Karl Knop

Zero-order diffractive microstructures for security applications

The optical characteristics of diffractive microstructures designed for zero-order read-out are discussed. Such devices offer new optical behaviour which can be exploited for applications in the field of visual and machine readable security features. They can be fabricated by low-cost embossing and evaporation techniques.

The Fabrication Of Fine Lens Arrays By Laser Beam Writing

Lenticular arrays with near diffraction-limited performance have been generated by laser beam writing. A photoresist film is exposed in a line raster mode by scanning under an intensity-modulated focused laser beam; subsequent development of the resist film produces the desired lenticular relief profile. The writing of exposure patterns by raster scanning is analyzed, and it is shown that very precise positioning of each raster line is required to avoid unwanted modulation terms. A low cost xy positioning table with better than 20 nm dynamic positioning accuracy was constructed to satisfy this requirement, and has been used to generate high quality fine lenticular arrays with lenslets of widths down to 20 μm and relief depths up to about 4 μm.

Passive focus sensor

A focus-sensor module for large-format photographic cameras has been developed that permits the measurement of defocus at any location of interest in the image field. The focus sensor employs passive triangulation through a split imaging aperture. The main difference between commercial autofocus modules with fixed-measurement positions and the new module is that the imaging aperture is subdivided into more than two fields to compensate for the unknown location of the defocus measurement. At ƒ/5.6 the focus sensor shows a maximum resolution in defocus of approximately 0.1mm at the image side at levels of illuminance in the recording plane ≥0.01 lx.

Optimum color filters for CCD digital cameras

As part of the ESPRIT II project No. 2103 (MASCOT) a high performance prototype color CCD still video camera was developed. Intended for professional usage such as in the graphic arts, the camera provides a maximum resolution of 3k X 3k full color pixels. A high colorimetric performance was achieved through specially designed dielectric filters and optimized matrixing. The color transformation was obtained by computer simulation of the camera system and non-linear optimization which minimized the perceivable color errors as measured in the 1976 CIELUV uniform color space for a set of about 200 carefully selected test colors. The color filters were designed to allow perfect colorimetric reproduction in principle and at the same time with imperceptible color noise and with special attention to fabrication tolerances. The camera system includes a special real-time digital color processor which carries out the color transformation. The transformation can be selected from a set of sixteen matrices optimized for different illuminants and output devices. Because the actual filter design was based on slightly incorrect data the prototype camera showed a mean colorimetric error of 2.7 j.n.d. (CIELUV) in experiments. Using correct input data in the redesign of the filters, a mean colorimetric error of only 1 j.n.d. (CIELUV) seems to be feasible, implying that it is possible with such an optimized color camera to achieve such a high colorimetric performance that the reproduced colors in an image cannot be distinguished from the original colors in a scene, even in direct comparison.