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## 5.2.2 Multispectral imaging for surface mapping of pigments<fnref n="102" />

### 5.2.2.1 Scope

This standard describes a method to record multispectral images of colour painted materials and artworks, which is a commonly used technique currently available to the scientist, conservator, archaeologist and art historian for the non-invasive investigation of works of art. This document will concentrate on the wavelength range that can be observed using modified commercially available cameras, which typically employ silicon based sensors sensitive from approximately 350 nm to 1100 nm. Cameras based on InGaAs sensors, which can record infrared radiation from approximately 700 nm to 1700 nm, can be used regularly in cultural heritage applications but due to their specialized technology they are out of the scope of this standard.

Concerning the choice of material/artefact, our suggestion, is to consider, for example, painted stone sculpture.

This method may be applied to:

* painted artefacts either untreated or subjected to any treatment or ageing

* representative surfaces of objects, indoors or outdoors.

### 5.2.2.2 Technical overview

Multispectral imaging is the procedure used to observe an object using selected ranges of wavelengths in the electromagnetic spectrum that include and extend beyond the capabilities of the human eye. A generic setup for multispectral imaging is composed of three main components:

1) Incoming radiation, which is generated by a radiation source and travels towards the object;

2) The object, which interacts with the incoming radiation;

3) Outgoing radiation, which, following the interaction between the incoming radiation and the object, travels from the object to the recording device.

The extent to which this radiation will penetrate the object under investigation will be dependent on its wavelength and on the absorbance of the materials which compose the object, with longer wavelengths of radiation generally penetrating further into the piece. For example, when examining a painting, shorter wavelengths (such as UV) are often

<footnote n="102">¹⁰² Mikel Sanz (CSIC)</footnote>
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readily absorbed by the outer layers (usually varnishes), while longer wavelengths can pass through the varnish and interact with the pictorial film and the under drawing. The radiation reaching any particular point in the object can be: (i) absorbed, (ii) reflected and/or (iii) absorbed and re-emitted as luminescence at longer wavelengths. Each outcome produces an image set which yields information specific to that point. Thus by selecting particular combinations of illumination and detection ranges, it is possible to gain insight about the distribution of materials in the object under study.

The test equipment for the acquisition of multispectral images has to be made up of a number of equipment components: 1) Radiation sources (RS) which provide the incident radiation to the object being studied, 2) a filter or set of filters in order to allow the transmittance of radiation in the wavelength range under study (FT) and exclude unwanted radiation from being recorded (FR); 3) a detector or recording device, a commercially available digital camera (modified by removal of the IR-blocking filter in the case of IR-induced or IR-reflected images and 4) a set of standards to enable the post-processing methods. The possible image set with the recommended equipment components are:

* Visible-reflected (VIS) image corresponds to standard photography and records the reflected light in the visible region (400-700 nm) from an object when this is illuminated with visible light. The image is collected in the range in which the object is usually observed and can serve as the reference point to interpret the other image sets. RS: Tungsten Halogen or Xenon lamps or LED source, FT: None, FR: Bandpass filters to allow only light in the range 400-700 nm.

* Infrared-reflected (IRR) images record the reflected radiation in the infrared region (700-1100 nm) from an object when this is illuminated with infrared radiation. This image can be valuable in revealing under drawings and concealed features. RS: Tungsten Halogen lamp, FT: None, FR: Bandpass filter to allow only light in the range 700-1100 nm.

* Ultraviolet-reflected (UVR) image records the reflected radiation in the ultraviolet region (200-400 nm) from an object when this is illuminated with ultraviolet radiation. RS: Black light fluorescent lamp or UV LED source, FT: Shortpass infrared filter (<700 nm), FR: Bandpass filters to allow only light in the range 200-400 nm.
<figure><img src="image_124.png" /></figure>

* Ultraviolet-induced luminescence (UVL) image records the emission of light (luminescence) in the visible region (400-700 nm) from an object when this is illuminated with UV radiation. This image is used to investigate the distribution of luminescent materials, such as organic binders and colourants. RS: Black light fluorescent lamp or UV LED source, FT: None, FR: Bandpass filters to allow only light in the range 400-700 nm.

* Visible-induced infrared luminescence (VIL) image records the emission of radiation (luminescence) in the infrared region (700-1100 nm) from an object when this is illuminated with visible light. RS: Incandescent lamp or Visible LED source, FT: Infrared shortpass filter (<700nm), FR: Longpass filter (>700 nm) to allow only infrared light (700-1100 nm).

* Visible-induced visible luminescence (VIVL) image records the emission of light in the visible region (500-700 nm) from an object when this is illuminated with visible light (400-500 nm). RS: Blue LED source, FT: None, FR: Bandpass filter (500-700 nm)

In all the cases, Standards and Calibration targets must be used. They are uniform reflective boards that should be a grey Lambertian reflector: a surface showing the same radiance when viewed from any angle. Several commercial available products are available :

1) Spectralon diffuse reflectance standards

* http://www.labsphere.com/labsphere-products-solutions/materials-coatings-2/targets-standards/diffuse-reflectance-standards/diffuse-reflectance-standards/

* http://www.labsphere.com/products/reflectance-standards-and-targets/reflectancetargets/spectralon-targets.aspx

2) Macbeth (X-rite) ColorChecker Chart

* http://xritephoto.com/ph_product_overview.aspx?catid=28

* http://xritephoto.com/ph_product_overview.aspx?id=1257&catid=28

3) Uniform reflective board

* http://xritephoto.com/ph_product_overview.aspx?id=944&catid=28
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* http://www.labsphere.com/labsphere-products-solutions/materials-coatings-2/targets-standards/diffuse-reflectance-standards/color-standards/

* http://www.labsphere.com/products/reflectance-standards-and-targets/reflectancetargets/spectralon-targets.aspx

### 5.2.2.3 Resources

For a general discussion and recommendations on the safe handling and positioning of objects for imaging in art historical and conservation contexts, the reader is referred to the AIC Guide to Digital Photography and Conservation Documentation

http://cool.conservation-us.org/coolaic/sg/emg/dtf/DTF_Online_Weblinks.pdf

For a free image processing system that includes a range of filters, arithmetic operations, colour processing, histograms, and geometric transforms

http://www.vips.ecs.soton.ac.uk/index.php?title=VIPS

* Data repository

http://www.labsphere.com/support/datasheets-library/

* Bibliography

https://www.britishmuseum.org/pdf/charisma-multispectral-imaging-manual-2013.pdf

* Blog entries

For information about new features in nip2 and how to report any problems encountered with nip2: http://libvips.blogspot.com.es/?view=magazine