technology Portable sPectroscoPy
The optical technique was verified by historical and iconographical information of the castle as well as through further chemical analysis. ‘Further research of these fragments and comparison with material from other sites will justify if this method is useful to date post-medieval material,’ says Meulebroeck.
rapid characterisation A big advantage of using spectroscopy is that it’s a quick classification method. In total 1,269 window glass fragments have been recovered from the Castle of Bladelin site and with such large sample sizes more detailed chemical analysis is impractical. Depending on the circumstances and the material, Meulebroeck says 50 to 150 measurements can be made in half a day using spectroscopy.
‘A lot of work has been done on classifying vessel glass and jewellery in the past, but less is known about plain window glass,’ Meulebroeck states. She says that some sites have large unstudied glass collections and optical spectroscopy could be a useful initial analysis method to evaluate them. The benefits of optical spectroscopy
compared to chemical analyses, are that firstly it’s straightforward to use by archaeologists and non- experts, says Meulebroeck.
She lists its virtues as that it can be portable,
it’s inexpensive, fast, and, importantly, it is non- or semi-destructive.
‘The optical technique cannot provide as
precise information as the [traditional] X-ray based techniques, but it can be used as a first-line analysis method to classify large collections in
sixteenth century window depicting ‘Mary with child’ from the holy Mary church in bruges, belgium. optical spectroscopy techniques developed at Vrije Universiteit brussel were used to date some of the glass fragments. Copyright: Monument – Vandekerckhove
groups and give a rough idea about the main composition,’ she says. Traditional X-ray based techniques, such as SEM-EDX and XRF laser ablation, are very accurate, but the equipment is complex requiring a skilled operator and cannot be taken out of the laboratory.
The tests are also time-consuming and expensive and most are destructive, with a small portion of the glass removed in order to test it.
Moving out of the lab Portable spectrometers are now commercially available for researchers like Meulebroeck. ‘The big advantage with these devices is that now you can bring the spectrometer to the sample,’ comments Rob Morris, director of marketing at spectrometer manufacturer Ocean Optics. Analysing a water sample from a lake, for example, can be carried out in situ and in real time with a small, low power spectrometer, rather than having to take the sample back to the lab, during which time the sample might have changed. Ocean Optics’ spectrometers have been used in diverse applications ranging from measuring the light penetrating a forest canopy, providing underwater fluorescence measurements of coral as an indicator of the health of the coral ecosystem, as well as for studying gases escaping from a volcano. The company’s Jaz spectrometer, which has a microprocessor on board, has been used to measure the lighting inside greenhouses to ensure the spectral luminance produced by indoor lights mimics that of sunlight, and by NASA scientists on the slopes of Mount Everest to measure UV radiation at high altitude.
Laser applications: • Ti:Sapphire pumping
• biotechnology • OEM integration
Ray Livingstone, a sales engineer at Hamamatsu, comments that spectrometer sensors and optics can be made smaller with nanofabrication techniques. Unlike some lab-based spectrometers, which are heavy and bulky and have moving parts, Hamamatsu’s MEMS spectrometer is fabricated as a single substrate. These fibre-optic portable mini- spectrometers have no moving parts and are robust. The diffraction slit is etched into the CMOS sensor using a femtosecond laser and
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info@laserquantum.com www.electrooptics.com May 2011 l electro oPtics 21
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