Published on 24-Oct-2020

Importance of 3D Modeling in High-Definition Remote Visual Measurement Systems

Importance of 3D Modeling in High-Definition Remote Visual Measurement Systems

Blog By Charles Janecka, Associate Product Manager at Olympus. This article appears in the September/October 2020 issue of Inspectioneering Journal.

How Stereo Measurement Works

Stereo measurement works in a similar way to how human eyes interact with our brains and our environment. We avoid bumping into things when we walk around because our brain is constantly calculating distance, based on triangulation and the parallax from our right and left eyes. For example, when we look at a building and close one eye and then the other (Figure 1), our brain knows that the building has not moved, merely the perspective from each eye, based on its position in relation to the object, has changed. Using these two different perspectives of a static object, measurement becomes possible. Lateral shift is inversely proportional to distance. Therefore, the “Z” distance is inversely proportional to lateral shift. In daily life, our eyes work with our brain to achieve this understanding of how far away various objects are (e.g., a table, a chair, a car, the moon, etc.) and how large or small they are.

Figure 1. Parallax lateral shift – left eye vs. right eye views.

Applications within Remote Visual Measurement Systems

In terms of remote visual measurement, the parallax determination is calculated in a slightly different way. It does, however, use the same fundamental principles. In remote visual inspection (RVI) equipment, the brain is replaced with a charge-coupled device imaging sensor and a processor. The eyes are replaced with the optical lenses in a tip adaptor. The tip adaptor has two lens systems with a preset offset in their distances to each other. Figure 2 demonstrates how the optical system works in conjunction with the charge-coupled device and processor. The parallax is used to determine the relevant distance of an object, based on the perspective shift across each side of the charge-coupled device.

Figure 2. Parallax of a stereo image on a charge-coupled device.

Using the X, Y, and Z position data and the parallax algorithm, RVI devices apply five different measuring modes to give automatic sizing and situational information after reference and/or measurement points are selected.

These measuring modes offer the following information:

  1. Distance: The distance between two points
  2. Point-to-Line: The perpendicular distance between a point and a user-defined line
  3. Depth: The perpendicular depth/height between a point and a user-defined plate
  4. Lines: Cumulative multiple points
  5. Area: The area of an enclosed shape (the circumference of the lines is also available here)

Measurement Environment and Setup Accuracy

Before an inspector undertakes measurements with any equipment, it is important that the measurement environment is correctly installed and understood. The optical characteristics of the lens system used may vary on a miniature scale from lens-to-lens during the manufacturing process. As such, specific “measurement environments” are created for each individual lens system. This informs the measuring device, or scope, of the exact optical characteristics of the optical adaptor being used. Even though these components are precision-made, when dealing with optics on this scale, even the slightest variance can have an impact on the imaging. Therefore, the optical characteristics of each tip adaptor should be considered. To obtain the highest accuracy, it is possible to carry out “one-to-one” stereo matching. This is where measurement data is created specifically for a certain tip (or tips) used with a certain scope.

Super Wide Stereo

Super wide field stereo videoscopes with a greatly extended measuring area can offer a wider field of view (FOV) and deeper depth of field (DOF). Measurement accuracy can be improved with advanced measuring algorithms and 1-to-1 measurement matching. Moreover, today’s processors, in conjunction with innovative optics and more sensitive charge-coupled devices, have made real-time display an option. For example, a five-point object distance meter with multispot ranging (Figure 3) and greater processing power can provide multiple measurement modes on one image.