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Identification of Pigments by Multispectral Imaging; a Flowchart Method Antonino Cosentino

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Identification of Pigments by Multispectral Imaging; a Flowchart Method Antonino Cosentino Cosentino Heritage Science 2014, 2:8 http://www.heritagesciencejournal.com/content/2/1/8 RESEARCH ARTICLE Open Access Identification of pigments by multispectral imaging; a flowchart method Antonino Cosentino Abstract The literature on the application of Multispectral and Hyperspectral imaging for identification of pigments on artworks is sparse. While these methods do not provide the analytical capability that spectroscopies do offer, the use of spectral imaging has the advantage of being a rapid and relatively low-cost solution for the examination of large areas. This paper presents a flowchart for the identification of historical pigments applied with gum Arabic using multispectral imaging (wavelength ranging from 360 to 1700 nm) performed with a modified digital camera for infrared, visible and ultraviolet photography; and an InGaAs camera for infrared reflectography. The flowchart method will be most successful on paint made of one layer of pure pigment, and it can selectively discriminate only a fraction of the 56 pigments analyzed. Though, considerably limited in its analytical capabilities, the low cost and speed of the workflow make the method worthwhile, even if only to localize retouching and areas appearing the same hue but painted with different pigments. The InGaAs camera is the only expensive instrument used in this study but its cost is relatively affordable for the average painting conservation studio since only a model with a low pixel count is required (320×256 pixels) rather than a more sophisticated InGaAs scanner system. Introduction will work for those which present peculiar behaviors in Multispectral imaging (MSI) [1,2] and Hyperspectral the range of the electromagnetic spectrum readily ob- Imaging [3-6], have been suggested as methods for the servable with an IR-VIS–UV modified digital camera non-destructive identification of pigments. Though, it is (360–1100 nm) and an InGaAs camera (900–1700 nm). mandatory to point out that these methods are problem- In this way, selected pigments are likely to be identified atic and the user may be subjected to draw conclusions by means of MSI examination. This simplified approach, that remain uncertain, essentially, because pigments are though demonstrated to be limited in its analytical diag- often mixed and overlapped in layers to make the de- nostic capabilities, has the benefit of being accessible sired color and effect. and easy to implement by professionals in the art con- To identify pigments with an acceptable degree of cer- servation and examination field. tainty, at least one other material specific technique This method is more likely to succeed when applied on must be used to complement hyper or multispectral im- artworks where pigments have been applied in one single aging diagnostics. The use of MSI to tentatively identify layer and not mixed; as is the case with miniatures [6,7], pigments has an important advantage justifying its appli- drawings [8] and prints. Unlike other references, which sug- cation: the rapid and low-cost survey of large areas. The gest the use of software algorithms to analyze the MSI im- intention of this paper is to show that with a flowchart ages, this paper proposes a more straightforward method based methodology it is possible to tentatively identify simply based on visual examination and the use of a photo- some historical pigments by means of MSI performed editing software for the characterization of features appar- with simplified equipment and without the aid of im- ent in the image. aging analysis software. This method doesn’t claim to allow the identification of all different pigments, but it Multispectral imaging Imaging methods This paper illustrates a flowchart method for pigment Correspondence: [email protected] Independent Scholar at “Cultural Heritage Science Open Source” Blog, identification based on the acquisition of MSI images in chsopensource.org, Piazza Cantarella 11, Aci Sant’Antonio 95025, Italy 4 spectral bands: Ultraviolet, UV (360–400 nm); Visible, © 2014 Cosentino; licensee Chemistry Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Cosentino Heritage Science 2014, 2:8 Page 2 of 12 http://www.heritagesciencejournal.com/content/2/1/8 VIS (400–780 nm); Infrared, IR (780–1100 nm) and In- the different wavelengths. In order to allow a compara- frared Reflectography, IRR (1000–1700 nm), Figure 1. tive examination through the different spectral ranges, The acronyms for the MSI methods presented in this those images are uploaded as layers of a single docu- paper highlight first the spectral band followed by R ment file in an image editing software, such as Adobe (Reflected), F (Fluorescence), FC (False Color). So the 8 Photoshop or GIMP, and manually resized to overlap imaging methods are called VIS (Visible), IR (Infrared), one another. Infrared Reflectography (IRR) was per- UVF (UV Fluorescence), UVF254 (UVC light source), formed with an InGaAs camera (320×256 pixels) Merlin UVR (UV Reflected), IRFC (Infrared False Color), NIR by Indigo Systems. Some suggested references on IRF (IR Fluorescence), IRR (Infrared Reflectography), art documentation and examination using each spectral Figures 1 and 2. band and the relative instrumentation for Infrared There are a number of studies on the application of each [1,2,20-23] and Ultraviolet [24,25] are given. of the above mentioned imaging methods specifically for the identification of pigments: UV Fluorescence (UVF) Filters [9-12], UV Reflected (UVR) [13], Infrared False Color The filters chosen for use in this paper are commercially (IRFC) [14,15], Infrared Fluorescence [16-18] and Infrared distributed for photography and so they are easy to find. Reflectography [19]. Though, there is no comparative study Their transmission spectra are available on the manufac- carried out using all of those methods, and therefore this turers’ websites, Schneider Optics for the B + W filters, paper intends to fill that void. Maxmax.com for the X-Nite filter and Heliopan for the infrared filter. This is the filter set used for the MSI: a) Instrumentation For Ultraviolet Reflected (UVR) photography, the B + W Imaging devices 403 filter is used together with the X-NiteCC1. B + W The MSI images presented in this paper were acquired 403 allows just the UV light to pass, and X-NiteCC1 is with a Nikon D800 DSLR (36 MP, CMOS sensor) digital necessary to stop the IR produced from the UV lamp; b) camera modified for “full spectrum”, ultraviolet–visible‐ For Visible (VIS) photography, just the X-NiteCC1 filter infrared photography (between about 360 and 1100 nm). is sufficient; c) For UV Fluorescence (UVF) photography, The CMOS sensor responds both to the near infrared the B + W 420 must be mounted to stop the reflected and near ultraviolet ranges of the spectrum, however UV, and the X-NiteCC1 is also necessary to exclude any manufacturers install an IR cut-off filter in front of the infrared from the UV lamp; d) For Infrared (IR), Infrared sensor to reduce infrared transmission. There are com- Fluorescence (IRF) and Infrared Reflectography, just the panies that will remove this filter in commercial cameras Heliopan RG1000 is used. for a small fee, and then the camera is said to be “full spectrum”. The camera is tethered to a computer to Lenses allow sharp focusing in non‐visible modes (IR and UV) A Nikon Nikkor 50 mm f/1.8D AF lens was used for all using live view mode. The MSI images created with this the MSI photos on the pigment swatches. Standard pho- camera do not precisely image the same area in each tography lenses are generally fine for MSI. Indeed, they waveband because it is necessary to refocus the lens at are transparent from 350 nm to 1700 nm. Though, it is Figure 1 Illustration of the spectral bands, imaging methods and imaging devices which contribute to the flowchart described in this paper. Cosentino Heritage Science 2014, 2:8 Page 3 of 12 http://www.heritagesciencejournal.com/content/2/1/8 Figure 2 Madonna and Child, Ingels Collection, Sweden. Example of Multispectral Imaging documentation and the acronyms used in this paper. recommended to test the lenses for hot spots in the in- Calibration frared imaging. Hot spots can be caused by a number of For the camera settings and image processing of visible factors, such as coatings and/or the interaction between images it is recommended to follow normal color man- elements within the lens and the image sensor. Lists of agement procedures [2]. For the purpose of this work other lenses tested for hot spots are available online the American Institute of Conservation Photo Docu- [26]. It is also recommended to use fixed focal lenses mentation (AIC PhD) target [27] was used for calibra- and avoid complex lenses that are likely to give flares in tion of visible photography. The camera has been the infrared and ultraviolet photography. Also it is rec- calibrated with the X-rite ColorChecker Passport and its ommended to avoid telephoto lenses over 200 mm for bundled software. Due to differences in technologies and MSI photography, so that the lens will be fast enough to variables in manufacturing processes every camera cap- work with the low intensity emission produced in the tures colors a bit differently. Even two identical cameras UV Fluorescence and IR Fluorescence imaging. from the same company are a bit different. Even if the Raw processing software could have included a profile for the specific model camera, it is possible to get even Lighting better results by creating a profile specifically for the Halogen lamps are used for VIS and IR photography. RAW output of the particular camera.
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