MW3060 Multi Aperture Camera
The Multi-Aperture Dual Focal Length MWIR seeker shown in the photograph has two imaging zones fixed in the optics; there are no moving parts to adjust the focal lengths. There are 4 apertures, all of which are visible at the front of the optics. The center aperture views the foveal field with a distortion free mapping function. The mapping function gradually changes as the fovea overlaps with the outer 3 apertures viewing the peripheral field.
The performance characteristics of the seeker are listed below:
· Foveal FOV: 30 deg with FtanΘ distortion free mapping function
· Focal Length: 10 mm with 1.2 mrad IFOV
· Peripheral FOV: 60 deg with FΘ mapping function
· Peripheral Focal Length: 8 mm with 1.5 mrad IFOV
· Spectral Band: 3.4 – 4.8 µm
Though the Multi-Aperture optical system has 4 contiguous apertures, it was designed to generate a continuous image across these apertures. This implies that there would not be gaps in the image between aperture sectors, nor would the overlap of sub-images appear as double images where the sectors adjoin. The expected image continuity is confirmed in the raw images shown below. The overlap region between foveal and peripheral fields is demarcated by a very faint ring pattern imposed over the image. This is an effect created by the mechanical ring holding the center lens in place. An inspection of both images reveals neither obvious gaps nor double images along the ring demarcation zone.
The optical system was designed to generate a distortion-free foveal image with an F tan θ mapping function. The distortion free nature of the foveal region is confirmed by viewing the rectilinear objects that appear inside the ring demarcation overlaying the Raw Lab images (straight lines and edges are not curved by the optics). The optical system was also designed to generate a demagnified peripheral field, with an F θ mapping function that introduces curvature, intentionally, to lower the pixel sampling resolution. However, the transformation from the rectilinear foveal mapping function to the curval peripheral mapping function is gradual. The transformation rate can be detected by observing the pillar to left of center in the Raw Lab images. Notice that the pillar is perfectly straight lower down but begins to bend as it enters the ring demarcation zone. The peripheral zone curvature is also evident in the cold (black) window frame on the right side of the image.
The ring intensity pattern is caused by emission from the mechanical ring structure, and it can either appear brighter or darker than the image depending on the mechanical ring’s relative temperature. The cryo-cooler dissipates heat that gradually warms the lens, causing the overlaid ring intensity pattern to appear brighter over time. An ambient or cold single point non-uniformity correction (NUC) is usually sufficient to reduce its effect.
A careful inspection of the two raw images below will reveal that fine structures, such as telephone poles, cables, fencing, and car parts are resolvable in both the foveal and peripheral regions, without any noticeable difference in quality between the two regions. This is an indication that the image quality is uniform across the field and is likely to be diffraction limited.
As noted above, curval mapping is necessary to introduce demagnification within the peripheral zone. The demagnification effect is readily apparent in the unprocessed image below, left. Notice that the fingers of the hand in the corner of the image are shorter than the fingers of the other hand, located in the foveal region. For comparison, the image on the right was warped (or stretched) Notice that warping stretches the fingers to normal length.
The real test is to see how the lens performs when viewing a variety of objects outside through a range of distances. The raw image on the left, below, shows two pickup trucks driving through the foveal and peripheral zones. The bed of the 2nd pickup truck is compressed slightly. The warped image on the right lengthens the bed to match that of the truck in the foveal zone.
In both sets of images, a simple 3rd order, pincushion warp function was applied with bilinear interpolation using the XCAP software developed by Epix. A more sophisticated adjustment can be applied using either 5th order pincushion warping, partitioning, or a user defined function to ensure that the foveal region does not become compressed as the periphery is stretched.