GB2198573A - Display apparatus - Google Patents

Abstract

Filter unit 1 has a liquid crystal display panel 2 having a plurality of sections 4-6, each electrically energisable independently of the others to be optically transmissive and having different transmission characteristics. The panel is sandwiched between two transparent glass substrates 3,3<1> one having a surface coating formed of three concentric regions each of a different colour transmissive material. A polarizing surface coating 7, oriented to block the light of an activated cell may be formed on one or both substrates. The sections with different transmission characteristics may alternatively be formed as separate sectors or as wedge-shaped radial sections. Each section may be further sub-divided into elements which can be separately energised to control the intensity level of each colour. The filter unit may be used in an imaging system to display slides, photographs on a TV receiver, in still photography and in video or film making. <IMAGE>

Description

COLOUR FILTER The present invention relates to a colour filter unit and to an optical imaging system incorporating such a colour filter unit.

European Patent Application Publication No. 126597 shows a system for displaying photographic slides or negatives on a television receiver. The system incorporates a rotatable filter wheel with different coloured areas which are sequentially placed in the path of a light beam, to prove colour-component representations of the slide/negative to be displayed.

The present invention provides an optical imaging system comprising a light source, a condenser lens arrangement to provide a beam of substantially parallel light passing through a region containing an image for display, a colour filter unit being located in the light beam, and a convergant lens arrangement to produce an image from the light beam, the colour filter unit comprising a liquid crystal display panel having a plurality of sections each of which is selectively electrically operable to be optically transmissive, each section having different transmission characteristics to the other sections.

Such an imaging system may be used in the display system of European Patent Application Publication No. 126597.

Preferably one section has transmission characteristics such as to pass a red-light component, another section has transmission characteristics such as to pass a blue-light component, while the third section has transmission characteristics such as to pass either a green-light or a luminance component.

In a filter unit particularly suited to a system for displaying slides/photographs on a television receiver, preferably the relative area of the three sections is chosen to effect compensation in any bias of spectral response inherent in the system. Thus for example, if the equipment in the system has a spectral response tending to increase sensitivity in the red portion of the spectrum, then the ratios of areas of the 'red', 'green' and 'blue' sections may be 4 : 5 : 20.

Preferably each section comprises a plurality of segments, selectably operable electrically either alone or in any combination as required; in this way the filter unit is selectably adjustable to permit transmission of a number of different intensity levels.

Preferably, the unit has a polarising filter over the panel, the polarising filter being oriented to block the light of an activated section.

The present invention also provides a colour filter unit comprising a liquid crystal display panel having a plurality of sections each of which is selectively electrically operable to be optically transmissive, each section having different transmission characteristics to the other sections.

In order that the invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawings, in which: Figure 1 is a schematic plan view of a filter unit embodying the present invention; Figure 2 is a cross-sectional elevation view of the filter unit of Figure 1; Figure 3 is a schematic diagram of an optical imaging system incorporating the filter unit of Figure 1; and Figure 4 is a schematic plan of another filter unit embodying the present invention and, Filter unit 1 shown in Figures 1 and 2 has a liquid crystal display panel 2 having three concentric cells, each of which is electrically energisable independently of the others such as to be optically transmissive.The panel is sandwiched between two transparent glass substrates 3 and 3' the latter having a surface coating formed of three concentric regions each of a colour transmissive material, namely a blue light transmission region 4, a red light transmission region 5 and a green light transmission region 6, these regions corresponding in area and boundary to the three cells of panel 2. Using a guest-host LCD material, the substrate 3 has a polarizing surface coating 7, oriented to block the light of an activated cell. For some types of liquid crystal material (for example ferro-electric and twisted-nematic materials), a second polarising layer is provided on substrate 3'. The planes of polarisation of the filter(s) are arranged to produce blocking of the light in either the driven or non-driven state, as required.

Unit 1 has a ground backplane (not shown) common to all regions located intermediate cell 2 and the substrate 3. Unit 1 also has a separate drive electrode (not shown) for each region, these drive electrodes being located at the opposite surface of substrate 3' to that which has the colour coatings.

One application of the filter unit 1 is in equipment to separately project the RGB components of an illuminated colour slide onto a TV pick up device, and to store the colour component signals in memory for simultaneous retrieval to display the full colour image on a TV display. Filter 1 has twisted-nematic LCD cells with an adequate switching speed for the particular application. Each section of the filter is addressed directly in succession with an ac driving waveform of pulses (typically of + 2 to 15 volts at a frequency in the range 125 Hz to 3KHz) applied to one electrode, with the common backplane being grounded; in a variant utilising ferroelectric LCD cells, pulses of + 20 to 60 volts are typically used. Each sector is covered by the appropriate colour filter.

Conventional video pick-up equipment and signal system can be used for frame storage of the information, the video system scanning continuously and data being routed into and out of the appropriate store on command.

With the illuminated slide in place and on receipt of an initiating trigger, the ac address voltage is applied to the red sector. If the polaroid filters are arranged to block light in the non-driven state, the addressed sector becomes transmissive.

This is inititated at the commencement of a frame and terminated at the end of a frame. Since the image is stationary the number of frames can vary from one upwards.

If it is required to complete the operation in one frame, then the LCD material must have 'on' and 'off' switching times to achieve this (ferro-electric materials having switching times of less than one video line).

During the red sector period of transmission, the video signal is stored in the red frame store. At the end of the nth frame count, there is a period of one or more fields during which no sector is driven to ensure that only one sector is ever transmissive at any one time. The green sector is then driven and the green signal stored; and at the end of a further n frames followed by a period in which all sectors are not driven, the blue sector is driven and the blue signal stored. The three frames stores are then accessed simultaneously and the RGB output signals used to drive the TV display.

Contrast ratios of 100 or more are achievabie by optimising choice of polarizers and LCD material; however any deficiency in contrast ratio which manifests itself in colour impurity due to transmission via the 'off' sectors can be corrected for by simple subtractive methods. One example of such a method is the use of primary and complimentary colour filters placed in the light path. The technique employed is to place a red filter in the light path and to record the red signal amplitude. A yellow filter is then substituted and the new red amplitude signal recorded, which will exceed the intial value by the error signal from the green sector signal, since with the green sector off but with a yellow light source, the light attenuation of the green sector is by virtue of the LCD cell alone, and is not aided by the green filter for the green component of the light source.Measurement of these error signals can be made using all three primary and secondary coloured filters for the red green and blue signal channels, and embodied in a summing amplifier for each output signal. Thus using three summing amplifiers, one for each output of the three frame stores, the three corrected colour signals are obtained for display on an RGB monitor. Table 1 gives typical values which may occur in this technique. Thus, in this example, the corrected signal for the R component would be achieved by inputting to a summing amplifier the uncorrected R component in parallel with a signal -5 -20 representing /100 the uncorrected B component and -20/100 the uncorrected G component. Similar processing occurs to obtain the corrected B and G components.

TABLE 1

COLOUR SOURCE R G G B RED 100 - YELLOW 120 (-20) ~ MAGENTA 105 - (-5) GREEN - - -- - 100 YELLOW (-3) ' 103 CYAN 1 - 106 (-6) BLUE - - 100 MAGENTA (-1) 101 CYAN (-4) 104 After correction, using a constant intensity white light source, the RGB outputs must be of equal amplitude.Due to the spectral distribution of the 'white' light source, and the spectral characteristics of the pick-up device, significant attenuation in light intensity may be required between colours.

This can be achieved by subdividing the sectors as shown, and selecting the number of subdivisions for the required sensitivity.

The panel 2 is divided up into three cells of equal area.

In an alternative arrangement, the relative area of the panel 2 attributed to each colour-component is chosen to take into account any colour-bias inherent in the optical system used.

Thus for example, if the type of system shown in Figure 3 were known to have a bias of increased sensitivity towards the 'red' end of the spectrum, then filters designed for use in that system would have the cells in a panel of differing area, with the 'red' cell having the minimum area. For example, the form of display system described in European Patent Application Publication No. 126597 has a significant bias towards the 'red' end of the spectrum, requiring attenuation equivalent to 4 stops for the green region and 5 stops for the red region.

Figure 3 shows an optical imaging system 10 in which the filter unit 1 might be used. If the filter unit is placed on or near the position of the aperture 'stop' in the imaging lens, then the unit operates in a similar manner to the lens stop; thus, irrespective of which segment or ring is activated, a uniform illuminated image is obtained. System 10 has a light source 11 which directs light to a condenser lens arrangement 12 to form a beam of parallel light incident on a mounted slide or negative 13 for display. The resultant light beam then passes through filter unit 1 in the principal plane of a convergant lens arrangement 14 (which, for simplification, is shown in Figure 3 as a single lens after the unit 1) and thereby forms an image 15. System 10 may be incorporated within a display system as described in European Patent Application Publication No.

126597.

Figure 4 shows another form of filter unit 30 in which the liquid crystal display panel 31 is again separated into three separately-energisable cells 32,33,34, each having a colour coating thereby to form a colour filter; however, in this case each cell is itself divided up into a number of elements (for example cell 32 having elements R1 to R5), any one or more of which can be separately energised. In a cell, the difference in area between adjacent elements is a factor of two, thus for example R2 is twice the area of R1 and half that of R3.

Accordingly, by appropriate inputting of electrical signals along control lines 35, 36, 37, the filter unit 30 can be instructed to operate at any one of a number of intensity levels for each colour, as required. In a variant to the embodiment of Figure 4, each cell is divided up into wedge-shaped elements rather than concentric elements.

Clearly, the present invention is not confined to the selection and combination of colours described hereinbefore, but can be used to provide a filter of different characteristics, for example a red-blue-luminance selection, or cyan-magentayellow selection or any other considered appropriate.

Any of the filter units described hereinbefore could be used as an adjustable stop system in an optical system, and especially as a colour separation filter and aperture control; any such unit could be used in combination with auto-control balance and gain control functions. Such a filter unit is applicable to still-photography, to video and to film-making.

Claims (7)

1. An optical imaging system comprising a light source, a condenser lens arrangement to provide a beam of substantially parallel light passing through a region containing an image for display, a colour filter unit being located in the light beam, and a convergent lens arrangement to produce an image from the light beam, the colour filter unit comprising a liquid crystal display panel having a plurality of sections each of which is selectively electrically operable to be optically transmissive, each section having different transmission characteristics to the other sections.

2. A system according to Claim 1, wherein one section of the panel has transmission characteristics such as to pass a red-light component, another section has transmission characteristics such as to pass a blue-light component, while the third section has transmission characteristics such as to pass either a green-light or a luminance component.

3. A system according to Claim 1 or Claim 2, wherein the relative area of the sections of the panel is chosen to effect compensation in any bias of spectral response inherent in the system.

4. A system according to any one of Claims 1 to 3 wherein each section of the panel comprises a plurality of segments, selectably operable electrically either alone or in any combination as required.

5. A system according to any one of Claims 1 to 4 wherein the unit has a polarising filter over the panel, the polarising filter being oriented to block the light of an activates section.

6. An optical imaging system substantially as hereinbefore described with reference to and as illustrated in one or more of the accompanying drawings.

7. A colour filter unit for use in a system according to any one of Claims 1 to 6, the unit comprising a liquid crystal display panel having a plurality of sections each of which is selectively electrically operable to be optically transmissive, each section having different transmission characteristics to the other sections.