The invention is particularly suited for the selective ablation of a surface, and is of particular use in the cosmetic treatment of skin defects and removal of unwanted tatoos. The invention is also suited to the field of corneal tissue ablation, particularly photorefractive keratectomy.

High intensity light, in particular laser light, is currently used in many applications for irradiating surfaces, for example in order selectively to ablate a surface. A particular field in which such high-intensity light sources are useful is that of cutaneous cosmetic surgery. In current such devices the light (usually laser light) is delivered as a focussed or collimated beam directly onto the skin of the patient. Such beams may have a circular profile or may be masked to produce, for example, a rectangular profile. The beam passes down a hand-held delivery"light pen"which is manually traced across the skin by the therapist or surgeon.

Some current designs use a high-power light source, such as a laser, masked in the shape of a square, rectangle or circle to illuminate and define the area of skin to be treated.

Delivery of the high-intensity light to the patient's skin is manually controlled by the therapist. Great care must be taken to trace the pen across the affected area of the patient's skin in a controlled manner, ensuring that exposure to the high-intensity light is as uniform as possible while avoiding overlap.

This method is not very satisfactory in that it results in an approximately controlled exposure under the manual control of the operator and is open to considerable human error. The exposure is not accurate or reproducible since there is no way of ensuring uniform illumination and avoiding overlaps across the area of the defect. This method of illumination often results in over-or under-exposure, the former causing burns in the overlapped regions and the latter necessitating a further course of treatment. Thus, inconsistent and variable results are obtained between patients, making it difficult quantitatively to assess the efficacy of a particular technique for a particular type of defect.

Another disadvantage of the present technique is that the therapist must be very conscious of any unexpected or inadvertent movement of the patient, and must react to any such movement promptly and accurately to avoid damage to the patient's skin.

There is therefore a requirement for an improved apparatus and method avoiding the above-mentioned disadvantages. In particular, there is a requirement for an apparatus and method which allows precise control of the high intensity light incident upon a surface. There is also a requirement for such a device that is simple to use and safe in operation. There is a particular requirement for such a device which is dynamic and can adaptively react to changes in the surface, such as for example inadvertent movement.

It is an object of the present invention to fulfil the above requirements.

According to one aspect of the present invention there is provided apparatus for selectively irradiating a surface, the apparatus comprising a light source for generating a beam of light directed towards the surface, a spatial light modulator

being interposed in the path of the light beam towards the surface, the spatial light modulator being selectively configurable so as to modulate the light which is incident upon the surface, whereby the surface can be selectively irradiated.

The apparatus may be provided with means for capturing data representing an image of the surface, the captured data being used to generate a particular configuration to be applied to the spatial light modulator, whereby the light incident upon the surface is controlled in response to the captured data.

According to another aspect of the present invention there is provided a method of selectively irradiating a surface, the method comprising the steps of: a. generating a beam of light; b. causing the beam of light to interact with a selectively configurable spatial light modulator so as to selectively spatially modulate the beam of light; and c. directing the modulated beam towards the surface.

The invention may comprise the following additional steps: d. capturing data representing an image of the surface; and e. using the captured data to generate a particular configuration to be applied to the selectively configurable spatial light modulator in step b. above, whereby the light incident upon the surface is controlled corresponding to the captured image data.

The spatial light modulator may modulate the amplitude, polarisation or phase of the light incident upon the spatial light modulator and hence the intensity of the light incident upon the surface to be irradiated. In particular where the phase of the light is being modulated, the intensity of light

may be determined by an optical (for example, Fourier or Fresnel) transform.

The light source may be a laser light source or any other suitable light source of predetermined wavelength and intensity.

The spatial light modulator may be a transmissive spatial light modulator, the light beam passing through the modulator and being modulated before being incident upon the surface.

Alternatively, the spatial light modulator may be a reflective light modulator, the light beam being reflected from the modulator and being modulated before being incident upon the surface.

Means may be provided for selecting the wavelength of the generated light beam. In this respect, a number of light sources may be selectively provided, each light source generating light of a different wavelength or range of wavelengths.

A number of spatial light modulators may be provided, means being provided to select a particular spatial light modulator to be used. Each spatial light modulator provided may be suitable for use with light of a particular range of wavelengths, means being provided to select a particular spatial light modulator compatible with the wavelength of the generated light beam.

The surface may comprise patient body tissue. For example, the body tissue may comprise the skin of a patient, the apparatus or method being used to selectively irradiate the skin of a patient.

Alternatively, the body tissue may comprise the cornea of a patient, the apparatus or method being used to selectively irradiate the cornea of a patient. The selective irradiation may result in selective ablation of the cornea. The configuration applied to the spatial light modulator may correspond to a pattern to be selectively ablated from the cornea, whereby the curvature of the cornea may be altered.

The irradiation may result in the selective ablation of the surface.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 shows schematically an embodiment of the present invention; and Figure 2 shows schematically another embodiment of the present invention.

Figure 1 shows a device according to one embodiment of the present invention. In Figure 1, a light source such as a laser 1 generates a beam of high-intensity light. The beam is expanded by beam expansion optics 2,3 and passes through a beam conditioning element 4 which may for example consist of diffractive optical elements or lenslet array homogenising optics. The uniform, expanded beam then passes through a transmissive spatial light modulator (SLM) 6. The SLM may be of any known type such as a magneto-optic SLM, a liquid crystal based SLM, a pixellated micro-mirror-based SLM or any other suitable type. Such SLMs are discussed, for example, in United States Patent No. 5,418,380.

The SLM 6 modulates the amplitude, polarisation and/or phase of the expanded beam of light (in a binary or multi-level manner) in accordance with the pattern displayed at that time on the SLM device, so that it acts as a mask for the light beam. The light then passes through a beam splitter 5, optionally a polarising beam splitter, and an imaging lens system 8, such as an achromatic imaging lens system, to a surface 9, such as a patient's skin, to be irradiated.

The surface is separately illuminated with ambient light and/or with a separate incoherent light source (such as a lamp), not shown. Light reflected from the surface passes back to the beam splitter 5 and, because it is unpolarised, is split and a proportion passes through a filter and/or shutter 13 and a relay lens 12 to a CCD or other sensor 11 which is positioned in the imaging plane of the lens system formed by the imaging lens system 8 and the relay lens 12.

The sensor 11 therefore acquires an image of the defect 10 on the surface 9, the configuration of which is to be applied to the SLM. To accomplish this, data from the CCD sensor 11 is passed to a digital camera interface 14 and into a computer 15. The image is processed in the computer 15 to provide data, which is then supplied to the SLM 6. In this way, the masking pattern displayed on the SLM 6 and used to modulate the incident light beam corresponds to the image of the defect 10 to be treated. The light beam can therefore be imaged precisely to the shape of the defect 10 so that only that particular area of the surface 9 is irradiated.

Furthermore, since the cycle time of image acquisition, processing and display on the SLM can be of the order of a few milliseconds, the beam profile can rapidly and dynamically be adapted to correspond to the view captured by the CCD sensor 11. In this way, the system can react rapidly to inadvertent or unexpected small movements of the surface

9, so that the surface to be treated does not have to be held absolutely rigid during treatment. The imaging lens system is also arranged to be of a high f number so that the lens to surface distance does not have to be absolutely constant in order to obtain a sufficiently well-focussed image on the surface.

The light source 1 may be pulsed and in such a case the sensor 11 can be protected against undesired scattered light by providing a shutter 13 to block the path between the beam splitter 5 and the sensor 11 when the light source 1 is energised and to allow light to pass for generating the mask when the light source 1 is de-energised. Alternatively or additionally a bandpass filter 13 may be positioned in the path between the beam splitter 5 and the sensor 11 to block radiation having the wavelength of the light source 1, but to allow the broader band scene illumination to pass.

In the device of Figure 1, alternative light sources 16,17 are also provided. This may be useful when it is required to use light of a number of different wavelengths. For example, when effecting laser removal of a coloured tattoo, light of different wavelengths may be required to remove different regions of the tattoo. In this situation, the CCD sensor 11 and digital camera interface 14 must be capable of capturing colour images. The colour image of the tattoo is then processed and a pattern corresponding to that part of the tattoo which is of a particular colour is supplied to the SLM 6. Irradiation is then initiated using light of a suitable wavelength to treat that particular colour. Thus, only the selected region of the skin surface is irradiated by light of that particular wavelength, which may vary from the ultra- violet to the infra-red. When it is desired to treat a different region, the image captured by the CCD sensor 11 is processed so that a different colour is selected, and a

corresponding light source is selected accordingly. The SLM to be used must then be capable of high-contrast binary transmission over a suitable bandwidth, for example several hundred nanometers. Substitution of a suitable alternative SLM (not shown) may be required for some wavelength ranges.

Since this technique offers accurate and uniform irradiation of the skin, beam energy may be selected to be very low and a number of exposures may be required so as to treat the skin without ablating, shocking, burning or other damage.

Alternatively a single exposure with greater beam energy may be required, either to ablate off a certain skin area or to illuminate the affected area once. The system provides precise, accurate, dynamic and easily reproducible irradiation.

Figure 2 shows an alternative device according to an embodiment of the present invention and the same references are used to denote the same or similar components. In the embodiment of Figure 2 a reflective SLM 7 is used in place of a transmissive SLM.

In Figure 2, a beam of high-intensity light from a light source such as a laser 1 is expanded by beam expansion optics 2,3 and passes through a beam conditioning element 4. The uniform, expanded beam then passes through beam splitter 5 which splits the beam, causing some light to be reflected towards reflective SLM 7.

As discussed above, the SLM 7 modulates the amplitude, polarisation and/or phase of the expanded beam of light in accordance with the pattern displayed at that time on the SLM device, so that it acts as a mask for the light beam. The light then passes through imaging lens system 8 towards the surface 9 to be irradiated. Thus, the SLM may be programmed

with a pattern corresponding to the defect 10 to be treated, so that only that particular area of the surface is irradiated.

In the device of Figure 2, the pattern to be supplied to the SLM 7 is acquired by sensor 11 which is positioned in the imaging plane of the lens system formed by the imaging lens system 8 and a relay lens 12 and therefore acquires an image of the defect 10 on the surface 9. Data from the sensor 11 is passed to interface 14 and into computer 15. The image is processed in the computer 15 to provide data which is supplied to the SLM 7. Thus, the masking pattern displayed on the SLM 7 and used to modulate the incident light beam corresponds to the image of the defect 10 to be treated. The light beam can therefore be imaged precisely to the shape of the defect 10 so that only that part of the surface is irradiated.

As with the device of Figure 1, alternative light sources 16 and 17 may be provided.

As with the embodiment of Figure 1, the surface is separately illuminated with ambient light and/or with a separate incoherent light source (not shown). Light reflected from the surface 9 passes back to the beam splitter 5 and is split and a proportion passes to sensor 11 by way of filter/shutter arrangement 13 which is positioned in the path between the beam splitter 5 and the sensor 11 in order to protect the sensor 11 from undesired light from the light source 1.

The device and method according to the invention may be useful in many fields which involve selective irradiation of a surface with light including selective removal of painted patterns or defects in such applications as paint striping signs or emblems on an undercoated painted or otherwise

fragile structure such as an aircraft body or valuable work of art, and selective error correction of particular patterns and colours on valuable printed matter. The invention is of particular application to those fields which require selective irradiation of a patient's body tissues, particularly the skin. Examples of such applications are the removal of tattoos, undesired skin hair, pigmented lesions, naevi, lentigos, freckles, port wine stains, café-au-lait and other skin marks and lesions. The invention may also be useful in many other fields of light illumination therapy or cosmetic surgery.

A further application of the invention is in the field of ocular surgery, for example in photorefractive keratectomy.

In this procedure, patterns are ablated from the surface of the cornea to change its curvature and correct various deficiencies of vision such as near-and far-sightedness.

The invention could be used to capture a dynamic image of the eye so that the required mask could accurately be aligned with the corneal surface. The mask pattern would be written to the SLM and the transmitted optical beam would then be imaged to the surface of the cornea with the desired pattern.

A further development of this technique would involve writing a Fourier Transform phase only hologram to the SLM. (In this case, the SLM would need to be capable of phase modulation).

A Fourier transform of this phase hologram would then be focussed onto the corneal surface. The resulting intensity pattern corresponds to the required pattern for corneal sculpturing. The phase distribution corresponding to the required intensity patterns may be determined, for example, by well-known iterative algorithms such as those of Gershberg and Saxton. Alternatively, the SLM may be a fully complex filter as explained, for example, by Florence and Juday in Proc. SPIE 1558 (1991) 487-498. This has the advantage that the distribution applied to the SLM has a direct Fourier Transform relationship to the desired intensity distribution and so can be quickly and easily calculated via a discrete Fourier Transform on a digital computer.