Optical inspection: introduction by the feature editor

Kristina M. Johnson

This 15 Nov. 1988 issue of Applied Optics features papers on the use of optoelectronic systems in the inspection of manufactured products and materials processing.

Optical inspection systems are essential to fully automated manufacturing. Optics offers the poten­tial for high-speed, high-resolution, fully parallel, and noncontact quality control of the manufacturing pro­cess. The main goal in the industrial environment is to differentiate between defective and nondefective objects. In many applications it is sufficient to detect defective objects, signaling their removal from the as­sembly line. Determining whether an object is defec­tive is often carried out in the frequency domain. A lens easily performs a transformation between the space and frequency domains, another advantage of using optics for industrial inspection. When it is im­portant to classify defects and associate them with a particular flaw in the manufacturing process, neural-like networks may be of use. Neural-like networks also have the potential for learning the difference be­tween defects and nondefects. This may be important when it is difficult or impossible to define clearly what constitutes a defective object.

Automated inspection is also important for nonde­structive evaluation of materials. Information about surface and interface roughness and dislocation densi­ties can be monitored in real-time using optical scat­tering and reflectance measurements. Here the pri­mary goal is process analysis.

In this 15 Nov. 1988 issue of Applied Optics, fourteen contributed papers present novel approaches to the optical and optoelectronic inspection of manufactured goods and material processing. The papers are grouped into four topical areas: optical inspection using information obtained in the space domain; sys­tems using Fourier and Hough transforms; optical

The author is with University of Colorado at Boulder, Optoelec­tronic Computing Systems Center, Boulder, Colorado 80309-0425.

Received 12 August 1988. 0003-6935/88/224613-02$02.00/0. © 1988 Optical Society of America.

scattering and reflectance measurements of surfaces and interfaces; and optical inspection using ultrasonic waves.

The first four papers describe methods for optical inspection of surfaces. They all use information ac­quired in the space domain, but the implementation and application vary considerably. The first paper by Ono, Sasaki, and Komatsu describes an optical sys­tem for detecting 10-µm or larger defects in the video high-density disk (VHD). The VHD tester classifies defects by size, position, and category. It uses reflect­ed light from a 10-µm diameter laser beam (λ = 800 nm) and positions sensors to detect the defects. A correlation between the reflected signals and the par­ticular defects is used for defect classification. The next paper by Baker proposes a new quantifying pa­rameter called the line equivalent width for defining surface flaws such as scratches and digs. The line equivalent width measures the amount of transmitted or reflected light removed from an illuminating beam by a surface flaw and compares this amount with the light removed by a line or slit of negligible depth and whose lateral dimensions are precisely known. The paper by Wygant, Almeida, and Soares describes the use of projection fringe interferometry to obtain 3-D topological maps of an aluminum block with grooves marked on the surface. The next paper by Häusler and Hermann presents work on optical range sensing by shearing interferometry measurements of diffusely reflecting objects. This is the first of two papers that discuss the influence of speckle on an optical range sensing interferometer which achieves a depth resolu­tion of 10 µm at a 300-mm working distance.

The next three papers use optical Fourier and Hough transform methods to assist in the optical in­spection process. Häusler, Hutfless, Maul, and Weissmann present an optical range sensor which uses the Fourier transform of a speckle shearing inter­ferometer to improve object depth resolution. The next paper in this group by Cielo and Vaudreuil describes the development of two optoelectronic sys­tems for the inspection of woven webs and extruded

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wires. These industrial materials are illuminated by a 1-D light source. Transmitted light is detected by a linear 1024 photodiode array and processed using a 1-D fast Fourier transform method. The emphasis of this work is on developing a rugged noncontact indus­trial inspection system to meet on-line manufacturing requirements. The paper by Casasent and Richards uses Fourier and Hough transforms to extract gross defect information from industrial products. Experi­mental results of a specific case study on product labels are presented.

The next group of papers approaches the problem of analyzing material defects that occur at thin-film in­terfaces and crystal dislocations. Roos, Bergkvist, and Ribbing present quantitative information about the SiO2/Si interface roughness from diffuse reflec­tance spectra. They use a modified Fresnel formalism for determining surface roughness by fitting calculated spectra to diffuse reflectance measurements obtained through experimentation. The paper by Cohn, Wag­ner, and Kruger presents a new technique—dynamic imaging ellipsometry—for high spatial resolution (~25 µm) studies of thin-film surfaces. The paper by Fre-derikse and Ying presents a method for determining the thermal properties of oxide coatings on stainless steel substrates at temperatures of 900°C. The paper by Sopori discusses optical scattering from a defect-etched semiconductor to characterize the dislocations

in the material. The integrated scattered light flux is shown to be proportional to the dislocation density. The paper by Engelhardt and Häusler presents a focus sensing technique for determining the 3-D shape of diffusely reflecting objects. A TV camera and ana­log electronics are used to find the locations in focus in a sequential series of positions in real time. This system has low resolution but may have uses to robotic vision.

The last two papers describe techniques using Nd:YAG pulsed lasers (λ = 1.06 µm) for the generation and detection of ultrasonic signals for nondestructive testing of materials. The paper by Bruinsma and Vogel presents a fiber-optic phased array to control the laser beam direction and focus the generated ultra­sound for maximum detection sensitivity. Wagner, Deaton, and Spicer report an improvement in detect­ing signal-to-noise ratio by reducing the acoustic signal spectral bandwidth. This is achieved by repetitively Q-switching a Nd:YAG laser (λ = 1.06 µm), which generates the ultrasonic signals.

The feature editor would like to thank the authors for their excellent contributions and the reviewers for their careful reading of these papers.

The author also works in the Department of Electri­cal & Computer Engineering.

4614 APPLIED OPTICS / Vol. 27, No. 2 2 / 1 5 November 1988