Optics for an extended depth of field
View Patent ↗An optical imaging assembly having cylindrical symmetry, comprising a plurality of lenses having surfaces with curvatures and spacings between the surfaces, such that an optical image formed by the plurality of lenses has a defocus aberration coefficient greater than 0.1 at a focal plane of the assembly.
1. An optical assembly, comprising:
a first lens having positive refractive power and at least two aspheric surfaces, the first lens is located at a first distance from an aperture on an optical axis common to the first lens and the aperture, the optical axis being substantially cylindrically-symmetric;
a second lens having negative refractive power and at least two aspheric surfaces, the second lens is meniscus-shaped near the optical axis and is located at a second distance from the first lens on the optical axis; and
a third lens having positive refractive power and at least two aspheric surfaces, the third lens is meniscus-shaped near the optical axis and is located at a third distance from the second lens on the optical axis, wherein
a real image of an object produced by the optical assembly has a defocus aberration coefficient greater than 0.1 at a focal plane of the optical assembly.
2. The optical assembly of claim 1 , wherein a defocus aberration coefficient of the optical assembly has a value greater than or equal to about 0.6.
3. The optical assembly of claim 1 , wherein a defocus aberration coefficient of the optical assembly has a value smaller than or equal to about 0.9.
4. The optical assembly of claim 1 , wherein a fifth order spherical aberration coefficient of the optical assembly has a value smaller than about −0.2 and greater than about −0.3.
5. The optical assembly of claim 1 , further comprising an infra-red filter located at a fourth distance from the third lens and a fifth distance from an image sensor, wherein the infra-red filter is contained in a plane with a normal substantially parallel to the substantially cylindrically-symmetric optical axis.
6. The optical assembly of claim 5 , wherein at least one surface of the image sensor defines the focal plane, the focal plane is substantially parallel to the plane that contains the infra-red filter.
7. The optical assembly of claim 5 , wherein the fourth distance is about 1170 μm and the fifth distance is about 861 μm.
8. The optical assembly of claim 5 , wherein the first lens, the second lens, and the third lens are made of plastic, and the infra-red filter is made of glass.
9. The optical assembly of claim 5 , wherein the first lens has a diffraction index of about 1.53, the second lens has a diffraction index of about 1.61, and the second lens has a diffraction index of about 1.53.
10. The optical assembly of claim 6 , wherein a first surface and a second surface of the at least two aspheric surfaces of the first lens are convex, the second surface faces the focal plane.
11. The optical assembly of claim 10 , wherein a first surface of the at least two aspheric surfaces of the second lens is concave, and a second surface of the at least two aspheric surfaces of the second lens faces the focal plane and is convex.
12. The optical assembly of claim 10 , wherein a pixel in the grid of pixels has a length of about 2.8 μm in the first direction and a length of about 2.8 μm in the second direction.
13. The optical assembly of claim 11 , wherein a first surface of the at least two aspheric surfaces of the third lens is convex, and a second surface of the at least two aspheric surfaces of the third lens faces the focal plane and is concave.
14. The optical assembly of claim 11 , wherein an image formed at the focal plane has a point spread function greater than or equal to about five pixels on the image sensor.
15. The optical assembly of claim 13 , wherein the image sensor includes a grid of pixels spanning about 1600 pixels in a first direction and about 1200 pixels in a second direction nearly orthogonal to the first direction.
16. The optical assembly of claim 13 , wherein the first distance is about 11 μm, the second distance is about 817 μm, and the third distance is about 631 μm.
17. The optical assembly of claim 16 , wherein the first surface of the at least two aspheric surfaces of the first lens has a semi-diameter of about 850 μm, and the second surface of the at least two aspheric surfaces of the first lens has a semi-diameter of about 1050 μm.
18. The optical assembly of claim 17 , wherein the first surface of the at least two aspheric surfaces of the second lens has a semi-diameter of about 1240 μm, and the second surface of the at least two aspheric surfaces of the second lens has a semi-diameter of about 1510 μm.
19. The optical assembly of claim 18 , wherein the first surface of the at least two aspheric surfaces of the third lens has a semi-diameter of about 2240 μm, and the second surface of the at least two aspheric surfaces of the third lens has a semi-diameter of about 2350 μm.
20. A method, comprising:
collecting optical radiation impinging on an aperture, a portion of the optical radiation emanating from an object;
propagating the optical radiation through an optical assembly comprising a set of at least three lenses and an infra-red filter, the optical assembly having substantially cylindrical symmetry with respect to a common optical axis, and a lens in the set having at least two aspheric surfaces;
detecting at least a portion of the optical radiation at an image sensor defining a focal plane; and
based on the detected portion of the optical radiation, producing an image of the object having:
a modulation transfer function for spatial frequencies greater than 50 cycles per millimeter having at least one value smaller than or equal to about 0.35; and
a point spread function greater than or equal to about 14 μm on the focal plane.