Tobias Wiesendanger

Chromatic confocal detection for high-speed microtopography measurements

The chromatic confocal approach enables the parallelization of the complete depth-scan of confocal topography measurements. Therefore, mechanical movement can be reduced or completely avoided and the measurement times shortened. Chromatic confocal point sensors are already commercially available but they need lateral scanning in x- and y-direction in order to measure surface topographies. We achieved a further parallelization in the x-direction by realizing a chromatic confocal line sensor using a line focus and a spectrometer. In a second setup, we realized an area measuring chromatic confocal microscope, which is capable of one-shot measurements without any mechanical scanning. The depth resolution of this setup can be improved by measuring in a small number of different heights. Additional information about the color distribution of the object is gained.

Chromatic confocal microscopy with a finite pinhole size

Chromatic confocal microscopy has the advantage of short measurement times because of its parallel depth scan. As most white-light sources have limited optical output power, light-efficient setups are necessary. Using an extended detection pinhole is one way to improve light efficiency. We have calculated the effect of extended pinholes in chromatic confocal setups. We found that, for certain pinhole sizes, the FWHM of the confocal signal is nearly constant over a large wavelength interval.

Signal evaluation for high-speed confocal measurements.

The confocal-detection principle is open especially for use in medical applications. For inspection systems applications for technical objects in reflection confocal setups are of growing importance. For such applications the confocal measurements need to have a very short measuring time. A fast detection system is needed and to satisfy this requirement only a small number of height levels are measured and a fast-evaluation algorithm is used. Drawbacks of the reduction of height levels are a greater influence of noise and additional systematic errors on the measured heights. Study the effects of the reduction are calculated, different evaluation algorithms are analyzed, and the optimization of the parameters is discussed.

Non-mechanically-axial-scanning confocal microscope using adaptive mirror switching

A non-axial-scanning confocal microscope employing a monochromatic light source has been developed. The system controls the defocus of an objective into three to .ve optimized states by using a membrane-adaptive mirror, and determines the axial height of an object according to the confocal output value with each defocus. A genetic algorithm is employed to optimize the adaptive mirror shape, with the information entropy of the spectrum of the lateral confocal spot pro.le used as a cost function in the genetic algorithm. Our experimental system successfully determined axial object height within 50 microm range with 0.64 % of error.

New approaches in depth-scanning optical metrology

Depth-scanning is an established technique in macroscopic and microscopic 3-D metrology. Representative in this context are the confocal technique and the white-light interferometry. A new fast depth-scanning technique has been applied to a confocal point sensor to be used in a laser-welding application for in-process measurement. The depth measurement range can be extended to about +/-1 mm at about 1500 measurement cycles per second. The possibilities and the potential of these techniques are described. Another principle of depth-scanning is the chromatic confocal technique. In connection with a new approach, an innovative confocal setup enables the parallelization of the complete depth-scan for the complete measurement of a line cut of moved objects. In the macroscopic scale, the new measurement techniques of depth-scanning fringe projection (DSFP) was introduced recently. In the microscopic scale, it has been implemented successfully in a stereo microscope.

Aberration measurement from confocal axial intensity response using neural network

We propose a high-speed, parallel system for lens aberration measurement employing a confocal optical setup. This method uses a non-interferometric, conventional confocal axial response to determine the spherical aberration coefficient of a confocal objective. The aberration coefficients are successfully calculated from the intensity axial response by employing a neural network. It is estimated that the system can find out the aberration coefficients of 10,000 microlenses in 20 seconds of measurement and 1 second of calculation time. Our experimental results also demonstrate the practicality of this system.

Non-axial-scanning confocal microscope by membrane mirror shape switching

A non-axial-scanning confocal microscope by membrane mirror shape switching has been developed. The system controls the defocus of an objective by membrane mirror, and determines the height of an object from the confocal output value of each defocus.