Warm filter configuration for reducing effects of reflected infrared radiation systems and methods

1 . A system comprising:

an integrated dewar cooler assembly (IDCA) configured to maintain an interior volume at a constant temperature;

a thermal imager disposed within the interior volume and configured to capture thermal images; and

an optical element external to the IDCA and configured to provide reflected infrared radiation in response to emitted infrared radiation from the thermal imager;

wherein the IDCA further comprises a coldshield comprising a coldstop configured to define the interior volume and a field of view of the thermal imager; and

wherein for each sensor of the thermal imager, within a solid angle viewed by the sensor through the coldstop, any ray reflected by the optical element returns back to the thermal imager through the coldstop;

wherein rays emitted by the thermal imager within the field of view of the thermal imager and reflected and provided by the optical element are received by the thermal imager.

2 . The system of claim 1 , wherein corner rays are reflected by the optical element in a uniform distribution over the thermal imager.

3 . The system of claim 1 , wherein:

the optical element comprises a curved concave surface configured to reflect the emitted infrared radiation to provide the reflected infrared radiation;

the optical element is a curved filter configured to pass filtered infrared radiation from an external scene to the thermal imager to be captured in the thermal images; and

the filter is configured to be selectively replaced to pass different filtered infrared radiation corresponding to different wavelength ranges.

4 . The system of claim 3 , wherein:

b

=

Γ

·

c

1

-

Γ

⁢

and

⁢

f

=

Γ

·

c

1

-

Γ

2

where:

b is a distance from the curved concave surface to the coldstop;

c is a distance from the coldstop to the thermal imager;

Γ is a magnification corresponding to a diameter of the coldstop divided by a diagonal dimension of the thermal imager; and

f is a focal length of the curved concave surface.

5 . The system of claim 3 , wherein:

the filter comprises a curved convex surface opposite the curved concave surface; and

the filter comprises a uniform thickness between the curved concave surface and the curved convex surface.

6 . The system of claim 1 , wherein:

the IDCA comprises a window sealing the IDCA and configured to pass the emitted infrared radiation and the reflected infrared radiation between the thermal imager and the optical element; and

the optical element exhibits a higher temperature than the interior volume.

7 . The system of claim 1 , wherein:

the system is a gas detection camera; and

the optical element is configured to be selectively replaced without disturbing the IDCA.

8 . A system comprising:

an integrated dewar cooler assembly (IDCA) configured to maintain an interior volume at a constant temperature;

a thermal imager disposed within the interior volume and configured to capture thermal images; and

a lens external to the IDCA and configured to provide reflected infrared radiation in response to emitted infrared radiation from the thermal imager;

wherein:

the IDCA comprises a coldshield comprising a coldstop configured to define the interior volume and a field of view of the thermal imager;

the system further comprises a filter external to the IDCA and configured to pass filtered infrared radiation from an external scene to the lens;

the lens is configured to pass the filtered infrared radiation to the thermal imager to be captured in the thermal images;

the filter comprises a flat surface facing the lens and configured to reflect the emitted infrared radiation to provide the reflected infrared radiation to the lens; and

corner rays emitted by a corner portion of the thermal imager pass through the lens and are reflected by the flat surface of the filter to provide reflected rays that pass back through the lens toward the thermal imager and are shaped by the lens to provide a uniform distribution over the thermal imager.

9 . The system of claim 8 , wherein:

the reflected rays which pass back through the lens toward the thermal imager are shaped by the lens to provide a uniform distribution over a field of view of the thermal imager; and

the filter is configured to be selectively replaced to pass different filtered infrared radiation corresponding to different wavelength ranges.

10 . The system of claim 8 , wherein the filter is flat.

11 . A method of using the system of claim 8 , the method comprising:

passing, by the filter, filtered infrared radiation from an external scene to the lens;

passing, by the lens, the filtered infrared radiation to the thermal imager to be captured in the thermal images;

emitting the corner rays by the corner portion of the thermal imager;

passing the corner rays through the lens; and

reflecting the corner rays passed through the lens by the flat surface of the filter to provide the reflected rays which pass back through the lens toward the thermal imager and are shaped by the lens to provide the uniform distribution over the thermal imager;

wherein the filter is configured to be selectively replaced to pass different filtered infrared radiation corresponding to different wavelength ranges.

12 . The method of claim 11 , wherein:

the reflected rays that which back through the lens toward the thermal imager are shaped by the lens to provide a uniform distribution over a field of view of the thermal imager.

13 . The method of claim 11 , wherein the filter is flat.

14 . A method comprising:

operating an integrated dewar cooler assembly (IDCA) of an imaging system to maintain an interior volume at a constant temperature, wherein the IDCA comprises a coldshield comprising a coldstop configured to define the interior volume and a field of view of the thermal imager;

operating a thermal imager disposed within the interior volume to capture thermal images; and

providing, by an optical element external to the IDCA, reflected infrared radiation in a uniform distribution over a field of view of the thermal imager in response to emitted infrared radiation from the thermal imager;

wherein for each sensor of the thermal imager, within a solid angle viewed by the sensor through the coldstop, any ray in the reflected infrared radiation returns back to the thermal imager through the coldstop;

wherein the interior volume and the field of view of the thermal imager are defined by the coldstop of the coldshield disposed within the IDCA; and

wherein the thermal imager does not receive extraneous infrared radiation emitted by portions of the imaging system external to the coldshield.

15 . The method of claim 14 , wherein corner rays are reflected in a uniform distribution over the thermal imager.

16 . The method of claim 14 , wherein:

the providing comprises reflecting, by a curved concave surface of the optical element, the emitted infrared radiation to provide the reflected infrared radiation in the uniform distribution;

the optical element is a curved filter;

the method further comprises passing, by the filter, filtered infrared radiation from an external scene to the thermal imager to be captured in the thermal images; and

the filter is configured to be selectively replaced to pass different filtered infrared radiation corresponding to different wavelength ranges.

17 . The method of claim 16 , wherein:

the curved concave surface exhibits a spherical contour;

b

=

Γ

·

c

1

-

Γ

⁢

and

⁢

f

=

Γ

·

c

1

-

Γ

2

where:

b is a distance from the curved concave surface to the coldstop;

c is a distance from the coldstop to the thermal imager;

Γ is a magnification corresponding to a diameter of the coldstop divided by a diagonal dimension of the thermal imager; and

f is a focal length of the curved concave surface.

18 . The method of claim 16 , wherein:

the filter comprises a curved convex surface opposite the curved concave surface; and

the filter comprises a uniform thickness between the curved concave surface and the curved convex surface.

19 . The method of claim 14 , further comprising:

passing, through a window sealing the IDCA, the emitted infrared radiation and the reflected infrared radiation between the thermal imager and the optical element; and

wherein the optical element exhibits a higher temperature than the interior volume.

20 . The method of claim 14 , wherein:

the method is performed by a gas detection camera; and

the optical element is configured to be selectively replaced without disturbing the IDCA.