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A spectroscope splits up white light into the spectral colours and reveals any wavelengths that have been absorbed by a gemstone (or other suitable substance) as a series of vertical lines or bands across a rainbow-like horizontal ‘spectrum’. Each sequence of lines or bands are produced by specific chemical elements within the structure and not only may cause the overall body colour of a gemstone they are frequently diagnostic.
There are two main methods of producing the spectrum. The earliest method, used by Isaac Newton in his experiments to discover the nature of light, uses a prism or series of slightly different prisms to split white light into the primary colours – Red, Orange, Yellow, Green, Blue, Indigo & Violet. The second and much more recent method is to use a diffraction grating, a piece of glass or plastic with possibly tens of thousands of fine lines scribed on its surface every millimetre. The rainbow-like spectrum is produced by the interference and re-enforcement of the light waves. If you look at the surface of a CD you will see rainbow colours – these are the product of a reflection grating, similar to a diffraction grating but the light is reflected from the surface rather than being transmitted through it. Spectroscopes with both of these methods are available today.
Before you use the spectroscope it is always a good idea to point it at the light source you intend to use to make sure that there are no absorption lines or bands and that there are also no bright emission lines, i.e. that you are using pure white light with a ‘clean’ spectrum. It is also a good idea to check all of the other lights in your work area as it is possible to see anomalous fluorescent lines in a spectrum from a typical office ‘strip light’ even if it is not directly overhead. If necessary work in a darkened environment to reduce the risk of light contamination. Note that even daylight has fine lines that may cause confusion. NEVER! look directly at the sun through a spectroscope.
A: Diffraction Grating spectroscopes produce evenly spaced, or linear, spectra so that individual absorption areas or lines may be easily assessed for wavelength. The almost imperceptible disadvantage is created because the spectrum is caused by the interference of the incoming light so some light is permanently lost, but in modern diffraction gratings this loss is reduced to a minimum and is far outweighed by the advantage of the linear spectrum. Another advantage is that an absorption line in the red end of the spectrum is exactly the same thickness and strength as similar lines in the green or violet regions of the spectrum; an important consideration, as we will see in the second part of this answer.
B: A Prism spectroscope has the advantage of transmitting virtually all of the light that enters the ‘slit’, but because the wavelengths are refracted by a greater amount as the wavelength decreases, from red to violet, the individual regions of colour are narrow at the red end and quite wide in the violet end. This means that for a given strength and thickness of absorption line, in the red it may be sharp and strong, in the green it may lose some definition and in the violet it maybe quite difficult to see.
The light entering a spectroscope does so through a very fine gap or ‘slit’. Spectroscopes of both types have fixed slits, meaning that they are not adjustable and therefore cannot be made to let more or less light in as required. Other spectroscopes, of both types, may have a pair of ‘knife-edge’ blades which can be separated or brought closer together as necessary to let more or less light into the spectroscope. Some spectroscopes of either type also have a ‘draw tube’ like a telescope which allows the user to adjust the focus for different regions of the spectrum. Fixed focus means that the spectroscope has been carefully adjusted so that the optimum focus is pre-determined for lines in the green region, thus achieving a very good focus for similar lines in both the red and the violet regions. Spectroscopes with adjustable slits and / or focus can be as much as 10 times more expensive that ‘fixed slit, fixed focus’ instruments. For most gemmological purposes the simple ‘fixed’ spectroscopes are perfectly adequate and in fact are used in most gemmological laboratories around the world.
OPL produces the ‘Teaching Diffraction Grating Spectroscope’ for use as a bench instrument with its dedicated spectroscope stand and the ‘Pocket Diffraction Grating Spectroscope’ which is smaller and intended for ‘field’ use, at the gem mines, at antique fairs or simply to put in your purse or pocket to carry around “just in case”. The Teaching or Bench model is used ideally with a powerful fixed light source while the Pocket model is ideal for use with a pen torch. Needless to say, both models can be used for either role and with either type of lighting. One great advantage of the Teaching model is that a student or gemmology teacher can set up a specimen and allow the spectrum to be studied as long as needed without removing the stone.
Ideally a bright, focused ‘incandescent’ light, such as that provided by a 20 or 50 watt Quartz Halogen bulb because modern ‘energy saving’ lights do not produce a complete spectrum, indeed some have quite astonishing emission lines which would completely destroy any subtle absorption spectrum likely to be present from a lightly coloured specimen such as a colourless zircon. Even some fibre optic light guides are unsuitable because they transmit a greenish light which may also be impure and prevent a full spectrum from being seen; for the same reason never use an LED light. Pen torches that use a small ‘lens’ bulb are normally quite good, but be sure that the batteries are fresh.
Usually people with glasses or contact lenses get along fine with a spectroscope. If you have very poor eyesight you may need to experiment by trying both with & without your glasses / contact lenses. In some cases it may be possible to use contact lenses instead of glasses, and sometimes by using non-prescription ‘reading glasses, but if you have serious problems using a spectroscope please check with your optician before using anything other than the glasses or contact lenses you have been prescribed.