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An f-number is a measure of the light-gathering ability of any optical system like a camera lens or even the human eye. It is calculated by dividing the system's focal length by the diameter of the entrance pupil. The f-number is also known as the focal ratio, f-ratio, or f-stop, and it is key in determining the depth of field, rate of light scattering, and exposure of a photograph. The f-number is dimensionless that is usually expressed using a lower-case hooked f with the format N, where N is the f-number. The f-number can also be known as the inverse of the relative aperture (the aperture diameter divided by focal length). The relative aperture indicates how much light can pass through the lens at a given focal length. A lower f-number means a larger relative aperture and more light entering the system, while a higher f-number means a smaller relative aperture and less light entering the system. The f-number is related to the numerical aperture of the system, which measures the r

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Field curvature correction in multichannel miniature imaging systems suited for wafer-level production

Nicolas Bongard, Hans Peter Herzig, Toralf Scharf

Using multiple optical channels increases the number of design possibilities for the objectives of mobile imaging devices. For easy wafer-level fabrication, we start from a single optical element-a monocentric plano-convex lens. The quality of the areal image is used to select the size of the field of each channel. Each channel optics is axially positioned to reduce the effect of the image field curvature. The resulting device has a small number of channels and it images a full field of view of +/- 40 deg with an f- number of 3. Details of the optical design, of the fabrication process, and of the device performance are reported. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Spie-Soc Photo-Optical Instrumentation Engineers2013

Multiple-Field Approach for Aberration Correction in Miniature Imaging Systems Based on Wafer-Level Production

Nicolas Bongard, Hans Peter Herzig, Toralf Scharf

IIn mobile imaging systems, the most difficult element to integrate is the objective lens. Here we present an intermediate approach between the costly traditional objectives and the low-resolution objectives inspired by the compound eyes of insects. Our multi-field approach uses a small number of optical channels each imaging a portion of the desired field of view. The full-field image is reconstructed digitally. The optics of each channel is kept simple for wafer-level fabrication and its size is sufficient to obtain a reasonable resolution. We present the design and fabrication of a prototype using 9 plano-convex lenses for 9 channels. Glass lenses glued on a wafer are used to image a full-field of ±40° with an f-number of 3. The images obtained shows field curvature correction. A simple image reconstruction scheme is presented. In conclusion, multi-field objectives fabricated with micro-optics technology are thin, simple to mount, robust, and easily replicated

SPIE Digital Library2013

Directly fabricated multi-scale microlens arrays on a hydrophobic flat surface by a simple ink-jet printing technique

Jürgen Brugger, Joo Yeon Kim

A shape-tunable approach is demonstrated for the fabrication of multi-scale polymer microlenses ([small mu ]-lenses) and microlens arrays (MLAs) using an ink-jet printing (IJP) technique. Also, the influence of the surface wetting conditions on the geometrical and optical characteristics of the printed [small mu ]-lenses is investigated. A photo-curable organic-inorganic hybrid polymeric resist (H-resist) is used; it is printed on a hydrophobic-treated glass substrate with different number of drops per [small mu ]-lens and cured using an ultraviolet lamp (UV lamp). High quality [small mu ]-lenses and MLAs with good uniformity and reproducibility were fabricated. The lens diameters and heights were controllable by changing the number of H-resist drops together with tuning the surface wetting conditions; these shape changes affect the optical properties of the [small mu ]-lenses and MLAs such as the numerical aperture (NA) and focal distance (f), as well as the f-number (f#), which indicates the light-gathering power. The optical properties of the tunable [small mu ]-lenses and MLAs are very attractive for application in optical systems such as interconnects and pixelated imagine sensors that use CCD or SPAD arrays, offering an efficient, simple and cheap alternative to the conventionally used photolithography technique.

Royal Society of Chemistry2012

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