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VSC6000/HS APPLICATIONS

Visualising Polarisation Features on Banknotes

Application NoteISSUE 1November 2014

VSC6000/HS: Visualising Polarisation Features on BanknotesISSUE 1November 2014

foster + freeman

There is a crossed linear polariser feature in the metallic ribbon on the 2010 Kazahstan 1000 Tenge note. Illuminatingthe ribbon with linear polarised light and viewing through another linear polariser at 90 degrees makes the featurevisible.

This is not something we can readily view in the VSC6000 but we could introduce linear polarisers into the VSC8000 ifneeded. However, the first step is to see what can be done with the VSC6000...

VSC6000Because the feature is on the metallic ribbon it is a specular reflection feature and so lighting geometry is important. Ifwe use coaxial illumination and put a circular polariser onto the feature so that we illuminate and view through the polariser, the feature is made visible (see figure 1). To avoid seeing a reflection of the coaxial illuminator the polariserneeds to be tilted. The highest contrast image is seen when the circular polariser is in its normal orientation and rotatingthe polariser makes no difference to the visibility of the bands.

However if we turn the circular polariser over (so it is now effectively a linear polariser) the pattern is still visible (with reduced contrast) but the visibility is now orientation dependent: if you rotate the polariser 90 degrees then the dark andlight bands swap over.

The National Bank of Kazakhstan issued a new 1,000-tenge note on 5 January 2010 to commemorateKazakhstan’s chairmanship of the Organization for Security and Co-operation in Europe in 2010.

Front: State Emblem of the Republicof Kazakhstan in left upper corner,decorative image of flying birds innational style, face value in Kazakhlanguage in the center, motives ofKazakh national patterns along thebanknote, holographic image ofBaiterek monument , color-changinglogo of National Bank of Kazakhstanon the right part

Reverse: State Flag of the Republicin the left lower corner, Akordapalace of President of the Republic ofKazakhstan in the center with facevalue in Russian language, image ofa bird and year of issue, numericalindication of face value in the leftlower part, motif of Kazakh nationalpatterns along the banknote.

VSC6000/HS: Visualising Polarisation Features on BanknotesISSUE 1November 2014

foster + freeman

Figure 1 Tilted circular polariser and coaxial illumination in the VSC6000

Figure 2 Images taken in the VSC6000 using right and left circular polarised lighting and a backwards circular polariser for viewing

It is also possible to use the left and right circular polarised light sources (see figure 2). If you use these then you mustuse the circular polariser in the backwards configuration (or use a linear polariser) and then the bands become visible. Byalternating between left and right circular polarisation the security pattern also switches. Because of the geometry of thelight sources the feature needs to be moved to near the edge of the zoomed out field of view to get the best image.Also note the red “1000” feature that is nicely revealed when using left circular polarised light.

VSC6000/HS: Visualising Polarisation Features on BanknotesISSUE 1November 2014

foster + freeman

VSC8000Using coaxial illumination and a circular polariser the feature is visible, the image is generally a bit nicer (see figure 3).Note that the polariser is not tilted in Figure 3 but the specular reflection of the coaxial illumination is just visible on theleft edge of the polariser. The appearance of the feature does not change if the circular polariser is rotated.

Figure 3 Image of the Kazkastan note under coaxial illumination, without and with a circular polariser.

If we use left and right circular polarised light sources, combined with the circular polariser held backwards then we getthe images in Figure 4. The LEDs are positioned in a better place on the VSC8000, making it easier to get a good image.By switching between the two types of circular polarisation we are able to move the switch the dark bands around.Alternatively we could use a single circular polarisation illumination type and by viewing through a linear polariser androtating it we can get the two feature states (see Figure 5). The image contrast is less good using this technique, morefiddly to achieve.Finally we could illuminate with linearly polarised light and by putting a linear polariser in the camera filter wheel viewthrough a linear polariser. Rotating the linear polariser by 90 degrees causes the dark bars to swap with the light bars(see Figure 6).There is also the option of fitting a fixed linear polariser in the camera filter wheel and using the 2 circular polarised illuminators to give the two feature states. The linear polariser must however be at the right angle.

Figure 4 Images taken in the VSC8000 using left and right circular polarised light sources and viewing through a backwards circular polariser

VSC6000/HS: Visualising Polarisation Features on BanknotesISSUE 1November 2014

foster + freeman

foster + freemansales@fosterfreeman.com Tel: +44 (0) 1386 768 050

Foster + Freeman Ltd. Vale Park | Evesham | WR11 1TD | United Kingdom www.fosterfreeman.com

Figure 5 Images taken in the VSC8000 using left circular polarised light and viewing through a linear polariser at two orthogonal orientations

Figure 6 Images taken in the VSC8000 using a rotating linear polariser in the linear spot illumination and a fixed linear polariser in the camera filter wheel.

Conclusions

This polarisation feature can be visualised with the VSC6000 using an additional circular polariser or a linear polariser. Inthe VSC8000 we could use the same techniques (that tend to give better images) or we could add a linear polariser to thecamera filter wheel to view the feature using either the already built-in circular polarised illuminators or by adding linearpolarised LEDs. The drawbacks with adding a linear polariser to the camera filter wheel would be that we would lose our“spare” slot and we would have to be careful to get the orientation of the polariser correct when it is mounted.