Spectrograph tests: Neon spectra.

                  The first tests of the spectrograph have been performed with a neon bulb as light source with an entrance optics of about f/8 to cover fully the grating surface. The image spectrum is reproduced below on figure 1 with the digitized spectrum reported in figure 2. The image shows a regular spectrum all over the image with no distortion nor vignetting. There is no distortion on the border of the image. See for instance the 585nm most intense neon light on the left border of figure 1.

                       The spectral resolution got with a 25µ inlet slit is about 0.13 - 0.14 nm full with half maximum ( FWHM)  that is around 3 cm^-1 in the wavenumber units used generally in vibrational spectroscopy.

Figure 1. CCD image of a Neon lamp. Slit = 25 µ.

Spectrograph placed on bench and illuminated with a f/8 neon source.

Figure 2. Neon spectrum obtained from the CCD image above. FWHM = 0.14 nm with a slit = 25µ.

Figure 3. Image of neon with the spectrograph installed in the Raman microscope.

         When the spectrograph is in place in the Raman microscope assembly, its input optics is the eyepiece at the exit of the flip mirror at the top of the microscope. This optics has been designed especially for the Raman signal coming out of the microscope. The illumination of the spectrograph with the neon bulb is not  ideal ( low f/value). Nevertheless, the fineness of the lines and the absence of vignetting and distortion are preserved. At wavelength above 600 mn, the chromaticity of the Olympus lens is not perfect, it is thus not possible to focus perfectly the whole spectrum on each side of the image. Focusing is a compromise. For the study of minerals, it is better to focus finely the left side under 1000 cm^-1  were the majority of Raman lines are situated.

Figure 4. Neon reference spectrum obtained with the JY H-20 monochromator used as a spectrograph. Slit 100µ.

This is the neon spectrum obtained with the first design of the Raman microscope. The neon emission lines are deformed by the field curvature and aberrations. In this case the focusing element is the grating itself. It can provide perfect focusing on an exit slit only.

Figure 5. Spectrum obtained from image in figure 3.

The spectral resolution (FWHM) is the same as measured in figure 2, around 0.14 nm. Note that in general for Raman spectra recording, I'm using a 100µ slit which provides much more signal and less integration time. The resolution is decreased ( about 0.38 nm FWHM) but still enough to get a good Raman spectrum and recognize mineral species.

Figure 6. Spectrum after digitizing image from figure 4. JY H-20 monochromator.

Notice the decreased sensitivity at the left of the figure due to the optics. The peaks are less symmetric and resolution lower.