FIG. 1 diagrammatically illustrates a known stereoscopic display system comprising a stereoscopic display device in association with a complementary viewing device in the form of spectacles comprising two orthogonal polarisers intended to be disposed so that each polariser is positioned immediately in front of a respective eye of the viewer. The stereoscopic display device comprises a matrix of pixels half of which emit light polarised in a first direction and the other half of which emit light polarised in a second direction orthogonal to the first direction. For example the pixels may be arranged in columns and rows in the matrix, and alternate pixels or alternate rows or columns of pixels may be of the two different polarisation types. Thus the two eyes of the user see different images determined by switching of the pixels of the two polarisation types, and in this manner the viewer perceives a three dimensional scene. Ideally light pertaining to only one of the different images should be incident on each eye of the user over as broad a range of wavelengths as possible if the three-dimensional image is to be of the best quality.
Referring to FIG. 1, such a known stereoscopic display device may comprise a layer of liquid crystal material 1 confined between two transparent electrodes 2 and 3 one of which is shown divided into two regions A and B denoting two pixels of the device. Furthermore the electrodes 2 and 3 and liquid crystal material 1 confined therebetween are disposed between two linear polarisers 4 and 5 having polarising axes whose relative orientation is determined by the mode of switching of the liquid crystal material in the device. For example, the polarising axes of the polarisers are normally aligned perpendicular to one another in a twisted nematic liquid crystal display (TNLCD) device. As is well known, the liquid crystal material of the device acts as a pixelated optical shutter enabling the emission of light to be controlled on a pixel-by-pixel basis, and accordingly it is not necessary to describe the switching operation of such a device in detail.
In the particular arrangement which is diagrammatically shown in FIG. 1 the polariser 5, which is positioned in front of the liquid crystal material 1 and associated electrodes 2, 3, has its polarisation axis 11 disposed vertically. Furthermore an optical retarder in the form of a quarter wave plate 6 is positioned in front of the polariser 5 and comprises two differently patterned regions A' and B' having optic axes 12 and 13 oriented respectively at -45.degree. and +45.degree. to the polarisation axis of the polariser 5. Thus light transmitted by the pixel A emerges from the region A' of the quarter wave plate 6 circularly polarised in a right-handed direction, as shown by the arrow 7, and light transmitted by the pixel B emerges from the region B' of the quarter wave plate 6 circularly polarised in a left-handed direction, as indicated by the arrow 8.
In order to view a stereoscopic image, it is necessary to ensure that the viewer's left eye is only aware of light transmitted by pixels of one type, such as the pixel A, whilst being permanently unaware of the state of pixels of the other type, such as the pixel B, whereas the viewer's right eye is only aware of light transmitted by pixels of the other type, such as the pixel B, whilst being permanently unaware of the state of pixels of the one type, such as the pixel A. To this end, viewing spectacles with different polarising arrangements are placed immediately in front of the viewer's eyes, only the arrangement in front of the viewer's right eye 14 being shown in FIG. 1 for the purpose of illustration.
In this case the arrangement comprises an optical retarder in the form of a quarter wave plate 9 having its optical axis 15 parallel to the optic axis 13 of the region B' of the quarter wave plate 6, and a polariser 10 having its polarisation axis 16 orientated at 90.degree. to the polarisation axis 11 of the polariser 5. Thus the region B' of the quarter wave plate 6 and the quarter wave plate 9 having parallel optic axis 13 and 15 behave as a half wave plate with its optic axis orientated at +45.degree. to the polarisation axis 11 and disposed between the crossed polarisers 5 and 10, thus allowing the transmission of light from the pixel B to the viewer's right eye 14. On the other hand, since the region A' of the quarter wave plate 6 and the quarter wave plate 9 have crossed optic axis and are disposed between the crossed polarisers 5 and 10, light transmitted from the pixel A is cancelled almost completely at all wavelengths (assuming the quarter wave plates 6 and 9 are made of the same material so that they have the same dispersion characteristics) so that substantially no light reaches the viewer's right eye 14 from the pixel A. Such fully achromatic behaviour due to summing of the effect of optical retarders to zero retardation is known, for example, from U.S. Pat. No. 4,719,507.
Although not specifically shown in FIG. 1, it will be appreciated that the converse arrangement associated with the user's left eye comprises a quarter wave plate having its optic axis parallel to the optic axis 12 of the region A' of the quarter wave plate 6 and a polariser having its polarisation axis at 90.degree. to the polarisation axis 11 of the polariser 5. In this case such a quarter wave plate and the region A' of the quarter wave plate 6 having parallel optic axes behave as a half wave plate with its optic axis at -45.degree. between the crossed polarisers, and thus light transmitted from the pixel A reaches the viewer's left eye, whereas the crossed optic axes of the quarter wave plate and the region B' of the quarter wave plate 6 ensures that substantially no light from the pixel B reaches the viewer's left eye.
However the two sequential quarter wave plates which form a half wave retarder (unlike a zero wave retarder) only act in the required manner at a single wavelength, so that their transmission properties between crossed polarisers are chromatic, rather than achromatic. This can be seen from the graph of FIG. 2 showing the transmission level as a function of wavelength for the light received by the viewer's right eye 14 from the region B', after passing through the analysing polariser 10, and from the region A' of the quarter wave plate 6. Whilst the light from the region A' is cancelled substantially completely at all wavelengths, the light from the region B', after passing through the analysing polariser 10, is substantially unattenuated at only one wavelength (about 525 nm) whereas the light at over wavelengths is attenuated to a lesser or greater extent. Of course, the light received by the viewer's left eye will have a similar transmission profile but with A' substituted for B' and B' substituted for A' in FIG. 2.
Instead of having an achromatic dark state and a chromatic bright state as in the case of the graph of FIG. 2, it is possible for the device to be adapted to have a chromatic dark state and an achromatic bright state by providing a polariser 10 with a polarisation axis parallel to the polarisation axis 11 of the polariser 5. FIG. 3 is a graph of the transmission profile of the light received from the regions A' and B' of the quarter wave plate 6 by the viewer's right eye showing, as expected, the wavelength dependence of the light received from the region B'. Again the transmission profile of the light received by the viewer's left eye is similar with A' substituted for B' and B' substituted for A' in FIG. 3.
FIG. 4 shows a graph illustrating the light levels received from the regions A' and B' of the quarter wave plate 6 by the viewer's right eye, but with polycarbonate retarders being used to form a quarter wave plate 9 in each part of the viewing spectacles. Such polycarbonate retarders are more conventional, and thus less costly, for such use. The performance of the system is not greatly compromised by the use of polycarbonate retarders.
S. Pancharatnam "Achromatic Combinations of Birefringent Plates" Proceedings of Indian Academy of Sciences Vol. XLI, no. 4, Sec. A, 1955, pp. 130-136 and pp. 137-144 discuss the use of stacked uniform retarder films to improve achromaticity. However these papers are not specifically concerned with stereoscopic imaging systems.