Wideband ambient light rejection

1. Optical apparatus, comprising:

an image sensor, which comprises an array of sensor elements, which generate respective signals, and outputs electronic images in which each pixel has a pixel value corresponding to a signal from a respective sensor element;

objective optics, which are configured to collect and focus optical radiation over a range of wavelengths along a common optical axis toward a plane of the image sensor; and

a dispersive refractive element, which is positioned to spread the optical radiation collected by the objective optics so that different wavelengths in the range are focused along different, respective optical axes toward the plane.

2. The apparatus according to claim 1 , wherein the dispersive refractive element is configured to cause the image sensor to capture a sharp image of the optical radiation emitted from an object at a target wavelength within the range, while spreading broadband background radiation within the range across the plane.

3. The apparatus according to claim 2 , and comprising a projection assembly, which is configured to project the optical radiation at the target wavelength onto the object, so that the emitted optical radiation collected by the objective comprises the projected optical radiation that is reflected from the object.

4. The apparatus according to claim 3 , wherein the projected optical radiation comprises a predetermined radiation pattern, and wherein the sharp image captured by the image sensor comprises an image of the pattern projected onto the object.

5. The apparatus according to claim 2 , and comprising a processor, which is configured to process the sharp image captured by the image sensor while rejecting interference caused by the background radiation.

6. The apparatus according to claim 5 , wherein the processor is configured to construct a 3D map of the object by processing the sharp image.

7. The apparatus according to claim 1 , wherein the dispersive refractive element is configured so that the optical radiation is focused with a chromatic aberration in excess of one micrometer per nanometer of wavelength at the plane of the image sensor.

8. Optical apparatus, comprising:

an image sensor;

objective optics, which are configured to collect and focus optical radiation over a range of wavelengths along a common optical axis toward a plane of the image sensor; and

a dispersive refractive element, which is positioned to spread the optical radiation collected by the objective optics so that different wavelengths in the range are focused along different, respective optical axes toward the plane,

wherein the dispersive refractive element has a first tilt relative to the optical axis, and wherein the apparatus comprises a wedged lens, which is configured to focus the optical radiation with a second tilt, opposite to the first tilt.

9. The apparatus according to claim 8 , wherein the wedged lens comprises a section of an axially-symmetrical lens.

10. Optical apparatus, comprising:

a projection assembly, which is configured to project optical radiation at a target wavelength onto an object;

an imaging assembly, comprising:

an image sensor; and

optics, which are configured to collect and focus the optical radiation reflected from the object, along with background radiation over a range of wavelengths, toward a plane of the image sensor, and which comprise a dispersive refractive element, which is configured to direct the collected optical radiation so as to form a sharp image of the optical radiation at the target wavelength in the plane of the image sensor, while spreading the background radiation across the plane; and

a processor, which is configured to process the sharp image captured by the image sensor while rejecting interference caused by the background radiation.

11. The apparatus according to claim 10 , wherein the projected optical radiation comprises a predetermined radiation pattern, and

wherein the sharp image captured by the image sensor comprises an image of the pattern projected onto the object, and

wherein the processor is configured to construct a 3D map of the object by processing the sharp image.

12. The apparatus according to claim 10 , wherein the dispersive refractive element is configured to spread the optical radiation collected by the optics so that different wavelengths in the range are focused along different, respective optical axes toward the plane of the image sensor.

13. The apparatus according to claim 10 , wherein the dispersive refractive element comprises a dispersive focusing element, which is configured to focus the radiation at the target wavelength onto the plane of the image sensor, while other wavelengths in the range are focused ahead of or behind the plane.

14. The apparatus according to claim 10 , wherein the dispersive refractive element is configured so that the optics have a chromatic aberration in excess of one micrometer per nanometer of wavelength at the plane of the image sensor.

15. The apparatus according to claim 10 , and comprising a motion mechanism, which is configured to shift a position of one or more elements of the optics relative to the plane so as to select the target wavelength that is to be imaged sharply in the plane.

16. A method for imaging, comprising:

collecting and focusing optical radiation over a range of wavelengths along a common optical axis toward a plane of an image sensor, which comprises an array of sensor elements, which generate respective signals, and outputs electronic images in which each pixel has a pixel value corresponding to a signal from a respective sensor element;

positioning a dispersive refractive element to spread the collected optical radiation so that different wavelengths in the range are focused along different, respective optical axes toward the plane.

17. The method according to claim 16 , wherein positioning the dispersive refractive element causes the image sensor to capture a sharp image of the optical radiation emitted from an object at a target wavelength within the range, while spreading broadband background radiation within the range across the plane.

18. The method according to claim 17 , and comprising projecting the optical radiation at the target wavelength onto the object, so that the optical radiation emitted from the object comprises the projected optical radiation that is reflected from the object.

19. The method according to claim 18 , wherein the projected optical radiation comprises a predetermined radiation pattern, and wherein the sharp image captured by the image sensor comprises an image of the pattern projected onto the object.

20. The method according to claim 17 , and comprising processing the sharp image captured by the image sensor while rejecting interference caused by the background radiation.