OPL Spectroscope — Hand-Held Absorption Spectrum Analysis
OPL Spectroscope — Hand-Held Absorption Spectrum Analysis
A prism or diffraction-grating spectroscope by Optical Products Ltd, used in gem labs and trade settings for absorption spectrum analysis
The OPL spectroscope is a hand-held prism or diffraction-grating spectroscope manufactured by Optical Products Ltd (OPL), widely used in gemmological laboratories and trade settings for absorption-spectrum analysis. The OPL teaching spectroscope, fitted with a diffraction grating, displays the wavelength bands from approximately 400 to 700 nanometres and is valued for its portability, ease of use, and reasonable price. Gemmologists use the instrument to identify diagnostic absorption lines and bands in gemstones, providing rapid species identification and treatment-indicator detection without the need for the more elaborate UV-Vis-NIR spectrophotometer found in research laboratories.
How an absorption spectroscope is used
The basic procedure for spectroscope use is straightforward. The gemstone is placed near a strong light source — typically a fibre-optic lamp or a high-intensity LED — so that light passes through or reflects from the stone before reaching the spectroscope's slit. The slit admits a narrow band of light, which the diffraction grating (or prism) disperses into its component wavelengths. The user views the dispersed light through the eyepiece and observes the colour spectrum from violet (400 nanometres) at one end to deep red (700 nanometres) at the other.
Absorption features appear as dark lines or bands in the spectrum, indicating wavelengths at which the stone has absorbed light. Diagnostic features include narrow lines (typical of transition-metal ion absorption), broad bands (characteristic of certain colour centres), and complete cutoff regions (where the material absorbs all light below a particular wavelength). Each gem species has its own characteristic absorption spectrum, and an experienced user can identify many common gem species and detect some treatments by spectroscope alone.
Diagnostic spectra
Several gem species display particularly diagnostic absorption spectra. Ruby and chromium-bearing red garnet show the characteristic chromium doublet near 694 and 692 nanometres in the deep red, with additional fluorescence in the same region. Almandine garnet shows broad iron absorption bands across the green and blue. Synthetic emerald often shows distinctive chromium lines that distinguish it from natural emerald. Sapphire shows iron-related bands that indicate origin in many cases. Zircon shows extremely narrow uranium-related lines characteristic of the species.
Some treatments are detectable by spectroscope. Beryllium-diffused sapphire often shows distinctive spectral features that distinguish it from naturally coloured sapphire of similar appearance. Heat-treated stones sometimes show spectral changes that indicate the treatment, although the diagnosis is rarely definitive without supporting tests. The spectroscope is therefore a screening instrument that often signals the need for further laboratory examination rather than a definitive identification tool for treatment status.
Skills and limitations
The OPL spectroscope, like all hand-held spectroscopes, requires practice to use effectively. Faint spectra can be difficult to interpret; small absorption features may require careful adjustment of the lighting and the orientation of the stone to become visible; and the relatively narrow magnification range of the instrument means that features need to be identified by their wavelength position and character rather than by detailed structural analysis.
The principal limitation is that the spectroscope shows only the visible-spectrum absorption (400 to 700 nanometres), missing the UV and IR regions that contain additional diagnostic information. Research-grade UV-Vis-NIR spectrophotometers, available in major laboratories, extend the analysis across a much broader wavelength range and provide quantitative absorption measurements. The hand-held spectroscope captures the qualitative essentials of the visible spectrum at a fraction of the cost and with full portability.
In the trade
The OPL spectroscope is a standard tool for working gemmologists, alongside the loupe, microscope, refractometer, dichroscope, and polariscope. It is particularly useful in the field-buying context, where rapid screening of stones for species identification and obvious treatment indicators is valuable, and where the alternative of laboratory submission would be impractical. For the cost of a few hundred pounds, the instrument provides absorption-spectrum analysis that would otherwise require thousands of pounds of laboratory equipment.
See also OPL gem microscope, UV-Vis-NIR spectroscopy, chromium absorption lines, and the broader gemmological instruments entries.