Work

Within my doctorate thesis, numerous industry projects and my work in my own start-up I have been developing and testing a wide range of today used optical measurement principles in practice. Here are shown some examples of the results of this work. The order is more or less arbitrary. Talking to some hr people, looking at my CV, the always says, but have ‚only‘ technical experience. So, but, … well I acquired and managed most of the projects I talk about below, … it’s just talking about technical details is way more interesting.

Programming

Optical metrology goes hand in hand with programming. This is due to its indirect measuring nature which requires to transform the recorded intensity fields into measurement values and usually a high degree of automation, recording a vast number of images usually in combination with some handling / actuator devices. Working with these systems for some time now, I got to program a wide bandwidth of languages and systems, e.g. C/C++, Qt, OpenCV, Python, Matlab, Labview, Embedded Devices (Arduino, Tiva, Beagle bone black), Xml, Java, Corba, …). Beneath this professional side of programming, in my spare time I did some work involving web-programming stuff (Html, Javascript, Php, (My)Sql, WordPress) and the usual M$ stuff (VBA, Access, Excel, …). One, in my opinion, interesting project is itom.

Fringe Projection Systems

At ITO (institute for applied optics / University of Stuttgart) fringe projection systems have been developed quite a long time. Starting with today state of the art macroscopic systems, moving on to microscopic systems using different types of spatial light modulators.
In general microscopic systems have a limited depth of focus. Using microscopic fringe projection on objects with a large aspect ratio this can become a serious limit. That’s the point where one of our projects started. We developed an algorithm for the „depth stitching“ of fringe projection measurements. This in first place sounds easier than it is. As we were using a stereo microscope resulting in a symmetric angle between illumination – object and object observation it is necessary to correct for lateral shift when moving the microscope relative to the object. Solving this issue it is possible to extend the measurement range of such a setup considerably.

Confocal Systems

Confocal microscopy was developed by Marvin Minsky in order to overcome the limits of conventional florescence microscopy. Later on the principle was successfully used as a measurement principle for 3D measurements of objects. The basic principle applied is a point illumination and detection, resulting only in a high intensity value on the detector, when the according object point is in focus.
For full field measurements usually a so called Nipkow disc is used, which basically is a disc with a number of pinholes distributed on it. Due to signal conditioning reasons, i.e. cross talk between pinholes, normally most of the illuminating light gets blocked on the disc (> 90%). As a consequence some manufacturers combine the Nipkow disc with a microlens disc, alleviating the problem a bit on the cost of complexity and size. Another possibility is to substitute the Nipkow disc completely with a microlens disc, offering a better signal to noise ratio, as also all of the reflected light is used and not increasing the system size.

A very compact system I was involved in the development can be found e.g. under http://www.twip-os.com.
One of the main drawbacks of confocal systems is the large number of images that have to be recorded for measuring objects with high aspect ratio. Accordingly different setups were developed overcoming this problem. One example are chromatic confocal systems, where the z-scan was replaced by a chromatic dispersion in object space. These systems in turn are ‚only‘ available as point or line measuring devices with higher lateral resolution. Full field measuring systems either require at least on scanning axis or, in case color cameras are used, have a limited z-resolution or for the case of laterally distributed spectrometers have reduces lateral resolution.

Polarization Optics

Nowadays spatial light modulators (SLMs) are quite commonly used in digital holography. When I started my PhD thesis the use of these modulators war not as common as today and the number of available modulators, especially for digital holography, was very limited. Accordingly modulators made for other purposes, i.e. projectors, were used. Doing so requires a polarization optical characterization of these modulator, as most of them are of TN (twisted nematic) type. Which makes their use way more complicated but also offers some interesting other possibilities, e.g. the creation of light fields with laterally varying intensity and polarization.

Digital Holography

Except using SLMs in digital holography it is possible to create different systems e.g. using static DOEs (diffractive optical elements). So it is possible measuring in-plane and out-of-plane deformations with only two camera images, by using Fresnel type holograms with different carrier frequency directions and two lasers.