Peter Schreiber

Thin compound-eye camera

An artificial compound-eye objective fabricated by micro-optics technology is adapted and attached to a CMOS sensor array. The novel optical sensor system with an optics thickness of only 0.2 mm is examined with respect to resolution and sensitivity. An optical resolution of 60 x 60 pixels is determined from captured images. The scaling behavior of artificial compound-eye imaging systems is analyzed. Cross talk between channels fabricated by different technologies is evaluated, and the influence on an extension of the field of view by addition of a (Fresnel) diverging lens is discussed. The lithographic generation of opaque walls between channels for optical isolation is experimentally demonstrated.

Artificial apposition compound eye fabricated by micro-optics technology

By exploring micro-optical design principles and technology, we have developed an artificial apposition compound eye. The overall thickness of the imaging system is only 320 microm, the diagonal field of view is 21 degrees, and the f-number is 2.6. The monolithic device consists of an UV-replicated microlens array upon a thin silica substrate with a pinhole array in a metal layer on the back side. The pitch of the pinholes differs from that of the lens array to provide individual viewing angle for each channel. Theoretical limitations of resolution and sensitivity are discussed as well as fabrication issues and compared with experimental results. A method to generate nontransparent walls between optical channels to prevent cross talk is proposed.

Artifical compound eyes - Different concepts and their application to ultra flat image acquisition sensors

Two different approaches for ultra flat image acquisition sensors on the basis of artificial compound eyes are examined. In apposition optics the image reconstruction is based on moire or static sampling while the superposition eye approach produces an overall image. Both types of sensors are compared with respect to theoretical limitations of resolution, sensitivity and system thickness. Explicit design rules are given. A paraxial 3x3 matrix formalism is used to describe the arrangement of three microlens arrays with different pitches to find first order parameters of artificial superposition eyes. The model is validated by analysis of the system with raytracing software. Measurements of focal length of anamorphic reflow lenses, which are key components of the superposition approach, under oblique incidence are performed. For the second approach, the artificial apposition eye, a first demonstrator system is presented. The monolithic device consists of a UV-replicated reflow microlens array on a thin silica-substrate with a pinhole array in a metal layer on the backside. The pitch of the pinholes differs from the lens array pitch to enable an individual viewing angle for each channel. Imaged test patterns are presented and measurements of the angular sensitivity function are compared to calculations using commercial raytracing software.

Theoretical analysis of an artificial superposition compound eye for application in ultra flat digital image acquisition devices

A new concept for a flat digital image acquisition device for large field of views (FOV) has been developed. Antetypes for the optical system are compound eyes of small insects and the Gabor-Superlens. A paraxial 3x3 matrix formalism is used to describe the arrangement of three microlens arrays (MLA) with different pitches to find the first order parameters of the system. These considerations are extended to arrays of anamorphic lenses with variable parameters to achieve homogeneous optical performance over the whole FOV. The model is validated by implementation of different systems into commercial raytracing software. A trade-off between system length, sensitivity and diffraction limited resolution as well as aberrations is discussed.

Design and fabrication of a chirped array of refractive ellipsoidal micro-lenses for an apposition eye camera objective

Apposition compound eye camera objectives are one approach for a vast reduction of the optical system length of an imaging optical sensor. Despite imaging the complete field of view through one aperture like in classical lenses, these objectives split the overall field of view in separated channels which are located adjoined like in insect eyes. Due to the splitting each channel can be optimized for reduction of aberrations occuring under oblique incidence. A correction for astigmatism, field curvature and distortion occurring under oblique incidence can be accomplished by the use of anamorphic micro-lenses leading to an improved resolution of the camera objective. In contrast to regular arrays of equally shaped and equidistant positioned micro-lenses the parameters of the lenses like radii of curvature, center position and angular orientation are functions of the position within the array. These functions can be derived analytically leading to a complete description of the array parameters. We present design considerations for a chirped array containing 130x130 individually shaped ellipsoidal micro-lenses. Melting of photo-resist is employed as fabrication technology for achieving diffraction limited performance. Detailed considerations for the semi-automated layout generation of the photo lithographical masks as well as characterization data of first realized prototypes of the array are given.

Micro-optically fabricated artificial apposition compound eye

An artificial apposition compound eye manufactured by micro-optics technology is demonstrated. The overall thickness of the imaging system is only 320 μm, the field of view is 21° on diagonal, the F/# is 2.6. The monolithic device consists of a microlens array on a thin silica-substrate with a pinhole array in a metal layer on the backside. The image formation can be explained by the moiré-effect or static sampling. The master structures for the microlens arrays are manufactured by lithographic patterning of photo-resist and a subsequent reflow process. These master structures are replicated by moulding into UV-curing polymer. The pitch of the pinholes differs from the lens array pitch to enable an individual viewing angle for each channel. The required precision is guaranteed by using a lithographic process also for the assembly. Thus, problems with accuracy of other attempts to develop similar systems using discrete components have been overcome. Imaging systems with different sizes of pinholes, numbers of channels and separation of the viewing direction of the channels are realized and tested. A method to generate nontransparent walls between the optical channels for prevention of crosstalk is proposed. Theoretical limitations of resolution and sensitivity are discussed. Imaged test patterns are presented and measurements of the angular sensitivity function are compared to calculations using commercial raytracing software. The resolution achievable with the fabricated artificial compound eye is analyzed.

Microoptical telescope compound eye

A new optical concept for compact digital image acquisition devices with large field of view is developed and proofed experimentally. Archetypes for the imaging system are compound eyes of small insects and the Gabor-Superlens. A paraxial 3x3 matrix formalism is used to describe the telescope arrangement of three microlens arrays with different pitch to find first order parameters of the imaging system. A 2mm thin imaging system with 21x3 channels, 70 masculinex10 masculine field of view and 4.5mm x 0.5mm image size is optimized and analyzed using sequential and non-sequential raytracing and fabricated by microoptics technology. Anamorphic lenses, where the parameters are a function of the considered optical channel, are used to achieve a homogeneous optical performance over the whole field of view. Captured images are presented and compared to simulation results.

Microoptical artificial compound eyes - from design to experimental verification of two different concepts

Two novel objective types on the basis of artificial compound eyes are examined. Both imaging systems are well suited for fabrication using microoptics technology due to the small required lens sags. In the apposition optics a microlens array (MLA) and a photo detector array of different pitch in its focal plane are applied. The image reconstruction is based on moire magnification. Several generations of demonstrators of this objective type are manufactured by photo lithographic processes. This includes a system with opaque walls between adjacent channels and an objective which is directly applied onto a CMOS detector array. The cluster eye approach, which is based on a mixture of superposition compound eyes and the vision system of jumping spiders, produces a regular image. Here, three microlens arrays of different pitch form arrays of Keplerian microtelescopes with tilted optical axes, including a field lens. The microlens arrays of this demonstrator are also fabricated using microoptics technology, aperture arrays are applied. Subsequently the lens arrays are stacked to the overall microoptical system on wafer scale. Both fabricated types of artificial compound eye imaging systems are experimentally characterized with respect to resolution, sensitivity and cross talk between adjacent channels. Captured images are presented.

Cartesian oval representation of freeform optics in illumination systems

The geometrical method for constructing optical surfaces for illumination purpose developed by Oliker and co-workers [Trends in Nonlinear Analysis (Springer, 2003)] is generalized in order to obtain freeform designs in arbitrary optical systems. The freeform is created by a set of primitive surface elements, which are generalized Cartesian ovals adapted to the given optical system. Those primitives are determined by Hamiltonian theory of ray optics. The potential of this approach is demonstrated by some examples, e.g., freeform lenses with collimating front elements.

Homogeneous LED-illumination using microlens arrays

Efficient homogeneous illumination of rectangular or circular areas with LEDs is a promising application for doublesided microlens arrays. Such illumination schemes employ a primary optics - which can be realized with a concentrator or a collimation lens - and a secondary optics with one or more double-sided microlens arrays and a collection optics for superposing the light from the individual array channels. The main advantage of this design is the achievable short system length compared to integrating lightpipe designs with subsequent relay optics. We describe design rules for the secondary optics derived from simple ABCD-matrix formalism. Based on these rules, sequential raytracing is used for the actual optics system design. Double-sided arrays are manufactured by polymer-on-glass replication of reflow lenses. With cylindrical lens arrays we assembled high-brightness RGB-illumination systems for rectangular areas. Hexagonal packed double-sided arrays of spherical lenslets were applied for a miniaturized circular spotlight. Black matrix polymer apertures attached to the lens array helped to avoid unwanted straylight.