Multispectral Imaging System 2017-05-25T17:39:20+00:00

Antonello – low-cost Multispectral Imaging (MSI) System

In Brief

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.

Antonello, MSI system. The simplest tool for multispectral imaging for Art! It has 3 main components: 18 bandpass filters, a filters adapter, and the MSI calibration card.

The labels on each filter indicate the center wavelength and the order number.

Filters adapter for lenses 52 mm thread.

DVD “Multispectral Imaging Software and demo data”

Features

  • user-friendly
  • 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.

Screw the filter adapter on the lens (52 mm filter thread). Slide in the filters. The adapter holds them in position. Just gently push them out when you are done shooting. The mirror-side of the filters must be facing the scene.

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 [22].

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.

 

Center

Wavelength (nm)

notes
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
560 Realgar inflection
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)
860 Ochre minimum
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.

A) MSI calibration card (large lab version). B) MSI calibration card gray swatches reflectance spectra (gray values). C) Reflectance of white swatch of MSI calibration card and of two color checkers for professional photography showing the absorption band in the violet region.

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.

Pigments Checker. Raw format spectral images and calibrated split components (gray values).

Registration

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).

Components

Full Spectrum modified Digital Camera (Nikon D800) (Not Included)

  

This camera is sensitive approximately to the 360-1100 nm range and it is the same camera we use for the Technical Photography methods, RTI and 3D Photo modeling. We save money and we make our Imaging Diagnostic equipment compact and lightweight.



HINT. Just one camera for all the Imaging Techniques!

18 Filters set



We use 18 filters just 1”diameter. Since they are small, they cost less. They have the same bandwidth (10 nm) but we chose the center wavelength of each filter based on historical pigments' spectral features.



HINT. You can add as many filters you want. You can tune up this system for your specific research goals.

  

Filters set: 3350 euro + 20 euro (shipping and handling)
Filters set Travel BOX



We 3D printed a travel box to host the 18 filters.



HINT. Compact and sturdy, it keeps the filters in the right order.

  

Filters set Travel BOX: 190 euro + 20 euro (shipping and handling)
MSI Calibration Card



Commercial gray cards for photography cannot be used as in-scene references for multispectral imaging since they absorb near UV and violet radiation. We developed a gray card to cover the 400-1000 nm spectral range.



HINT. It comes free with Pigments Checker v .2.1 and later.

  

MSI Calibration card: 130 euro + 20 euro (shipping and handling)
1" Filters adapter



We 3D printed an adapter to mount the bandpass filters on any photographic lens.



Filters adapter : 310 euro + 20 euro (shipping and handling)
Software



A complete workflow for spectral images calibration, registration, and pigments mapping.



 

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.