“Everything in life is vibration.”— Albert Einstein

RamanSpectroscopy
Optical filters and the value they bring to Raman spectroscopy

“I’m pickin’ up good vibrations.”— Brian Wilson, the Beach Boys

Optical filters and the value they bring to Raman spectroscopy

Abstract:
Raman spectroscopy is a powerful analytical technique that provides valuable
insights into the molecular composition and structure of materials.
To achieve accurate and precise Raman measurements, optical filters play a crucial role in enhancing signal-tonoise ratio, and reducing background interference. This white paper explores the fundamentals of optical filters,
their types, and their indispensable role in Raman spectroscopy applications. It will also discuss the importanceof selecting appropriate optical filters to optimize Raman spectroscopy experiments and enhance the overall
analytical performance.

Introduction to Raman
spectroscopy
Raman spectroscopy is a non-destructive
analytical technique that uses the inelastic
scattering of monochromatic light to probe
molecular vibrations in a sample. The Raman
scattering process yields unique spectral
information, allowing researchers to identify
chemical compounds, study molecular structures,and monitor chemical reactions.
The technique finds applications in diverse
areas, including pharmaceuticals, forensics,environmental science, materials science,geology, and biology, among others. Its benefits
are many and varied. For example, it enables
the identification of molecules and compounds
present in a sample, including organic andinorganic substances, pharmaceuticals, polymers,minerals, and biomolecules. This makes it avaluable tool in analytical chemistry and materialscience. It is also a non-destructive technique that
requires little to no sample preparation, allowing

In most cases, Raman spectroscopy can be
performed with little or no sample contact,
reducing the risk of cross-contamination andpreserving the sample’s integrity. It can also
provide real-time or near-real-time results, making
it suitable for fast analysis in various applications,
including pharmaceutical quality control andprocess monitoring.
Its versatility allows Raman spectroscopy to be
applied to a wide range of samples, including
solids, liquids, and gasses, and it can analyze
small regions within a sample. This capability isuseful in identifying heterogeneous distributions,
mapping molecular structures, and studying
surface properties. Raman spectroscopy can also
be combined with other analytical techniques,
such as infrared spectroscopy and mass
spectrometry, or microscopy to provide a morecomprehensive understanding of a sample’scomposition and structure. However, Raman
signals can be weak or obscured by fluorescenceand other background interferences.

for the investigation of delicate or valuable
samples.

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Chroma Technology Corp

Optical Filters in Raman
spectroscopy
Optical filters selectively transmit or block specificwavelengths of light based on their spectral
characteristics. They can be broadly categorized
into two types, edge filters and bandpass filters,
though other optics may be required for proper
Raman measurements depending on the systembeing used. Both edge and bandpass filters areused in all Raman spectroscopic instruments, buteach serves different purposes. To function as
an effective Raman filter, they all must have steep
edges so that Raman signals with wavenumbers of“clean-up” filter to limit the spectral output of theselasers, which, if left unchecked, can supersede orveil the Raman signal being measured and create
an impure signal.
2.3 Dichroic mirrors
Dichroics are typically used in a microscopebased Raman system at 45 degrees to redirect
and reflect the laser output towards the sample
under investigation, as well as transmit any signal
generated back to a detector. Though these optics
can add to the rejection of Rayleigh scatteringin their reflection ranges, they do so much less200cm-1 or lower can be detected.

efficiently than edge filters and so should not

2.1 Edge filtersin the absence of an edge filter. The transitional

Edge filters, also known as longpass filters, aredesigned to transmit light with wavelengths longer
than a specific cut-on wavelength. In Raman
spectroscopy, longpass edge filters are employed
to block Rayleigh scattered light, which has thesame wavelength as the incident laser light. This
rejection of the intense Rayleigh scattering allows
for isolation and detection of the much weaker
Raman-scattered photons. These are typicallyused at normal incidence (0deg) to the incoming

be relied upon for appropriate signal detection
slope of these dichroic mirrors between maximum
reflection of the laser and transmission of theexpected Raman signal should also be optimized
for collection of expected wavenumbers.
2.4 Notch filters
Notch filters are specialized optics that possessa very narrow rejection range with maximaltransmission efficiencies for both shorter andlonger wavelengths. Used at normal incidence,light path.

these can allow for detection of Raman signals

2.2 Bandpass filters

also attenuating the incident laser excitation. One

Bandpass filters work by passing a specific narrow
range of wavelengths and rejecting all otherwavelengths outside of that band and are typicallyused at normal (0deg) incidences. Many modern
diode laser sources emit multiple wavelengths orgenerate less-than-monochromatic light. Bandpass
filters are almost always required to act as a laser

in both Stokes and anti-Stokes directions, while
application for these filters is in Coherent AntiStokes Raman (CARS) imaging.

Optical filters and the value they bring to Raman spectroscopy

Factors affecting filter selection:

4.1 Capability

Several key factors must be considered when

Does your supplier offer the range of optical filters

selecting optical filters for Raman spectroscopy.

you require, whether you need higher transmission

3.1 Laser excitation wavelength

keep out unwanted photons? Are you collecting

The choice of laser excitation wavelength isfundamental to Raman spectroscopy. The opticalfilters used must be compatible with the laser’s
excitation wavelength to effectively block theRayleigh scattering and transmit the Ramanshifted wavelengths. Depending on the sample
being probed, the choice of laser wavelength canlead to a fluorescence signal on top of a Raman
signal, giving confusing results. Often, choosing
a different wavelength, dual wavelengths, oremploying special techniques can reduce oreliminate that fluorescence output.
3.2 Filter’s optical properties
The optical filters must have high transmission
efficiency for the desired Raman spectral region,
provide a steep cut-on to reject the lasers, andvalues, steep transitions, or superior blocking to
the proper wavenumbers to efficiently identify yourexpected samples?
4.2 Experience and expertise
Find out if your supplier has a proven trackrecord of expertise in optical filter design andmanufacturing. An experienced supplier will have a
deep understanding of the unique challenges andrequirements of Raman spectroscopy applications
and can even provide tailored solutions to meetyour specific needs. They should also demonstrate
excellent customer support, technical expertiseand value for your money.
4.3 Customization
Raman spectroscopy setups can vary significantly
based on the laser excitation wavelength, sample

transmit low wavenumber Raman output.

characteristics, and experimental requirements.

3.3 Signal-to-noise ratio (SNR)

options to tailor filters to your specific application,

An ideal filter should enhance the SNR by reducing
background noise. Deep blocking characteristics
along with very high transmissions create highvalue SNR necessary in Raman filter sets.

A good supplier should offer customization
ensuring optimal performance.
4.4 Reputation and customer reviews
Research the supplier’s reputation. Look forcustomer reviews and testimonials to gauge theiroverall satisfaction with the filters and services

Sourcing optical filters for Raman
spectroscopy

provided.

It is important to ask the right questions to ensurethat the chosen supplier can effectively meetthe filter requirements for your particular Raman
spectroscopy application or project.

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Chroma Technology Corp

About Chroma Technology

5.3 Cost-effective Raman sets

Chroma Technology is a leading manufacturer

As well as the popular standard Raman filterof Raman filter sets of very durable, and highly
precise optical filters using 1st surface hard coat
sputter technology for OEM and researchers
alike. The company has been supplying solutions
for industries ranging from the life sciences andagriculture to manufacturing, inspection, security
and aerospace for more than 30 years. Chroma
Technology’s reputation is built on dedicatedcustomer service, including free technical andsets, Chroma also offers a range of costeffective versions of 785nm and 532nm
Raman sets, designed to deliver exceptional
performance without breaking the bank. These
cost-effective Raman sets boast the same orsimilar designs as the standard catalog sets,
ensuring uncompromised quality and accuracy,
but they are tailored to offer slightly relaxed
specifications for applications that do not requireapplications support.

overengineering, particularly in the flatness of

5.1 Chroma Technology’s optical filter solutions forlong-pass filters (LPs).

Raman spectroscopy
Chroma Technology offers Raman filters, and setsthat are designed for the most common Raman
wavelengths. These include: UV (266nm), Visible(488nm, 532nm and 633nm) and NIR (633nm,785nm and 1064nm).
5.2 Standard Raman filters and sets
Chroma’s standard filters offer high transmission
for detection of weak signals, as well as broadtransmission band, excellent image quality, ultrasteep edges and deep blocking. These are some of
the most advanced optical filters available, thanks
to Chroma’s expertise and experience in precision
thin-film design and deposition technology.
Standard Raman filter sets include a narrow-band
laser clean-up filter matched with a laser dichroic
paired to a long pass emission filter, each withsheer spectral slopes. Together, these filters allow
for the detection of even the faintest Raman
signals.

dichroic beamsplitters (DCs) and the steepness of

OEM customers in China have placed their trust inthese filters, using them in portable and handheld
Raman devices for use in security checks, effective
detection of explosives and drugs, food safety, andempowering laboratory applications. The ultrasteep filters are widely used by materials science
researchers in Raman microscopes.
5.5 Customization capability
If you’re looking for something a bit different, or
you aren’t sure what might best work for yourparticular application, the expert team at Chroma
is here to help. Simply get in touch and let theteam know what you are looking to achieve, andif the existing range of products don’t quite fit,
they will take you through the process of creating
something custom that will work.

Optical filters and the value they bring to Raman spectroscopy

The process is straightforward. Send us adrawing or let us know what you are looking toachieve. Once the engineering team has all of thespecifications, they will design a custom set foryour application. Already have something that isn’tfunctioning like you think it should, our team has
the breadth of experience to troubleshoot and findout why, without overengineering a solution.
5.6 How can Chroma Technologies help me
find the right filter for my Raman spectroscopy
application?
Get in touch with the team at Chroma today to
get a head start on your Raman spectroscopy
application without delay.
Phone: +1 800.824.7662
Email: sales@chroma.com
Website: www.chroma.com/request-a-quote
For more information on Chroma Technology’s
range of optical filters for Raman spectroscopy,visit: https://www.chroma.com/raman

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“Ask the right questions and nature
will open the doors to her secrets.”— Dr. C. V. Raman, Nobel Prize recipient fordiscovery of the Raman Effect

US Corporate Headquarters
10 Imtec Lane
Bellows Falls, Vermont 05101 US
info@chroma.com
Tel: + 1-800-824-7662+1-802-428-2500

Chroma Technology GmbH
Maximilianstrasse 33
D-82140 Olching, Germany
europe@chroma.com
Tel: +49-8142-2847525

Chroma Technology Japan
8F Yokohama Onoecho Building
4-57 Onoecho Naka-ku
Yokohama 231-0015 Japan
Phone: +81 (0) 45 285 1583
Fax: +81 (0) 45 285 1501
japan@chroma.com

Chroma China
Rm 803, Building No.16
Yuanbo Wuli
Xingjin Road, Jimei District
Xiamen City, Fujian Province 361021
Phone: +86-0592-5062089
china@cn.chroma.com

Aug 2023