GB2286056A - Electrically controllable wavelength filter - Google Patents
Electrically controllable wavelength filter Download PDFInfo
- Publication number
- GB2286056A GB2286056A GB9401192A GB9401192A GB2286056A GB 2286056 A GB2286056 A GB 2286056A GB 9401192 A GB9401192 A GB 9401192A GB 9401192 A GB9401192 A GB 9401192A GB 2286056 A GB2286056 A GB 2286056A
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- GB
- United Kingdom
- Prior art keywords
- filter
- wavelength
- light
- wavelengths
- light source
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on liquid crystals, e.g. single liquid crystal display cells
- G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
- G02F1/139—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
- G02F1/1393—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent the birefringence of the liquid crystal being electrically controlled, e.g. ECB-, DAP-, HAN-, PI-LC cells
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F2203/00—Function characteristic
- G02F2203/05—Function characteristic wavelength dependent
- G02F2203/055—Function characteristic wavelength dependent wavelength filtering
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- Physics & Mathematics (AREA)
- Nonlinear Science (AREA)
- Chemical & Material Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Color Television Image Signal Generators (AREA)
- Liquid Crystal (AREA)
Abstract
A controllable optical filter comprises an electrically controllable phase plate 6 interposed between polarisers 4, 8. The phase plate is preferably a liquid crystal pi cell. A retardation plate may be placed in series with the phase plate. Different parts of the spectrum can be transmitted and blocked simultaneously, thus light of a given colour may be selected from a broadband light source. <IMAGE>
Description
ELECTRICALLY CONTROLLABLE WAVELENGTH FILTER.
The present invention relates to an electrically controllable wavelength filter. Such a filter may be used with an image sensor to form a colour camera or with a monochrome display to form a colour display.
According to a first aspect of the present invention, there is provided an electrically controllable wavelength filter, comprising a first polariser, an electrically controllable wavelength dependent phase plate, and a second polariser, the phase plate being in a light path between the first and second polarisers.
It is thus possible to provide a filter which has electrically controllable peak transmission versus wavelength characteristic. The filter may be controlled to transmit a range of wavelengths, and the range of wavelengths may be varied
Preferably the electrically controllable wavelength dependent phase plate is a liquid crystal device. Advantageously the liquid crystal device is a z cell.
Advantageously, the filter may further comprise a fixed retardation plate.
Preferably the first and second polarisers are plane polarisers.
Advantageously the polarisers may be crossed or parallel.
Preferably control means are provided for supplying control signals to the filter such that the filter response has a peak of transmission at a first wavelength when a first signal is applied to the electrically controllable wavelength dependent phase plate and a peak at a second wavelength when a second signal is applied to the electrically controllable wavelength dependent phase plate.
The control means may be arranged to control the filter such that the filter is arranged to pass one of first, second and third ranges of wavelengths. For instance, the first range may have a transmission maximum corresponding broadly to red light, the second range may have a transmission maximum corresponding broadly to green light, and the third range may have a transmission maximum corresponding broadly to blue light.
According to a second aspect of the present invention, there is provided a wavelength dependent sensor comprising a photodetector and a filter according to the first aspect of the present invention for selecting wavelengths to be transmitted to the photodetector.
Advantageously the photodetector is an image sensor. It is thus possible to provide a colour camera.
According to a third aspect of the present invention, there is provided a wavelength controllable light source, comprising a source of light and a filter according to the first aspect of the present invention for selecting wavelengths to be transmitted.
Advantageously the light source may be spatially modulated. It is thus possible to provide a colour display.
The present invention will further be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram of a camera comprising an image sensor and a filter constituting an embodiment of the present invention;
Figure 2 is a graph of a transmission characteristic of a filter constituting an embodiment of the invention and having crossed polarisers;
Figure 3 is a graph of a transmission characteristic of a filter constituting an embodiment of the present invention and having parallel polarisers;
Figure 4 is a schematic diagram of a display constituting an embodiment of the present invention;
Figure 5 is a graph of control voltage versus time for the display of Figure 4; and
Figure 6 is a graph of the spectrum of a fluorescent tube suitable for backlighting a spatial light modulator.
A wavelength tunable filter 2 comprises a first polariser 4 arranged to polarise light along a first direction, a s cell 6, and a second polariser 8 arranged to polarise light along a second direction. The polarisers 4 and 8 and the s cell 6 are optically arranged in series, with the n: cell 6 between the first and second polarisers. The 7e cell 6 is arranged to act as a variable retarder. The optic axis of theft cell 6 is at an angle of substantially 450 with respect to the first direction.
The second polariser 8 may be crossed or parallel with respect to the first polariser 4.
The wavelength tunable filter 2 is situated in the optical path to a detector array 10, such as a charge coupled device. The array 10 is positioned in an image plane of focusing means, such as a lens or lenses (not shown) so that an image is formed thereon. The detector array comprises a plurality of broad band photodetectors.
When the first and second polarisers are crossed, the s cell 6 can be controlled so as to either select a wavelength for maximum transmission, or to cause the filter to transmit substantially no light. A large voltage, for example 25V, applied to the Tc cell 6 causes the cell 6 to have substantially no optical activity. Thus the plane of polarisation of light passing through the cell 6 remains crossed with respect to the second polariser 8 and is not transmitted thereby.
When lower voltages are applied across the = cell 6, it functions as a half-wave plate. However, the wavelength XH at which the cell 6 functions as a half-wave plate is voltage dependent. Wavelengths longer and shorter than xH experience phase changes between components of light parallel and perpendicular to the optic axis of the phase plate which are not equal to (2m + 1 )x radians, where m is an integer which defines the order of the half-wave plate, (e.g. m = 1 for second order).
The action of a half-wave plate is such that the angle between the plane of polarisation of light exiting the plate and the plane of polarisation of light entering the plate is twice the angle between the plane of polarisation entering the plate and the optic axis of the half-wave plate.
Thus, since the optic axis of the Jr cell 6 is at substantially 45" to the plane of polarisation of the first polariser, the light of wavelength ?.H leaving the cell 6 is rotated by substantially 90" and consequently is parallel to the axis of the second polariser 8.
The transmission coefficient T of a filter having crossed polarisers is given by:
T = sin2 ( 7r Afleff d/X) (1) where Afleff represents voltage dependent birefringence,
d is the thickness of the cell, and X represents wavelength.
Maxima in transmission occur when ( /niff d/X) = (2x +1)/2 (2) where x is an integer.
Thus, for a given thickness d of the cell 6, the voltage dependent birefringence Aneff can be varied to select the wavelength X at which maximum transmission occurs. For devices such as a colour camera and a colour display, the filter 2 is controlled so as to sequentially select three transmission bands having maximum transmission at wavelengths falling within the red, green and blue regions of the electromagnetic spectrum, respectively. Furthermore, it is advantageous that the transmission bands be overlapping but that, for any given voltage applied to the cell 6, only one peak in the transmission spectrum falls within the range of human colour vision.
A suitable spectral response can be obtained by operating the device as a second order filter, i.e. X=1. Such an arrangement allows three transmission bands, as shown in Figure 2, each of which has only one peak in the visible spectrum. The cell 6 should be sufficiently thick so that reasonably large voltage differences are required in order to select each of the centre wavelengths of the transmission bands.For a cell 6 which is 14 microns thick and which contains E7 (E7 is the name given to a liquid crystal material manufactured by Merck Limited) as the liquid crystal, so that Aneff at V=0 has a value of 0.21, then solving equation (1) for (Afleff.dIX) =1.5 gives transmission peaks as 630nm (red) for an applied voltage Vr of 1.3 volts, 540nm (green) for an applied voltage Vg of 2 volts, and 450nm (blue) for an applied voltage Vb of 2.5 volts. The above applied voltages give rise to values of Aneff equal to An/3, 0.28AnOff and 0.23AnOff, respectively, where anOff is the value of Aneff at zero volts.The cell 6 can also be switched to a substantially nontransmitting state by application of a voltage of approximately 20 to 30 volts. The transfer characteristics for the applied voltages Vr, Vb and Vg are shown in Figure 2 by curves 14, 16 and 18, respectively.
In order to function as a colour camera, the filter must be switched at three times the video rate. Thus, the filter characteristic must be changed approximately every 6ms. Consequently the switching time from one filter response to the next must be less than 3ms.
A retardation plate may be placed in series with the cell 6 (between the polarisers 4 and 8) so as to reduce the thickness of the cell that is required. This is advantageous since a thinner cell operates more quickly. For example, a fixed retardation plate arranged to produce a retardation of 0.6 microns requires that the values of (neff.d) for maximum transmission at 450, 540 and 630nm be 0.075, 0.21 and 0.345, respectively. These can be realised by a 6 micron thick cell operating at voltages Vr=2.25, Vg=2.5 and Vb=10 volts.
The polarisers 4 and 8 may also be arranged with their directions of polarisation parallel to one another. The transmission coefficient T of a filter having parallel polarisers is given by:
T = cos2 (7: Afleff d/h) (3) where Afleff represents voltage dependent birefringence,
d is the thickness of the cell, and
X represents wavelength.
Maxima in transmission occur when
( Aneff dlh) = x (4) where x is an integer.
The parallel polariser filter does not have an opaque state, since at V=0 there is a colour dependent fixed retardation. For a 14 micron cell filled with E7 (i.e. of the type whose characteristics are illustrated in Figure 2) but having parallel polarisers, the value of Afleff for a red filter is 0.42 AnOff. This would require operating the cell 6 at very low voltages where it may transfer into a 7: twist state. However a 18.5 micron thick cell requires larger driving voltages to be used and would prevent the cell transferring to a 7e twist state. Figure 3 shows an example transmission characteristic for a filter having a 18.5 micron thick cell filled with E7 and having parallel polarisers. The cell is operated in a second order, i.e. X=2.The cell thickness was chosen so as to obtain peaks in the transmission characteristics 14', 16' and 18' at the same wavelengths and operating voltages as for the filter having crossed polarisers whose transmission characteristics 14, 16 and 18 are illustrated in Figure 2.
As an alternative to using a thicker cell, a fixed retarder may be used to reduce the required cell thickness to obtain second order operation. A fixed retarder having a value of (Afleff.d) =0.825 microns allows the use of a 6 micron cell with operating voltages of Vr= 1.5, Vg=2.75 and
Vb = 10 volts.
A device having parallel polarisers has a transmission characteristic which is more suited for use in a display apparatus since the overlap between adjacent characteristics is reduced. A display apparatus incorporating an electrically controllable filter is illustrated in Figure 4.
A light source 20, a spatial light modulator 22, such as a pixelated liquid crystal device, and a filter 2 are arranged in series. The spatial light modulator and the filter are controlled by a controller 24 such that the image data presented to the spatial light modulator 22 is in synchronism with the colour selected by the filter. Thus, red, green and blue images are presented in a time multiplexed manner in order to build up a full colour image. Alternatively the display apparatus may be used to provide a monochrome display having an arbitrary colour.
Figure 5 shows an example of how the voltage to the cell 6 may be controlled to provide time division multiplexing of the light transmitted by the cell 6 in order to build up a full colour image.
Figure 6 shows the spectral power distribution of a fluorescent tube suitable for use as a backlight for a spatial light modulator in a colour display. The phosphors used within the tube are selected to have relatively narrow peaks in intensity at or near the maximum transmission wavelengths for the red, green and blue filters formed by the electrically controllable filter 2. The peaks in the fluorescent tube spectrum are of a narrower width than the passbands of the coloured filters formed by the electrically controllable filter 2, thereby allowing each single colour to comprise a relatively narrow range of wavelengths.
It is thus possible to provide a robust and fully controllable electrically controllable filter. Such a filter may be used in cameras and displays and allows an effective increase in resolution compared to systems requiring three monochrome spatially distinct pixels to form one colour pixel; i.e.
by providing time-multiplexed colour separation, spatial resolution need not be sacrificed to provide the colour separation. The filter may be used in a bandstop mode as well as a bandpass mode.
Claims (21)
1. An electrically controllable wavelength filter, comprising a first polariser, an electrically controllable wavelength dependent phase plate, and a second polariser, the phase plate being in a light path between the first and second polarisers.
2. A filter as claimed in Claim 1, in which the electrically controllable wavelength dependent phase plate comprises a liquid crystal device.
3. A filter as claimed in Claim 2, in which the liquid crystal device is a 7C cell.
4. A filter as claimed in any one of the preceding claims, further comprising control means for applying a control signal to the electrically controllable phase plate for controlling a transmission characteristic of the filter.
5. A filter as claimed in any one of the preceding claims, further comprising a substantially fixed retarder.
6. A filter as claimed in any one of the preceding claims, in which the first and second polarisers are parallel.
7. A filter as claimed in any one of Claims 1 to 5, in which the first and second polarisers are crossed.
8. A filter as claimed in any one of the preceding claims in which, in use, the electrically controllable wavelength dependent phase plate is operated in a second order mode.
9. A wavelength dependent sensor, comprising a photodetector sensitive to a first band of wavelengths, and a filter as claimed in any one of the preceding claims for selecting a narrower band of wavelengths for transmission to the photodetector.
10. A sensor as Claimed in Claim 9, in which the photodetector is an image sensor.
11. A sensor as claimed in Claim 10, in which the filter is sequentially controlled to pass first, second and third ranges of wavelengths to the image sensor.
12. A sensor as claimed in Claim 11, in which the first range of wavelengths has a transmission maximum corresponding broadly to red light, the second range has a transmission maximum corresponding broadly to green light, and the third range has a transmission maximum corresponding broadly to blue light.
13. A wavelength controllable light source comprising a light source for emitting light over a range of wavelengths and a filter as claimed in any one of Claims 1 to 8 for selectively transmitting a range of wavelengths produced by the light source.
14. A wavelength controllable light source as claimed in Claim 13, in which the light source is spatially modulated.
15. A wavelength controllable light source as claimed in Claim 13 or 14, in which the source of light comprises a backlit spatial light modulator.
16. A wavelength controllable light source as claimed in Claim 15, in which the spatial light modulator is a liquid crystal device.
17. A wavelength controllable light source as claimed in any one of
Claims 13 to 16, in which the filter is sequentially controlled to pass first, second and third ranges of wavelengths.
18. A wavelength controllable light source as claimed in Claim 17, in which the first range of wavelengths has a transmission maximum corresponding broadly to red light, the second range has a transmission maximum corresponding broadly to green light, and the third range has a transmission maximum corresponding broadly to blue light.
19. An electrically controllable wavelength filter substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.
20. A display substantially as hereinbefore described with reference to
Figures 1 to 6 of the accompanying drawings.
21. A camera substantially as hereinbefore described with reference to
Figures 1 to 3 of the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9401192A GB2286056A (en) | 1994-01-21 | 1994-01-21 | Electrically controllable wavelength filter |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9401192A GB2286056A (en) | 1994-01-21 | 1994-01-21 | Electrically controllable wavelength filter |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9401192D0 GB9401192D0 (en) | 1994-03-16 |
| GB2286056A true GB2286056A (en) | 1995-08-02 |
Family
ID=10749161
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9401192A Withdrawn GB2286056A (en) | 1994-01-21 | 1994-01-21 | Electrically controllable wavelength filter |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2286056A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5990991A (en) * | 1996-06-26 | 1999-11-23 | Sharp Kabushiki Kaisha | Twisted liquid crystal surface mode device |
| US6144433A (en) * | 1996-06-26 | 2000-11-07 | Sharp Kabushiki Kaisha | LCD in which two regions of liquid crystal layer act as two optical retarders having inclined optic axes |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1469638A (en) * | 1973-07-18 | 1977-04-06 | Secr Defence | Liquid crystal display device |
| GB2011641A (en) * | 1977-12-27 | 1979-07-11 | Hughes Aircraft Co | Electro optic tunable optical filter |
| WO1984004402A1 (en) * | 1983-04-21 | 1984-11-08 | Beckman Instruments Inc | Liquid crystal tuned birefringent filter |
| GB2196751A (en) * | 1986-09-03 | 1988-05-05 | Gen Electric Plc | Optical switching device |
-
1994
- 1994-01-21 GB GB9401192A patent/GB2286056A/en not_active Withdrawn
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1469638A (en) * | 1973-07-18 | 1977-04-06 | Secr Defence | Liquid crystal display device |
| GB2011641A (en) * | 1977-12-27 | 1979-07-11 | Hughes Aircraft Co | Electro optic tunable optical filter |
| WO1984004402A1 (en) * | 1983-04-21 | 1984-11-08 | Beckman Instruments Inc | Liquid crystal tuned birefringent filter |
| GB2196751A (en) * | 1986-09-03 | 1988-05-05 | Gen Electric Plc | Optical switching device |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5990991A (en) * | 1996-06-26 | 1999-11-23 | Sharp Kabushiki Kaisha | Twisted liquid crystal surface mode device |
| US6144433A (en) * | 1996-06-26 | 2000-11-07 | Sharp Kabushiki Kaisha | LCD in which two regions of liquid crystal layer act as two optical retarders having inclined optic axes |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9401192D0 (en) | 1994-03-16 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |