Improving the calibration of phase measuring deflectometry by a polynomial representation of the display shape

Background: Phase measuring deflectometry is a highly precise and full field metrology technique for specular surfaces based on the distortion of known reference patterns observed as a reflection at the surface under test. Typically, liquid crystal displays are employed to provide the required patterns. Due to a lack of research, these displays are used without sufficient calibration. Methods: In this work, we present an enhanced calibration for phase measuring deflectometry, taking flatness deviations of the display surface into account. The display shape is modelled as a polynomial surface whose coefficients are determined by minimizing the retrace error in a global optimization procedure during calibration. This approach does not require any additional measurements or hardware. Improvements due to the enhanced calibration model are qualified experimentally using a flat and a spherical concave mirror. Results and conclusion: The model-based parameterization of the display surface yields significant improvement on both samples. The peak to valley (PV) of measured deviations on the plane mirror are reduced by 67% to 0.55 μm. Measuring the spherical sample without the display parameterization leads to a rather large shape deviation of 33.40 μm PV which is reduced by 94% to 1.98 μm. The viability of our approach confirms the dominant role of flatness deviations of the display surface as an error source in absolute shape measurement using phase measuring deflectometry

Optical In-Process Measurement: Concepts for Precise, Fast and Robust Optical Metrology for Complex Measurement Situations

Optical metrology is a key element for many areas of modern production. Preferably, measurements should take place within the production line (in-process) and keep pace with production speed, even if the parts have a complex geometry or are difficult to access. The challenge for modern optical in-process measurements is, therefore, how to simultaneously make optical metrology precise, fast, robust and capable of handling geometrical complexity. The potential of individual techniques to achieve these demands can be visualized by the tetrahedron of optical metrology. Depending on the application, techniques based on interferometry or geometrical optics may have to be preferred. The paper emphasizes complexity and robustness as prime areas of improvement. Concerning interferometric techniques, we report on fast acquisition as used in holography, tailoring of coherence properties and use of Multiple simultaneous Viewing direction holography (MultiView), self reference used in Computational Shear Interferometry (CoSI) and the simultaneous use of several light sources in Multiple Aperture Shear Interferometry (MArS) based on CoSI as these techniques have proven to be particularly effective. The use of advanced approaches based on CoSI requires a transition of the description of light from the use of the well-known wave field to the coherence function of light. Techniques based on geometric optics are generally comparatively robust against environmental disturbances, and Fringe Projection (FP) is shown to be especially useful in very demanding measurement conditions

1 kHz 3.3 µm Nd:YAG KTiOAsO4 optical parametric oscillator system for laser ultrasound excitation of carbon-fiber-reinforced plastics

We present a new laser prototype for laser ultrasonics excitation. The fundamental wavelength of a Q-switched Nd:YAG laser with a repetition rate of 1 kHz is converted to 3.3 μm with a KTiOAsO4 optical parametric oscillator. The achieved pulse energy at 3.3 μm is 1.7 mJ, and the pulse duration at the fundamental wavelength of 1.06 μm has been measured to be 21 ns. The ultrasonic excitation efficiency is about 3.5 times better compared to the application of state-of-the-art CO2 lasers

Single-shot terahertz field driven X-ray streak-camera

A few femtosecond X ray streak camera has been realized using a pump probe scheme that samples the transient response of matter to ionizing soft X ray radiation in the presence of an intense synchronized terahertz field. Borrowing its concept from attosecond metrology, the femtosecond X ray streak camera fills the gap between conventional streak cameras with typical resolutions of hundreds of femtoseconds and streaking techniques operating in the sub femtosecond regime. Its single shot capability permits the duration and time structure of individual X ray pulses to be determined. For several classes of experiments in time resolved spectroscopy, diffraction or imaging envisaged with novel accelerator and laser based short pulse X ray sources this knowledge is essential, but represents a major challenge to X ray metrology. Here we report on the single shot characterization of soft X ray pulses from the free electron laser facility FLAS