An example of Raman spectrum obtained with the instrument described in
previous pages is reproduced below. An apatite single crystal (picture
on the right) has been used as a target to get this spectrum. The CCD image
shows a series of faint Raman lines. The laser line which should appear
to the left of the image has been nearly eliminated by the beam splitter
and blocking filter and is barely visible in the picture.
Raman spectrum of apatite single crystal from CCD
To convert the CCD image
to a spectral curve, I have used Visual Spec, the freeware from Valérie
http://astrosurf.com/vdesnoux/ . This software written for data processing
of astronomical spectra can also been used for Raman spectroscopy image
conversion and for
spectral calibration with neon light to get a wavelength x-scale in
nanometers. The raw spectra reported on this page illustrate the
first steps in the Raman spectra processing : image conversion to
spectrum and spectral calibration. Generally, the Raman spectra appear
above a more or less intense continuous background which must be
subtracted from the spectrum. As usually done in vibrational
spectroscopy, the wavelength scale is converted from wavelength in
nanometers to wave numbers in cm-1. These two additional processings of
spectra will be explained later. The spectrum reproduced
in figure 2 was obtained with the dichroic plate beam splitter in place.
The laser line peak in the spectrum is very low compared to the
Raman intensity due to the 2 steps removals of direct laser light. The
combined absorption of both filters can be seen in figure 2 between 637
and 640 nm.
The spectrum of figure 3 has been
recorded with a traditional beam splitter (50-50). It means that 50 % of
the light is lost on each pass through the splitter. In the spectrum we
can see that the laser intensity is now very high which is a problem
when recording very weak Raman spectra like pyrite for instance due to
spurious reflections inside the spectrograph.
Comparing figure 2 and 3, it can be seen that the background is much
more noisy in figure 3 obtained with a traditional beam splitter ( much
less expensive). This is due to the light loss of this splitter in
comparison with the dichroic mirror. I can also be noticed in those
figures that the steepness of the absorption at 635-640 nm is much
higher if the laser rejection filter is used alone. A small part of the
spectrum is thus lost if the dichroic mirror and laser rejection filter
are used together. However due to the higher intensity, the accumulation
time for detection is much reduced when both filtering devices are used.
The shape of the background in figure 3 requires further comments. This
background curve is approximately bell shaped. This is a consequence of
the vignetting of the spectrograph already mentioned previously. From
that curve it can be inferred that the sensitivity of the Raman signal
is wavelength dependant. At 700 nm (1500cm-1) the intensity
of the signal is close to zero; the CCD is not fully covered by the
spectrum. The spectrum presented here were recorded at a H20 wavelength
of 670nm. Of course it is always possible to change that position to
record spectra at higher wavelengths. The sensitivity variation with
wavelength must be taken into account when comparing to literature
Neon spectrum from CCD camera
calibration of the Raman spectra is made by comparing the raw Raman
curve with a known line spectrum of a neon bulb. After each recording of
the Raman data, the top flip mirror is rotated so that the neon light
can reach the spectrograph. An example of such a neon spectrum on the CCD is reproduced in figure 4. This image is converted
to spectrum in the usual way with the V Spec program.
On the image, the imperfections of the H20 used as a spectrograph are
obvious. As the grating is spherical, the aberrations are not
fully corrected and a neon emission line appears somewhat curved. Due to
field curvature, it is not possible to bring each part of the spectrum
in perfect focus; a compromise should be found. For our calibration
purpose, only the central horizontal part of the image where
the lines are narrower is used. This procedure gives the spectrum in
figure 5 without too much line broadening.
Figure 6 Final spectrum.
To get the final spectrum as
presented here in figure 6, two additional steps must be performed:
baseline correction and recalculation of the x-axis in wave numbers (cm-1). The baseline correction was performed with
an old version of the Lab Control Spectacle software (1995) which allows
to draw a multi points background. The recalculation of the abscissa and
spectrum presentation and comparison is made in MS Excel.
The apatite spectrum obtained with the described microscope is similar
to the data of the literature except the relative intensities of the
peaks as mentioned above. My spectra always have higher intensities in
the low wave numbers range and lower intensities around 1000 cm-1.
Nevertheless, the spectra from the present work can be used for the
identification of the minerals.