Advanced Filters & Monochromators to Optimise Spectrometers

Spectroscopy is a technique frequently used by chemists and biochemists to examine the structure of unknown compounds. This word has been derived from the Latin noun spectrum meaning “apparition” or “an image” and the Greek verb skopein meaning “to look”.

In spectroscopy, a sample is submitted to some form of energy in the form of light or radiation and this is followed by studying the way the sample interacts with that energy. Newton and Goethe, the great German writer, philosopher and scientist, were among the earliest scientists examining the spectral properties of light.

Spectrometers are made up of four basic units:

  • Detector
  • Sample holder
  • Wavelength selector
  • Energy/light source

This article focuses on wavelength selectors.

The two major types of wavelength selectors are as follows:

  • Monochromators
  • Interference filters

Monochromators

Monochromators are provided with an entrance, two slots, and an exit slit through which light enters and exits the device with a dispersing unit, either a grating or a prism and mirrors to maneuver the light as it enters and leaves the grating or prism.

Block diagram of monochromator

Block diagram of monochromator

The spectral quality of light emitted by a monochromator is the dispersive capability of the grating/prism and a function of the width of the slits. A double grating is used for applications requiring the highest levels of performance, and the light from the first grating passes through a second unit, decreasing scatter and providing even higher resolution. However, each optical unit on which the incident light impinges on leads to energy loss. Thus, there is a trade-off between wavelength resolution and the intensity of light that is emitted.

Interference Filters

When compared to monochromators, interference filters are considerably lighter and smaller and also provide technological benefits, in particular highly increased potential grasp of energy ('light grasp') compared to monochromators.

An interference filter that is properly designed has the potential to collect several hundred or even several thousand times the quantity of light collected by a monochromator with the same bandwidth. Interference filters are generally made up of up to several 100 optical layers deposited on a glass substrate or transparent quartz.

The filter’s specific performance characteristics are determined by the thickness of the optical layers. It is possible to design interference filters to meet the requirements of spectroscopists, for example Linear Variable Filters in which the wavelength of light transmitted and reflected from the optical layers changes linearly as it travels along the length of the filter. In general, this is referred to as scanning and is achieved by depositing the optical layers as a wedge.

Filters and their Applications

The performance of filters and their applications with and without monochromators has been extended by the recent developments in filter design and production. Edinburgh Biosciences has collaborated with Delta Optical Thin Film A/S. Coupling long wavelength pass (LWP) and short wavelength pass (SWP) filters enables users to select over the full wavelength range from ~300 nm – 850 nm.

The bandpass (the width of the wavelength range) transmitted by the pair of filters can be selected by users by altering their relative positions. The filters do for wavelength selection just the same thing done by semiconductors for electronics in decreasing volumes by factors of several thousand and permitting significant cost savings through effortless access to high volume production.

Thin-Film Interference Filters

Edinburgh Biosciences will be launching its Dynamic VariChrome (DVC) system very soon. Delta Optical Thin Film's interference filters will be integrated into a chassis with a programmable control module and motorized drives and switches.

Users will be able to select either a wavelength range or a single wavelength, scan speed, and bandpass through a software interface. This will indeed allow users to attain wavelength selection performance characteristics comparable to those obtained with a conventional 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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