Dark reference in CCD image sensors

1. An image sensor comprising:

an imaging array of optically active pixels each comprising (i) a photosensitive area and (ii) a vertical CCD (VCCD) region associated therewith;

proximate the imaging array, a dark-reference region of optically inactive pixels each comprising (i) a photosensitive area and (ii) a VCCD region associated therewith;

a first substantially opaque light shield (i) disposed over the dark-reference region of optically inactive pixels and (ii) defining a plurality of openings therein; and

a second substantially opaque light shield (i) defining a plurality of openings therein and (ii) disposed over the first light shield,

wherein:

within the dark-reference region, the openings in the first light shield are disposed over photosensitive areas or VCCD regions but not both,

if the openings in the first light shield are disposed over photosensitive areas, then the openings in the second light shield are disposed over photosensitive areas but not VCCD regions, such that each photosensitive area in the dark-reference region has no more than one opening thereover, and

if the openings in the first light shield are disposed over VCCD regions, then the openings in the second light shield are disposed over VCCD regions but not photosensitive areas, such that each VCCD region in the dark-reference region has no more than one opening thereover.

2. The image sensor of claim 1 , further comprising a first dielectric layer disposed between the first and second light shields.

3. The image sensor of claim 2 , further comprising a second dielectric layer disposed above the second light shield.

4. The image sensor of claim 1 , wherein (i) the openings in the first light shield are disposed over photosensitive areas and (ii) along a row of pixels in the dark-reference region, photosensitive areas over which the first light shield has an opening are separated by at least one photosensitive area (a) over which the first light shield does not have an opening and (b) over which the second light shield has an opening.

5. The image sensor of claim 4 , wherein the row of pixels in the dark-reference region comprises photosensitive areas over which the first light shield has an opening alternating with photosensitive areas over which the second light shield has an opening.

6. The image sensor of claim 1 , wherein (i) the openings in the first light shield are disposed over photosensitive areas and (ii) there are no openings in the first and second light shields disposed over the VCCD regions in the dark-reference region.

7. The image sensor of claim 1 , wherein (i) the openings in the first light shield are disposed over VCCD regions and (ii) along a row of pixels in the dark-reference region, VCCD regions over which the first light shield has an opening are separated by at least one VCCD region (a) over which the first light shield does not have an opening and (b) over which the second light shield has an opening.

8. The image sensor of claim 7 , wherein the row of pixels in the dark-reference region comprises VCCD regions over which the first light shield has an opening alternating with VCCD regions over which the second light shield has an opening.

9. The image sensor of claim 1 , wherein (i) the openings in the first light shield are disposed over VCCD regions and (ii) there are no openings in the first and second light shields disposed over the photosensitive areas in the dark-reference region.

10. The image sensor of claim 1 , wherein each photosensitive area in the imaging array of optically active pixels comprises a light-sensitive device selected from the group consisting of a photodetector, a photodiode, a photocapacitor, and a photoconductor.

11. The image sensor of claim 10 , wherein one or more of the photosensitive areas in the dark-reference region of optically inactive pixels lack at least one region of dopants that at least partially defines the light-sensitive device in each photosensitive area in the imaging array of optically active pixels.