Jorge Marina

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.

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.

Toward online non-contact roughness profile measurements with a sensor based on conoscopic holography: current developments

On-line non-contact roughness metrology is still an open problem. Usual methods involve either contact (stylus-type devices) or perform indirect evaluations of some roughness parameters, such as Ra, with light scattering techniques or speckle measurement (among the most common optical techniques), inductance (only for magnetic materials) or ultrasound methods. However, a generic method able to obtain every roughness parameter (what means recording the real distance profile), able to work with a variety of surface types, and able to be installed in production lines is still to be developed. In this article, the ongoing work towards the construction of a non-contact optical profile measuring sensor that could be used for roughness measurements is presented. Our approach is based on Conoscopic holography, a common-path interferometric technique which is a good candidate for industrial applications. Current research effort is focused in enhancing accuracy in these systems, by both reducing the coherence of the illuminating source (laser) and changing the hardware and software setup, with the aim of building a sensor able to capture a profile of an objects surface in a single shot with high precision from a relatively long standoff (several cm).

Conoscopic holography based profilometers for defect inspection : improvements in speed, resolution and noise reduction

One of the more challenging applications of optical metrology is real-time dimensional control and surface inspection in industrial applications, where strong requirements of cost, speed of operation, ease of setup and applicability in adverse environments, greatly limit the number of applicable technologies. An optic profilometer, based on Conoscopic holography, has been designed specifically for this purpose and used in several on-line inspection systems. This device is able to obtain a distance profile of a target in a single shot; works at long distance standoff (700-1200mm) and still keeps good resolution (under 0.2mm) with a very easy and reliable setup. However, there are still some drawbacks that should be addressed. The first one is the signal processing, which is a relatively expensive process and limits the acquisition rate at no more than 70 profiles per second. The second one is speckle noise, which is an inherent problem in systems that use coherent-light illumination and triangulation, and therefor could be extrapolated to many other optical inspection systems. This paper shows the current lines of research to solve both problems and presents some initial, yet very interesting, results. These improvements can be applicable to other ranges of devices using this technology in adverse environments, for roughness and vibration measurement or surface defects detection.

On-line non-contact measuring of synchronizer hubs

In the industry of sintered automobile synchronizer hubs, as in many others, a fast on-line measurement system of the production is a very important tool for production and quality control, enabling among others: extended quality control to all the production, lower start-up times, improved knowledge of the process and influence of its parameters and database management of the information. In this paper we describe an on-line optical measuring system that is able to inspect 100% of the production, making special emphasis on how the resolution and accuracy of the measurement could be compromised by factors that are not related to the sensor itself, as well as how to correct them. The paper also describes how to validate such a system according to the ISO 5725 standard.

Toward extended range sub-micron conoscopic holography profilometers using multiple wavelengths and phase measurement

Conoscopic Holography proved to be a very adequate solution for in-situ optical measurement in industrial inspection and quality control systems, offering high-precision with a wide range of standoff distances, while being quite insensitive to the harsh environmental conditions often encountered in industry, as it is a common-path technique. With the aim of extending their applicability, we have already addressed, with good results, several issues that improve sensors based on this technology which include: the use of phase information to obtain one-shot profile measurements at frame rate with higher precision; new signal processing techniques; and speckle reduction to diminish measurement errors. However, the undesirable effect of using the phase information is that it reduces the maximum steep that can be measured without ambiguity, which becomes an issue when working with high precisions. In this article we present our ongoing work towards using the concepts of multiple-wavelength interferometry to extend the measurement range, something that, to our knowledge, has not been done for this technology before.