Original concept

It was initially envisaged that the mark would be illuminated with polarised light as shown in Figure 7-1 and two spatially varying voltage patterns would be written to the LCTVD. These voltage patterns would match the security mark and would change the phase difference o f the x and y electric field components of the incident light by two different amounts. It was hoped that these voltages would have the effect so that for one voltage the azimuth angle o f the resultant polarisation ellipse would be parallel to, and for the other voltage the azimuth angle would be perpendicular to, a fixed analyser, as shown in Figure 7-5.

These voltage patterns would be unique to the particular security mark, and would give a resultant maximum and minimum intensity transmitted by the analyser respectively. This would only happen for the true mark, and would allow it to be automatically verified or rejected. Figure 7-5 shows how such a security mark reader could be used.

@ 0 0 Unique pattern

^ D B D ^ applied to LCTVD to

D D B verify marking

Pattenied biréfringent securitv' Light

Polarisation absorbed/transm itted

producing different polarisations) polarisation pixel by pixel

Figure 7-5 Proposed use o f LCD to verify birefnngent security mark

It has been seen in Chapter 3 (s.3.4.4) that the LCTVD is too thin to achieve the full range o f phase change for a wavelength of 632.8nm. However, even if a full range o f phase change was available it would not be possible to change any input polarisation to linear polarisation at a specified angle purely by changing the birefringence o f the LCTVD (see s.7.3.2 below). To do this the input polarisation (but not the mark or the LC) would have to be rotated. One o f the advantages of using a liquid crystal device as a variable retarder is the lack of mechanically moving parts, so it is not desirable to have to rotate the input.

7.3.2

Flaws in original concept

Examination of equations 2-4 - 2-6 show that to obtain complete control of the azimuth angle of the polarisation ellipse and its ellipticity both the phase difference (between the X and y electric field components) (p, and the magnitudes of those components {a and b), must be controlled. If the input polarisation is fixed, a and b cannot be changed. For the light to be linearly polarised, (p must be a multiple of n. If co s^= l, equation 2-4 shows that the value of a is determined by the particular values of a and b. Without changing a and b, the best that can be hoped for is to induce an appropriate (p so that the azimuth angles of the output polarisation are aligned with (or orthogonal to) the chosen analyser angle. The ellipticity cannot be controlled as well, so (because the light is not linearly polarised) any minimum intensity measured will not be zero. The effect o f changing the birefringence o f a material on the polarisation of light passing through it, when the input polarisation state is constant, is shown in Figure 7-4.

Chapter 7. Security device: introduction and literature survey

If the light passing through the LCTVD cannot be converted to linear polarisation aligned with the analyser, the resultant intensity transmitted will not be the maximum possible. When the intensity is not at a maximum (or a minimiun) there are many different polarisations which can lead to the same intensity being transmitted by the analyser. The different values of azimuth angle and ellipticity of polarisation that would transmit the same intensity through a fixed analyser can be found from equation 2-25: the intensities are normalised so that a'^^b'^= \,so the relationship between h and y becomes

1- 26'

cos^ Y equation 7-1

Values of (0-<x) and ellipticity required to pro d u ce a c o n sta n t intensity through analyser. 100 n 9 0 - 8 0 - 7 0 - 6 0 -

?

Figure 7-6 Values of ellipticity and y needed to transmit a constant intensity (as a proportion of the incident intensity) through an analyser. Missing values of y are when cos^y>l. {j=(0-a)

where ^=angle of analyser relative to jc axis and a=azimuth angle of polarisation ellipse).

Figure 7-6 shows the graph of ellipticity against y for a various values of intensity (relative to input intensity of 1). It can be seen that the range o f values for the ellipticity and azimuth angle o f the polarisation ellipse, which can lead to the same intensity being transmitted by the analyser, depends on the particular transmitted intensity chosen. For example, if the desired intensity is 1, the only output polarisation which will achieve this is linear polarisation with the azimuth angle parallel with the analyser. If the azimuth angle or ellipticity changes from this, the intensity measured through the analyser will decrease (as

(equation 7-1 confirms this because when 7=1, the only real solution for b ’^ is zero. This occurs when c o s ^ /= l. A smaller value for cos^y gives a negative value for b

When 7<1, there are several values o f b ’^ and / which give solutions. An interesting case occurs when 7=0.5. At this position the solution for y is 45°, regardless of the ellipticity of the polarisation passing through the analyser. This occurs because, when the ellipse is oriented at 45° to the analyser, the analyser transmits equal components of the major and minor axes of the ellipse. The normalisation of the sum of the squares of the major and minor axes leads to the transmitted intensity remaining constant regardless of the ellipticity "i. Figure 7-6 shows lines of constant intensity that would be transmitted through an analyser. The range of ellipticities and azimuth angles that this represents can be read from the graph.

If the reading device could be arranged so that a near minimum or near maximum intensity was transmitted through the analyser, the range of polarisations emitted by the security mark that would cause this would be very restricted. This would mean that the reader could distinguish between a false and a true mark by simply registering this one intensity (as no other polarisation pattern would produce this result). However, as has been shown, this is not possible. Consequently, a false mark may transmit the same intensity as the true mark even though its polarisation pattern was quite different. This could lead to a false positive result.

The security mark would need to be verified using at least 2 LCTVD settings. This is to avoid having to measure the absolute intensity o f the source (as one reading is divided by the other to normalise it), and to avoid a false mark being made using simply a linear polariser oriented so as to give the required intensity. In fact, if this were not done, an opaque object could be used to give a minimum intensity.

7.3.3

Improved concept

As previously discussed, in order to fully distinguish between different states of polarisation which may be emitted by the security mark it is necessary to do more than just measure the intensity which is transmitted by an analyser. It was felt that, in order to analyse the transmission of each part of the security mark individually, a pixellated detector should be

From equation 2-25: I(y)=a'^cos^y+b’^sm^y If /=45® then cos^/=sin^/=0.5. Therefore I(y)=0.5{a’^+b’^'). As a ’^+b’^ is constant, the Intensity transmitted will be constant regardless of the individual values of a and b

Chapter 7. Security device: introduction and literature survey

used. This also had the advantage of enabling an image to be viewed and saved. It has already been shown that the Stokes vector of an incident beam can be fully characterised by passing the beam through the LCTVD at four separate grey levels, and comparing the four intensities transmitted by an analyser. It was felt likely that a similar method would work for verifying the polarisation pattern produced by the security mark. As uniform voltages were applied to the LCD, a pixellated LCD was not necessary, so the reader was designed using the single LC cell rather than the LCTVD (although comparison with the LCTVD was made). A single LC cell is considerably cheaper to produce than the LCTVD, and requires simpler drive electronics. It will be shown in the next chapter that it was possible to verify even the complex photographic slides by passing linearly polarised light through the security mark, LC device and analyser to the CCD detector (as shown in Figure 7-1). The mark was verified by analysing the resultant intensity pattern produced for each of four voltages of the LC device. The method is automatic, quick and simple, and the components of the reader can be obtained for modest cost. The reader was robust to image distortion such as rotation, and sensitive enough to distinguish between slides whose optical path length, {And) differed by only 25nm.

7.4

Relevant patents published

Published patents fall into two categories: those that describe a security mark, and those that describe a reading system. There are many patents relating to security marks, but very few which describe an automatic reading system.