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Lecture Hall ICM - Internationales Congress Center München SPIE Optical Metrology > Optical Measurement Systems for Industrial Inspection XI > Digital Holography
12:00-12:20 h | ICM - Internationales Congress Center München ICM Room 14c
Subjects: Optical Measurement Systems / Optical Metrology
The high-speed topography of both specular reflective and diffuse reflective surfaces with one single instrument represents a major challenge in optical metrology. Metrology techniques exist, which can cope with both types of surfaces, such as white light interference microscopy (WLI) and confocal microscopy (CM). However, the measurement is very time-consuming ranging from multiple seconds to minutes. Structured illumination, on the other hand, recovers the shape only, but is restricted to the inspection of rough scattering surfaces. Dual wavelength holography can overcome the aforementioned shortcomings. The measurement range and the axial resolution can be adjusted via the choice of the dual wavelength pairs. Moreover, a flexible and lightweight setup can be enabled, without the necessity to employ imaging lenses due to the recovery of complex information (amplitude and phase) and numerically refocusing the recorded wavefield to the object plane. In the last years, Vertical Cavity Surface Emitting Lasers (VCSEL) have received more attention. The single mode VCSEL offers excellent coherent properties, with a coherence length of a few tens of cm, an isotropic angular radiation profile and dimensions < mm, allowing a very compact and lightweight setup arrangement. In this paper, we describe the application of VCSEL for the dual wavelength digital holography for the inspection of diffuse scattering and specular reflective surfaces. At first, the spectral response of the VCSEL is determined in a calibration process. A wavemeter with sub-Angstrom spectral resolution has been employed to investigated stability and repeatability. For the sake of a lightweight setup and in order to efficiently use the power provided by the VCSEL, an off-axis arrangement has been chosen. The shape measurements are taking on referenced surfaces. The measurement system, the evaluation of the data and an error analysis based on the referenced surfaces will be shown and discussed in this paper.