Inverse Fringe Projection Research Articles

A new method for high dynamic range 3D measurement combining adaptive fringe projection and original-inverse fringe projection

For fringe projection profilometry (FPP), high reflections on metal, plastic, and ceramic surfaces cause intensity saturation, resulting in three dimensional (3D) measurement errors. The measurement efficiency using the multiple exposure method (MEM) is low and the exposure time is difficult to quantify. The original and inverse fringe projection method (OIFPM) suffers from saturation of the opposite fringes on highly reflective surfaces. The adaptive fringe projection method (AFPM) is still limited by image dynamic range and pixel mapping accuracy. To this end, this paper proposes a new scheme combining OIFPM and AFPM, called the original and inverse adaptive fringe projection method (OIAFPM). Firstly, an adaptive intensity template in the camera coordinate system is generated. Secondly, a pixel mapping between the camera and projector images is established. Thirdly, original and inverse adaptive fringes are generated and projected. Finally, 72 wrapped phase calculation equations are given in detail. Experiments confirm that the 3D measurement employing the proposed OIAFPM is much better than that employing AFPM and OIFPM. Although the 3D measurement employing MEM is similar to that employing OIAFPM, the measurement efficiency employing OIAFPM is much higher than that employing MEM.

Optical inline inspection detecting 3D defects on complex free-form surfaces in harsh production environments

Abstract In today’s industrial manufacturing, quality monitoring is one of the key elements to ensure continuous improvement and to trigger innovations. Especially inline inspection presents considerable challenges to the underlying measurement system. Besides the – in most cases – already not easily satisfiable demands concerning resolution, measurement uncertainty, and cycle time, the robustness of the chosen solution within the overall production environment is an essential requirement for a successful operation in the field. Furthermore, a sufficient ease of operation for the production staff plays a crucial role for the acceptance of the measurement technology. In sheet metal forming, for example, vibrations, ambient light variations and short cycle times as well as oiling and surface variations of the specimen under test challenge and simultaneously limit currently available measurement systems in their application. Especially in automotive manufacturing, visionary design concepts increase the complexity of components with respect to shape and forming capability, leading to critical areas on the object prone to constrictions and cracks. Varying geometry and surface defect characteristics call for an automated 3D inspection to maintain constant product quality for each single part. In this paper, an applied method for the inline inspection of free-form metal surfaces is presented, which allows for quality monitoring even in a harsh environment. So-called inverse fringe projection in combination with single-sideband demodulation facilitates the inspection of free-form metal surfaces in 3D, based on a single camera image (‘single-shot’), which allows for measurement even in a vibrating environment. Only one single camera and a projector are needed, simplifying the integration into series production considerably. Calibration-free, fast setup possibilities enable flexible handling and maintenance in a short-cycled production. First inline tests in a press shop at BMW proof the capability of this method. As an example, constrictions with a depth of about 80 microns could be detected automatically during series production within the press.

Advanced Characterization Techniques for Turbine Blade Wear and Damage

This paper presents four complementary non-destructive measurement techniques for material characterization and damage detection of turbine blades. The techniques are macroscopic fringe projection with inverse fringe projection algorithms, robot guided microscale fringe projection, high frequency eddy current and pulsed high frequency induction thermography, both in the megahertz range. The specimen on which the measurements were carried out is a blade of the 1st stage high pressure turbine of a modern airplane jet engine. The turbine blade was characterized with regard to the macroscopic and microscopic geometry, cracks in the base material as well as the condition of the protective layer system.

Turbine blade wear and damage – An overview of advanced characterization techniques

Abstract This paper gives an overview of four measurement techniques that allow to extensively characterize the status of a worn turbine blade. In addition to the measurement of geometry and surface properties, the condition of the two protective coatings needs to be monitored. Fringe projection was used to detect and quantify geometric variances. The technique was improved using newly developed algorithms like inverse fringe projection. A Michelson interferometer was employed to further analyze areas with geometric defects and characterize the surface morphology of the blade. Pulsed high frequency induction thermography enabled the scanning of the blade for small cracks at or close to the surface. High frequency eddy current testing was used to determine the protective layers status and their thickness.

Single image geometry inspection using inverse endoscopic fringe projection

Fringe projection is an important technology for the measurement of free form elements in several application fields. It can be applied for geometry elements smaller than one millimeter. In combination with deviation analysis algorithms, errors in fabrication lines can be found promptly to minimize rejections. However, some fields cannot be covered by the classical fringe projection approach. Due to shadowing, filigree form elements on narrow or internal carrier geometries cannot be captured. To overcome this limitation, a fiberscopic micro fringe projection sensor was developed. The new device is capable of resolutions of less than 15 µm with uncertainties of about 35 µm in a workspace of 3 × 3 × 3 mm³.<br />Using standard phase measurement techniques, such as Gray-code and cos²-patterns, measurement times of over a second are too high for in-situ operation. The following work will introduce a new approach of applying a new one image measuring method to the fiberscopic system, based on inverse fringe projection. The fiberscopic fringe projection system employs a laser light source in combination with a digital micro-mirror device (DMD) to generate fringe patterns. Fiber optical image bundles (FOIB) are used in combination with gradient-index lenses to project these patterns on the specimen. This advanced optical system creates high demands on the pattern generation algorithms to generate exact inverse patterns for arbitrary CAD-modelled geometries. Approaches of the optical simulations in the context of the complex beam path, together the drawbacks of the limited resolutions of the FOIBs shall be discussed. Early results of inverse pattern simulations using a ray tracing approach of a pinhole system model are presented.<br />

Multiscale Optical Inspection Systems for the Regeneration of Complex Capital Goods

The inspection of capital goods with complex geometries is a challenging task due to the limited maneuvering and measuring space available. We developed a set of optical measurement systems for the inline inspection of such machines and capital goods. At first we introduce a borescopic fringe projection system based on of-the-shelf components. It is capable of detecting geometric variances in hard to reach areas, e.g. inside machines or in between parts with highly complex geometries like blade integrated discs (blisks). Single parts like compressor blades are measured with an inverse fringe projection system, which uses fully adaptable fringe patterns. The adaptable patterns lead to a high sensitivity and high speed. Results can be achieved with only one fringe pattern per measurement.In order to perform microscopic measurements and surface characterizations, we use a michelson interferometer with advanced 3D reconstruction algorithms to detect microscopic variances of the objects surface. These newly developed algorithms lead to higher sensitivity and improved results.Together these three inspection systems enable us to detect and to quantify geometric defects or variances of different industrial parts. Based on this information the prediction of the reliability of a part can be improved and the lifetime of an industrial part can be extended leading to a reduction of maintenance costs.

Remote metrology by comparative digital holography

A method for the remote comparison of objects with regard to their shape or response to a load is presented. The method allows interferometric sensitivity for comparing objects with different microstructure. In contrast to the well-known incoherent techniques based on inverse fringe projection this new approach uses the coherent optical wave field of the master object as a mask for the illumination of the sample object. The coherent mask is created by digital holography to allow instant access to the complete optical information of the master object at any place desired. The mask is reconstructed by a spatial light modulator (SLM). The optical reconstruction of digital holograms with SLM technology allows modification of reconstructed wavefronts with respect to improvement of image quality, the skilled introduction of additional information about the object (augmented reality), and the alignment of the master and test object.

Object adapted pattern projection—Part I: generation of inverse patterns

Fast and robust 3D inspection is of big importance for industrial quality control. Therefore, reliable optical techniques are needed that use as few images as possible for measurement. One promising technique for this aim is the inverse fringe projection which has the following advantages: The technique includes the information of a preceding measurement into the projected inverse pattern. Therefore, it is possible to do differential measurement using only one camera frame for each state. Because the camera takes images with user-defined patterns (e.g. fringes with constant frequency), one always has optimized patterns for sampling independently of the object shape. The hardware needs are as low as just a programmable projector and a standard camera. Till now the technique had drawbacks concerning the robust pattern generation and the quantitative (instead of only qualitative) evaluation of differential phases. In this paper, we concentrate our investigations on the robust pattern inversion. We show how the process can be simplified by separating the different inversion problems. Different methods and examples for generating the inverse pattern will be shown and compared to each other.

Comparative digital holography

We described a method for direct holographic comparison of the shape or the deformation of two objects when it is not necessary that both samples be located at the same place. In contrast to the well-known incoherent techniques based on inverse fringe projection, this new approach uses a coherent mask that is imaged onto a sample object that has a microstructure different from that of the master object. The coherent mask is created by digital holography to permit instant access to complete optical information on the master object at any wanted place. Transmission of the digital master holograms to the relevant places can be made with a broadband digital telecommunication network such as the Internet.