Benchtop Raman spectroscopy instruments and—more recently—handheld Raman spectroscopy instruments, which are more frequently used for forensic screening, historical research, and pharmaceutical quality control, rely heavily on the design of their optical filters.1
There are additional factors that need to be taken into account with handheld Raman spectroscopy, such as the stability and size of the optical components, even though many of the optical components in a handheld Raman spectroscopy instrument are similar to those in a benchtop instrument.
Raman spectroscopy equipment can be made smaller by eliminating as many moving parts as possible, using monolithic optical mounts, and using optics, including filters, made to be as small as possible for the instrument.
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This reduces the amount of space taken up by optical mounts in the Raman spectroscopy instrument and eliminates the need for large actuators and motorized translation stages.
As the instrumentation could be used in a real-world setting with little to no environmental controls, additional factors for handheld Raman spectroscopy instruments could include the temperature and environmental stability of optical components.
Iridian Spectral Technologies are experts in providing customized optical filters, such as a selection of filters for Raman spectroscopy. They can assist in producing filters that have the ideal optical characteristics and dimensions for the user’s instrumentation needs.
Raman Filter Properties
Raman spectroscopy equipment uses a wide range of filters, such as notch, laser line, and long and short wave pass filters. The removal of any remaining beam transmission from the typically very powerful laser source and the selection of the Stokes or anti-Stokes Raman signal are the two primary uses of spectral filters in Raman spectroscopy.
The desired range of properties must be carefully considered, as the proper selection of filters can improve the performance of a handheld Raman spectroscopy instrument. The operating wavelength range, also known as the cut-on and cut-off wavelengths, is one of the most crucial.
Bandpass and notch filters can selectively block or permit a bandwidth centered on a specific wavelength. The central wavelength of laser line filters will typically match the laser.
The notch filter blocking range for narrowband laser sources might need only be a few nanometers wide to cover the entire laser spectrum. However, a high optical density (OD) will be essential because the laser intensity will probably be many orders of magnitude higher than the signal contribution in Raman spectroscopy.
Edge steepness is yet another essential characteristic of Raman spectroscopy filters. The spectral width on the slope at the filter’s edge defines edge steepness.
A steep edge prevents the loss of any desired signals while obstructing the region of unwanted light, especially for long and short-pass filters where there is sometimes little wavelength separation between the elastic scattering lines and Stokes or anti-Stokes to be detected. However, for handheld instruments, the cost of the components is often a key parameter to consider - striking a balance between cheaper versus steeper filters.
Other factors to consider in Raman spectroscopy include the angle of incidence at which the optical components operate, the purity of the polarization, and the characteristics of the light beam. To get the best performance, it may be necessary to correct optical aberrations in the light path because many filters lose efficiency as the laser beam becomes more cone-shaped.
References
- Sorak, D., Herberholz, L., Iwascek, S., Altinpinar, S., Pfeifer, F., & Siesler, H. W. (2012). New developments and applications of handheld Raman, mid-infrared, and near-infrared spectrometers. Applied Spectroscopy Reviews, 47(2), pp. 83–115. https://doi.org/10.1080/05704928.2011.625748
This information has been sourced, reviewed, and adapted from materials provided by Iridian Spectral Technologies.
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