Monochromators in Spectroscopy

Spectroscopy is a technique that is often used by chemists and biochemists to study the structure of unknown compounds. In the Latin noun, spectrum means “an apparition” or “image” and the Greek verb skopein means “to look”.

In spectroscopy, a sample is subjected to light or radiation and its interaction with that energy is examined. Goethe and Newton were among the earliest scientists who examined the spectral properties of light.

Spectrometers include four basic units: energy/light source, detector, wavelength selector and sample holder. This article focuses on wavelength selectors.

Wavelength Selectors

There are 2 main types of wavelength selectors - monochromators and interference filters.

Monochromators

Block diagram of monochromator

Figure 1. Block diagram of monochromator

Monochromators (Figure 1) include two slits - an entrance slit and an exit slit - via which light enters and exits the device with a dispersing unit, either grating and mirrors or a prism to manipulate the light as it enters and leaves the grating prism.

The spectral quality of light produced by a monochromator is a property of the dispersive capability and the width of the slits of the grating or prism. A double grating is utilized for applications that demand the highest levels of performance, whereby the light from the first grating is passed via a second grating. This reduces scatter and gives greater resolution.

However, when the incident light impinges on each optical unit it leads to a loss of energy. Hence, there is a trade-off between the intensity of the emitted light and the wavelength resolution.

Interference Filters

Interference filters contain several optical layers deposited on a glass substrate or transparent quartz. The specific performance characteristics of the filter are determined by the thickness of the optical layers.

Interference filters are relatively lighter and smaller than monochromators, and also provide technological advantages - when an interference filter is properly designed, it can collect several hundred or several thousand times the quantity of light collected by a monochromator of the same bandwidth.

Interference filters can be custom-designed to meet the specific needs of spectroscopists. For instance, Linear Variable Filters, in which the wavelength of light reflected and transmitted from the optical layers changes linearly as it passes along the filter’s length. This is often called scanning, and is accomplished by depositing the optical layers as a wedge.

However, innovative developments in the design and production of filters are expanding the performance of filters and their applications with and without monochromators. Combining long wavelength pass (LWP) with short wavelength pass (SWP) filters enables users to select frequencies over the full wavelength range from ~300 to 850nm.

Moreover, by altering their relative positions, users can select the bandpass i.e. the width of the wavelength range transmitted by the filters. Similar to what semiconductors have done for electronics, the filters enable wavelength selection, reduce volumes by factors of several thousand, and allow considerable cost savings via easy access to high volume production.

Conclusion

Edinburgh Biosciences has collaborated with DELTA and will be introducing its Dynamic VariChrome (DVC) system shortly.

Interference filters designed by DELTA will be integrated into a chassis with a programmable control module and motorised drives and switches. Users will be able to choose a single wavelength or wavelength range, bandpass and scan speed through a software interface.

This way, users can accomplish wavelength selection performance characteristics similar to those obtained with a traditional monochromator but in a much smaller device and with higher energy transmission.

This information has been sourced, reviewed and adapted from materials provided by DELTA Optical Thin Film.

For more information on this source, please visit DELTA Optical Thin Film.

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