gemlab.co.uk at J K Harris for professional gemmologists and students of gemmology.

Gemlab.UK Gemmological Spectroscopy PHOTOGRAPHY

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The Gemstone Images

I have photographed the gemstones in daylight in order to establish a standard resolution of colour throughout and any reference to the colour of a gemstone relates to this lighting unless stated otherwise. This includes those images showing a variation in colour due to pleochroism. In the comparison Excel files colour change of a gemstone in tungsten light is also shown where it has been photographed in scattered light along with the appropriate spectrum. Otherwise any reference to colour change, or colour shift, is given in the “Comparison Notes". In the Principal Gemstones section of the database and in the pages entitled “Through my spectroscope” colour change is illustrated in more detail where relevant.

The Spectra Images

The spectra I have photographed using a tungsten fibre optic lamp. The images were captured through a small diffraction type spectroscope attached to a digital camera and coupled to the body tube of a monocular microscope.

The advantage of this set-up is twofold. In photography it ensures maximum control to acquire an image and in teaching it provides a stable image which students can view without having to handle the specimen or the spectroscope. This is hands free spectroscopy.

The use of a monocular microscope in conjunction with a spectroscope is not a new idea, but often the spectroscope was simply balanced on top of the eye tube and the light source was directed vertically from below via a sub-stage condenser into the gemstone. This works adequately with a flat tablet with two parallel faces or perhaps even a cabochon, but not with a faceted stone. In this method the majority of the light is scattered or lost by total internal reflection, especially with stones of high refractive index, and fails to enter the spectroscope. To overcome this I have developed a method by which the light is directed horizontally above the microscope stage and enters the faceted stone via the pavilion. This is set at a suitable angle to enable internal reflection from the table facet and maximum light is then directed up into the body tube of the microscope and on to the slit of the spectroscope. In the case of very small stones I use an objective lens with a magnification of 3.2x and a working distance of 50mm as a light gathering accessory in which case true optical alignment is essential.

CALIBRATION

In most cases calibration was checked using a Beck prism spectroscope No.2522 with built in wavelength scale and tolerance allowed due to parallax limitation. In the case of some unusual, rarely encountered materials, spectra of known wavelength were introduced into the optical path during photographing. This was done using thin sections of materials doped with rare earth elements, (neodymium, erbium and thulium) certain liquids (Potassium permanganate and olive oil); and certain light sources (sodium, mercury vapour and tri-phosphor coated tubes and sunlight.) The method used was to superimpose one of these spectra on to the spectrum of a specimen being photographed. The main Fraunhofer lines in the solar spectrum, combined with the emission lines in a triphosphor lamp produced a useful spectrum in this respect. From there calculations were made between these known and the unknown positions of lines in the other spectra, using a nanometre - pixel related formula by computer to compile a comparison scale.

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