Single Grating Phase-Shift (SGPS): a novel approach to phase demodulation of fringes

This project was developed in the context of the National Italian Project: Low-cost 3D imaging and modeling automatic system (LIMA3D). The Laboratory aim was to design, develop and perform a metrological characterization of a low-cost optical digitizer based on the projection of a single grating of non-coherent light.

SGPS (Single Grating Phase-Shift) is a whole field profilometer based on the projection of a single pattern of Ronchi fringes: a simple slide projector can be used instead of sophysticated, very expensive devices, to match the low-cost requirement.

A novel approach to the phase demodulation of the fringes has been developed to obtain phase values monotonically increasing along the direction perpendicular to the fringe orientation. As a result, the optical head can be calibrated in an absolute way, very dense point clouds expressed in an absolute reference system are obtained, the system set-up is very easy, the device is portable and reconfigurable to the measurement problem, and multi-view acquisition is easily performed.


3D-Optolab: optical digitization based on active triangulation and non-coherent light projection

As shown in Fig. 1, the system is composed by the optical head and two moving stages. The optical head exploits active stereo vision. The projection device, LCD based, projects on the target object bi-dimensional patterns of non-coherent light suitable to implement the SGM, PSM GCM and GCPS techniques for light coding. Each point cloud is expressed in the reference system of the optical head (XM, YM, ZM).

The optical head is mounted on a moving stage, allowing its translation along the vertical and the horizontal directions (Tv, To); as shown in Fig. 2, areas large up to two square meters can be scanned and large surfaces can be measured by acquiring and aligning a number of separate patches. The objects presenting circular symmetry can be placed on a rotation stage: it enables the acquisition of views at different values of the rotation angle.

The system is equipped with stepped motors, to fully control the rotation and the translation of the optical head. Each view is aligned by means of rototraslation matrices in the coordinate system (XR, YR, ZR), centerd on the rotation stage. The sequence of acquisition of the range images is decided on the basis of the dimension and the shape of the object, and is performed automatically.

Fast calibration is provided, both for the optical head and for the rotation stage.

Fig. 1 - Schematization and image of the prototype.
Fig. 2 - Example of acquisitions of a large object (top) and a smaller one (bottom).

System Performances

  • Optical, non contact digitization based on active triangulation and non-coherent light projection;
  • Adjustable measuring area, from 100 x 100 mm to 400 x 400 mm (single view);
  •  Measuring error from 0.07 mm to 0.2 mm, scaled with the Field of View;
  • Automatic scanning, alignment and merging, mechanically controlled;
  • Color/texture acquisition;
  • PC based system, with 333 Pentium II, Windows 98 and Visual C++ software
  • Import/Export formats for CAD, rapid prototyping, 3D-viewers, graphic platforms;
  • Software interface for the handling, the processing and the visualization of both partial and complete point clouds.

OPL-3D: a portable system for point cloud acquisition

OPL-3D has been specifically designed for applications of reverse engineering and rapid prototyping, as well as for applications of measurement and quality control.

The system exploits active stereo vision (the absolute approach is implemented) using time-multiplexing based on the Gray-Code-Phase-Shifting method.

The projector-camera pair

OPL-3D can host a wide variety of projectors. In the left figure in Fig. 1 the device is the ABW LCD 320: it is a microprocessor-controlled and column-driven projector, specifically intended to be used in this class of systems. Alternatively, those devices currently available for video projection can be succesfully used, as that one shown on the right figure in Fig. 1 (Kodak DP 900, based on DLP technology).

The detector is a commercial CCD video camera. In the configurations shown in Fig. 1, the camera is an inexpensive colour Hitachi KP D50, with standard resolution (752 x 582 px). However, any type of camera (black/white or colour, with single or multiple CCDs for colour separation, and with different pixel densities) can be mounted on the system, depending on the application and on the projector used. In Fig. 2, for example, a 1300 x 1030 px digital video camera (Basler model) is mounted, to acquire at the required resolution large fields of views

The mount

The projector and the camera are mounted onto a rigid bar, that can be easily moved around the scene by means of a tripod, and that holds the adjustment units for proper orientation. The mount is fully reconfigurable: all parameters can be varied according to the distance from the target, the required measurement resolution and the FoV (Fig. 3).

Given the fact that through sophisticated calibration procedures the system is able to finely estimate the operating parameters, no accurate positioning equipment (micropositioners, microrotators) is required, the only requirement being stability of the mount during the measurement procedure.

Fig. 4 shows two examples of on-site measurements of complex shapes where the full flexibility of the system was mandatory to perform the acquisition.

Fig. 3 - Images of the tripods used and of the equipment of the prototype.
Fig. 4 - Two on-site acquisition campaigns carried out by the Laboratory: the Winged Victory point cloud acquisition (left) and the Ferrari point cloud acquisition (right).

The electronic hardware

OPL-3D is equipped with a PC, that has the purpose of (i) driving the projector with the appropriate pattern sequence, (ii) acquiring the image sequences from the target, and (iii) elaborating the images. In addition, it contains all the features to perform sophisticated procedures for setting up and reconfiguration.

The PC is in the current configuration a Pentium III 900 MHz, 1 GB Ram, equipped with a Matrox Meteor II Frame Grabber. The Projector is operated by the PC through the Serial Connector.


OPL-3D exhibits low-measurement uncertainty (120 mm) over large measurement areas (450 x 340 mm), linearly scalable in the case of smaller areas. Special care has been devoted to flexibility of use, in-field measurement setting, reconfigurability and robustness against environmental light changes and surface colour and texture

Fig. 5 shows the acquisition of the blue car already seen in Fig. 2. Multiview alignment and registration is performed by either purposely designed software or by means of commercially available products, depending on the complexity of the process.

Fig. 5 - Point Cloud obtained with every acquisition aligned to form a complete and dense reconstruction.

Technology transfer

OPL 3-D has been put into the market by Open Technologies s.r.l., Italy, a start-up company of the University of Brescia, under the Trade Name of 3DShape, in a manifold of versions, including sophisticated software for multi-view combination, point cloud manipulation and transformation, up to surface generation.