d Repeatability

F or a lin e a r ly p o la r ise d input o f 6 0 ° , rep ea t r e a d in g s o f in t e n s it ie s w e r e ta k en ,

a n d t h e p o l a r i s a t i o n , u s i n g th e S t o k e s p o l a r i m e t e r w a s c a l c u l a t e d . T h is i s s h o w n in F ig u r e 6 - 3 b e l o w .

Reproducibility of LCTVD Stokes polarimeter for linear input polarisation along 60 deg.

■10 -100 -0 .7 ■0 6 -0 5 -0 .3 -0 .9 -0.1 -02 -0 3 -0 4 -0 5 - 0 6 75d e g -0 .7 -0 .9 O Input 60 d e g lin ea r 60 d e g 45 d e g

Figure 6-3 Variation of predicted input polarisation for repeat intensity measurements through LCTVD for input polarisation 60°.

These measurements were taken on a single day. The mean and standard deviation values are shown in Table 6-4. Sample size: 16.

a ° e P

Ideal value 60 Infinity 1

Mean 60.9177 10786.52 0.9972

Standard deviation 0.2855 13598.28 0.0095

Table 6-4 Mean and standard deviation values for a, e and P for the polarisations shown in Figure 6-3. Repeat readings for input linear polarisation of 60°.

It can be seen that there is little variation in values for a and P. The large standard deviation in the value for ellipticity is a result of the small denominator. The practical effect of this variation is best understood by referring to Figure 6-3. It can be seen that the points

Chapter 6 Algorithm to determine unknown polarisation

closely grouped, and hence represent similar polarisations. The actual values for ellipticity varied from 40852 to 693.

6.3.4. e Using single pixel LC cell

Following on from the success of the method using the LCTVD, the method was repeated using a single pixel TNLC cell. This cell had a similar LC material and thickness to the LCTVD, and had the advantage that it was cheaper and simpler to produce than the LCTVD. It also did not produce the diffraction and drive signal effects discussed in Chapter 5. It was driven with a 5GHz square wave from a calibrated function generator.

The results using the single pixel cell were not as good as using the LCTVD. This was because the total intensity transmitted by the cell (without an analyser present) varied with voltage, and the degree o f this variation depended on the polarisation of the incident light. To investigate this variation in intensity transmission polarised light was passed through the LC cell, and the intensity recorded by the photodiode, without an analyser present, was recorded. The voltage of the LC cell was varied. The results are shown in Figure 6-4, which shows the change in this total intensity with LC voltage for several incident polarisations. This variation in total intensity transmitted by the LC invalidates the assumption made in Equation 6-6 and this cannot be corrected without leaving more unknowns than equations.

V a ria tio n o f to ta l in te n s ity reading w ith s in g le pixel LC v o lta g e fo r d iffe re n t in c id e n t p o la risa tio n s . N o a n a ly s er present.

50 49 _Odeg 45 deg 90 deg 2 9.5 48 2 9 JS 47 Circular Input 46 28.5: 45 44 43 42 2 7 41 40 26.5 0 0.5 1 1,5 2 2,5 3 3.5 4 4,5 . 5 ? LC cell voltage (rms) V

Figure 6-4 Variation in total intensity transmitted by single pixel LC cell

Five different input polarisations are shown in Figure 6-4. The maximum degree of intensity change is 11.04% (of the maximum), when the polarisation of the incident light is at 45°, and

the minimum variation is 5.05% when the polarisation is 90°. The reasons for this variation will be discussed in the next section.

6.4

Reflection

Whenever there is a boundary between two layers of different refractive index some light will be transmitted, and some will be reflected. It has recently been reported that the total intensity transmitted by a pixellated parallel-aligned nematic LC SLM varies with LC voltage, even when the incident light is polarised along the extraordinary axis of the SLM. This variation can be nearly 25%, and is suggested as being caused by diffraction and interference effects. Similar effects have been observed with TNLCDs [Davis et al., 1999]. The single LC cell used in this thesis did not produce a diffraction pattern, so only the effect of interference upon reflection from the LC/ITO boundary is considered in this section.

Examination of the reflections produced from the single LC cell showed that one (or more) of the reflections was varying with cell voltage, and the degree of this variation depended on the input polarisation. To prove it was the reflection that was causing the change in transmitted intensity that is shown in Figure 6-4, an extra photodiode was inserted to intercept this reflection. As expected, it was found that the variation in transmission was opposite to the variation in reflection. The variation in total intensity (reflected + transmitted) was minimal: the same polarisations were tested as in Figure 6-4 and the maximum variation in the (maximum) intensity was only 1.26 %.

Chapter 6 Algorithm to determine unknown input polarisation