GB2403812A

GB2403812A - Electrically controllable colour filter

Info

Publication number: GB2403812A
Application number: GB0316011A
Authority: GB
Inventors: Justin Noel Staines
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-07-09
Filing date: 2003-07-09
Publication date: 2005-01-12
Also published as: GB0316011D0

Abstract

This electrically controllable tunable colour filter comprises a polarizer (6) and quartz plate (8). The angle of the polarization plane is rotated depending on the wavelength of the light. The desired portion of the spectrum can then be selected by matching the rotational angle of the polarization plane of the LCD shutter (10) with that of the desired wavelength created when passing through a uniaxial crystal (such as quartz) plate (8), or a high index refractive glass with an inductive coil (11). The desired portion of the spectrum then passes onto the CMOS or CCD image sensor (4). This process is repeated three times with a different voltage applied to (10) to capture each of the R, G and B parts of the image. These can then be combined to give a 100% spatial image with no degradation in display quality. The quartz plate can be replaced by a Faraday rotator. The inductive coil of the Faraday device is linked to the control voltage of the liquid crystal device.

Description

1 240381 2

ELECTRICALLY CONTROLLABLE TUNABLE COLOUR FILTER AND

MANUFACTURE THEREOF

This invention relates to the design of a tunable colour filter for the optical sensor of a digital camera, and the temporal integration of sequentially presented image fields of alternating primary colours (R,G,B) .

The problem with existing colour filters are that they are made of high density glass, but the transmitting spectal band is usually fixed. Either a different colour filter has to be used to select a different spectral band, or a Bayer pattern colour filter such as described in Figure 3. When a colour filter is used with a single sensor, each individual cell in the sensor gathers light of only one particular colour. To reconstruct a complete colour image, an interpolation is needed. The red, green and blue information is interpolated across several adjacent cells to determine the total colour content of each individual cell. Figure 3 shows a portion of the Bayer filter. The problem with a single sensor camera using a Bayer filter is lower then full spatial resolution (65% of the full horizontal resolution and 80% of the full vertical resolution) It is also possible two have a 3 sensor camera, and split the incident beam into its constituent ROB parts using a prism (as detailed in Figure 2). You get 100% spatial, but this is quite expensive and only possible in professional cameras.

One solution to have full spatial resolution (1 pixel, 3 colours) is to use the wavelength light absorption properties of silicon (see Figure 4). A semi-conductor multi-colour tunable filter is created by multiple epitaxial layers of varying composition. An AC modulation voltage is superimposed on a sweep bias to periodically shift the fundamental absorption edge of each individual epitaxial layer. This arrangement permits a unique voltage to be associated with a particular colour. A lock-in filter matches colours observed at a detector with the voltage present for a given observation. See US4350413 1982-09-21 BOTTKA NICHOLAS; LESSOFF HOWARD (Multi-Colour tunable filter) and US4492434 1985-01-08 BOTTKA NICHOLAS (US); HILLS MARIAN E (US); LESSOFF HOWARD (US) (Multi-colour tunable semiconductor device). However one problem ! associated with this is that as the incident beam passes through the semiconductor the intensity of light reduces and passes through each layer of silicon, so although if the top layer is detecting say blue this will be at 100% light intensity, the bottom layer which is detecting say red will be at a much lower light intensity.

An object of this invention is to create a tunable colour filter by using the optical activity dispersion in a uniaxial crystal (such as quartz) plate and the birefringence of a liquid crystal shutter with an electric field applied. Optical dispersion activity was discovered in 1811 by Blot, and US4305046 (LE FLOCH ALBERT; LE NAOUR ROGER 1981-12-08) describes how a tunable laser uses these properties. Prior to this US2834254 1958-05-13 SAGE STANLEY J discloses a similar methodology, but with no means to then electronically select the resultant beam.

Colour dependent birefringence filters have been used with image sensors before, but not as temporal integration of sequentially presented image fields of alternating primary colours (R,G,B).merely to resolve green colour imbalance (see US4605956 1986-08-12 COK DAVID R (US) ).

Also in information displays, it has been found to use electrically controlled birefringence of liquid crystals (albeit using a dye compound) to change pixel colours in displays (see EP0336351 1989-10-11 SILVERSTEIN LOUIS D; BERNOT ANTHONY J).

The preferred embodiment of this invention will now be described together with the accompanied drawing (Fig 1). The incident beam passes through a polymer polarizer (6) so the light is only in one polarity, which then is dispersed through the uniaxial crystal such as quartz (8). The optical dispersion and birefringence properties of a thin quartz plate (less than 5mm, greater than lmm, in this instance 1.7 mm) then rotates the polarization plane depending on the wavelength of the light. The resultant beam then passes through the liquid crystal shutter (10). The rotational angle of the polarization plane of the liquid crystal shutter is directly proportional to the voltage across the liquid crystal shutter, thus controlling the desired portion of the spectrum.

So for example if the light is of wavelength \, and it is rotated by the uniaxial crystal to a - 3 polarization plane of angle Q then the liquid crystal shutter will only allow this wavelength through when the applied voltage across the liquid crystal shutter is equivalent to the same polarization plane angle 0. Light of different wavelengths will have different angles of rotation for the polarization plane.

The transmission coefficient can be described by the Jones Matrix in Figure 5.

The desired portion of the spectrum then passes onto the CMOS or CCD image sensor (4).

For a single image, this process is repeated three times with a different voltage applied to (10) to capture each of the Red, Green and Blue parts of the spectrum.

A further embodiment of this invention uses a high density glass or similar medium surrounded by a coil - Figure 6(9) - instead of the uniaxial crystal (8). An electric current flows through the coil that is enhances by a ferrite core (11) and this generates an electromagnetic field. This electromagnetic field will also alter the polarization plane of the light passing through the medium as defined by the laws described by Faraday.

Claims

1. A tunable colour filter that uses the optical activity dispersion in a uniaxial crystal (such as quartz) plate to rotate the polarization plane of the light proportional to the wavelength of the incident light.

2. A tunabled colour filter as claimed in 1 where the uniaxial crystal (such as quartz) plate has a thickness no greater than 5mm, no thinner than Imm, and an optimum thickness of 1.7mm.

3. A tunable colour filter as claimed in any preceding claim that uses the birefringence properties of a liquid crystal shutter, to select the desired portion of the spectrum by the applied electric field and only allowing the desired wavelength which has the equivalent rotation angle of the polarization plane.

4. A tunable colour filter as claimed in any preceding claim, but instead of using the optical activity dispersion properties of a uniaxial crystal (such as quartz), has a high density glass or similar medium surrounded by an inductive coil which sets up an electromagnetic field, which in turn rotates the polarization plane of the light in accordance with the Faraday effect.

5. A tunable colour filter as claimed in any preceding claim, whereby the applied voltage across the inductive coil used to rotate the polarization plane ofthe light, is linked either directly using the same circuit, or indirectly using coupled bifilar secondary windings, to the control voltage of the LCD shutter, which in turn controls the secondary polarization plane and subsequently the desired portion of the spectrum automatically. Further more the electromagnetic field can be enhanced by a ferrite core. - s -

6. A tunable colour filter as claimed in any preceding claim and the temporal integration of sequentially presented image fields of alternating primary colours (R. G. B).

7. A tunable colour filter as claimed in any preceding claim, comprising a thin optical stack as illustrated in Figure 1, which fits over a standard monochrome CMOS or CCD based optical sensor.

8. A tunable colour filter as claimed in any preceding claim, comprising an optical stack as illustrated in Figure 1, which uses the shape of the lens (8) to determine the focal length of the aforementioned optical stack incident on the image sensor (4), thus removing the need to have an external lens on the camera.

9. A tunable colour filter substantially as herein described above and illustrated in the accompanying drawings.