Academic journal article Heritage Science

Identification of Pigments by Multispectral Imaging; a Flowchart Method

Academic journal article Heritage Science

Identification of Pigments by Multispectral Imaging; a Flowchart Method

Article excerpt

Authors: Antonino Cosentino (corresponding author) [1]

Introduction

Multispectral imaging (MSI) [1, 2] and Hyperspectral Imaging [3, 4, 5, 6], have been suggested as methods for the non-destructive identification of pigments. Though, it is mandatory to point out that these methods are problematic and the user may be subjected to draw conclusions that remain uncertain, essentially, because pigments are often mixed and overlapped in layers to make the desired color and effect.

To identify pigments with an acceptable degree of certainty, at least one other material specific technique must be used to complement hyper or multispectral imaging diagnostics. The use of MSI to tentatively identify pigments has an important advantage justifying its application: the rapid and low-cost survey of large areas. The intention of this paper is to show that with a flowchart based methodology it is possible to tentatively identify some historical pigments by means of MSI performed with simplified equipment and without the aid of imaging analysis software. This method doesn?t claim to allow the identification of all different pigments, but it will work for those which present peculiar behaviors in the range of the electromagnetic spectrum readily observable with an IR-VIS?UV modified digital camera (360?1100?nm) and an InGaAs camera (900?1700?nm). In this way, selected pigments are likely to be identified by means of MSI examination. This simplified approach, though demonstrated to be limited in its analytical diagnostic capabilities, has the benefit of being accessible and easy to implement by professionals in the art conservation and examination field.

This method is more likely to succeed when applied on artworks where pigments have been applied in one single layer and not mixed; as is the case with miniatures [6, 7], drawings [8] and prints. Unlike other references, which suggest the use of software algorithms to analyze the MSI images, this paper proposes a more straightforward method simply based on visual examination and the use of a photo-editing software for the characterization of features apparent in the image.

Multispectral imaging

Imaging methods

This paper illustrates a flowchart method for pigment identification based on the acquisition of MSI images in 4 spectral bands: Ultraviolet, UV (360?400?nm); Visible, VIS (400?780?nm); Infrared, IR (780?1100?nm) and Infrared Reflectography, IRR (1000?1700?nm), Figure?1. The acronyms for the MSI methods presented in this paper highlight first the spectral band followed by R (Reflected), F (Fluorescence), FC (False Color). So the 8 imaging methods are called VIS (Visible), IR (Infrared), UVF (UV Fluorescence), UVF254 (UVC light source), UVR (UV Reflected), IRFC (Infrared False Color), IRF (IR Fluorescence), IRR (Infrared Reflectography), Figures?1 and 2.

Figure 1: Illustration of the spectral bands, imaging methods and imaging devices which contribute to the flowchart described in this paper. [see PDF for image]

Figure 2: Madonna and Child, Ingels Collection, Sweden. Example of Multispectral Imaging documentation and the acronyms used in this paper. [see PDF for image]

There are a number of studies on the application of each of the above mentioned imaging methods specifically for the identification of pigments: UV Fluorescence (UVF) [9, 10, 11, 12], UV Reflected (UVR) [13], Infrared False Color (IRFC) [14, 15], Infrared Fluorescence [16, 17, 18] and Infrared Reflectography [19]. Though, there is no comparative study carried out using all of those methods, and therefore this paper intends to fill that void.

Instrumentation

Imaging devices

The MSI images presented in this paper were acquired with a Nikon D800 DSLR (36 MP, CMOS sensor) digital camera modified for ?full spectrum?, ultraviolet?visible?infrared photography (between about 360 and 1100?nm). The CMOS sensor responds both to the near infrared and near ultraviolet ranges of the spectrum, however manufacturers install an IR cut-off filter in front of the sensor to reduce infrared transmission. …

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