Antonello – low-cost Multispectral Imaging (MSI) System
Conservators and art historians need non-invasive methodologies to identify and map pigments on works of art and archaeology. These tools allow them to select appropriate conservation procedures, acquire information on the workshop practices, distinguish original sections from inpaints and to enhance visualization of faded pigments and inks.
Antonello is a simple system composed of 18 bandpass filters and a full spectrum DSLR camera, covering the 400-925 nm spectral range. Using a DSLR camera rather than a monochromatic scientific camera has the advantage that the same camera can be used for other technical photographic methods, making the overall imaging equipment lighter, compact and affordable.
We developed Antonello to spread the use of this methodology across Art Conservation professionals and institutions.
- compact and lightweight
- easy to understand, tune up and upgrade
- cover the 400-920 nm spectral range
- can be used with digital cameras modified “full spectrum” to cover the 360-1100 nm range.
The filters set
The system is composed of 18 bandpass filters whose labels clearly indicate their center wavelengths and their order number. This is the list of filters (center wavelength (nm)): 405, 430, 450, 467, 480, 500, 532, 560, 580, 610, 640, 671, 700, 730, 760, 840, 860, 920. The 18 filters are not equally spaced but have the same bandwidth (10 nm). The center wavelengths of these filters have been chosen in order to better represent the spectral features of historical pigments and they correspond to specific absorption or inflection points.
Whites. Titanium white has a strong UV absorption band (filter at 405 nm) that goes until the violet region and distinguishes it from the other white pigments. Lithopone features the absorption bands (filters at 670 nm and 725 nm) in the red- infrared region due to its zinc sulfide component .
Blues. Prussian blue and cobalt violet have maxima in the blue region (respectively, filters at 450 nm and 700 nm). Maya blue and indigo have inflection points in the infrared (respectively, filters at 760 nm and 840 nm).
Smalt features a maximum in the blue region (filter at 430 nm) and a large absorption band in the yellow region (filter at 580 nm). Phthalo blue has a maximum in the blue region (filter at 465 nm) and an absorption band in the infrared (filter at 920 nm). Cobalt blue features an inflection point in the green region (filter at 525 nm). Azurite and ultramarine large maxima in the blue region are covered by all the 5 filters already mentioned from 405 nm to 480 nm.
Greens. Malachite, verdigris and phthalo green have their maxima in the green region (respectively filters at 525 nm and 500 nm). Viridian has its inflection point in the infrared (filter at 760 nm). Green earth has weak absorption bands which cannot be resolved with this system and consequently it will produce a flat spectrum. Cadmium green is identified by its infrared fluorescence and no filter was selected for its spectral features. Cobalt green maximum is represented by the filter at 532 nm. The filter at 405 nm outlines the chrome green maximum in the violet region.
Yellows. Cadmium yellow has its sharp inflection point in the blue-green region (filter at 480 nm) while cobalt yellow has a week absorption band in the red region (filter at 610 nm). Orpiment and saffron have their inflection point outlined by the filter at 532 nm. Lead tin yellow I has a sharp inflection point covered by the filter at 467 nm. Lead tin yellow II, naples yellow and gamboge inflection points are represented by the filter at 532 nm. Yellow and red ochre as well as sienna show an S-shape spectral feature with maxima at 600 nm and 770 nm and minima at 645 nm, 860 nm and 915 nm (respectively filters at 610 nm, 760 nm, 640 nm and 920 nm). Massicot has an inflection point at 450 nm (filter at 450 nm) and yellow lake reseda at 490 nm (filter at 500 nm). Realgar has its inflection point at 560 nm and a filter was added to cover it.
Reds. Vermilion, cadmium red, alizarin and madder lake have inflection points at 600 nm (filter at 610 nm). Lac dye spectrum is heavily dependent on the binders with an inflection point varying widely and so no specific filter was added for it. Carmine lake has an inflection at 630 nm (filter at 640 nm). Red lead has its inflection point at 575 nm (filter at 580 nm).
Browns and black. They don’t have spectral features useful for their identification.
|405||Titanium white absorption; chrome green maximum|
|430||Smalt, malachite maxima|
|450||Prussian blue maximum, massicot inflection|
|467||Phthalo blue maximum (465), lead tin yellow I (465) inflection|
|480||Cadmium yellow inflection|
|500||Verdigris, phthalo green maxima; yellow lake R. (490) inflection|
|532||Cobalt blue inflection; cobalt green maximum; orpiment, lead tin yellow II (525), naples yellow (515), gamboge (535) and saffron inflections|
|580||Red lead (575) inflection|
|610||Cobalt yellow absorption (615); ochre (600) maxima, vermilion, cadmium red, alizarin, madder lake inflection|
|640||Ochre (645) minimum; carmine lake (630) inflection|
|671||Lithopone absorption (670)|
|700||Cobalt violet maximum|
|730||Lithopone absorption (725)|
|760||Maya blue, viridian (770) inflections; ochre (770) maximum|
|840||Indigo inflection (830)|
|920||Phthalo blue, ochre (915) minima|
Table 1. 18 filters center wavelength and the respective pigments’ spectral features that they outline.
MSI calibration card
Commercial gray cards for photography cannot be used with Antonello since they absorb near UV and violet radiation and cannot be used for the filter at 405 nm. The MSI calibration card covers the 400-925 nm spectral range. It has 6 swatches, pure white, 4 grays and pure black.
Splitting raw images
The photosensors of each pixel of a DSLR camera cannot distinguish the wavelength of the incoming light and they are covered with a CFA; tiny color filters placed over the pixels to select only red green or blue light. The CFA on the Nikon D800, as on the majority of digital cameras, implement the Bayer CFA scheme. The raw image is converted to a full-color image by a demosaicing algorithm run by the on camera image processor. This image cannot be used to reconstruct reflectance spectra. The raw images are split into their 4 (BGRG) components using ImageJ (Fiji distribution) and the dcraw plugin. Based on the spectral response of the Bayer filter the split images are selected among the B channel in the range 400-480, G for 500-560 and R for 580-920.
Registration is the operation of aligning and scaling in order to make all the images overlap to each other. Registration is performed within ImageJ (Fiji). The 18 spectral images are registered taking the first spectral image as a reference for all the others (differently than progressive methods which register one image to the previous).
Our Research on Multispectral Imaging
|Publications on Multispectral Imaging|
|A. Cosentino “Imaging Multispettrale low-cost con filtri interferenziali” Archeomatica, 2, 12-17, 2015..|
|A. Cosentino “Multispectral imaging and the art expert” Spectroscopy Europe, 27 (2) 6-9, 2015.|
|A. Cosentino “Multispectral Imaging of Pigments with a digital camera and 12 interferential filters” e-Preservation Science, 12, 1-7, 2015..|
|A. Cosentino “Panoramic, Macro and Micro Multispectral Imaging: An Affordable System for Mapping Pigments on Artworks” Journal of Conservation and Museum Studies, 13(1): 6, 1–17, 2015|
|A. Cosentino “Multispectral imaging system using 12 interference filters for mapping pigments” Conservar Património 21, 25-38, 2015.|
|A. Cosentino “Crowd funded research: low-cost multispectral imaging” RECH3, International meeting on Retouching of cultural heritage. 2015.|
|A. Cosentino “Transmittance spectroscopy and transmitted multispectral imaging to map covered paints” Conservar Património 24, 37-45, 2016.|