Hybrid telecentric triangulation sensor system with real-time field-dependent deconvolution

High-resolution optical inspection techniques typically require expensive imaging systems. One possible approach for reducing the overall cost of such systems is to combine optics of limited performance with digital image restoration techniques. In this contribution, we introduce a new hybrid system based on the combination of a field-programmablegate-array (FPGA), a large telecentric lens with strong field dependent chromatic aberrations and different types of object-adapted projections for 2D and 3D inspection tasks. The optical design tries to minimize cost and weight by replacing the – typically very large – first refractive group of the object-sided telecentric lens by one thin diffractive optical element. This approach can correct the field-dependent aberrations very well. Unfortunately, strong chromatic aberrations are unavoidable. For restoration, we use digital image processing by means of advanced deconvolution algorithms. The necessary number of operations for the strong fielddependent aberrations is extremely high. We combine algorithmic approaches (convolutions using singular-value decomposition) and parallelization directly on the FPGA to achieve live restoration (15 Hz) for a custom-built image sensor with three megapixels. The power consumption is low and a compact processing unit is realized. This hybrid (digital-optical) imaging system is used for 2D- and 3D inspection based on adaptive triangulation. One approach is based on inverse fringe projection. Ray-tracing based on the sensor setup and a reference CAD-model of the part to be tested is used for computing a fringe mask. This mask is inscribed on a spatial light modulator, which then projects the fringes towards the object. If the object under test is without faults, high-frequency equidistant straight-line fringes will result on the image sensor and low height measurement uncertainties at low lateral resolutions, even for non-cooperative objects, are achievable. The importance of a large depth-of-field and constant image-scale along the optical axis for the inverse fringe projection lead to the decision of a telecentric objective.

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