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the grating itself is fabricated on the lens by nanoprinting. ‘Everything is on a single substrate manufactured by MEMS techniques and, because of this, it is very small, has a good alignment, and low light losses,’ Livingstone says. The device weighs only 9g and measures 27.6 x 13 x16.8mm. At Vrije Universiteit Brussel, data captured by
Avantes’ portable spectrometer is being compared against the lab-based optical spectroscopy techniques to determine the validity of the portable equipment. It is being used in situ to characterise material from a stained glass window in a church in Bruges in tandem with lab-based analyses.
This particular study focused on identifying the fluxing agent present in coloured glass with UV- VIS-IR spectroscopy to date the material. During Roman times, sodium was the predominant fluxing agent. This changed to potassium around 900 BC when ash began to be used and medieval and post-medieval window glasses are rich in the element. This changed again in the second half of the 19th century when industrial soda became more common. The technique relies on the presence of a colouring agent, such as cobalt used in blue glass. Cobalt gives a distinctive three-band spectrum using optical spectroscopy. The position of the first cobalt absorption band will change depending on the sodium, potassium and calcium composition of the glass and therefore acts as an indicator of the fluxing agents used and subsequently the time period the glass originates from – sodium-rich glass will have a cobalt band at 434nm, while with potassium-rich glass this is at around 527nm. ‘So far, we have identified certain parts of the window that is sodium-rich,’ explains Meulebroeck, meaning it is 19th or 20th century material.
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A further indicator is the metal oxide colouring agents, which, for certain colours, also vary depending on the period. Chromium, for instance, was only used from the mid-19th century onwards. ‘It’s very easy to identify the colouring elements using spectroscopy just by the presence or absence of absorption bands characteristic for those elements,’ Meulebroeck says. The technique is not only useful for
archaeologists Meulebroeck says, but could be utilised by art historians and those conserving stained glass, for example.
building design From ancient buildings to those not built yet, portable spectrometers are playing their part in architectural design and research at the Taubman College of Architecture and Urban Planning at the University of Michigan, USA. Mojtaba Navvab,
22 electro oPtics l May 2011
hilde Wouters, a PhD student at Vrije Universiteit brussel, characterising ancient glass using optical spectroscopy
a professor at a faculty within the Building Technology Area at the college, is using portable spectrometers from Ocean Optics to study colour variance within a CAVE, a computer-assisted virtual environment. The CAVE within the college allows architects, engineers and designers to view their designs in an immersive virtual environment. A mannequin head containing various sensors, including a spectrometer, as well as acoustic sensors to measure sound levels, is placed within the CAVE allowing real-time measurements to be taken of lighting, colours and spectral variation as the user moves through the simulation. Colours and lighting spectral characteristics and luminance within the CAVE are simultaneously recorded and analysed along with the visual response of the user experiencing the simulation, such as pupil dilation. ‘The sensing equipment allows the researchers to measure the dynamics of light and colour within the CAVE as experienced by the user,’ Navvab says. One of the uses for the CAVE is to generate a virtual model of house interior designs for assisted living. By using the sensing setup, the colours of walls and surfaces can be optimised for eyesight of the elderly.
‘The portable equipment provides a lot of
flexibility,’ Navvab says. ‘It’s very difficult to set up a full spectrometer outside of the laboratory where everything has to be controlled. The portable equipment allows you to measure specific angles, specific directions of light, and very awkward positions that otherwise wouldn’t be possible.’ Objects like lights are difficult to simulate,
according to Navvab, because, even though they look bright on the screen, the eye doesn’t get a true sensation of brightness. It is only as bright as the projector. ‘In virtual reality, we’re trying to see how realistic we can make perceived brightness by making the intensity ratios as accurate as possible,’ he says.
‘Using the spectrometers and other sensors
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