Multispectral Imaging of Heritage Artefacts with VideometerLab 3

analytikLtd

VideometerLab 3 Multi-Spectral Imaging

Rapid Non-destructive Surface Analysis

Adrian Waltho, Analytik Ltd (Cambridge, UK)

adrian.waltho@analytik.co.uk

www.analytik.co.uk/multispectral-imaging

• Light reflectance at separate colours

• Statistical image processing

• Chemical, physical and spatial properties

• Differentiate | Quantify | Report

Traditional colour imaging uses three broad bands of colour:

Red, Green and Blue

Normal Colour Imaging

• RGB photographs have limited spectral resolution

• Chlorophyll a and b give almost the same RGB signal and are not spectrally separated

Chloro-a High Low High

Chloro-b High Low High

Multispectral Imaging

Chloro-a High Low Low High

Chloro-b Med High Med Low

• Using just 4 wavelength bands with tightly defined ranges, Chlorophyll a and b can easily be distinguished

• VideometerLab 3 uses 19 wavelength bands

Multispectral Imaging

• Many images obtained at selective wavelength bands

• Each image pixel contains spectral data points

• Spectral signature reveals chemo-specific information

• See spatial location of surface chemical variation

Ultraviolet Near-Infrared

Infra-Red Red Yellow Green Blue Ultra-Violet

VideometerLab 3 Schematic

• Narrowband illumination provided by 19 LEDs between 375nm-970nm (UV-Vis-VNIR)

• Integrating sphere diffuses light onto sample

• 2056x2056 CCD camera captures reflectance at each LED wavelength for each pixel

• Emission filter wheel for longpass/bandpass filtering

• Bright-field or dark-field lighting for translucency or diffuse scatter imaging

Camera

Integrating sphere

Emission filter wheel

Sample

• Precise lighting control for optimal illumination of each sample across whole spectrum

• Diffuse light on sample ensures even lighting from all directions

• Broadband camera spectrally and spatially calibrated (NIST)

• Emission filters enable multispectral reflectance and fluorescence imaging abilities

• Powerful, user-friendly image analysis and quantification software for data and modelling

Camera

Integrating sphere

Emission filter wheel

Sample

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Zoom area for next slide

Bodleian Library – Papyrus Hieroglyphics

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Same area at 850nm (infrared) illumination (false

coloured) – note smudge (fingerprint?) just above tear

in paper, invisible in RGB image

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nCDA transformation of same area

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Palimpsest sRGB image

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Close-up of zoomed area

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nCDA transform highlights original text

Historic Scotland – Pigment analysis

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• Circled are cadmium red and lead white pigment samples

• Circled is a mix of cadmium red and lead white

• The VideometerLab can build up a library of the spectral responses of each pigment

• The VideometerLab can then recognise this spectrum in unknown samples. We see that the pure cadmium red and the cadmium red mix are both ‘hits’ for the cadmium spectrum

• We can also see that the more pure spectrum registers as a closer fit than the cadmium-lead mix. We see this because the pure cadmium is much more orangey in the false-colour image

• This technique can be applied to paintings, for example this portrait of a captain from Leith

• By using false-colouration, we can see that the yellow pigment used on the captain’s hatband is the same (or very similar) to the yellow on his buttons, but different from the yellow on his braiding

Royal Armouries Fencing Manual

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This image shows the redness on the left fighter’s cheek is spectrally the same as the scribbling in the middle of the picture and the spot in the bottom left

Perhaps more importantly, it shows there is nothing else of a similar pigmentation in the whole image

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This image shows the redness on the left fighter’s cheek is spectrally the same as the scribbling in the middle of the picture and the spot in the bottom left

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The Videometer can prove this quantitatively as well – the next slide shows the spectral properties of the points in the center of the small crosses below

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You can see that 3 and 4 (teal and brown, page and sword) are significantly different from 1 and 2 in terms of their spectral profile.

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Visually, the Sten gun looks scratched arbitrarily. In fact, it is known there is an engraving on the side

Royal Armouries Sten Gun

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The Videometer can isolate an blow up this engraving

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The Videometer can also spectrally enhance the contrast

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Ordinary RGB imaging cannot do this - it is only possible to get the clarity of the enhanced image at 850 nanometers where the scratches start to respond differently to the metal under illumination, and RGB cameras only go up to around 700nm

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The Videometer can false-colour images to aid understanding

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It can also focus tightly on specific sections of the image to contrast them better – it is possible to improve signal to noise almost without limit at the expense of spending more time on the analysis

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ΧΡΙΣΤΗΦΗΣ ΗΛΙΑ

“Christofis Elias”

ΛΟΧΟΣ ΣΤ

“Sixth Company”

ΛΕΥΚΩΣΙΑ

“Lefkosia”

• The gun belonged to a Greek Cypriot named Elias Christofis who fought in World War II. (Lefkosia is the capital of Cyprus)

• Some of the decisions between similar looking letters might appear odd, but make a lot of sense when looked at multi-spectrally:

– For example, this letter looks like it could easily be a lambda or an aleph, but when viewed at 920 rather than 850nm, the line in the middle disappears

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• The translations were done by a Greek-speaker, who said that the transliteration of the third line that was given to her didn’t make sense. Its unclear if that means what Mr Christofis wrote was literally nonsense or if I transliterated it badly, but I would guess the latter

• This could be solved by a more guided transformation process – for example taking a more careful image of the third section of writing, finding out more about the sample so you can ‘guess’ letters and have the Videometer fill in the blanks or by a more complicated transformation calibration

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Museum Conservation Service

• Faded ‘Elizabeth R’ signature on official declaration of war communication document to overseas UK forces (sRGB image)

• Highlight training set of pixels to teach the software the spectral pattern of interest

• Faded ‘Elizabeth R’ signature on official declaration of war communication document to overseas UK forces

• Software scores every pixel in the image using the trained nCDA algorithm

• nCDA score determines pixel false colour value

• Signature’s spectrum is highlighted against all other dis-similar spectra on the page

• Faded Officer’s co-signature on official declaration of war communication document to overseas UK forces (sRGB image)

• Faded Officer’s co-signature on official declaration of war communication document to overseas UK forces

• Faded pencil inscription on the rear of a photograph of unknown British officer in 1858 (sRGB image, date circled)

‘1858’ written in pencil

• Faded pencil inscription on the rear of a photograph of unknown British officer in 1858

• Pencil spectrum is highlighted against all other dis-similar spectra on the page

• Suspected signature painted over in final composition

• Almost invisible in sRGB image

Multispectral Image Analysis

• A variety of image processing tools are available in VideometerLab’s analysis software

• Analysis protocols and models can be saved and applied to new samples for routine analysis

• Multiple analysis models can be run on the same image data to quantify separate parameters

• Analysis can be batch-run to aid in high-throughput data acquisition

• Image archiving provides a record of samples for future data mining and regulatory compliance

• Nuanced, human-like analysis of heterogeneous samples

• Objective, accurate, repeatable measurements

• Non-destructive analysis allows further sample assay work

• No sample preparation is required

• Rapid data acquisition and analysis enables high throughput screening

Why use Multispectral Imaging?

• Hardware options

• Emission filter wheel (4 x 25mm Ø) for fluorescence macro-imaging

• Dark- or Bright-field backlighting for scattering or translucency analysis

• Bespoke solutions available

• Software options

• Image Processing Toolbox for filtering and segmentation of image data in individual spectral bands

• Multispectral Imaging Toolbox for multivariate image analysis and modelling

• Blob Toolbox for granular object separation, analysis and data output reporting

• Matlab multispectral imaging toolbox integration

Hardware and Software Options

• Nuanced, human-like analysis of heterogeneous samples

• Sophisticated data analysis tools for the novice and power user

• Objective, repeatable data collection with easy export

• Rapid, non-destructive testing with no sample preparation

VL3 Multispectral Imaging

adrian.waltho@analytik.co.uk

Multispectral imaging at Analytik.co.uk

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