Seeing in a new light

We delve into the newest applications for multispectral imaging

Multispectral imaging provides valuable information on the quality and safety of a vast array of materials from pharmaceuticals to raw meat and burned biscuits. The same imaging techniques can be used to measure skin sensitivity to sticking plaster, detect counterfeit drugs and packaging and gain insight into historical artefacts such as mediaeval manuscripts and weapons. Some of the newest applications in multispectral imaging converge as they all in some way have parameters measureable by the technique such as colour, texture, gloss, shape and size which when combined give previously unseen information.

[caption id="attachment_27548" align="alignright" width="200" caption="Figure 2: Spectra from three genuine and one counterfeit (blue) samples"][/caption]

The non-destructive investigation of materials with non-uniform colour and texture can be difficult, tedious and expensive. Conventional techniques such as NIR spectroscopy only measure a single point or average over a fixed area and do not give an objective overall assessment of visual quality. Multispectral imaging can be described as a trade-off sacrificing spectral resolution to increase spatial information giving a ‘snapshot’ of the combined bulk properties of a sample, handling natural variation and inhomogeneity.

[caption id="attachment_27530" align="alignleft" width="200" caption="Figure 3: Left: RGB image of counterfeit (left) and genuine tablet, visible differentiation is difficult. Right: Multispectral false colour image of the same samples, orange is genuine"][/caption]

Traditional colour imaging uses three broad bands of colour red, green and blue and is known as RGB imaging. As a consequence of the broad bands RGB imaging has very limited spectral resolution and is unsuited to differentiating samples showing variation within a single broad band. Multispectral imaging refers to multiple wavelengths over the whole range from UV through visible to NIR (230 to 1050 nm). VideometerLab 2, a lab based multispectral imager from the Danish based company Videometer A/S, is based on a high-intensity integrating sphere illumination featuring light emitting diodes (LED) and a black and white high resolution CCD camera (2056x2056 pixels) (Figures 1 and 2). Measurements are combined at up to 20 different wavelengths into a single high resolution multispectral (2056x2056x20) image with every pixel in the image representing a spectrum.

[caption id="attachment_27532" align="alignright" width="200" caption="Figure 4: Textured logo on counterfeit packaging breaks down in the NIR (bottom left area) whereas the genuine packaging remains consistent (bottom right)"][/caption]

The choice of illumination wavelength ensures each application can be optimised and is not restricted to the wavelengths spanned by traditional RGB technology. Using LEDs in the UV or NIR adds information not visible to the human eye. As an example, most objects are white or transparent in the NIR region, which allows for the separation of the colour and surface properties of the measured object. Uniform and non-uniform samples alike are simply placed in the target area and custom designed PC software for data capture and analysis means results are available in less than 10 seconds; including sample handling time. A radiometric and geometric calibration procedure with NIST traceable standards is available to ensure accuracy and repeatability, and automatic diagnostic tests can be performed routinely to ensure instrument stability.

[caption id="attachment_27534" align="alignleft" width="200" caption="Figure 5: Top: visible images of skin before and after repeated application and removal of sticking plaster. Bottom: VideometerLab 2 images"][/caption]

Anti-counterfeiting

The presence of counterfeit pharmaceutical products poses an obvious hazard to human health.  Counterfeits may take the form of missing or incorrect active pharmaceutical ingredient, re-packaged authentic or re-packaged expired product. Characterising an incorrectly formulated counterfeit product can be achieved entirely non-destructively and in a few seconds using multispectral imaging.

Figure 3 shows spectra from three genuine and one counterfeit sample. The instrument response of the three genuine samples is similar across all illumination wavelengths whereas the counterfeit is clearly differentiated. Data treatments based on these differences can be applied automatically, counterfeit products are shown below as blue, genuine as orange. Figure 4 shows and RGB image of counterfeit and genuine tablets, plus a multispectral false colour image of the same sample.

[caption id="attachment_27536" align="alignright" width="200" caption="Figure 6: Top: skin and hair. Middle: hair only. Bottom: skin with hair shown as average skin colour"][/caption]

Multispectral imaging techniques can also be used to identify counterfeits through the analysis of their packaging. Although visually it may be difficult to detect counterfeit packaging it is a trivial matter to automatically compare the shape and size of characters, trademarks, company names and logos on good/bad packaging, and specialised inks and texture effects on authentic packaging are simple to detect (Figure 5) when imaged. Subtle changes in size, shape, colour, and markings can all be combined and used to automatically detect counterfeits often without removing the product from the original blister packaging.

Dermatology

Colophony, a plant extract used in the manufacture of sticking plaster, can cause skin irritation. Recently, multispectral imaging has been used to detect skin irritation caused by band aids even before the ‘redness’ became apparent to the patient. By simply imaging skin before and after the sticking plaster was applied and removed a clear ‘redness’ due to the removal of skin and the presence of haemoglobin was apparent (Figure 6). Expanding the study showed a clear correlation between application/removal count and irritation. This objective measure of the degree of skin irritation is useful as it allows for faster and more reliable skin testing.

[caption id="attachment_27537" align="alignleft" width="200" caption="Figure 7: Testing the ‘staying power’ of lipstick. Lipstick on skin Left: daylight view Right: VideometerLab 2 image."][/caption]

Multispectral imaging is also useful in studying skin pigmentation both in making a diagnosis and in assessing the effectiveness of treatment. Since the presence of hair can make some diagnoses difficult NIR wavelengths can be used to separate hair and skin image contributions. In Figure 7, the hair is ‘replaced’ by average skin colour allowing subtle changes in skin condition to be monitored more easily.

Cosmetics

The efficacy of topical pharmaceuticals, cosmetics, skin care products and sunscreens can also be evaluated using multispectral imaging as shown in the wear testing of (Figure 8). In summary, it is possible to objectively measure the resistance to smearing of lipsticks applied to the skin and then repeatedly wiped off. The resistance of long wearing products can be tested objectively and the point determined when the product would require re-application.

[caption id="attachment_27539" align="alignright" width="200" caption="Figure 8: Lipstick wear study."][/caption]

Figure 9 shows a lipstick wear study. A single application of lipstick on skin is wiped off repeatedly as multispectral images were taken. As the product is removed the band at position 1 on the x-axis is shifted towards -0.5. New applications of lipstick can also be monitored (the band at position -0.5 shifts towards 1 as applications increase – not shown). It is then trivial to calculate intensity per pixel and to predict when re-application is required. Examples showing sunscreen and foundation cosmetics are shown below in Figure 10.

[caption id="attachment_27541" align="alignleft" width="200" caption="Figure 9: Various sunscreen and foundations images. Top: immediately after application Bottom: after repeated wear testing"][/caption]

Forensics

The examination of fingerprints on banknotes requires a method to make prints visible before comparison; treated prints are viewed as purple in colour. Unfortunately, this is not a useful methodology for all notes since UK £20 banknotes are purple and treated prints are still not well distinguished. By employing various data treatments and noise reduction techniques the VideometerLab 2 can be used to highlight very faint prints even on red/purple backgrounds (Figure 11).

The RGB image shows a main print and a very faint secondary print to the left. This faint print starts to become visible as the contrast is enhanced using 630 nm (orange) illumination. Transforming with a Minimum Noise Fraction analysis then automatically contrasts regions of interest, and finally normalised Canonical Discriminant Analysis shows the greatest contrast.

[caption id="attachment_27542" align="alignright" width="200" caption="Figure 10: A single set of fingerprints shown after various data treatments. From left to right; RGB image; image at 630 nm; MNF; nCDA"][/caption]

The forensic examination of documents such as disputed wills and altered cheques for potential fraud can be achieved by careful analysis of materials such as ink and paper as well as the individual characters in handwritten, type and commercial printing.

Historical Artefacts

In the study of historical artefacts it is essential that the analysis techniques used should be non-destructive. Multispectral imaging offers a non-invasive, non-destructive method for the investigation of artefacts such as historical parchments and engraved metal objects. Figure 12 shows a Tudor Rose emblem on a knife. A careful selection of wavelengths and false colour images will be evaluated to bring clarity. By focusing on small sections it is possible to further enhance the contrast. Figure 13 shows text from the ‘Elder Westrogothic Law’, the oldest complete extant manuscript in Swedish which was made readable using multispectral imaging. Analysis of four partially or largely unreadable pages was possible using the VideometerLab 2.

[caption id="attachment_27544" align="alignleft" width="200" caption="Figure 11: A Tudor Rose emblem on a knife comes into view"][/caption]

There is incredible variety in the applications of multispectral imaging using the VideometerLab 2. Developed initially to support food technologists ensure the quality and safety of processed foods it does not require highly skilled operators, results are intuitive, it is fast and very easy to use. As a result multispectral imaging can now be found in the most unlikely places such as museums and hospitals as well as forensics departments and anti-counterfeiting labs. The ability to see the unseen, and to find the important detail ensures the continued rapid uptake of multispectral imaging.

[caption id="attachment_27545" align="alignright" width="200" caption="Figure 12: Left: RGB image Right: VideometerLab 2 image."][/caption]

The Authors Heather Murray, Applications Scientist, Alex Bates, Product Specialist and Tom Greenwell, Marketing Manager of analytic

Contact: t: + 44 (0) 870 991 4044 e: tom.greenwell@analytik.co.uk w:  www.analytik.co.uk