WO1997043688A1

WO1997043688A1 - Colour selective filters

Info

Publication number: WO1997043688A1
Application number: PCT/GB1997/001252
Authority: WO
Inventors: Stephen Paul Mcgeoch
Original assignee: Barr & Stroud Limited
Priority date: 1996-05-10
Filing date: 1997-05-08
Publication date: 1997-11-20
Also published as: GB9609737D0; AU2707997A

Abstract

A colour selective filter comprises neutral linear polarisers (6, 7) and continuously tunable waveplates (A, B) arranged in alternating sequence along an optical axis (OA). At least two waveplates (A, B) are provided.

Description

COLOUR SELECTIVE FILTERS

The present invention relates to colour selective filters and in particular to colour selective filters which are continuously tunable across a range of colours.

Colour filters are used in many applications ranging from the entertainment industry, for example scenery lighting, to electronic display devices and cameras. Colour filters range from basic coloured plastic sheets to complex electrically tunable filters which allow the colour passed by the filter to be selected by an electric control signal .

The human eye is capable of perceiving not only monochrome or single wavelength light, e.g. green and blue, but can also perceive complex mixtures of light wavelengths such as give rise to the colours magenta and orange. It is desirable that a tuneable colour selective filter can be tuned to pass not only a wide range of monochrome colours but also a wide range of the complex colours which the human eye can perceive.

One known type of electrically tunable colour filter is a liquid crystal cell comprising an aligned liquid crystal sandwiched between two glass plates . The glass plates are each provided with an electrode so that a voltage can be applied across the liquid crystal to alter the alignment of the liquid crystal. The liquid crystal cell is in turn sandwiched between a pair of crossed or parallel polarisers . The transmission of the filter varies harmonically with the inverse of wavelength. For example, where the polarisers are parallel polarisers, the transmittance spectrum T(λ) is described by the equation ;

T(λ) <* cos² [2 π dΔn { V) / λ ]

where λ is the wavelength, d is the thickness of the liquid crystal layer, and Δn is the effective birefringence of the liquid crystal which can be tuned by varying the voltage V applied to the cell.

The range of birefringence over which the cell can be tuned provides a corresponding range of states each of which permits the transmission through the filter of a different colour of light. However, the range of states is extremely limited and filters of this type do not provide a range of colours which is sufficient, for example, to provide a good quality colour display Indeed, because of the cos-squared dependence of the transmission spectra, filters of this type cannot easily be tuned to transmit substantially monochrome colours. An alternative electrically tunable colour filter is described in US5,347,378 and comprises a pair of surface stabilised ferroelectric liquid crystal (SSFLC) devices. Arranged m sequence along the optical axis of the device are- a first pleochroic polariser which passes vertically polarised blue and green light and horizontally polarised red light, a first of the SSFLC devices,- a second pleochroic polariser which passes vertically polarised green and red light and horizontally polarised blue light; the second SSFLC device; and a third polariser which is an ordinary neutral polariser which passes vertically polarised light of all colours. The SSFLC cells are bistable devices in which the ferroelectric liquid crystal can be switched between a state m which the crystals are vertically aligned and a state in which the crystals are aligned at an angle of 45° to the vertical. When both SSFLC devices have their crystal axis aligned vertically, the filter transmits green light. Switching the crystal axis of the first SSFLC device by 45° causes the filter to transmit red light. With the crystal axis of both the first and second SSFLC cells switched by 45°, the filter transmits blue light. It will be apparent that at any instant in time, the filter of US5,347,378 can transmit only red, blue or green light However, by rapidly switching the filter between these three colour transmission states, an observer is able to perceive other colours resulting from the mixture of the three primary colours . It is an object of the present invention to overcome or at least mitigate certain of the disadvantages of known electrically tunable colour filters.

It is a further object of the present invention to provide a colour selective filter which can be used to generate a wide range of colours from a broadband or white light source.

According to the present invention there is provided a colour selective filter comprising m sequence along an optical axis of the filter; a first linear polariser; a first continuously tunable waveplate; a second linear polariser; a second continuously tunable waveplate; and a third linear polariser, wherein said first, second and third linear polarisers have their polarisation axes aligned substantially parallel or substantially perpendicular to one another and are neutral over the waveband of interest.

The term 'waveplate' is used here to mean an optical element which produces a phase shift between the ordinary and extraordinary waves propagated through it.

By providing two continuously tunable waveplates in combination with neutral linear polarisers, the present invention enables a wide range of colour transmission states to be achieved, covering a significant proportion of the visible colour spectrum when the linear polarisers are neutral over the waveband of the visible spectrum.

In order to further extend the range of colours within the visible spectrum, the number of continuously tunable waveplates may be increased to three or more, with each waveplate being separated by a neutral linear polariser and with neutral linear polarisers arranged at each end.

Preferably, the waveplates are liquid crystal cells m which liquid crystals are aligned m the unbiased state at an angle to the polarisation axis of the polarisers, for example at an angle of 45°. When a bias voltage is applied to the liquid crystal cells, the alignment angle of the liquid crystal cells is changed, generating a corresponding change in the birefringence of the cell More preferably, the liquid crystal cells comprise non-twisted nematic liquid crystals although other suitable continuously tunable liquid crystals may be used. For example, certain forms of smectic or cholesteπc liquid crystals are continuously tunable. Examples of suitable nematic crystals include E44 and E7™ manufactured by Merck™

As an alternative to liquid crystal cells, the waveplate may comprise some other material having a birefringence which is continuously electrically tunable. A suitable material is lithium niobate .

The colour selective filter may be provided by a pair of filter units each comprising a continuously tunable waveplate sanαwiched between a pair of linear polarisers which are neutral over the waveband of interest, with the two filter units being aligned side by side along the optical axis, said second linear polariser referred to previously being provided by the two innermost or confronting polarisers of the filter units . The linear polarisers used in the present invention are neutral over the waveband of interest and enable colour variation within that waveband to be achieved. The waveband of interest could be limited to only a portion of the visible spectrum, for example that portion containing only red tones or hues.

For a better understanding of the present invention and in order to show how the same may be carried into effect reference will now be made, by way of example, to the accompanying drawings, in which- Figure 1 shows a longitudinal cross-section through a colour selective filter embodying the present invention,

Figure 2 shows a typical transmittance spectrum of a conventional colour selective filter unit in one operating state; Figure 3 shows the chromaticity range of a conventional colour selective filter unit,

Figure 4 shows transmittance spectra of the colour selective filter of Figure 1 for four operating states; and Figure 5 shows the chromaticity range of the colour selective filter of Figure 1.

There will now be described with reference to Figure l a continuously tunable colour selective filter. The filter comprises a pair of liquid crystal cells or filter units indicated generally by the reference letters A and B and lying in a plane at right angles to the optical axis OA of the filter. Each cell A, B is known per se and comprises a layer of non-twisted nematic liquid crystal 1 sandwiched between a pair of glass plates 2, 3. On the surface of each glass plate 2, 3 which faces the liquid crystal layer 1 is an electrode layer of indium tin oxide 4 which provides a light-transparent electrical conductor. On the surface of each electrode layer 4 there is provided a further layer of polyimide 5 which is treated to cause the contacting liquid crystal layer 1 to align with a predetermined orientation when there is no voltage applied to the electrodes, i.e. in the unbiased state. Typically, the polyimide 5 is treated by rubbing its surface with a velvet brush, causing the liquid crystals to align at an angle of around 45° to the vertical.

Provided on the outwardly facing surface of each outer glass plate 2 is a neutral linear vertical polariser 6 which may be a plastic sheet adhered to the glass plate. A suitable polariser material is NITTO G1220™. Sandwiched between the two inner most glass plates is a further neutral linear polariser 7 which is also vertically aligned. This inner polariser 7 may be provided by a single polariser plate or may be composed of two separates polariser plates, e.g. plastic sheets, abutted together.

The device of Figure 1 effectively provides two continuously electrical tunable filter units in series, such that the transmission of the complete device is proportional to the product of two cos-squared terms, i.e. T( λ ) « cos² [ 2 π d_AΔn_A ( V_A) / λ ] . cos² [ 2π d_BAn₃ ( V_B) / λ ]

where the subscripts A and B identify the two filter units. Figure 2 shows a typical transmittance spectrum for only one of the filter units A or B at a selected operating state, i.e. where the liquid crystal cell bias voltage is fixed, and illustrates the cos-squared variation discussed above. By varying the bias voltage the transmittance spectrum is correspondingly varied such that the filter can be tuned across the chromaticity range 8 illustrated in Figure 3. Figure 3 is a CIE, international standard, chromaticity diagram on which can be identified colours which the human eye can perceive. The y-axis corresponds to the normalised intensity of green light and the x-axis corresponds to the normalised intensity of red light. The normalised intensity of the third primary colour, blue, is given by one minus the sum of the normalised intensities of red and green light and so all perceivable colours can be represented on the two-dimensional chromaticity diagram. The outer curve indicated by reference numeral 9 in Figure 3 corresponds to the chromaticity of monochromatic light which is in fact perceived by the human eye as a complex colour because there is some residual stimulation by monochromatic light of receptors other than receptors specific to that light. All colours within the curve 9 correspond to colours produced by complex light spectra and which can be perceived by the human eye. It will be apparent that the single filter unit arrangement of the prior art offers only a very limited selection range (i.e. curve 8) within the overall perceivable colour range (curve 9) .

Using the dual filter arrangement illustrated in Figure 1, it is possible to obtain more complex transmittance spectra (i.e. non-sinusoidal) than are achieved using a single filter unit arrangement because, as can be seen from the above equation, both amplitude and phase modulation of the transmission spectrum can be achieved. Figure 4 illustrates four such transmittance spectra obtained at four different operating (bias) states of the device. These four states correspond to the transmission of magenta, blue, orange, and green light. Figure 5 shows the chromaticity range 10 of the filter of Figure 1 which can be contrasted with the chromaticity range 8 for the single filter arrangement shown in Figure 3. The data shown in Figure 5 was obtained by varying the bias voltage applied to a first of the filters A, B in discrete steps whilst, at each said step, varying the voltage applied to the second of the filters in discrete steps across the entire range.

It can be seen from Figure 5 that the dual filter arrangement can be tuned to transmit a much greater variety of colours that is possible with a single filter unit arrangement. Furthermore, the dual filter arrangement can be tuned to transmit a greater range of substantially monochrome colours, for example green and blue as shown in Figure 4, because the tunable range 10 approaches the outer curve 9.

It will be appreciated that various modifications may be made to the above described embodiment without departing from the scope of the present invention. For example, three or more filters units may be arranged in series to further increase the range and choice of selectable colours .

Claims

1 A colour selective filter comprising in sequence along an optical axis (OA) of the filter; a first linear polariser (6) ; a first continuously tunable waveplate (A) ,- a second linear polariser (7) ; a second continuously tunable waveplate (B) ,- and a third linear polariser (6) , wherein said first, second and third linear polarisers (6,7) have their polarisation axes aligned substantially parallel or substantially perpendicular to one another and are neutral over the waveband of interest .

2 A colour selective filter as claimed in Claim 1, wherein the waveplates (A,B) are liquid crystal cells in which liquid crystals are aligned in the unbiased state at an angle to the polarisation axis of the polarisers (6,7) .

3 A colour selective filter as claimed in Claim 2, wherein the liquid crystal cells comprise non-twisted nematic liquid crystals.

4 A colour selective filter as claimed in Claim 1, wherein the waveplates (A,B) are made of lithium niobate.