Measure Better with Borescopes
July 31, 2007
The strength of any RVI product is its ability to resolve fine detail. Although charge coupled device (CCD) pixel count is a critical component of resolution, it is not the only factor. Lens design, lens coatings, color rendition, image clarity and back-end processing complement and enhance what the CCD is able to capture.
Viewing and quantifying defect damage during a visual inspection has become a critical function of quality control engineers and all borescope operators. This need to quantify inspection data has driven borescope manufacturers to achieve higher levels of system performance.
Unlike conventional stereo methods or those options that require the use of a known object size in the field of view for calibration, the laser true measurement method allows the videoscope operator to easily capture critical measurement data without the need for special tip adapters or interruption of the inspection for an optical tip adapter change. This saves time during inspection and the cost associated with functionally specific tip adapters.
On the front end, the videoscopes are designed for maximum versatility. Articulating videoscopes offer 360-degree field viewing coverage, range in diameter from 4 to 8 millimeters and are available in lengths up to 7.5 meters. Application specific videoscopes include working channel videoscopes designed to deliver active tools, via internal working channels, to the work site for foreign object debris (FOD) removal or other in-situ work.
Borescopic measuring technologies seek one critical ingredient to achieve an accurate measurement: the distance from the target inspection object to the imaging gathering element in the borescope system, typically referred to as the working distance.
After the working distance is defined via triangulation, known imaging system optical characteristics facilitate the measurement of target object dimensions including length, area, cumulative line length, point to line and depth. Tip adapters are available for direct and side viewing applications, with most industrial applications dictating only one direct and one side viewing tip to perform measurement and general inspection. This represents a reduction in cost based on minimizing optical tip requirements as well as a much less frequent tip change, maximizing service life of the videoscope.
Laser true measurement achieves a quick, effective and easy means to quantify data. This is the unique imaging capability of the processor that is directly compatible with multiple CCD sensors. A standard videoscope with a general inspection optical tip, designed for maximum brightness, can now perform accurate measurements live or on stored images for post inspection image analysis. The videoscope is equipped with laser illumination via an optical fiber that readily projects a “grid” onto a target worksite. This diverging pattern is passively observed and captured by the CCD imager in the videoscope by simply pressing a button on the videoscope control body. The observed pattern (relative dot position) varies from image to image as a function of the videoscope working distance and its orientation to the target surface. The processor passively interpolates the data and permanently stores this data history with the image. The second and final step is to position the video cursor for measurement with text, specific annotation or other relevant information easily superimposed via the integral QWERTY keyboard. Critical to the measurement’s accuracy is one’s ability to accurately position a video cursor on the target image. Another critical element of the measurement process is the actual displacement of the video cursor during the measurement and its own resolution of position. A larger monitor with a smaller measured distance increment per cursor move, or pixel travel, will yield the most detailed and accurate measurement-it is desirable to resolve 0.001-inch per cursor move without electronically “zooming” the image, which ultimately degrades resolution.
What was once a cumbersome task involving multiple steps-videoscope removal from the worksite, tip change and defect relocation-can be reduced to a single snap shot during the course of a routine inspection. NDT