Non-axially-symmetric Lens with extended depth of focus for Machine Vision applications

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Coral Simpson
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Non-axially-symmetric Lens with extended depth of focus for Machine Vision applications Category: Sensors & Measuring Techniques Reference: TDI0040 Broker Company Name: D Appolonia Broker Name: Tanya

it Abstract: A Polish SME proposes an innovative non-axially-symmetric lens, originally developed for human vision that is able to extend the field of view from near field (0.

1 Non-axially-symmetric Lens with extended depth of focus for Machine Vision applications Category: Sensors & Measuring Techniques Reference: TDI0040 Broker Company Name: D Appolonia Broker Name: Tanya Scalia Telephone: tanya.scalia@dappolonia.it Abstract: A Polish SME proposes an innovative non-axially-symmetric lens, originally developed for human vision that is able to extend the field of view from near field (0.3 m) to infinity without requiring any mechanical moving parts (i.e. lens opening). It can be applied into space for satellite navigation, ISS external operations (e.g. improving astronauts and robots' optical capability during EVA operations), Intra-Vehicular Activities and for Rovers applications (e.g. Mars sample return missions). Description: The proposed lens geometry is based on an innovative optical concept, which was originally developed for the ageing human population. Due to the mechanisms that occur in the human eye, its ability to change its optical power decreases at a rate of 0.2D per year. To compensate for this process, an innovative lens optic geometry was proposed. The main differences with respect to common lenses are: non axialsymmetric lens geometry concept curvature radius rising (or decreasing) with angular coefficient each angular segment in section has the shape (radial cross-section) of a lens with optical power corresponding to actual angular coefficient The optical element has a sharp step for transition between 0 and 2π.

Figure 1 - Optic power mapping across optic element and its scaled image To obtain such an optical geometry, a theoretical model was transformed into a numerical one so that the requested shape could

The optical bench tests confirmed the expected theoretical features as well as the variation of geometry parameters possible, when different optic powers and MTF (Modulation Transfer Function) are

2 Figure 1 - Optic power mapping across optic element and its scaled image To obtain such an optical geometry, a theoretical model was transformed into a numerical one so that the requested shape could be implemented. The optical bench tests confirmed the expected theoretical features as well as the variation of geometry parameters possible, when different optic powers and MTF (Modulation Transfer Function) are requested. Such an innovative optical concept has a large potential for machine vision (CCD Charged Coupled Device camera). The non-axially-symmetric geometry of the lens permits an extended depth of focus of the CCD camera. As a matter of fact, when the offered optical element is combined with dedicated software, which embeds an inverse model of the specified lens, the camera structure can be extremely simplified; specifically the lens, a CCD matrix and the embedded processor, in which a sharp image of the extended depth of field is processed in real time. Obviously such a camera structure does not need any moving parts either for focus or for the aperture/iris adjustment. This feature ensures extreme resistance to any damage, while preserving most of the functionality of the main camera enabling a higher reliability, since no mechanical component is needed. Figure 2 Optic element integration with a camera Figure 3 Image comparison for fixed aperture and focus (infinity) with and without the LightSword innovative lens The principle of the proposed lens functionality is illustrated in Figure 3, where a common lens of a camera set-up for infinity vision is compared with the same system, but integrated with the innovative proposed lens concept. The observed label with LightSwords text observed with the commercial camera loses its sharpness and contrast, when the label is placed closer and closer to

3 the camera lens. When the proposed lens is integrated, the same label is observed with good quality along the whole range of distance investigated between the observed object and the camera lens. The following identified features of this lens are considered important for potential space applications: - camera is small and light, - power consumption is decreased because of lack of mechanical components - extended focus is a part of the optics geometry - thanks to the focus extension it is possible to measure distances between observed objects with the use of a single lens (instead of two as currently done) such a feature may be integrated through the lens mathematical model contained in the camera software - low lighting environment is accepted without need of auxiliary power consumption Some potential space missions already identified include: Rovers explorations: One of the rovers main system is the observation camera for navigation purposes as well as for geological characteristics observations (for example the ExoMars PanCam-Panoramic Camera). EVA/IVA (Extra-Vehicular and Intra-Vehicular Activities) operations: Astronauts outside a space vehicle need to move in open hazardous space where the increased radiation, stress, storage of gaseous mixture, extreme temperature and other physical parameters impose a limitation on the length of the space walk. Therefore the planned actions (repairs, inspections, installations of outside equipment etc.) have to be performed as quickly as possible, but without reducing the quality of the final work. The integration of a network of small and robust cameras equipped with the proposed asymmetric lenses would provide a real-time distributed visual scenario and 3D mapping of all the surrounding objects, regardless of the lighting conditions. Moreover such information is crucial for automated robots capable of performing e.g. external and internal repairs and installations. Innovations and advantages of the offer: The asymmetric lens shifts the functionality of a camera from fragile and complicated mechanical elements to a solid-state functional material and dedicated real-time software. The concept of this technology combines two fundamental breakthroughs: Non axially-symmetric optics, which embeds the extended depth of field in a single solid state piece of material; When applied to machine vision, the underlying theory can be easily programmed into the processing software for the CCD matrix, where an image processing algorithm can be loaded on a FPGA (Field Programmable Gate Array) environment to make real time processing possible (expected 100 frames of 2 mega pixels with frequency up to 50 Hz).

4 The consequence of the above breakthroughs is the following: small size: down to smartphone camera size; low weight and low power consumption thanks to the removal of the electromechanical focus and aperture; rangefinder embedded in a single optic path; extended depth of focus constant intensity of transmitted light - imaging with low intensity illumination is supported. Further Details: The primary objective of this technology offer is the delivery of the novel lens itself. Presently it is called LSOE which stands for LightSword Optical Element. The preliminary production process of this lens has been mastered by the donor of this technology. The lens can be produced from suitable polymeric material. When a potential space application is considered, glass material should be considered more suitable with appropriate technological adjustments. The optical bench tests already carried out demonstrated successfully both the geometrical and optical LSOE functionality. The lens produced can respect different requirements on depth of field and mechanical fixing systems. The technology offer of the lens can be also integrated with the delivery of a complete machine vision solution, where the requested depth of field extension is combined with proper processing software of scene imaging and parameterization of distances. This aspect was implemented at laboratory level and tests were carried out; however the system was assembled in an extended way, where image processing was performed on the side computer and not on FPGA (Field Programmable Gate Array). The proposed optical structure is presently refractive, however it may be extended to diffractive option, thus selected ranges of spectrum could be exploited for different purposes. Application: This technology was originally developed for the ageing human population and its main application is in improving the optics for elderly people. The space industry may however take advantages of this optical technology namely for the following reasons: Replacement of the focus ring. a robust surveillance camera for rover exploration can be manufactured without having any moving focal ring; it will generate the image with an extended depth of field starting from 33 cm. Such a surveillance camera will operate at all times since it requires only a passive optical element without suffering from the light intensity. This function is a perfect application for the asymmetric lens element, which will provide sharp imaging between 33 cm (e.g. for self-inspection) and infinity, preserving the light throughput of the lens;

5 Replacement of the mechanical aperture. The moving aperture of the camera can be replaced by a combination of a wide-dynamic-range CCD sensor, an electronic shutter and time-gating of the image intensifier (I2). The principle of auto-gating is based on the fact that it changes the pulse width of the power supplied to the photo-cathode in the case of illumination that is too strong (as a matter of fact it was originally invented to overcome the frequent saturations of the photo-multiplier and phosphor screen during light bursts). Potential domain of application in Space: EVA and IVA operations Rover exploration Broker comments: D Appolonia has been a partner of the FP7 project that led to the first demonstration of the proposed lens. The laboratory tests have confirmed that the concept is sound. The manufacturing process, although obviously not yet optimized, has been established for PMMA (polymethylmethacrylate) lenses and can be brought into commercial level without great efforts. It will require adaptation for other different materials. The concept is demonstrated in the visible range, but it is possible to adapt it to other wavelengths. This technology description was downloaded from

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