Image sensor having a variable aperture and mono-color focal plane array for dual color detection

1. An image sensor, comprising:

a focal plane array;

a cold shield having a top wall disposed over the focal plane array and defining an aperture operatively configured to be selectively switched between a first diameter and a second diameter that is larger than the first diameter; and

a filter element disposed between the cold shield aperture and the focal plane array, the filter element including a first band-pass filter and a second band-pass filter,

wherein the first band-pass filter has a diameter equal to or greater than the first diameter; the first band-pass filter being disposed in coaxial alignment with the cold shield aperture and directly beneath the cold shield aperture when the cold shield aperture is switched to the first diameter or the second diameter,

wherein the second band-pass filter is covered by the top wall of the cold shield when the cold shield aperture is switched to the first diameter and is disposed directly beneath the cold shield aperture when the cold shield aperture is switched to the second diameter,

wherein the first band-pass filter is adapted to pass to the focal plane array a first signal having a first wavelength within a first band of wavelengths and the second band-pass filter is adapted to pass to the focal plane array a second signal having a second wavelength within a second band of wavelengths, the second band of wavelengths being different than the first band of wavelengths, and

wherein the focal plane array is a mono-color focal plane array having a plurality of pixels, each of which is adapted to detect the first signal passed by the first band-pass filter and the second signal passed by the second band-pass filter.

2. An image sensor according to claim 1 , wherein the second wavelength in the second band is longer than each wavelength in the first band.

3. An image sensor according to claim 1 , wherein the first band-pass filter is a medium wavelength infrared (MWIR) band-pass filter and the second band-pass filter is a long wavelength infrared (LWIR) band-pass filter.

4. An image sensor according to claim 1 , wherein the focal plane array is a LWIR focal plane array.

5. An image sensor according to claim 1 , wherein the second band-pass filter is adjacent to the first band-pass filter.

6. An image sensor according to claim 1 , wherein the second band-pass filter surrounds the first band-pass filter.

7. An image sensor according to claim 1 , wherein the first band-pass filter and the second band-pass filter are concentrically aligned with the cold shield aperture.

8. An image sensor according to claim 1 , wherein the cold shield aperture has a higher f/# when switched to the first diameter than when switched to the second diameter, where f/# corresponds to a height from the focal plane array to the cold shield aperture divided by the respective cold shield aperture diameter.

9. An image sensor according to claim 1 , wherein the first diameter of the cold shield aperture is equal to or less than f/4 and the second diameter of the cold shield aperture is greater than f/4, where f corresponds to a focal length of the filter element.

10. An image sensor according to claim 9 , wherein the second diameter of the cold shield aperture is equal to or greater than f/2.