Holographic projector

1. A projector arranged to project an image within a display area on a display plane, wherein the image comprises a light feature comprising a dark region surrounded by a light region, and the projector comprises a light sensor spatially separated from the display plane, wherein the display area and the light sensor are aligned such that light forming the light feature of the image on the display plane is at least partially disposed around the light sensor, without substantially impinging thereon, when operated in an aligned state, wherein the aligned state defines a selected alignment between the display area and the display plane; and wherein, when operated in the aligned state, a reference point in the dark region of the light feature is coincident with a reference point of the light sensor.

2. The projector as claimed in claim 1 wherein the light sensor is disposed upstream of the display plane.

3. The projector as claimed in claim 1 wherein the image is substantially focused on the display plane.

4. The projector as claimed in claim 1 wherein the light feature comprises a hollow light feature or a part thereof.

5. The projector as claimed in claim 4 wherein the light feature comprises one of: a hollow circle or ring of light; a hollow polygon of light; an arc of light, and an intersection of edges of a polygon of light.

6. The projector as claimed in claim 1 further comprising an alignment controller arranged to change a position of the display area on the display plane.

7. The projector as claimed in claim 6 wherein the alignment controller is arranged to change the position of the display area on the display plane if the light sensor detects light of the light feature.

8. The projector as claimed in claim 6 wherein the alignment controller is arranged to change the position of the display area on the display plane by translating the display area.

9. The projector as claimed in claim 8 wherein the alignment controller is arranged to translate the position of the display area in order to restore the aligned state.

10. The projector as claimed in claim 6 wherein the alignment controller is arranged to continually assess, and change as necessary, the relative alignment of the display area and light sensor during projection of a sequence of images.

11. The projector as claimed in claim 1 wherein the projector comprises: a spatial light modulator arranged to display a diffractive pattern comprising a hologram of the image; and a light source arranged to illuminate the diffractive pattern in order to form a holographic reconstruction, corresponding to the image, on the display plane.

12. The projector as claimed in claim 11 wherein the diffractive pattern further comprises at least one phase-ramp function and translating the display area comprises changing a phase gradient of the at least one phase-ramp function.

13. The projector as claimed in claim 1 wherein the projector is further arranged to change a shape of the light feature if the light sensor detects light of the light feature in order to determine a translation required to restore the aligned state.

14. A method of operating a projector arranged to project an image comprising a light feature within a display area on a display plane, wherein the light feature comprises a dark region surrounded by a light region, the method comprising:

providing a light sensor spatially separated from the display plane;

projecting the image within the display area on the display plane, wherein light of the light feature is at least partially disposed around the light sensor, without substantially impinging thereon, when operated in an aligned state, wherein the display area is correctly positioned with a selected alignment on the display plane in the aligned state, and wherein, when operated in the aligned state, a reference point in the dark region of the light feature is coincident with a reference point of the light sensor;

detecting, by the light sensor, light of the light feature during a light sensing interval, and

determining a misaligned state when the detected light is greater than a threshold, wherein the display area is not correctly positioned in the display plane in the misaligned state.

15. The method as claimed in claim 14 further comprising:

translating the display area relative to the display plane to restore the aligned state, in response to determining a misaligned state.

16. The method as claimed in claim 15 further comprising:

changing the projected image so as to change the light feature;

detecting, by the light sensor, light of the changed light feature during a light sensing interval, and

determining a direction of a translation of the display area required to restore the aligned state.

17. The method as claimed in claim 15 wherein translating the display area is performed incrementally.

18. The method as claimed in claim 15 , wherein the projector comprises a spatial light modulator arranged to display a diffractive pattern comprising a hologram of the image; and a light source arranged to illuminate the diffractive pattern in order to form a holographic reconstruction, corresponding to the image, on the display plane, and wherein the diffractive pattern further comprises at least one phase-ramp function and translating the display area comprises changing a phase gradient of the at least one phase-ramp function.

19. The method as claimed in claim 14 , wherein the method is performed to continually assess, and change as necessary, the relative alignment of the display area and light sensor during projection of a sequence of images.

20. A projector arranged to project an image within a display area on a display plane, wherein the image comprises a light feature comprising a dark region surrounded by a light region, and the projector comprises a light sensor spatially separated from the display plane, wherein the display area and the light sensor are arranged such that, when operated in an aligned state, light forming the light feature of the image on the display plane is at least partially disposed around, without impinging on, a reference point of the light sensor, wherein the aligned state defines a selected alignment between the reference point of the light sensor and a reference point on the display plane; and wherein, when operated in the aligned state, a reference point in the dark region of the light feature is coincident with the reference point of the light sensor.