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.

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.

A common-path interferometric profilometer using a Savart plate as a lateral shearer has been successfully tested under harsh environmental conditions to measure the shape of a surface, detecting defects and characterizing surface properties. The whole profile is obtained from a single image and its depth sensitivity is easily scalable, making this technique suitable for many different applications. Although this system has been successfully used for surface inspection and defect detection, some behaviors cannot be explained by the usual simple model for fringe formation, which, amongst other things, considers normal incidence of the incoming rays into the Savart plate. These deviations from the ideal case are more noticeable for high resolutions from short distances. This paper studies the formation of the fringe pattern, which is crucial for understanding the behavior of the system and proper calibration.