Boris Pretzel
Higher Scientific Officer, Conservation Department

The first issue of this journal included an article discussing the use of uv - visible - near infra-red spectroscopy and describing our choice of instrument.'  ¹   We chose the Hitachi U4001 spectrophotometer which has a large sample compartment allowing measurements to be taken even with fairly bulky samples. However, the maximum possible sample size is still very limited compared to the size of many of the objects in the Museum's collection!

One way of overcoming the problem of limited sample size is to set up a flexible link between the light sources in the instrument and a remote detection arrangement. The link between the instrument and the remote detector can be achieved using a combination of optical fibre bundles to carry the radiation from the instrument and mirrors and lenses to focus the radiation into the fibre bundles. The detector arrangement itself needs to collect the radiation reflected from the sample at each wavelength (regardless of the angle at which it is reflected) and focus this onto a detector. Here the intensity is
compared to that of radiation reflected from a (white) reference reflector. To ensure
efficient collection of all the reflected radiation, the detectors are normally housed in a spherical arrangement ² called an 'integrating sphere'.  A flat section is cut out of the sphere to form a sample port (the point at which the sample surface is placed in contact with the sphere) and, if desired, another to form a reference port. Barium sulphate is usually chosen as the reference white from which the inside of the sphere is made, as this is relatively inexpensive, stable, and has suitable reflection properties, at least for radiation in the visible region (400nm to 765nm).

Combinations of integrating spheres with fibre optics are well known. Due to the nature of the design and the materials used, their use is normally limited to colour measurement. The losses from the optical fibres and the comparatively poor reflectance properties of barium sulphate in the near infra-red region lead to unreliable and unusable spectral information for wavelengths far outside the visible region; standard external integrating spheres have an effective spectral range of 300nm to 8OOnm. Even in this rather limited spectral range, it can be difficult to obtain reproducible results. Errors can be caused by extraneous light entering the sphere, the difficulty of holding the sphere still during the measurement, and the lack of suitably large flat surfaces on objects of interest.

In the present design, the spectral range is greatly increased by using a mixture of low water content fused silica fibres (having relatively good light transmission in the near infra-red region) and high water content fused silica fibres (with better transmission in the ultra-violet region) as the optical fibre bundles. Further, the internal surface of the collector arrangement is coated with Spectralon, a thermoplastic based on Polytetrafluoroethylene with very good reflectance characteristics in the spectral range of interest (250nm to 2500nm).

Concave surfaces are not accessible for measurement using traditionally shaped integrating spheres. In the current design the sample port has been elongated, resulting in a conical nozzle as can be seen in the photograph. The collector is now no longer spherical and therefore has been called the 'external integrating head'. It has a measuring area with 10mm diameter and built in infra-red and visible/ultra-violet radiation detectors (a Peltier cooled PbS detector and a photomultiplier tube detector, respectively). The photomultiplier tube doubles up as a handle for supporting the integrating head. Cables connect the output from the detectors to the spectrophotometer for amplification and generation of the spectrum being measured.

The integrating head was built by Labsphere Inc ³  and has been fitted with a viewer and eye-piece through which the sample area may be viewed before measurement. This allows accurate relocation of areas for subsequent measurements. To help exclude extraneous radiation and to protect objects while they are being measured, the sample port has a 2mm thick foam rubber gasket around it.

Initial tests with the integrating head showed that it was difficult to obtain reproducible results due to the difficulty in holding the integrating head still throughout the measurement. Further errors were caused by moving the sphere between taking the background scan and the sample scans as the transmission characteristics of the optical fibre bundles changes as their curvature is changed. To overcome these difficulties without impairing the flexibility of the attachment, a stand allowing highly flexible positioning and rigid clamping of the integrating head was commissioned from Focon Equipment ⁴ . The stand, an initial version of which can be seen in the photograph, will enable reproducible measurements to be taken even on difficult sample surfaces.

The combination of the movable spectrophotometer, the integrating head with a wide spectral band width and the stand provides a highly flexible measuring arrangement and opens up the vast collection in the Museum for study by uv-vis-nir spectroscopy. It is planned to use this equipment, amongst other things, to monitor the growth of stains in white Carrara marble statues. These stains are thought to be due to bacteria living in the marble ⁵ .  If this is so, the stains will only enlarge under conditions favouring the growth of the bacterial colony. With the increased spectral range, the integrating head can be used not just to measure the colour changes occurring with time but also to assess the amount of water contained in the marble to see whether the aforementioned conclusion is reasonable.

The cost of the integrating head attachment was £18,000 and that of the stand £2,300. The purchases were made with the financial support of the Gabo Trust.

References

1 Martin, G and Pretzel, B,'UV-VIS-NIR Spectroscopy: What is It and What Does It Do?', V&A Conservation Journal 1(1), October 1991, pp.13 -14.
2 Clarke, FJJ and Compton, JA, 'Correction Methods for Integrating-Sphere Measurement of Hemispherical Reflectance', Color Research and Application 11(4), Winter 1986, pp.253-262.
3 Labsphere Inc., PO Box 70, North Sutton, NH 03260, USA, tel:(0101603) 9274266.
4 Focon Equipment, 1 Eastcote Avenue, S Harrow, Middlesex HA28AJ, tel:(081) 864 9612.
5 Cook, R and Martin, G, 'Preliminary Investigations into Discolourations Occurring in White Marble' in: the Institute of Archaeology Jubilee Conservation Conference-Recent Advances in the Conservation and Analysis Artefacts', 1987, pp. 359-364.