A wide variety of three dimensional (3D) measurement systems that can extract shape information’s with sub millimetric accuracy is available in the industry. However, they generally are of macroscopic size and measuring on confined areas is not feasible. To miniaturize such systems, the step proposed is the integration of flexible image guides combined with compact optical probes. This miniaturization process is tested on an active stereoscopic measurement system. In the projection channel of the system, a digital micro-mirror device (DMD) generates structured binary patterns from an incoherent white light source and injects them into a first image guide. Then, a compact optical system projects the pattern on the measurement area. The same configuration principle is applied to the acquisition channel and allows the capture of the measurement area through a second image guide and finally to a digital camera. In this miniaturized system, image guides have lower resolution than in standard imaging devices. Indeed they are equivalent of 70k pixels devices to compare to the almost 800k pixels of the DMD and camera. That implies lower axial and lateral resolutions and consequently the shape reconstruction method must be carefully chosen. In this paper, several reconstruction strategies such as tuning the projected patterns frequency and also phase-shfit versus gray code based methods were compared considering the best axial resolution criteria.
In three-dimensional (3-D) measurement systems based on triangulation, a stereoscopic angle between two distinct viewpoints encodes the depth information. This angle generally generates some distortion known as keystone distortion. In this paper an original 3-D optical configuration in addition with a digital light processing system (DLP) is presented which prevents keystone distortion, resulting in less calibration and post-processing work. The DLP is used to project incoherent white light fringes pattern at high frame rate (30 Hz) on a specimen and a CCD camera dynamically captures this projected pattern. Using a phase shifting algorithm, the reconstruction of the 3-D shape of the specimen is finally computed.
This optical configuration is based on an « out of axis » aperture method combined with an afocal design for both projection and acquisition. With the combination of these two properties, the stereoscopic effect is obtained without any keystone distortion and a unique objective lens instead of two in a classical 3-D measurement system is used. As a result of this unique objective lens, the global volume of the measurement device can be easily minimized.
This system was designed with the optical software Zemax to limit geometric and chromatic aberrations and to control the diffraction effect. Experiments showed that high surface profile accuracy can be obtained on a variety of surfaces, allowing reverse engineering on micro-scaled objects or precise 3-D measurements of macro-scaled objects. Also a depth resolution value under 2 μm for a scanned area around 7.5x5.5mm is obtained under experimental conditions.