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Light incident on a non-transparent medium is predominantly scattered without changing its wavelength. This effect is termed Rayleigh scattering. A small part of this visible light is, however, scattered in a different wavelength. This phenomenon is called Raman scattering or the Raman effect, after the Indian physicist and Nobel laureate C. V. Raman. But what exactly is Raman scattering?
The tool also provides a new way to perform multi-scale analysis, as an alternative to wavelet analyses or scale-sensitive fractal analyses, both already available in Mountains®. Bandpass-filter banks have the advantage of using tools that are easier to understand, based on the well-known Gaussian filter.
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Above. Combined transmission characteristics, compensated with a factor 4, form the elementary bandpass filter. Transmission is on the vertical axis, wavelengths on the horizontal axis, in μm.
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When both low-pass and high-pass filters are applied consecutively with the same cut-off value, wavelengths below and above the cut-off are attenuated and only the wavelengths around the cut-off value are transmitted.
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The applications of Raman spectroscopy in medicine are very varied. In this way, for example, the chemical composition of kidney stones can be analysed immediately after their removal. Then the patient can be given tailored recommendations for the prevention of new stones without complex analyses in specialist laboratories. It is also possible to analyse a living biological sample using its Raman spectrum. Neither of these methods has become established as standard yet.
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However, these disadvantages are minimised to a large extent by using modern lasers such that they are increasingly irrelevant for the practical use of Raman spectroscopy.
The basic prerequisite for Raman spectroscopy is a monochromatic light source. Because the light scattered during Raman scattering is of relatively low intensity, the light source must also have a very high radiation intensity. Lasers have both characteristics, lasers are available on the market with different fixed frequencies or as tunable devices.
For years we have used Raman spectroscopy reliably and routinely to analyse samples for our customers. For this reason, we are also able to obtain exact measurement results in challenging conditions. The spectrometers and databases we use are from renowned brand-name manufacturers and as such guarantee not only precise results, but also maximum protection for the material.
Both types of filter have an attenuation factor of 50 % at the cut-off, so the transmitted wavelength is multiplied by ¼. By construction, this factor can be compensated by multiplying the result by 4 to obtain a transmission of 100 % at the cut-off. This is what is done in the VDA2007 standard to detect dominant wavelengths.
The Raman spectrum of each substance has certain areas with higher and lower areas of Raman intensity (so-called bands); these areas produce a characteristic image. This image can be compared to known patterns in a spectral library and the type of sample and its characteristics determined beyond doubt.
In our modern laboratory, we use Raman spectroscopy for residual dirt analysis for the assessment of fibres, plastics or salts, for the verification and the identification of filmic contamination as well as particulate contamination and in chemical analytics, in particular in plastics analytics. There the method is used, e.g. for the identification of deposits, residues, inclusions, media, substances, additives and materials (plastics).
Band pass filtercircuit
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Raman spectroscopy is suitable for the analysis of a large number of substances. It is possible to analyse liquids, gases and solids.
Compared to other spectroscopic methods, for example FTIR spectroscopy, Raman spectroscopy offers a few advantages that result above all from the usage of different lasers in the visible to near IR range for a very wide range of materials. Specifically, these include:
At Quality Analysis we offer a broad spectrum of measuring and analytical services. These services also include the use of Raman spectroscopy to identify inorganic and organic samples as well as their composition and crystal orientation. Having spectroscopic analysis undertaken by us offers you a whole string of technical and commercial advantages.
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The new Bandpass-filter bank Operator, released with Mountains® 9.2 decomposes a profile or a surface into a series of bands that can then be analyzed using surface texture parameters. Functional correlation may be carried out to identify active bands, where correlation is higher. This can help in identifying the range of scales to be used to control a particular function on a workpiece.
Raman scattering describes the interaction of the photons (particles of light) with the molecules of the medium. The photons create molecular vibration in the sample. During this process the photons lose energy. Because the wavelength of the light is dependent on its energy, the wavelength is reduced by the loss of energy, in other words: the frequency changes compared to that of the incident light. The frequencies produced by Raman scattering are dependent on the material on which the light is incident. The frequency differences are dependent on various energies in the material such as the rotation, spin-flip and vibration processes. Part of this energy is transferred from the material to the light and changes the frequency of the light. This is the so-called Raman effect.
The Raman spectrum is characterised by the bands mentioned. These areas of higher Raman intensity are characteristic for every substance. In this way the spectrogram of an unknown substance can be compared to samples from a spectral database. If the bands are in the same places, the classification is unambiguous, as in our example for the comparison of a sample to the spectrum for polypropylene.
The combination of a high-pass filter and a low-pass filter may be used to establish a narrow bandwidth which can help to study surface behavior at a particular scale. Digital Surf’s senior surface metrology expert François Blateyron provides further explanations on this type of filtering, a tool for which is released with Mountains® 9.2.
A waviness profile is filtered using a low-pass filter, which means that low frequencies (long wavelengths) pass and high frequencies (short wavelengths) are attenuated.
Highpass filter
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Physical phenomena are usually multi-scale in nature, so complex surfaces should be analyzed using multi-scale tools, to capture this complexity, and Mountains® software now offers a full range of these.
Symmetrical filters can be cascaded to obtain second, fourth or eighth orders that have more selective transmission curves with less overlap. They can be used to decompose the spectrum into more bands.
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Since the invention of more powerful lasers that are at the same time less aggressive on the material, Raman spectroscopy has become established in almost all areas of chemical analytics. Thanks to the high information density, chemicals can not only be reliably identified, pure material concentrations can also be assessed in complex mixtures. Raman spectroscopy offers many other possible applications.
A roughness profile is obtained by the opposite process, using a high-pass filter which attenuates long wavelengths. Each profile receives a sub-set of the frequencies contained in the original profile. In this sense, they are band-limited, or scale-limited.
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This is the Raman spectrum for a particle of polypropylene (red) compared to a reference from a spectral database (blue). The identification is unambiguous.
Due to the shape of the Gaussian transmission characteristics, the bandpass curve is non-symmetrical. This is due to the fact that roughness is in fact obtained by subtracting waviness from the original profile. However, in signal and audio processing, low-pass and high-pass filters are symmetrical by construction. It is possible to apply the same principles to surface analysis and obtain a symmetrical filter.
Multi-scale analysis using bandpass filters can generate a series of profiles or a series of surfaces, each filtered at a different cut-off value. This illustrates the concept of “filter bank” which decomposes the spectrum into individual bands, like an audio graphical equalizer. The number of scales can be defined by the number of bands per octave, an octave being the interval between a wavelength and its double or its half.
Using Raman spectroscopy, it is possible to draw conclusions about the following material characteristics, among others:
For the analysis of the reflected, scattered light, first all the light at the excitation wavelength (that is the Rayleigh scattering) must be removed using an optical filter. The remaining scattered light (the Raman scattering) is guided to an optical grid and split into its individual wavelengths. A CCD sensor produces a spectrum from this light.
Raman spectroscopy is a method for the analysis of the inelastic scattering of light at molecules or solids and is used for the analysis of material characteristics, among other aspects.
If a material is treated thermally or mechanically, its internal stress can change. If you now compare the Raman spectra of a sample of the treated and untreated material, the changes in the stress can be detected in the form of frequency shifts. Higher frequencies are indicative of a compressive stress, while lower frequencies indicate an increase in the tensile stress.
Microscope calculations are a range of formulas used for digital microscopy applications to calculate the depth of field in microscope, field.
During the analysis of the structure of chemical substances, the process determines the structure of the molecules in a chemical substance. This process, important in chemistry and pharmaceuticals, determines the polarisation of the Raman scattered light. If this light is completely polarised, the molecules are isotropically polarised; on the other hand if the polarisation of the scattered light is incomplete, the molecules are anisotropically polarised. The exact degree of depolarisation is determined by placing various polarisation filters in the beam path.
Depending on the specific characteristics of the material to be analysed (e.g. the area of the excitation wavelength), Raman spectroscopy also has disadvantages. In particular, these include:
Above. Classic Gaussian transmission characteristics for a cut-off of 800μm (blue curve for roughness, red one for waviness).
This type of filter is known as a bandpass filter. It isolates a narrow bandwidth at a particular cut-off and allows us to analyze the behavior of the surface at that scale. Sweeping the value of the central cut-off, from the smallest to the largest wavelengths, enables multi-scale analysis.