Enabling & Support

Imaging optics and optical device for mapping a curved image field

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ESA / Enabling & Support / Space Engineering & Technology
561 - Abstract:
The European Space Agency provides a solution for mapping a curved image field onto a flat imaging detector array. The technology reduces the cost and weight compared with similar solutions and works by pixelating the image at the focal plane, therefore where its sharper, and guiding it to specific elements on the detector. ESA is looking to licence the technology to companies which develops optical imaging systems.

Description

When trying to capture an interesting image it is necessary to use at least a simple lens to focus the image of the object onto the light sensitive media, in order to store it. Nowadays, the digital era, these media are usually CCD or CMOS detector arrays, with light detecting pixels arranged typically in a flat, rectangular shape.

However, simple lenses have a curved plane of focus; when projected onto a flat surface, the edges of the image appear out of focus when the centre is focused clearly, an aberration called Petzval field curvature.

This problem is well known in the optical instrument industry and several solutions were proposed to overcome this hurdle and register a sharp image all throughout the detector. The main one consists of correcting through the use of compensating optical elements, complex combinations of lenses to flatten the plane of focus and address other aberrations. Others mimic nature and the eye by establishing a curved detector surface.

Both of these solutions have some issues of their own, a complex lens system is inherently of complicated design and assembly, with every additional lens adding more weight and volume to the final assembly. Curved detectors are built by mosaicking individual detectors onto curved surfaces, by fabricating curved arrays or by manufacturing thin and flexible detectors that can be laid upon a desired shape template. Current technology limits the feasibility of the last two and the first delivers detectors only suitable for large telescopes and with lower performances than “off-the-shelf” flat counterparts.

The present invention addresses previous shortcomings by coupling a mass produced flat focal plane array, such as a common CCD or CMOS detector, with a focal plane adapter. This focal plane adapter can be described as a plurality of waveguides bundled together with one flat end that faces the detector array and the other end shaped to match the focal plane. Waveguides are hollow conduits layered with highly light-reflective coating and measuring some hundreds of micrometers long and a few micrometers thick.

On the curved front surface, where the light is incident, the focused image is pixelated at the focal plane, therefore where its sharper, and guided to specific elements on the detector, as ideally, each conduit is matched with a light processing component of the flat sensing array. This reduces the effects of the Petzval field curvature. As such the adapter should be aligned with the detector array and the cross-section of each waveguide should match the shape of the detector elements, for the correct mapping of each image segment.

The advances in micro fabrication technologies make this approach feasible, as the thin walls required to map each individual waveguide into a single detector unit without covering them, are only possible to fabricate with the latest advancements.

Innovations and advantages

The hollow waveguide array capable of reducing the effects of field curvature optical aberrations has some advantages over traditional systems. It is very light weight and simple when compared to complex multiple lens configurations, while remaining robust enough when properly encased to withstand reasonable shocks. One other issue with the lens solutions is the amount of light lost in the process of flattening the focus plane, a phenomenon not as critical with the present solution.

In terms of cost it is much cheaper to manufacture the curved focal plane adapter than the presented alternatives, namely the lens systems and curved detector arrays, the former derived from its inherent complexity and required manufacturing precision and the latter from its novelty and lack of experience and know how to drive its production costs to acceptable values.

Domains of application

This novel imaging optics are suited for applications that require wide fields of view, and therefore are more prone to suffer from loss of resolution due to field curvature aberration. Example of such systems are wide angle cameras (as used in portable devices such as mobile phones), on-vehicle cameras to detect obstacles, space telescopes (for whom the technology was first developed), robotic vision (industrial monitoring applications) and spectrometers. Along with all the previous is every other imaging tool or device that could benefit from cheap, simple and light curvature aberration correction optics.

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