José M. Enguita

On-line metrology with conoscopic holography: beyond triangulation.

On-line non-contact surface inspection with high precision is still an open problem. Laser triangulation techniques are the most common solution for this kind of systems, but there exist fundamental limitations to their applicability when high precisions, long standoffs or large apertures are needed, and when there are difficult operating conditions. Other methods are, in general, not applicable in hostile environments or inadequate for on-line measurement. In this paper we review the latest research in Conoscopic Holography, an interferometric technique that has been applied successfully in this kind of applications, ranging from submicrometric roughness measurements, to long standoff sensors for surface defect detection in steel at high temperatures.

Fast stereo vision algorithm for robotic applications

Autonomous navigation applications demand sensors with a low sample time to be able to increase speed. We have developed a stereo vision algorithm, capable to deliver dense disparity maps for single, high-resolution scanlines at high speed (40 ms/line), even for wide disparity ranges. We have tested the algorithm with synthetic and real images. Our algorithm is based on a dynamic programming schema with a cost function based on a weighted sum of squared intensify errors. Weight factors are based on gradient values. The algorithm includes explicitly detection of occlusion. Occlusion cost changes dynamically depending on gradient values of matched points.

Conoscopic holography-based long-standoff profilometer for surface inspection in adverse environment

One of the more challenging applications of optical metrology is real-time dimensional control and surface inspection in industrial ap- plications, where strong requirements of cost, setup, and applicability in adverse environments greatly limit the number of applicable technolo- gies. This work shows an optic profilometer developed specifically for this purpose, the algorithms developed for efficient and accurate dis- tance map calculation, and an example of its use in a harsh steelmaking environment. This device, based on conoscopic holography, is able to obtain a distance profile of a target in a single shot, works at long dis- tance standoff 700 to 1200 mm, and still keeps good resolution under 0.2 mm with a very easy and reliable setup. The second part of the work focuses on a real example of this technology applied in an on-line in- spection system in steel continuous casting funded by the European Coal and Steel Community, and which is currently working in the Aceralia LDA steelmaking factory in Asturias Spain. The system is placed in the process line and performs on-line detection of surface defects over hot steel slabs above 800°C in the inspection point from a distance of 1200 mm. 100% of the production can be inspected without interfering with the process and without adding any delay.

Industrial online surface defects detection in continuous casting hot slabs

Early surface defects inspection in hot steel products is a difficult task, but can help to reduce significantly production costs. This is the case of steel slabs when they are produced in the continuous casting line. Conoscopic holography phase-based long stand-off profilometers have shown to be a great tool for this kind of inspection, and a breakthrough system based on them is being used for more than 2 years in production conditions with high reliability and economical impact. This paper presents the results of this system and the challenges it has overcome: hot material up to 900°C, dust, scale over the inspected surface.

Common-path two-wavelength interferometer with submicron precision for profile measurements in on-line applications

We propose a common-path two-wavelength interferometric system based on a single optical element, a Savart plate, that is able to obtain single-shot profile measurements with submicron precision from safe working distances (beyond 100 mm). These characteristics make this sensor ideal for surface inspection in on-line applications. For the illumination branch, two lasers with close wavelengths are combined and then passed through a rotating holographic diffuser for drastic speckle reduction. In the acquisition branch, the interferometric signals of both wavelengths are captured simultaneously by a camera, and their phase signals are combined to extend the measurement range.

On-line submicron profile measurements from safe distances with conoscopic holography: feasibility and potential problems

On-line noncontact surface inspection with high precision is still an open problem. Usual methods are, in general, not applicable in hostile environments or not adequate for on-line measurement, as they are either slow in nature or need to work from very short, unsafe distances, providing small depths of field and apertures. The ongoing work toward the development of a noncontact optical profile measuring sensor that could be used for submicron measurements in on-line applications is presented here. Our approach is based on conoscopic holography and triangulation, and uses a very simple method for removing speckle noise, which is key for obtaining high precisions from safe distances (several centimeters).

Denoising of conoscopic holography fringe patterns with orientational filters : a comparative study

One of the most important problems to address when applying interferometric techniques to industrial applications is the prevalence of noise, which results in poor fringe patterns and thus poor measurements. One of the techniques that suffers most from this problem is conoscopic holography. Though this interferometric technique is ideal for industrial inspection, the poor quality of fringe patterns obtained in adverse environments may make measurement impossible. Classical filtering techniques based on one-dimensional filters or general speckle removal filters, such as the Frost or the gamma, may not suffice in adverse conditions. Therefore, a new approach, based on the nature of the fringe pattern information itself, is worth looking into. In this paper we propose the use of orientational filters to develop a filtering method that not only removes noise of any nature, but also enhances the fringe-pattern information. Several approaches to these algorithms are implemented and evaluated using synthetic conoscopic fringe patterns under different noise conditions, showing how they clearly outperform classical filters with negligible distortion even in the worst conditions. Examples with real data acquired with the latest prototype of a conoscopic long-standoff profilometer are also provided.

Face recognition using binary thresholding for features extraction

The paper deals with a system for the analysis and automated identification of a human face. A face can be recognized when the details of individual features are resolved. The idea is to extract the relative position and other parameters of distinctive features such as eyes, mouth, nose and chin. The overall geometrical configuration of face features can be described by a vector of numerical data representing position and size of main facial features. At first, from sequential images, eye coordinates are extracted by detecting eye winking. The interocular distance and eye position can be used to determine size and position of the areas of search for face features. In these areas binary thresholding is performed, the system modifies the threshold automatically to detect features. To find their coordinates, discontinuities are searched for in the binary image. Experimental results show that the proposed method is robust, valid for numerous kind of facial images in real scenes, works in real time with low hardware requirements and the whole process is conducted automatically.

Thickness quality control

This article describes a series of methods to detect thickness defects in DWI tinplate. These methods provide information to identify the origin of the defects, allowing fast corrections and, therefore, improving mill performance. In this approach, an adapted thickness signal is spatially sampled from the input stages of the mills last X-ray gauge. The system is also able to detect other problems related to thickness quality such as third-octave chatter using only the thickness measures from an X-ray gauge. The usage of this system resulted in an improvement in the factory performance and considerable money savings.

In-Situ Waviness Characterization of Metal Plates by a Lateral Shearing Interferometric Profilometer

Characterizing waviness in sheet metal is a key process for quality control in many industries, such as automotive and home appliance manufacturing. However, there is still no known technique able to work in an automated in-floor inspection system. The literature describes many techniques developed in the last three decades, but most of them are either slow, only able to work in laboratory conditions, need very short (unsafe) working distances, or are only able to estimate certain waviness parameters. In this article we propose the use of a lateral shearing interferometric profilometer, which is able to obtain a 19 mm profile in a single acquisition, with sub-micron precision, in an uncontrolled environment, and from a working distance greater than 90 mm. This system allows direct measurement of all needed waviness parameters even with objects in movement. We describe a series of experiments over several samples of steel plates to validate the sensor and the processing method, and the results are in close agreement with those obtained with a contact stylus device. The sensor is an ideal candidate for on-line or in-machine fast automatic waviness assessment, reducing delays and costs in many metalworking processes.