Continuously Variable Filters in spectroscopy

Biochemists and chemists wishing to investigate the structure of unknown compounds often apply a technique known as spectroscopy. The word derives from the Latin noun spectrum meaning “an image” or “apparition” and the Greek verb skopein meaning “to look”. Spectroscopy involves submitting a sample to some form of energy in the form of radiation or light and examining how the sample interacts with that energy. Newton and Goethe, the great German scientist, philosopher and writer were among the earliest scientists investigating the spectral properties of light.

Spectrometers consist of 4 basic units:

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

Here, we will focus on wavelength selectors.

There are 2 major types of wavelength selectors:

  • Monochromators
  • Interference filters



Monochromators with a dispersing unit, either a prism or a grating, have 2 slits, an entrance and exit slit through which light enters and leaves the device, and mirrors to manipulate the light as it enters and leaves the prism or grating.

Block diagram of monochromator.

The spectral quality of light emitted by a monochromator is a function of the width of the slits and the dispersive capability of the prism / grating. For applications requiring the highest levels of performance, a double grating is used whereby the light from the first grating is passed through a second unit, reducing scatter and giving even greater resolution. However, each optical unit which the incident light impinges on results in a loss of energy. Hence there is a trade-off between wavelength resolution and the intensity of the emitted light.


Interference filters

Interference filters are considerably smaller and lighter than monochromators and also offer technological benefits, in particular greatly increased potential grasp of energy ('light grasp') compared to monochromators. When properly designed, an interference filter is capable of collecting several hundred or even several thousand times the quantity of light collected by a monochromator with the same bandwidth. In general, interference filters are made of up to several 100 optical layers deposited on a transparent quartz or glass substrate. The thickness of the optical layers determines the specific performance characteristics of the filter. Interference filters can be designed to meet the needs of spectroscopists, e.g. Continuously Variable Filters in which the wavelength of light transmitted and reflected from the optical layers changes continuously as it passes along the length of the filter. This is usually referred to as scanning and is achieved by depositing the optical layers as a wedge.

For a more in depth discussion of the advantages and uses of interference filters for wavelength selection, please refer to the accompanying article Upgrade of diffraction grating spectrometers for multiple purposes.

Recent developments in filter design and production are extending the performance of filters and their applications with and without monochromators. Edinburgh Biosciences has partnered with Delta Optical Thin Film A/S. Coupling LWP (long wavelength pass) and SWP (short wavelength pass) filters allows users to select over the full wavelength range from ~300 nm – 850 nm. In addition, by adjusting their relative positions, users can select the bandpass (the width of the wavelength range) transmitted by the pair of filters. The filters do for wavelength selection what semiconductors have done for electronics in reducing volumes by factors of several thousand and allowing significant cost savings through easy access to high volume production.

Edinburgh Biosciences will be releasing its Dynamic VariChrome (DVC) system shortly. Delta Optical Thin Film's custom optical filters will be incorporated into a chassis with motorised drives and switches and a programmable control module. Users will be able to select either a single wavelength or wavelength range, scan speed and bandpass via a software interface. In this way, users can achieve wavelength selection performance characteristics comparable to those achieved with a conventional monochromator but with higher energy transmission and in a much smaller device.

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