WO1999009351A1

WO1999009351A1 - Liquid crystal shutter for a lighting fixture

Info

Publication number: WO1999009351A1
Application number: PCT/GB1998/002434
Authority: WO
Inventors: David Anderson; Janos Hajto; Ian Thomson
Original assignee: Strand Lighting Limited
Priority date: 1997-08-14
Filing date: 1998-08-13
Publication date: 1999-02-25
Also published as: EP1015812A1; GB9717267D0; AU8740898A

Abstract

The present invention concerns a high intensity luminaire (15) for stage, theatre and architectural illumination, comprising: means for mounting a high intensity light source (35); an optical system for focusing light from a mounted high intensity light source through an exit aperture of the luminaire; a liquid crystal (LC) panel (70) mounted in the optical path of the luminaire; means for applying a variable voltage to said crystal panel; dichroic means (144) adapted to reduce the effect of heat generated in operation by the high light source on the operation of said panel; and means for controlling the contrast ratio of light emitted by said panel in response to variable ambient conditions; said control means comprising means for providing feedback to said panel either in response to the measured temperature of the panel or to measured contrast ratio of light transmitted through the panel.

Description

LIQUID CRYSTAL SHUTTER FOR A LIGHTING FIXTURE

The present invention relates generally to lighting fixtures and more particularly, to luminaires configured to image a high-intensity beam of light at a distant location .

High intensity lighting fixtures (often referred to as "spotlights", "stage lights") are commonly used in theatre, location, studio and architectural lighting applications. Such fixtures typically include a substantially ellipsoidal concave reflector positioned along the reflector's longitudinal axis. The reflector defines two focal regions and the lamp is usually positioned with the centre of its filaments located at or near the first of the two focal regions, so that light emitted from the filaments is reflected toward the second focal region. A gate is located between the first and second focal points of the reflector. The gate is designed to hold shutters, shields, patterns and the like which are used to shape the projected beam of light. Filters of colour glass or plastic are used to colour the beam of light.

While luminaires of this kind are useful, they do have several shortcomings. In particular, the mechanical shutters used to shape and control the beam of light are most often manually operated. This means that a spotlight operator or stagehand must physically move the shutters on a fixture in order to control the light beam. In addition, mechanical shutters offer a relatively limited variety of light-beam shaping options . Straight- edged shutters may be used to create linearly shaped beams, and devices known as irises and gobos may be used to create curved and circular shapes. However, presently available devices have only a limited range of motion and shape .

Mechanical shutters also suffer from other deficiencies. First, mechanical shutters may only be moved as fast as an operator can adjust them. The relatively slow movement means that it takes a relatively long time for a shutter or other components to change the shape of a beam of light. While it is possible to move the shutters automatically using mechanical means, such a lighting fixture would be complex and expensive, and shutter movement would not be considerably faster than a manual shutter. Second, mechanical shutters are subject to heat damage and when so damaged must be replaced or repaired.

Colouring light with mechanical filters is also less than desirable. Like mechanical shutters, presently available filters must be manually mounted on a lighting fixture. Automatic devices used to change colours or filters are relatively complex and expensive. Filters are also subject to heat damage and, therefore, have a limited life. Accordingly, it would be desirable to have a shutter for a lighting fixture that overcomes the problems associated with mechanical shutters. Furthermore, it would be desirable if the shutter was capable of producing a wide variety of shaped or patterned light beams and also provide coloured light. Further still, it would be desirable if the shutter was automatic and could create a predetermined program of changing and varying light beams . Therefore, a concern of the present invention to provide a device for controlling and shaping a beam of light from a lighting fixture.

Another concern of the present invention to provide a luminaire which can shape and colour a beam of light without using mechanical shutters, colour filters, or similar types of devices.

Thus an embodiment of the present invention provides a device that can be programmed to automatically provide a series of changing light beams. A further embodiment of the present invention provides a device that can rapidly change the colour and/or pattern of a beam of light.

In accordance with a first aspect of the present invention there is provided a luminaire as set out in claim 1.

A voltage source is coupled to the liquid crystal shutter unit and a means for controlling the voltage source, such as a microprocessor or micro-controller, is used to adjust the voltage that is applied across the elements of the LC shutter and unit thereby controlling whether each element transmits or blocks light. The means for controlling the voltage source is coupled to the light sensor and receives its output signal. The transmissivity of the LC shutter is affected by temperature, and the temperature of the unit is directly affected by the energy in the light beam from the lamp. When the lamp is turned on, relatively little heat energy is imparted to the shutter unit and its temperature is relatively low. As the lamp heats up during use, great amounts of thermal energy are imparted to the shutter unit and its temperature rises. If the lamp is switched on and off, the temperature of the shutter unit will change. In order to maintain a constant contrast ratio, the voltage applied to the elements is adjusted to compensate for temperature changes .

As the contrast ratio of the LC shutter is dependent on the applied voltage and the ambient temperature of the LC shutter (not room ambient), it can be controlled by measuring the ambient temperature and altering the applied voltage accordingly. With this method, a thermocouple of other electro-thermal device is located close to the lamp side or the lens side of the LC shutter. This thermocouple/temperature sensor is linked to the micro-processor or micro-controller which, in turn, is coupled to the voltage source for the LC shutter. As the fixture heats or cools, the magnitude of the electrical signal from the thermocouple/sensing device is changed and the microprocessor/micro-controller passes a modulated signal to the voltage source which in turn adjusts the applied voltage to the LC shutter in order to maintain a constant contrast ratio.

The liquid crystal shutter can be one of substantially conventional design modified to operate with one of the voltage control mechanisms mentioned above. Such shutter units have a plurality of voltage controlled pixels or elements. In an alternative embodiment of the invention, the liquid crystal shutter unit is a solid polymer matrix having a first and second transparent electrodes on each flat surface. The solid polymer has a plurality of liquid crystal droplets encapsulated within it. Each of these function in a similar manner to the voltage controlled pixels in a more conventional type of display unit. When no voltage is applied to the polymer sheet, the molecules in the liquid crystal are aligned to allow light to pass through the polymer sheet. When a voltage is applied, the molecules align so that the refractive index is different than that of the polymer, causing the light to be effectively blocked. It should be noted that the polymer sheet can be designed such that the molecules of the liquid crystal are misaligned with the polymer when no voltage is applied, causing light to be blocked. When the polymer sheet is so designed, the application of an electric field will cause the refractive indices to be similar such that light is transmitted. Similar performance can be obtained by using an LC shutter where the polymer matrix is replaced by a plurality of glass or polymer beads suspended in a liquid crystal film. When no electric field is applied to the liquid crystal film, the molecules of the liquid crystal are aligned such that the refractive index of the film is substantially different from that of the glass or polymer beads, thus causing incident light to be scattered. When an electric field is applied, the molecules of the liquid crystal material align such that the refractive index of the two materials is substantially similar, thus allowing incident light to be transmitted. Like, the polymer sheet described above, the polymer beads can be configured such that when no electric field is applied, light is blocked and when a field is present, light is transmitted.

As should be understood, the elements of the shutter unit, whether pixels or a plurality of droplets or beads, can be individually activated and, therefore, can individually transmit or block light. Since the voltage source is coupled to a programmable microprocessor which can control the individual elements, a virtually infinite variety of different shapes and patterns can be generated and the shape and configuration of the light beam from the lamp readily adjusted. In addition to adjusting the shape, size, and pattern projected by the light beam, its colour can also be controlled. In conventional spotlights, coloured gels, usually made of a polymer material, are used to impart colour to the white light emitted by the lamp. In the present invention, one or more liquid crystal colour filters can be used. In one embodiment of the invention, red, green, and blue liquid crystal colour filters are used. Since each of them is electro-optical, each can be controlled with the microprocessor and a wide range of colours and shades can be created. Thus, an enormous variety of combined colours and coloured patterns can be very easily projected by a lighting fixture fitted with the present invention .

Heat generated by intense light sources is always a problem in luminaires .

Thus in accordance with a second aspect of the present invention there is provided a luminaire as set out in claim 17.

These are just some of the features and advantages of the present invention. Many others will become apparent by reference to the detailed description of the invention taken in combination with the accompanying drawings, which will now be described by way of example, and in which: Fig. 1 is a perspective, exploded view of an axial lighting fixture or luminaire including a shutter designed in accordance to the present invention.

Fig. 2A is a cross-sectional taken along line 2-2 of Fig. 1.

Fig. 2B is an enlarged view of a portion of Fig. 2A. Fig. 3A is a cross-sectional view of a right angled luminaire comprising another embodiment of the present invention . Fig. 4 is a schematic illustration of one embodiment of the present invention showing a polariser, colour filters, and analyser.

Fig. 5 is a schematic illustration of one embodiment of the present invention including the microprocessor- based controller used to control the voltage source to the liquid crystal shutter unit.

Fig. 6 is an illustration of the contrast ratio to voltage relationship of a liquid crystal shutter unit at various temperatures . Fig. 7 is a greatly enlarged, cross-sectional view of a solid polymer sheet used as a liquid crystal shutter unit in one embodiment of the present invention.

Figures 8 and 9 of the accompanying drawings are cross sections of the present invention. Figure 10 shows how S and P polarised light components can be recombined.

A high intensity lighting fixture or luminaire 15 in accordance with the present invention is shown in Fig. 1. The lighting fixture 15 is designed for use in theatre, location, studio, and architectural lighting applications and to produce an intense beam of light for imaging at a distant location. The lighting fixture 15 includes a concave reflector 17 which may be ellipsoidal, parabolic, spherical, or constructed of a combination of one or more spherical, elliptical, or parabolic curves. This reflector is located generally within a rear housing 19. The reflector 17 has a base 21 (Fig. 2) at its first end, with the first focal region 25 being located some distance from this base along an optical axis 26 toward a projection system comprising lenses 55 and 60. The reflector has a mouth 27 at its second end which is located between the first focal region 25 and a second focal region 31. A lamp 35 is located within the reflector 17 with the centre of its filament(s) or light emitting portion located substantially at the first focal region 25. This lamp 35 is substantially aligned with the first focal region 25 so that substantially all points on the reflector 17 reflect the light emitted from the lamp 34 toward the second focal region 31. The lighting fixture also includes a forward housing 50 having a first lens 55, a second lens 60, and a shutter 70. Figure 2 also shows a control system for maintaining the contrast ratio in the shutter unit 70. This control system will be described hereinafter in greater detail with respect to Figure 4, where similar integers have the same reference numbers .

Another embodiment of the invention is shown in Fig. 3A. This embodiment comprises a luminaire 80 with a rear housing 85. The rear housing 85 is designed with features to promote heat transfer to its surroundings. Specifically, the housing 85 is right angled, having a first section 140 and a second section 142 with a mirror 144 positioned between these two sections. Preferably, the mirror 144 is a cold mirror having first and second surfaces 145 and 146. The first surface 145 of the mirror 144 is coated with a dichroic material which will reflect the incident light within the visible spectrum towards section 142, but will allow infra-red radiation (heat) to be transmitted to a heat sink 86 to dissipate the heat to the ambient atmosphere. The lighting fixture also preferably includes a means of removing the ultraviolet (UV) component of the light by means of a UV filter to enhance the lifetime of the liquid crystal material in the shutter 70 if shutter 70 is a liquid crystal panel and the polarisers . The lighting fixture 80 includes a similar forward housing 50 as the lighting fixture 15, except that the second lens 60 is moved slightly toward the first lens 55. It will be appreciated that the angle between the optical axes passing through the luminaire need not be an actual right angle. The angled embodiment of Figure 3A is advantageous because of the reduction in heat energy projected onto the liquid crystal shutter carried by the Dichroic mirror and its associated heat sink. As heat is always a problem in high intensity luminaires this angled approach is also advantageous in non-LC panel luminaires having conventional shutters and gobbos so that shutter 70 can be a conventional mechanical shutter.

In all the embodiments so far mentioned where shutter 70 is a liquid crystal panel, the shutter 70 modulates light using a liquid crystal ( "LC" ) shutter unit . In the embodiments shown in Figs . 1, 2 and 3 , the shutter 70 includes an LC panel which is designed to operate at ambient temperatures as high as about 300°C though as will be described a preferred operating range for the LC material is -10°C to 140°C. The LC shutter unit 70, in its simplest form, is monochromatic and its purpose is to adjust the amount and configuration of the light produced by the lamp 35 of the lighting fixture. The LC shutter unit 70 has a side facing the lamp 35 (referred to herein as "lamp side 162") and a side facing the lens 55 (referred to herein as the "lens side 164"). As with a conventional liquid crystal display ( "LCD" ) , the LC shutter unit 70 as shown in Figure 3B includes two glass plates 166 and 168, spaced about 2 to 30 microns (μm) apart. The glass plates 166 and 168 have a liquid crystal -material 170 held between them. This liquid crystal material 170 can be of many types, such as cholesteric, smectic, ferroelectric, anti erroelectric and nematic. As is known, with the addition of a chiral component, liquid crystal molecules from a helical structure. However, in the presence of an electric field, the molecules can align with the electric field causing deformation or untwisting of the helix. Each of the plates 166 and 168 have a plurality of transparent electrodes (conductors) 173 (Fig. 2B) which provides a means of applying an electric field to individual pixels or elements in the LC shutter unit 70. Contrary to the operation of the chiral material molecules, the twisted nematic and supertwisted nematic liquid crystal materials utilise the positioning of the molecules by application of an electric field from parallel to perpendicular positions in order to accomplish light modulation. Liquid crystal materials suitable for use in the present invention can currently be obtained from: Merck Ltd., Merck House, West Quay Road, Poole, Dorset BH15 1TD, UK.; and Trident Microsystems Ltd., Perrywood Business Park, Honeycrock Lane, Salfords, Surrey, RH1 5JQ. Preferable materials will be described hereinafter.

In addition to the various types of LC material just described it is possible for the liquid crystal shutter to use twisted nematic (TN) and super-twisted nematic

(STN) liquid crystals in the shutter. In the case of STN liquid crystals the light is rotated through an angle of up to 270°, as opposed to the 90° of TN materials. This provides a higher contrast ratio than can be obtained with TN materials .

Cholesteric LC materials have the ability to selectively reflect or transmit incident light according to its wavelength depending on the direction of the circular polarisation - i.e. 50% of the incident light can be transmitted and 50 % can be reflected. Therefore, if the transmitted wavelength range of the cholesteric liquid crystal material can be controlled accurately, the device can be used as a colour filter. For example 50% of the incident light can be transmitted and 50% reflected. Thus if the transmitted wavelength range of the cholesteric liquid crystal material can be controlled accurately the device can be used as a colour filter. Also due to the polarisation effect the cholesteric can be used as a polariser or analyser.

When ferroelectric liquid crystals are used in the LC shutter unit 70, each plate 166 and 168 has a series of grooves (not shown) made in the plate surface which serves to anchor the liquid crystal material to the plate surface - this is termed the alignment layer. When the plate 166 's conductors are set at zero potential (or voltage) and the plate 168 's conductors are set at a positive potential (or voltage), which should not normally exceed 60 volts, the molecules in the LC material..170 align with the electric field. In operation, light from the lamp 35 is polarised by a polariser 175. The polarised light is then transmitted to the LC shutter unit 160. When the molecules in the LC material are aligned with the applied electric field, the incident polarised light is no longer twisted by the helical structure of the liquid crystal molecules and in turn is blocked by an analyser 177. This results in the blocking of the light over the region where the electric field is being applied, i.e., the elements 174. As best seen in Figs. 2 and 5, the polariser 175 is positioned adjacent to the lamp side 164 and an analyser 177 is adjacent to the lens side 166. The polariser 175 transmits only electromagnetic waves which have planes of polarisation orientated in a particular direction. Specifically, the polariser 175 transmits only those waves that are aligned with its plane of polarisation. The polariser element can be one of various designs, including a prism, polymer or dichroic dye sheet polarisers or by stacks of parallel transparent plates . Method of polarising light suitable for use in the present embodiments will be described in greater detail hereinafter. Although polarising the light from the lamp 35 reduces the overall luminous output of the lighting fixture 15, polarised light is presently required for proper operation of the LC shutter unit 70 (as discussed below, a polariser and analyser are not required for all LC shutters). Once the light is polarised, and orientated by the liquid crystal molecules, the analyser 177 is used to either transmit or block the plane-polarised light.

As with conventional LCD's, the elements of the LC shutter unit 70 can be individually controlled. Th s, one or more may be activated, or turned on, causing light to be blocked. Various patterns can be created by turning on only selected elements 174, while the remainder of the elements are inactive or off. One way of controlling the patterns and creating a series of changing patterns is to use a programmable microprocessor, as found in a theatre control desk (discussed hereinafter) to control the voltage applied to the elements . In most lighting applications, it is critical that the desired patterns be extremely sharp, that is, that the LC shutter unit 70 pass only the light that creates a desired pattern while blocking all other light. However, the ability of the individual elements to block light is affected by operating temperature. In particular, while a certain voltage is sufficient to cause a single element in the shutter unit 70 to block light at a certain temperature, at other temperatures the same voltage is not sufficient to cause an element to block light.

It should be noted that if, instead of a parallel aligned .LC cell, a crossed LC cell is used (i.e. a twisted nematic material operating between crossed polarisers), the cell will operate in such a way as to reduce the intensity of the light transmitted by the cell, as a voltage is applied. In this way, the cell can be used as a dimmer or dowser to gradually reduce the intensity of the projected light beam. The main structure of the cell will be the same as described with reference to Figure 2B. The principle of operation of the LC panel dimmer or dowser is as follows: Incidence unpolarised light that is linearly polarised by the entrance polariser is rotated by the 90° twisted-nematic molecular configuration of the LC cell because the twisting pitch of the director of the liquid crystal layer is relatively large as compared with the wavelength of visible light.

It is very important to note that the rotation of polarisation produced by the liquid crystal layer is independent of the wavelength of the light that is used, provided that the wavelength is much shorter than the product of the helical pitch of the molecular twist and the optical anisotropy of the liquid crystal material. Therefore the following inequality must be satisfied in order for light travelling through the liquid crystal molecular layer to be linearly polarised: d.Δn >> /2 where: d = optical path distance

(plate separation)

Δn = optical anisotropy

Λ = wavelength where d is equal to the spacing between the two glass substrates and nearly equal to k times the helical pitch.

In a typical cell with approximately lOμm separation, the pitch is approximately 40 to lOμ and the optical anisotropy may vary from approximately 0.2 to 0.4 making the product of these numbers much larger than the wavelength of light in the visible range (approximately

0.4 to 0.7μm) - so, the inequality is satisfied.

Consequently, the direction of the polarised light is rotated by 90° after passing through the liquid crystal. The orientation of the polarisation of the second polariser can be parallel or perpendicular to the direction of the first polariser. In the first case, the light is blocked (DARK STATE) when no voltage is applied to the device, and in the second case, the light is transmitted (BRIGHT STATE) when no voltage is applied to the device. This is because of the 90° twist in the polarisation direction of the light. The first type of cell is called a "negative type TN-FE mode cell" and the second type is called a "positive type TN-FE cell" - where TN refers to Twisted Nematic and FE refers to the

Field Effect which causes the cell to perform in the way described. Thus, it can be seen that by applying a voltage to the cell and modulating the amount of twisting of the polarisation direction and thus the amount of light which is transmitted, the cell can be used to modulate the intensity of the light beam - i.e., it can be used as a liquid crystal dimmer or liquid crystal dowser.

For practical applications in a luminaire, the positive TN-FE mode cell should be used (although both types are applicable) because, in this case, no external voltage is required in order to maintain the bright state.

In order to control the elements in the LC shutter unit 70 to achieve properly sharp patterns, the transmissivity of the elements must be measured. Transmissivity refers to the amount of light that an individual element on a liquid crystal shutter will transmit. One measure of the LC shutter unit 70 's transmissivity is its contrast ratio. The contrast ratio of a liquid crystal shutter is defined as the ratio of maximum intensity of transmitted light to the minimum intensity of transmitted light. Of course, the temperature of the LC shutter unit 70 is directly affected by the infra-red radiation (heat) emitted with light from the lamp 35. The relationship between contrast ratio and applied voltage at various temperatures is best seen with reference to Fig. 6.

When the lamp 35 is first turned on, relatively little thermal energy is imparted to the shutter unit and its temperature remains relatively close to the ambient temperature of its surroundings. As the lamp 35 heats up during use, large amounts of thermal energy are imparted to the LC shutter unit 70 and its temperature rises. If the lamp 35 is switched on and off, the temperature of the unit will change. In order to compensate for these temperature fluctuations, the applied voltage to the liquid crystal shutter unit 70 is varied, thus maintaining a constant contrast ratio. In order to vary the voltage, the present invention preferably includes a feedback control mechanism which is discussed below.

As best seen by reference to Fig. 4, the shutter 70 includes a light source 200, preferably an LED, which is independent of the light beam produced by the fixture. The light source 200 is positioned adjacent to the lamp side 164 of the LC shutter unit 160 and, preferably, out of the centre of the light beam produced by the lamp 35. Thus, the light source 200 is offset from the center longitudinal axis of the LC shutter unit 70.

Opposite the light source 200, on the lens side 164 of the LC shutter unit 70, is a light sensor 210. The light sensor 210 may be a photo-diode or a photo- transistor and sensors suitable for use in the present invention are available from Radio Spares (RS Worldwide) . Photo-diodes operate such that their electrical output changes proportionately with the amount of incident light. The light sensor 210 produces an output signal 212 which is proportional to the amount of light it senses from the light source 200. The output signal 212 is sent via a communication link 213 (such as a cable, wire, fibre optical or a transmitter/receiver pair) to a microprocessor 214. The microprocessor 214 is of conventional design and produces an output control signal which is sent via a communication link 216 to a voltage source 220. Preferably, the voltage source 220 is a conventional direct current voltage source as commonly used in liquid crystal display applications. The microprocessor 214 is used to adjust the voltage that is applied to the elements 174 of the LC shutter unit 70 to maintain a constant contrast ratio. Specifically the microprocessor 214 calculates the contrast ratio based on the output signal 212 it receives from the sensor 210 and instructs the voltage source 220 by sending it a signal to deliver a certain voltage to the elements 174 of the LC shutter unit 70 via a communication link 222. It should be noted that the above LED light source and light sensor can be replaced by an electronic component such as an opto-isolator or thermocouple 225. An opto-isolator consists of an LED light source and a photo-diode or photo-transistor which are mounted within the same encapsulation. The thermocouple 225 is located adjacent. to the lamp side 162 or the lens side 164 of the LC shutter unit 70 and measures the ambient temperature. The output of the thermocouple 225 is delivered to the microprocessor 214 which uses the information regarding temperature to adjust the voltage to the elements in the LC shutter unit 70. Figure 4 shows an arrangement using both a thermocouple and a light source. As explained either of these systems can be used individually.

The microprocessor 214 can be programmed to control the voltage source 220 so that a variety of patterns, such as circles, ovals, rectangles and other geometric patterns are produced on the LC shutter unit 70. Numerous more complex patterns can be created by individually controlling the pixels or, more broadly, elements 174 on the LC shutter unit 70. Various standard pixel combinations and patterns can be stored in the memory of the microprocessor 214, a computer, or a control desk for transmission to the LC shutter unit 70 via a communication link.

In one embodiment of the present invention, in order to introduce colour into the white light produced by the lamp 35, one or more colour filters, preferably LC colour filters may be positioned between the polariser 175 and the LC shutter unit 160 or between the polariser 175 and the analyser 177. This arrangement is shown in Figure 5. Specifically, a red LC colour filter 250, a green LC colour filter 252, and a blue LC colour filter 254 may be used... Each LC colour filter operates in a similar manner to the LC shutter unit 70 and to one another, except for the colours they filter. For purposes of brevity, only the red LC colour filter 250 will be described in detail. A liquid crystal colour filter is constructed from an array of liquid crystal pixels - each pixel being made up of three small sub-pixels. The three sub-pixels each contain a colour filter - either red, green or blue. In the case of the red colour filter 250, each pixel will only allow light with a wavelength within the red part of the visible spectrum to be transmitted. In order to transmit red light, only the sub-pixels with the red filter are activated. By mixing of the different coloured sub-pixels, different colours of light may be transmitted. Each individual sub-pixel is activated by means of a thin film transistor. There is an array of such transistors fabricated onto a layer of glass which is used to address the array of sub-pixels. The colour filters are an array of red, green and blue dots of dyed or coloured material on a second sheet of glass. Both glass plates are coated with a polymer material and rubbed to create tiny grooves which are used to anchor the liquid crystal material. The two plates are aligned accordingly, spacers are added to maintain a constant plate separation, and a liquid crystal material is added to the space between the plates . The plates are then sealed and means of conveying power to the thin film of transistors is added.

The filters may be controlled by the voltage source 220 and microprocessor 214 or by additional independent voltage sources and microprocessors. In addition, one or more colour sensitive feedback control mechanisms (similar to the light sensor 210) may be coupled to the microprocessor in order to provide information regarding temperature change effects on the colour filters. The microprocessor may then adjust the voltage supplied to the filters to produce appropriate and desired colours. One of the advantages of using LC colour filters is that since the filters are electro-optical, slight variations in voltage (which can be precisely controlled by the microprocessor 214) will cause slight variations in colour. Therefore, a wide variety of colours can be created by mixing the colour filtering of each LC colour filter.

The LC colour filters may also be incorporated directly into the LC shutter unit so that the LC shutter is a pixellated LC display panel. This gives the possibility of projecting coloured images. This has the advantage of eliminating the need for multiple filters and their supporting structure.

In another embodiment of the present invention, the LC shutter unit is a polymer sheet shutter 300 with encapsulated liquid crystal droplets or beads. As is best seen by reference to Fig. 7, the shutter 300 consists of a first transparent electrode 302 and a second transparent electrode 304. Located between the transparent electrodes 302 and 304 is a solid polymer material 306 having a plurality of voids 308 in which liquid crystal beads or droplets 310 are encapsulated. The liquid crystal droplets 310 in the polymer material 306 are often referred to as polymer dispersed liquid crystals. Polymer dispersed liquid crystals ("PDLCs") consist of an optically isotropic transparent polymer matrix containing liquid crystal droplets - the materials used are electro-optical materials similar to other liquid crystal devices. The presence of an applied electric field causes the liquid crystal director to align in the direction of the applied field. When the molecules in the liquid crystal droplets are aligned so that the refractive index of the liquid crystal is similar to that of the polymer matrix, the light entering the film is not scattered and the device will transmit light, specifically, when no voltage is applied to the material, the liquid crystal molecules are randomly dispersed and the mismatch in the refractive index of the liquid crystal and the polymer matrix causes scattering/blocking of the incoming light. When an electric field is applied, the molecules align so that light is transmitted. It should be noted that the molecules may initially be arranged such that when no electric field is present (i.e. no voltage applied) they transmit light and when a field is present they block light. The latter arrangement is generally preferable. There are two preferred methods of achieving this scatter/non-scatter result: (1) by using liquid crystals encapsulated in a polymer film by micro-encapsulation or emulsification techniques; or (2) by using liquid crystals dispersed in a polymer binder by phase separation from a homogeneous solution.

One of the benefits of using a polymer sheet 300 is that it does not require a polariser or analyser. However, higher contrast ratios can be obtained by using a polariser in combination with a PDLC unit. This combination allows the PDLC unit to be used as an optoelectronic analyser, thus giving higher contrast ratios and is suited to applications where higher light intensities are used.

The embodiment shown in Fig. 8 of the accompanying drawings discloses one type of polariser and shows how polarised light can be obtained by using a stack 400 of glass plates 401 with air gaps 402 between them which split the light into P and S polarised components.

Figure 8 actually illustrates two possible variants and for the purpose of describing the first variant the second (right hand) set of polarising plates should initially be ignored. In this particular embodiment (and in both the variants) the glass plates are 1.0 mm thick

(but can be of any practical thickness) and there is an air gap of 1.0 mm width between adjacent plates although any practical width can be used. Whilst the number of plates can be varied in accordance with the optical requirements a suitable number is ten plates. The stack 400 is oriented to the optical axis 410 of the unpolarised light generated by a lamp 411 mounted in a dichroic reflector 412. The stack is oriented to the optical axis such that the angle formed between the axis and the incident surface of the stack is equal to the Brewster's angle for the glass. At this angle the reflected P component of the light is transmitted and so only a part of the S polarised light is reflected. The Brewster's angle varies for different materials and for glass is around 56°. At this angle approximately 15% of the S polarised light is reflected. Thus by using more glass plates - and thus increasing the number of reflections of S polarised light - the efficiency of the glass plate stack as a polariser can be increased.

The reflected polarised light is then passed via a collimating lens 414 through a liquid crystal panel 413. This liquid crystal panel can be similar to the panels already described in the present specification and will also have associated control circuitry again as already described . As this system of polarisation means that the incident light is split into two polarised components, an alternative method of using a glass stack as a polariser is possible. This is the second variant shown in Figure 8 in which a dichroic cold mirror similar to the mirror in the embodiment of Figure 3A is used to reflect the light from the reflector 412 in place of the left hand stack 400. In this variant the glass stack 400 is located further along the optical axis as shown in the portion A, then the transmitted component of the light through the glass stack can be used as polarised light at the LC device 413. Figure 9 of the accompanying drawings is identical to Figure 8 and has the same components but discloses a non-right angled version. It will be appreciated that as in the Figure 3A embodiment the LC shutter in the Figure 8 and Figure 9 embodiments can be replaced by a conventional shutter. In this case the polarising stack would not be required.

In addition to the glass plate stack just described it is possible to use high efficiency polarising prisms such as Glan-Taylor prisms, Glan-Thompson prisms or Wollaston prisms which separate the S and P polarised components. Such a prism is shown in Fig. 10 of the accompanying drawings . With these types of polarisers high efficiency can be achieved due to the fact that, instead of throwing away one component of the polarised light, this previously discarded component is rotated and then recombined with the remaining component. Thus using a prism 800 of the type just described one component of the light, for example the P polarised component, will be transmitted along the main optical axis of the lighting fixture whilst the S component is reflected by the prism at an angle to the optical axis. This S component can then be passed through a twisted nematic liquid crystal cell or another prism 801 in order to change its plane of polarisation. Once the polarisation plane has been changed this component can be recombined with the P polarised beam using lenses 802, 803 and 804 thus obtaining higher efficiency. This arrangement can be used with any of the embodiments described which require polarised light for their operation.

In the lighting fixture already described it is also possible to add a colour changer anywhere in the optical path of the luminaire between the light source and the exit aperture of the luminaire. The colour changer operates in the following manner:

White light from a light source is projected onto the polariser which converts unpolarised light from the source into plane polarised light. This polarised light is rotated through an angle of 90° by the molecules of the twisted nematic liquid crystal and then is allowed to pass through an analyser which is oriented at 90° to the polariser. When an electric field is applied across the pixels of the liquid crystal array, the molecules of the LC .material untwist (due to the di-pole moment) such that the incident polarised light is no longer rotated through 90° and so cannot be transmitted by the analyser. This assembly allows for the modulation of the light beam from the lamp. Thus in accordance with a further embodiment of the present invention the changing of the colour of the light beam in a lighting fixture can also be done by using a tunable nematic liquid crystal panel.

Figure 11 of the accompanying drawings shows schematically such a system. This figure shows a white light source 600, a polariser 601, a liquid crystal panel 602 and an analyser 603. This liquid crystal panel 602 uses a nematic LC material as already described with the difference that the LC material is placed between plates with the "rubbed grooves" aligned parallel to one another as opposed to being crossed as with the twisted nematic panel. The principle of the operation of this colour changer is as follows. Light from the white light source 600 is incident on the polariser 601 which passes only plane polarised light. This light passes through the LC panel (or cell) 602 before being either transmitted or blocked by the analyser depending on its orientation with respect to the polariser.

When a voltage V is applied to the LC panel 602 the molecules of the LC material are tilted due to the di-pole moment on each molecule. This tilting of the molecules causes a change in refractive index of the extraordinary component and in turn causes a phase difference between the two components. This difference in phase is resolved by the analyser, causing certain wavelengths to be rotated though a given angle, θ. Since the white light is made up of many different wavelengths of light, certain wavelengths will be rotated more than others (due to the relationship with the refractive index) and these selected wavelengths can be either transmitted or blocked by rotating the analyser. The LC panel is thus in effect acting as a tunable λ÷2 or ÷4 plate .

The operation of the LC panel can be described by the following equation:

{n o - n_{( v)} ) x d = λ/2

where, n₀ = ordinary refractive of LC material n_(v) = extraordinary refractive index of LC material as a function of applied voltage d = distance between plates of panel λ = wavelength of light By varying the applied voltage the difference in the optical anisotropy can be changed which in turn changes the wavelength of the light which is transmitted or blocked by the analyser 604. As the voltage is increased further the bandwidth of the selected wavelength ( s ) will be changed according to the right-hand side of the equation becoming 3λ÷2 or 5λ÷2. A more effective colour changing performance can be obtained by the addition of another liquid crystal panel adjacent to the first one. Hence, the polariser, lc panel and analyser described above will become: first lc polariser, lc panel, second polariser, lc panel and analyser. The application of variable voltages to these panels as described above results in an improved range of colours being imparted to the light beam. In order for this method to work effectively that the incident light will need to be collimated. Figure 12 shows a suitable collimation arrangement. In this case the lamp 600 is shown in a reflector 605 reflecting the light onto a collimating lens 606 followed by a long focal length lens 607 and a suitable gate 608. The gate

608 is followed by projection optics shown generally at

609 which as shown includes a field stop 610 and lenses 611, 612. Collimated light can also be obtained by using a number of conventional and well documented optical methods such as the use of a spherical reflector and aspheric lens system.

It is also possible for the colour filter or changer just described to be replaced with a diffraction grating or gratings . These gratings are effectively composed of thin slits of approximately 5-10 micron separation in a plate.

The slits have the effect of allowing certain wavelengths of light to pass through whilst others are reflected or refracted. It can be seen that by varying the slit width the wavelength and therefore the colour of light which is passed can be controlled. The device can thus be used as a colour changer.

It is envisaged that three different diffraction gratings would be used in each pixel of the LC gate in order to select the three primary colours of red, green and blue. By doing this any colour can be imparted to each pixel of the light beam.

Having now described a colour filter embodiment in which a liquid crystal panel is used to provide a controllable range of output colours it is of course possible to combine such a colour filter arrangement with a pixellated liquid crystal panel of the kind previously described in the present specification.

Such an embodiment is shown in Fig. 13 It will be seen that this embodiment is a variant of the embodiment shown in Fig. 3A in that the optical axis through the luminaire is right angled though as mentioned the right angle is not essential. In Fig. 11 an ellipsoidal, non- faceted dichroic reflector is shown at 900. This directs light onto a dichroic cold mirror 901 which reflects the light via a hot mirror 902 onto a collimating lens 903. The light then passes into a LC colour filter 904 of the type described with reference to Fig. 11 of the drawings. The light then passes through pixellated LC panel 905 which is located in the focal plane of the collimating lens 903. The pixellated LC panel can be one of the types as described hereinbefore. The light finally passes through a suitable projection lens 706,707. It will be appreciated that each of the two liquid crystal devices will be controlled by a system similar to that described with reference to Fig. 4.

Reference has already been made in the present specification with regard to the liquid crystal material to be used in the various liquid crystal panels which have been described. As will be appreciated an important consideration is that the liquid crystals will be able to operate over a relatively wide range of temperatures and in particular, due to the intense light source which also generates a considerable amount of heat energy, at temperatures well in excess of 100°. Normal nematic materials such as E46 sold by Merck which has a maximum temperature limit of approximately 89°C. One approach to providing an elevated temperature limit is to add a second material having a much higher clearing point. Unfortunately liquid crystal materials with high clearing points tend to have high melting points which in turn has a deleterious effect by increasing the melting point of the new mixture. It is thus difficult to provide the correct balance.

As a result of considerable experimentation two mixtures have been discovered which provide adequate performance over a wide temperature range. The first mixture is formed from a nematic material MLC-6233 sold by Merck and a terphenyl T15 also sold by Merck and having a nematic range of 130 - 239°C. In the first mixture the ratio of the two substances was 84% MLC-6233 to 16% T15. The resultant mixture had an operative range of approximately 0°C to 131°C.

A second mixture had 80% of E49 and 20% of T15 and a clearing temperature of approximately 140°C.

It is accordingly believed that mixtures of MLC-6233 and T15 where the percentage of T15 is between approximately 10% and 20% by weight are useful. Also very useful is the standard, commercially available Merck liquid crystal material called MLC-6428 which has nematic operating temperature range of -39°C to 139°C.

It is also possible to use nematic materials MLC- 6428 or MLC-6233 manufactured by Merck without adding another substance or a mixture .of 77% MLC-2233 and 23% T15.

It will be appreciated that the preceding description discloses a number of embodiments and variants and that a large number of permutations are possible using the various integers which have been described. Thus a simple form of the present invention could incorporate a tuneable LC colour filter of the type described located either in a straight-through or angled optical path luminaire. This colour filter can be replaced by a normal non-LC gobo arrangement. Additionally the colour filter LC arrangement can be used with a monochromatic pixellated LC array, or it can be replaced by a coloured LC pixellated array.

Claims

1. A high intensity luminaire for stage, theatre and architectural illumination, comprising: means for mounting a high intensity light source; an optical system for focusing light from a mounted high intensity light source through an exit aperture of the luminaire; a liquid crystal (LC) panel mounted in the optical path of the luminaire; means for applying a variable voltage to said crystal panel; dichroic means adapted to reduce the effect of heat generated in operation by the high light source on the operation of said panel; and means for controlling the contrast ratio of light emitted by said panel in response to variable ambient conditions ; said control means comprising means for providing feedback to said panel either in response to the measured temperature of the panel or to measured contrast ratio of light transmitted through the panel .

2. A luminaire according to claim 1, wherein the control means comprise: a light source independent of the light beam produced. y the high intensity light source when mounted and positioned adjacent to the lampside of the liquid crystal shutter; a light sensor for monitoring the contrast ratio of the panel and positioned opposite the light source on the other side of the panel; and a microprocessor for modifying the voltages applied to the liquid crystal panel in response to the monitored contrast ratio.

3. A luminaire according to claim 1, wherein the control means comprise: a temperature sensor mounted adjacent to said panel; and a microprocessor for modifying the voltages applied to the liquid crystal panel in response to the monitored temperature.

4. A luminaire according to any one of claims 1 to 3 , wherein said panel provides an addressable array of pixels whereby light from said light source can be modulated to display images .

5. A luminaire according to any one of claims 1 to 3 , and comprising a polariser interposed between the panel and the light source when mounted and an analyser mounted on the other side of the panel; and wherein the panel comprises a tunable nematic crystal panel the optical anisotropy of which is changed in operation by selecting said applied voltage thereby changing the wavelength of light which is transmitted or blocked by said analyser so that the panel can act as a variable colour filter.

6. A luminaire according to any one of claims 1 to 3 , wherein the LC panel can be controlled to act as a dimming arrangement .

7. A luminaire according to claim 5 or claim 6, and including a second liquid crystal panel arranged in said optical path, and wherein said second liquid crystal panel has an addressable array of pixels whereby light from the light source when mounted can be modulated to display images.

8. A luminaire according to claim 7, wherein feedback means are provided to maintain the contrast ratio of said second panel .

9. A luminaire according to any one of the preceding claims, wherein the or each liquid crystal panel includes first and second transparent electrodes, and an optically isotropic, polymer-dispersed liquid crystal matrix with encapsulated drops of liquid crystal material positioned between said first and second transparent electrodes.

10. A luminaire according to any one of the preceding claims, wherein the liquid crystal material has a nematic range between - 10o c to approximately 140O c.

11. A luminaire according to claim 10, wherein the liquid crystal material comprises a combination of a nematic liquid and a terphenyl liquid having an operative range of approximately -10┬░C to 140┬░C.

12. A luminaire according to claim 11, wherein the percentage of the terphenyl component to the nematic component is between 10% and 18% by weight.

13. A luminaire according to claim 5 and any one of claims 6 to 12 when dependent on claim 5, wherein said polarising means comprise a stack of transparent plates having air gaps therebetween so as to split the light into p and s polarised components; the stack being oriented with respect to the optical axis of the unpolarised light from the light source such that the angle formed between the optical axis and the incidence surface of the stack is equal to the Brewster's angle of the transparent plates .

14. A luminaire as claimed in claim 5 and any one of claims 6 to 12 when dependent on claim 5, wherein the polarising means comprise at least one prism.

15. A luminaire according to any one of claims 5 to 14 and comprising means for recombining the s and p components of light separated by said polarising means so as to increase the contrast ratio of light emitted by the combination of the polarising means, the panel and the analyser.

16. A luminaire according to any one of the preceding claims, wherein said dichroic means comprise a dichroic mirror so as to reflect light on a first optical axis from the light source onto a second optical axis at an angle with respect to the first optical axis.

17. A high intensity luminaire for stage, theatre and architectural illumination, comprising means for mounting a high intensity light source so as to project a beam of light along a first optical axis onto a mirror arranged to reflect the light beam from the light source along a second optical axis angled with respect to said first optical axis; shutter means in said second optical path for controlling patterns of light emitted from the luminaire; and lens means for focusing light reflected by said mirror.

18. A luminaire according to claim 17, wherein said mirror is coated with dichroic material adapted to reflect light substantially within the visible spectrum towards said lens system.

19. A luminaire according to claim 17 and claim 18, wherein the first optical axis is at right angles to the second optical axis .

20. A luminaire according to any one of claims 17 to 19 and including a liquid crystal panel mounted in the optical path of the luminaire.

21. A luminaire according to claim 20, and including means for maintaining the contrast ratio of light paving through the panel .

22. A luminaire according to claim 20 or 21, wherein a polariser is interposed between the panel and the light source, when mounted, and an analyser is mounted on the other side of the panel; and wherein the panel comprises a tunable nematic crystal panel the optical anisotropy of which is changed in operation by selecting said applied voltage thereby changing the wavelength of the light which is transmitted or blocked by said analyser so that the panel can act as a variable filter or a colour dimmer.

23. A luminaire according to claim 22, wherein said polarising means comprise a stack of transparent plates having air gaps therebetween so as to split the light into p and s polarised components ; the stack being oriented with respect to the optical axis of the unpolarised light such that the angle formed between the optical axis and the incident surface of the stack is equal to the Brewster's angle of the transparent plates .

24. A luminaire according to claim 23, wherein said polarising means also act as said mirror to reflect the light from said first optical axis onto said second optical axis.

25. A luminaire according to claim 22, wherein the polarising means comprise at least one prism.

26. A high intensity luminaire for stage, theatre and architectural illumination, comprising means for mounting a high intensity light source, a lens system for focusing light from the light source through an exit aperture of the luminaire, a liquid crystal shutter mounted in said optical path; feedback means for monitoring the contrast ratio of said shutter, and at least one diffraction grating mounted subsequent to said shutter and adapted to act as a colour filter.

27. A luminaire according to claim 26, wherein said liquid crystal shutter comprises an addressable array of pixels whereby light from said light source can be modulated to display images .

28. A luminaire according to claim 27, wherein said liquid crystal shutter is monochromatic.