Holographic image display system

1. A method of displaying an image holographically, the method comprising:

inputting display image data defining said image for display;

processing said image data to determine first image data representing a first spatial frequency portion of said image data and second image data representing a second spatial frequency portion of said image data, wherein said second spatial frequency is higher than said first spatial frequency;

displaying a hologram of said first image data on a spatial light modulator (SLM) to form a holographically generated intermediate real image;

modulating said intermediate real image using said second image data to display said image;

wherein said displaying of said hologram comprises phase modulating light from a laser light source using said spatial light modulator, and wherein said modulating of said intermediate real image comprises intensity modulating said intermediate real image using a second spatial light modulator.

2. A method of displaying an image holographically as claimed in claim 1 wherein said modulating comprises determining, from said second image data modulation data for driving said second spatial light modulator, and wherein said determining of said modulation data comprises scaling said second image data to increase pixel values of said second image data.

3. A method of displaying an image holographically as claimed in claim 2 further comprising controlling an intensity of said light from said laser light source to compensate for said scaling.

4. A method of displaying an image holographically as claimed in claim 1 further comprising projecting light from said modulated intermediate real image, said projecting including forming a second intermediate real image onto a diffuser.

5. A method of displaying an image holographically as claimed in claim 1 wherein said displaying of said hologram of said first image data comprises generating and displaying a plurality of temporal holographic subframes, wherein each of said subframes after a first said subframe includes noise at least partially compensating for noise in one or more previous said subframes.

6. A method of displaying an image holographically as claimed in claim 1 wherein said image comprises a full colour image.

7. A holographic image projection system comprising:

at least one laser light source;

a first spatial light modulator (SLM) to phase modulate light from said at least one laser light source;

intermediate optics to provide an intermediate real image plane at which a real image produced by a hologram on said first SLM is formed;

a second SLM located at said intermediate real image plane to intensity modulate said real image; and

output optics to project an image of said intensity modulated real image; and

wherein a resolution of said second SLM is greater than a resolution of said first SLM.

8. A holographic image projection system as claimed in claim 7 wherein one or both of said first and second SLM comprises a reflective SLM, wherein one or both of said first and second SLM has an associated beam splitter, and wherein an optical path from said laser light source to said output optics comprises a folded optical path.

9. A holographic image projection system as claimed in claim 7 wherein said output optics are configured to provide a second intermediate real image plane, and wherein said output optics include a diffuser in said second intermediate real image plane.

10. A holographic image projection system as claimed in claim 9 wherein said diffuser is a piezoelectrically actuated diffuser.

11. A holographic image projection system as claimed in claim 7 wherein an active area of said first SLM is smaller than an active area of said second SLM.

12. A holographic image projection system as claimed in claim 11 wherein said active area of said first SLM has a maximum lateral dimension of less than 1 mm or of less than 0.5 mm.

13. A holographic image projection system as claimed in claim 7 wherein said first SLM comprises a multiphase SLM with at least three quantised phase levels.

14. A holographic image projection system as claimed in claim 7 comprising three said laser light sources of different wavelengths, and including optics such that each said three wavelengths is provided along a common optical path to said first SLM, whereby said holographic image projection system is able to provide a full colour display.

15. A holographic image projection system as claimed in claim 7 further comprising a controller to input image data defining an image for display and to control said first SLM to display a hologram to form said real image at said intermediate image plane with a first spatial resolution and to control said second SLM to intensity modulate said real image at a second higher spatial resolution to reproduce said image for display.