Conoscopic probes - a new solution to old problems 3 D measurement problems

These days, precision industrial point-distance mapping typically is performed by conventional interferometry, triangulation or dynamic focusing methods. Stereoscopic techniques are employed for measurements at longer distances. Miniaturization and limitations in current metrology systems have begun to be felt as manufacturing tolerances continue to tighten and minimum feature dimensions shrink particularly in the semiconductor and microelectronics industry.

A novel interferometric distance probe that performs high accuracy distance measurement over length scales encompassing 0.04 microns to several meters has been developed and commercialised by a company called Optimet in Israel.

Optimets new 3D non-contact measurements probe the Conoprobe is protected by 8 patents and based on a technique called Conoscopic Holography. Conoscopy , is the optical interference effect produced by doubly refracting crystals illuminated with convergent polarized light rays. This new technology gives rise to a number of unique features of great use in industrial measurement applications.

Unique features

The Conoprobe can measures diffuse reflecting surfaces such as machined metals and other relatively shiny objects. Objects with variation in reflectivity usually cause no problem. Measurements on mirror like surfaces are limited but can be circumvented if applying a thin layer of talk powder on the surface.
Distance measurements are done collinearly, in the optical axis thus enabling measurements at angles very close to grazing incidence (>5 degrees) and in narrow and deep holes of diameter less than 1 mm and depth ratios of 25:1.
Due to the fact that there are no moving parts or critical mechanical set-up in the head, the measurements are done with very high repeatability.
Measurement can be adapted over a large scale simply by changing the objective lens.
Accuracy is then ranging from 1 to 20 micron depending of lens and working distance.

How it works

The technology basis for the conoscopic probe is as follows. A ray from incoherent monochromatic point light source is directed at an uniaxial crystal where it is split into two rays that propagate within the crystal at different velocities along practically identical geometrical paths. This fact gives rice to the techniques collinear arrangement and associated high stability. The bi-refrigent crystal produces a propagation velocity differential between the two rays. The velocity of the so-called ordinary rays is anisotropic. Consequently, two separate wave fronts emerge from the crystal with different relative phase and cross-polarization angle, depending on the angle of the single incident at the crystal. Polarizing plates then align the directions of the electrical field component of the rays that are divided and recombined in such a manner as to form an interference fringe pattern (Gabor Zone Lens) at the output. As in classical interference, the fringe spacing is proportional to distance from the point of reflection.

In short - the whole light spot carries distance information in form of an angle difference between the two rays. The resulting pattern is then analysed in the frequency domain to give the converging distance from the surface.

The Conoprobe can sample up to 700 points/s at intervals down to 1 mm.

Fig. 1.Interferometric basic set-up

The Conoprobe equipment emits a laser beam through a prism behind the lens and the reflected beam is measured according to the interformetric set up in fig. 1

Applications

A multitude of industrial metrology applications will benefit from instruments employing Optimet s conoscopic technology.

Typical applications are on-line surface profile and distance measurement of mechanical and electronic components.

With its ability to measure in holes and crevasses Conoscopy really gives Machine Vision in plastic and mechanical industry a 3:d dimension. Highly accurate Robotic 3-D Vision is another potentially important application.

Reverse engineering is a growing and very suitable area for this technology.

Products

The Conoprobe measurement head comes with a number of DLL s for easy integration with other measurement systems or in-line inspection applications.

For the reverse engineering market and other scanning applications the Mini Conoscan 3000 with x-y scanning tables and software to export into CAD/CAM file formats is a suitable configuration.

New developments

The MicroConoprobe can be attached to the camera port of a standard microscope to perform surface inspection applications and fast auto focusing. Adaptation of the optical path is not necessary and precision and depth of field can be adjusted simply by changing the microscope s objective barrel. Distance measurement accuracy for a 10x objective will be around 0,1 micron over a 200 micron range.

We believe that there are number of applications where Conoscopy is the only available alternative to measure with. There are also a number of existing applications where Conoscopy can offer better performance and reliability of measurement data.