WO2007049231A1

WO2007049231A1 - System for shifting at least one light spot using liquid crystal cells

Info

Publication number: WO2007049231A1
Application number: PCT/IB2006/053920
Authority: WO
Inventors: Levinus Pieter Bakker; Robert Frans Maria Hendriks
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-10-28
Filing date: 2006-10-25
Publication date: 2007-05-03
Also published as: TW200820232A

Abstract

A system for reading an information carrier (501), the system comprising probe array generation means for generating an array of light spots (503). Phase modulating means (506) are provided for shifting the light spots (503) relative to the information carrier (501), the phase modulating means (506) comprising a liquid crystal cell in respect of each light spot (503), and means (922a,b) for applying a variable electric field to each respective liquid crystal cell so as to vary the optical path-length thereof. Each liquid crystal cell comprises a first liquid crystal layer (720, 920) having a plurality of wedge-shaped sub- regions (723), and a second, wedge-shaped liquid crystal layer for fine tuning.

Description

SYSTEM FOR SHIFTING AT LEAST ONE LIGHT SPOT USING LIQUID

CRYSTAL CELLS

FIELD OF THE INVENTION

The invention relates to a system for shifting at least one light spot, for example over an information carrier in view of data recovering.

The invention may be used in the field of optical data storage or microscopy.

BACKGROUND OF THE INVENTION Use for optical storage is nowadays widespread for content distribution, for example in storage systems based on the DVD (Digital Versatile Disk) standards. Optical storage has a big advantage over hard-disk and solid-state storage in that information carriers are easy and cheap to duplicate.

However, due to the large amount of moving parts in the drives, known applications using this type of storage are not robust to shocks when performing read operations, considering the required stability of said moving parts during such operations. As a consequence, optical storage cannot easily be used in applications which are subject to shocks, such as in portable devices.

Thus, a system for shifting at least one light spot, for example over an information carrier, in view of data recovering has been proposed, as described in the following.

Referring to Fig.l of the drawings, such a system for reading data stored on an information carrier 101 comprises an optical element 102 for generating an array of light spots 103 from an input light beam 104, said array of light spots 103 being intended to scan the information carrier 101.

The optical element 102 corresponds to a two-dimensional array of apertures to the input of which the coherent input light beam 104 is applied. The apertures correspond for example to circular holes having a diameter of lμm or much smaller. The input light beam 104 can be realized by a waveguide (not represented) for expanding an input laser beam, or by a two-dimensional array of coupled micro lasers.

The light spots are applied to transparent or non-transparent areas of the information carrier 101. If a light spot is applied to a non-transparent area, no output light beam is generated in response by the information carrier. If a light spot is applied to a transparent area, an output light beam is generated in response by the information carrier, said output light beam being detected by the detector 105.

The detector 105 is thus used for detecting the binary value of the data of the area on which the optical spot is applied.

The detector 105 is advantageously made of an array of CMOS or CCD pixels. For example, one pixel of the detector is placed opposite an elementary data area containing a data of the information carrier. In that case, one pixel of the detector is intended to detect one data of the information carrier.

Advantageously, an array of micro-lenses (not represented) is placed between the information carrier 101 and the detector 105 for focusing the output light beams generated by the information carrier on the detector, improving the detection of the data.

The array of light spots 103 is generated by the array of apertures 102 in exploiting the Talbot effect which is a diffraction phenomenon working as follows. When a coherent light beam, such as the input light beam 104, is applied to an object having a periodic diffractive structure (thus forming light emitters), such as the array of apertures 102, the diffracted lights recombine into identical images of the emitters at a plane located at a predictable distance zθ from the diffracting structure. This distance zθ is known as the Talbot distance. The Talbot distance zθ is given by the relation zθ = 2.n.d²/λ, where d is the periodic spacing of the light emitters, λ is the wavelength of the input light beam, and n is the refractive index of the propagation space. More generally, re-imaging takes place at other distances z(m) spaced further from the emitters and which are a multiple of the Talbot distance z such that z(m) = 2.n.m.d²/λ, where m is an integer. Such a re-imaging also takes place for m = ¹A + an integer, but here the image is shifter over half a period. The re- imaging also takes place for m = ¹A + an integer, and for m = ³A + an integer, but the image has a doubled frequency which means that the period of the light spots is halved with respect to that of the array of apertures.

Exploiting the Talbot effect allows to generate an array of light spots of high quality at a relatively large distance from the array of apertures 102 (a few hundreds of μm, expressed by z(m)), without the need for optical lenses. This allows to insert for example a cover layer between the array of aperture 102 and the information carrier 101 to prevent the latter from contamination (e.g. dust, finger prints...). Moreover, this facilitates the implementation and allows to increase in a cost-effective manner, compared to the use of an array of micro-lenses, the density of light spots which are applied to the information carrier.

Fig.2 depicts a more detailed view of the system, including a detector 205 which is intended to detect data from output light beams generated by the information carrier 201. The detector comprises pixels referred to as 202-203-204, the number of pixels shown being limited to facilitate the understanding. In particular, pixel 202 is intended to detect data stored on the data area 206 of the information carrier, pixel 203 is intended to detect data stored on the data area 207, and pixel 204 is intended to detect data stored on the data area 208. Each data area (also called macro-cell) comprises a set of elementary data. For example, data area 206 comprises binary data referred to as 206a-206b-206c-206d. One pixel of the detector is intended to detect a set of data, each elementary data among this set of data being successively read by a single light spot generated by the array of apertures. This way of reading data on the information carrier is called macro-cell scanning in the following.

Fig.3, which is based on Fig.2, illustrates by a non-limitative example the macro-cell scanning of an information carrier 301.

Data stored on the information carrier 301 have two states indicated either by a black area (i.e. non-transparent) or white area (i.e. transparent). For example, a black area corresponds to a "0" binary state while a white area corresponds to a "1" binary state. When a pixel of the detector 305 is illuminated by an output light beam generated by the information carrier 301, the pixel is represented by a white area. In that case, the pixel delivers an electric output signal (not represented) having a first state. On the contrary, when a pixel of the detector 305 does not receive an output light beam from the information carrier, the pixel is represented by a cross-hatched area. In that case, the pixel delivers an electric output signal (not represented) having a second state.

In this example, each set of data comprises four elementary data, and a single light spot is applied simultaneously to each set of data. The scanning of the information carrier 301 by the light spots 303 is performed for example from left to right, with an incremental lateral displacement which equals the distance between two elementary data.

In position A, all the light spots are applied to non-transparent areas so that all pixels of the detector are in the second state.

In position B, after displacement of the light spots to the right, the light spot to the left is applied to a transparent area so that the corresponding pixel is in the first state, while the two other light spots are applied to non-transparent areas so that the two corresponding pixels of the detector are in the second state.

In position C, after displacement of the light spots to the right, the light spot to the left is applied to a non-transparent area so that the corresponding pixel is in the second state, while the two other light spots are applied to transparent areas so that the two corresponding pixels of the detector are in the first state.

In position D, after displacement of the light spots to the right, the central light spot is applied to a non-transparent area so that the corresponding pixel is in the second state, while the two other light spots are applied to transparent areas so that the two corresponding pixels of the detector are in the first state.

The scanning of the information carrier 301 is complete when the light spots have been applied to all data of a set of data facing a pixel of the detector. Fig.4 depicts a three-dimensional view of the system as depicted in Fig.l. It comprises an array of apertures 402 for generating an array of light spots applied to the information carrier 401. Each light spot is applied and scanned over a two-dimensional set of data of the information carrier 401 (represented by bold squares). In response to this light spot, the information carrier generates (or not, if the light spot is applied to a non-transparent area) an output light beam in response, which is detected by the pixel of the detector 403 opposite the set of data which is scanned. The scanning of the information carrier 401 is performed in displacing the array of apertures 402 along x and y axes.

The array of apertures 402, the information carrier 401 and the detector 403 are stacked in parallel planes. The only moving part is the array of apertures 402.

The scanning of the information carrier by the array of light spots is done in a plane parallel to the information carrier. A scanning device provides translational movement of the light spots in the two directions x and y for scanning all the surface of the information carrier and, although such scanning may be performed mechanically, in order to make the system as robust as possible, scanning of the information carrier is more preferably realised without moving components.

Referring to Fig.5 of the drawings, which is based on Fig.l but additionally comprises a phase-modulator 506 in the light path of the input light beam 504, such non-mechanical scanning is realized by applying a phase profile defined by the phase-modulator 506 to the input light beam 504, and in varying this phase profile. The phase-modulator 506 varies the phase of the input light beam 504 with respect to the lateral distance x (and/or y).

It is noted that the phase-modulator 506 can also be placed between the array of apertures 502 and the information carrier 501 (not represented).

When the phase-modulator 506 acts so as the phase Φ(x) varies in a linear way with respect to the position x, this leads to a lateral shift Δx of the array of light spots 503 along the lateral axis x. It can be defined that the phase Φ(x) and the lateral position x are linked by the following relation:

where x is the lateral position (taken for example from the extreme left side of the phase-modulator 506), λ is the wavelength of the input light beam 504, a is a variable parameter.

It can be shown that if a phase profile as defined by Eq.1 is performed by the phase- modulator 506, the lateral shift Δx of the array of lights spots 503 is given by the following relation:

Δx = a.Z Eq. 2 where Z is fixed value corresponding advantageously to the Talbot distance zθ, or to an integer multiple or a sub-multiple of the Talbot distance zθ.

The parameter a allows modifying the linearity factor of the phase profile in view of changing the lateral shift Δx. For each value of the parameter a, a different phase profile is defined. A variation of the parameter a results as a consequence in a shift spots in x.

For scanning all the surface of the information carrier 501, each macro-cell data of the information carrier must be scanned by a light spot of the array of spots. The scanning of a macro-cell data thus corresponds to a two-dimensional scanning along the x and y axis.

This two-dimensional scanning is performed in defining simultaneously a linear phase modulation according to the x and y axis, the defined phase profile resulting from a linear combination of a linear phase profile according to the x (as defined by Eq. 1) axis and a linear phase profile according to the y axis (similarly as defined by Eq. 1 in substituting x by y). The phase-modulator 506 comprises advantageously controllable liquid crystal (LC) cells, in which case, each of the apertures of the array of apertures 502 has its own LC cell.

A known solution is to use a wedge-shaped LC cell, where a uniform electrode on both sides is used to change the orientation of the LC-molecules, and hence the optical path- length in the wedge. Since each aperture of the array of apertures has its own LC cell, the phase profile defines a ramp having incremental steps, the ramp globally fitting a linear equation defined by Eq. 1 above. However, this is not necessarily the most practical solution, since the required phase profile that has to be applied requires a very thick, and hence very slow LC-layer.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a system for shifting at least one light spot wherein a thinner layer of liquid crystal material (relative to the prior art) can be used to effect the required scanning, thereby increasing the speed of the system.

- In accordance with the present invention, there is provided a system for shifting at least one light spot generated from an input light beam, said system comprising

- phase modulating means comprising a liquid crystal cell comprising a first liquid crystal layer having a plurality of regions, each region having a variable thickness, said phase modulating means further comprising means for applying a variable electric field to said first liquid crystal cell so as to vary the optical path-length thereof.

Thus the above-mentioned object is achieved by having an LC-cell consisting of a number of small wedges, preferably in combination with a relatively thin, wedge-shaped LC-cell (for fine tuning) as a phase modulator in a scanning system.

Beneficially, each of said regions of the liquid crystal cell is substantially wedge-shaped.

Preferably, the liquid crystal cell comprises a second liquid crystal layer of variable thickness, that can be used to fine tune the phase profile of the liquid crystal cell according to the electric field applied thereto. The second liquid crystal layer may also be substantially wedge-shaped.

The system may comprise means for generating a plurality of light spots for scanning the information carrier, wherein the phase modulating means comprises a liquid crystal cell in respect of each light spot, wherein each respective liquid crystal cell comprises a liquid crystal layer having a plurality of regions, each region being of variable thickness.

The system may comprise an array of apertures to which is applied an input light beam for generating an array of light spots, wherein the phase modulating means comprises a liquid crystal cell in respect of each aperture.

The present invention extends to a phase modulator for a system as defined above, the phase modulator comprising a liquid crystal cell comprising a first liquid crystal layer having a plurality of regions, each region having a variable thickness, and means for applying a variable electric field to the first liquid crystal layer so as to vary the optical path-length thereof.

These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

Fig.1 illustrates schematically a system for reading an information carrier;

Fig.2 is a schematic diagram illustrating a more detailed view of components dedicated to macro-cell scanning used in the system of Fig.1 ;

Fig.3 is a schematic diagram illustrating the principle of macro-cell scanning; Fig.4 illustrates a three-dimensional view of a system for reading an information carrier;

Fig.5 illustrates schematically a principle component of a non-mechanical scanning arrangement for use in a system for reading an information carrier;

Fig.6 is a schematic cross-sectional view (a) of a single wedge LC cell and a graphical representation (b) of the optical path-length as a function of the position in the wedge;

Fig.7 is a graphical representation of the optical path-length as a function of the position in the LC cell in respect of a multi- wedge LC cell for use in a system according to an exemplary embodiment of the present invention;

Fig.8 is a graphical representation of the optical path-lengths at two different electric field strengths as a function of the position in the LC cell in respect of a multi-wedge LC cell for use in a system according to an exemplary embodiment of the present invention;

Fig.9 is a schematic cross- sectional view of a multi-wedge LC cell for use in a system according to an exemplary embodiment of the present invention; and

Fig.10 illustrates various apparatus and devices comprising a system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The system according to the following exemplary embodiment of the invention aims at reading data stored in an information carrier. The information carrier is intended to store binary data organized according to an array, as in a data matrix. If the information carrier is intended to be read in transmission, the states of binary data stored on the information carrier are represented by transparent areas and non-transparent areas (i.e. light absorbing).

Alternatively, if the information carrier is intended to be read in reflection, the states of binary data stored on the information carrier are represented by non-reflective areas (i.e. light-absorbing) and reflective areas. The areas are marked in a material such as glass, plastic or a material having magnetic properties. The scanning system according to the present invention can be used in any information carrier reader requiring at least one light spot to be shifted over a data layer for example, the Talbot ROM system described in principle above or a holographic reader with angular multiplexing, and the present invention is not necessarily intended to be limited in this regard.

Thus, the present invention proposes a relatively inexpensive optical scanning system, based on liquid crystal (LC) materials. As explained above, instead of the relatively thick, wedge-shaped LC-cell proposed in the past, it is proposed herein to use an LC-cell consisting of a number of small wedges, in combination with a relatively thin LC-cell with a wedge shape as a phase modulator in, for example, a system for reading an information carrier, such as that described above with reference to Figures 1 to 4.

Referring back to Fig.5, the phase modulator 506 comprises a plurality of LC-cells such that each aperture of the array of apertures 502 generating the probe array has its own LC- cell to shift the respective light spot relative to the information carrier 501. For each liquid crystal cell, it is proposed to use a (non-twisted) liquid crystal layer with the director profile parallel to the polarisation of the incoming light 504.

Using this configuration, the index of refraction of the layer can be tuned by applying an electric field in the layer. In this way, the optical path-length (phase profile) in the layer can be controlled.

Fig.6(a) illustrates an LC-cell having a liquid crystal layer, single wedge (of maximum height H) in a substrate 621 having two electrodes 622 by means of which a voltage can be applied across the cell.

Fig.6(b) illustrates graphically, the optical path-length as a function of the position in the wedge 620 for a certain electric field in the liquid crystal layer.

As explained above, a major problem of the single wedge is the slowness of the switching of the LC layer 620, due to its large thickness. In order to make the LC-layer thinner, in accordance with the invention, the single wedge can be divided into a number of regions, and the thickness of the wedge -regions (and hence the optical path-length) can therefore be reduced by a certain amount. This is a similar principle to that of the construction of Fresnel lenses.

Fig.7 shows these regions 723 of the liquid crystal layer 720 of an LC-cell for use in an exemplary embodiment of the invention and the resultant reduction of the optical path- length.

There is a constraint for splitting the single wedge into a number of small sub- wedges 723. This constraint is that only an integer times the wavelength can be subtracted from the optical path-length. This means that at every discontinuity of the profile i.e. at the edges of the sub-wedges, the optical path-length in the structure should make a step equal to an integer times the wavelength. This may be implemented into a static structure as a glass Fresnel lens.

However, when the structure consists of a liquid crystal layer of which the properties change with the electric field in the layer, then the constraint mentioned above is not met for all electric field strengths. This is shown in Fig.8, where first and second optical path- lengths are shown corresponding to different respective electric fields. When the constraint as given above holds for the profile defined by h, it does not necessarily hold for the profile defined by h.

It will now be explained how a profile of sub-wedges can be used in a Talbot-ROM scanner for a system such as that illustrated schematically in Fig.5 and described above. For the Talbot-ROM system, it is necessary to scan (tilt) an input light (e.g. a laser) beam over an angle of at maximum α=12.5 mrad. For a beam diameter d of 16mm, this tilt can be given to the laser beam by guiding it through a structure that imposes a wedged phase profile of maximum height h defined by :

h = ^{d Λ}^^a) = 500 wavelength, λ for λ=400mm. The index of refraction of a typical liquid crystal layer can be varied approximately An=0.3. The thickness H of a single wedge phase shifter should hence be :

d - tan(a) .

H = ^^ « 667 micron

An

This is very thick for an LC layer, it results in a very slow switching time. It is therefore proposed herein to split the single wedge with height h wavelengths into yfh sub-wedges, as already shown in Fig.7 above. Then every sub-wedge has a height of yfh wavelengths.

For h=500, the wedge can be split into 23 sub-wedges with each a height of 22 wavelengths. In this structure, there are 22+1=23 optical path-length (or phase) profiles that meet the constraint as given above. In other words, there are 23 electric field strengths, which result in a phase profile that meets the above-mentioned constraint. The other electric field strengths cannot be used. The 23 phase profiles can be used as a course setting of the phase profile.

In order to have a fine-tuning of the phase profile, it is proposed to use a second LC-layer.

The optical path-length in this layer should be wedged. The height of this layer is again

4h , which is considerably smaller than h. For the example given above, the height of the layer should be 23 wavelengths. In combination with the multi- wedge structure, this single wedge LC-layer can be used to have any value in between the course settings of the multi- wedge structure.

The maximum thickness of the LC-layer is reduced from h to 4h . This means that the switching time of the LC-layer, which is a quadratic function of the thickness, reduces with a factor h. For the 667 micron thick layer the switching time is approximately 178 seconds, so the switching time of the proposed scanner is approximately 0.36 seconds. Note that the combined switching time of the two layers is minimised by giving them the same maximum thickness.

Referring to Fig.9, the multi-wedge LC layer can be made by using a planar first substrate

921a, and a multi- wedge-shaped second substrate 921b with an LC-layer 920 in between and transparent electrodes 922a-922b on the flat sides of the substrates 921a-921b. For manufacturability reasons, it might be beneficial to have the director of the LC -layer 920 in the same direction as the grooves in the upper electrode 922a, since the alignment of the LC molecules by rubbing the substrate is easier in the direction of the grooves. The grooves can be produced in the same way as blazed gratings.

The same principle could be used to have an even faster system consisting of more than 2 layers.

One of the principle additional advantages of a scanning device according to the invention relative to the prior art is that, in the prior art described above, the LC-cell comprises an LC-layer with patterned electrodes, whereas the electrodes for the LC-layer in the present invention do not need to be patterned. Controlling the scanner in one direction only requires two voltages. Note that the single- wedge LC-layer can be implemented for instance by using a wedged LC-layer, with a wedge shaped electric field produced with patterned electrodes, or with a non-plane-parallel electrode system.

As illustrated in Fig.10, the system according to the invention may advantageously be implemented in a reading apparatus RA (e.g. home player apparatus...), a portable device PD (e.g. portable digital assistant, portable computer, a game player unit...), a mobile telephone MT. Each of these devices comprises an opening (OP) intended to receive an information carrier IC (e.g. referred to as 501 in Fig.5), in view of a data recovery.

The shifting system in accordance with the invention may be used in a microscope. Microscopes with reasonable resolution are expensive, since an aberration-free objective lens with a reasonably large field of view and high enough numerical aperture is costly. Scanning microscopes solve this cost issue partly by having an objective lens with a very small field of view, and scanning the objective lens with respect to the sample to be measured (or vice-versa). The disadvantage of this single-spot scanning microscope is the fact that the whole sample has to be scanned, resulting in cumbersome mechanics. Multi- spot scanning microscopes solve this mechanical problem, since the sample does not have to be scanned over its full dimensions, the scanning range is limited to the pitch between two spots. In a microscope in accordance with the invention, a sample is illuminated with the spots that are created by the probe array generating means, and a camera takes a picture of the illuminated sample. By scanning the spots over the sample by means of the shifting system of the invention, and taking pictures at several positions, high-resolution data are gathered. A computer may combine all the measured data to a single high-resolution picture of the sample.

The focus distance can be controlled manually, by looking at a detail of the picture of the sample. It can also be performed automatically, as is done in a digital camera (finding the position in which the picture has the maximum contrast). Note that the focusing of the imaging system is not critical, only the position of the sample with respect to the probes is important and should be optimized.

A microscope in accordance with the invention consists of an illumination device, a probe array generator, a sample stage, optionally an imaging device (e.g. lens, fiber optic face plate, mirror), and a camera (e.g. CMOS, CCD). This system corresponds to the system of Fig. 5, wherein the information carrier (501) is a microscope slide on which a sample to be imaged may be placed, the microscope slide being deposited on a sample stage. Light is generated in the illumination device, is focused into an array of foci by means of the probe array generator, it is transmitted (partly) through the sample to be measured, and the transmitted light is imaged onto the camera by the imaging system. The sample is positioned in a sample stage, which can reproducibly move the sample in the focal plane of the foci and perpendicular to the sample. A position measurement system can be implemented into the stage, or it can be implemented in the system. In order to image the whole sample, the information carrier is scanned by means of the shifting system in accordance with the invention so that all areas of the sample are imaged by an individual probe. Instead of a transmissive microscope as described above, a reflective microscope may be designed. In a reflective microscope in accordance with the invention, light that has passed through the sample is reflected by a reflecting surface of the microscope slide and then redirected to the camera by means of a beam splitter.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice- versa. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A system for shifting at least one light spot generated from an input light beam (104), said system comprising: - phase modulating means (506) comprising a liquid crystal cell comprising a first liquid crystal layer (720, 920) having a plurality of regions (723), each region (723) having a variable thickness, said phase modulating means (506) further comprising means (922a,b) for applying a variable electric field to said liquid crystal cell so as to vary the optical path-length thereof.

2. A system according to claim 1, wherein each of said regions (723) of said liquid crystal cell is substantially wedge-shaped.

3. A system according to claim 1, wherein said liquid crystal cell comprises a second liquid crystal layer of variable thickness.

4. A system according to claim 3, wherein said second liquid crystal layer is substantially wedge-shaped.

5. A system according to claim 1, comprising means (504, 502) for generating a plurality of light spots (503) for scanning an information carrier (501) wherein said phase modulating means (506) comprises a liquid crystal cell in respect of each light spot (503), each respective liquid crystal cell comprising a liquid crystal layer (720, 920) having a plurality of regions (723), each region (723) being of variable thickness.

6. A system according to claim 5, comprising an array of apertures (502) to which is applied an input light beam (504) for generating an array of light spots (503), wherein said phase modulating means (506) comprises a liquid crystal cell in respect of each aperture.

7. A phase modulator (506) for a system according to claim 1, said phase modulator comprising a liquid crystal cell comprising a first liquid crystal layer (720, 920) having a plurality of regions (723), each region (723) having a variable thickness, and means (922a,b) for applying a variable electric field to said first liquid crystal layer (720, 920) so as to vary the optical path-length thereof.

8. A portable device comprising a system as claimed in anyone of claims 1 to 6.

9. A mobile telephone comprising a system as claimed in anyone of claims 1 to 6.

10. A game player unit comprising a system as claimed in anyone of claims 1 to 6.

11. A microscope comprising a system as claimed in anyone of claims 1 to 6.