Prior to 3D Phase Measurement, videoscope measurement was based on stereo or shadow techniques, which are complex techniques, and the tip of the probe had to be changed from a viewing tip to a measurement tip, adding time to the inspection process. For example, with stereo measurement, it is first necessary to spot the defect using an optical tip. This tip must then be replaced with a stereo tip, the defect must be re-located, the image is frozen, the cursors are matched and the measurement is taken. With 3D Phase Measurement, the defect is located, the image is frozen and measurement is carried out. There is no need to change the tip. Greater ease of use means fewer operator mistakes and more repeatable and accurate results.

Auto Tip Detection is another productivity feature that allows the probe to identify the attached measurement tip and provide the relevant calibration information, which is stored in the system memory.

Unlike conventional stereo, shadow or laser-based measurement systems that operate on a point-by-point basis, 3D Phase Measurement processes the image data to generate a full 3D map of the viewed surface before beginning the measurement process itself. Inspectors can simply place measurement cursors on a normal, full screen image without the point matching, shadow identification or dot selection steps that can be challenging with other measurement techniques. Areas where measurements cannot be made because of shadowing or excessive distance to object are clearly indicated by a red overlay.

3D Phase Measurement imagery also provides a great deal of information about a flaw. For example, with a dent, the inspector can perform an initial depth measurement by placing three cursors outside the area of the dent to establish a reference plane, and then placing a fourth cursor within the dent reference plane in a position where the greatest depth would seem to be located. If the point cloud is then viewed, the system will indicate the location of the measurement cursors, focus on the region around the measurement and can optionally color code this region, using a depth scale relative to the reference plane. This can assist in more precise measurement by indicating whether or not the cursors are appropriately placed around the dent and if the fourth cursor is in fact at the deepest point.

The versatility of 3D Phase Measurement imagery is also demonstrated by fact that the 3D map can also be rotated, zoomed and viewed to provide further information on the shape of a defect and the location of the measurement cursors. The system also has the facility to provide a Profile View of the defect. This is achieved when the user positions cursors on either side of an area of interest and the 3D Phase Measurement system draws a line between them. Profile View is then selected and a cross section of the part along that line is displayed, helping to visualize the shape of a pit or crack or corrosion area. While working in Profile View, the user can move a cursor to obtain accurate and fast measurement of depth at points along the cross-section relative to the reference surface. This profile can then be graphically displayed regardless of viewing angle.

• 3D imagaery - 3-dimensional surface scans enabling inspectors to rotate and zoom an image for a more accurate view of the defect.
• Profile View - helps inspectors assess the size, shape and depth an indication by supplying a cross section view.
• On-demand measurement - inspectors can view and measure a defect using a single probe tip, eliminating the extra steps required to back out, change the tip and then relocate the defect. Inspectors get measurement when they need it--on-demand.

Even with the current range of measurement techniques, measurement remains the most difficult aspect of using video borescopes. Inspectors must be highly trained and practiced to obtain reliable and repeatable results. This expertise level has been addressed as RVI is now professionalized as an official NDT discipline and is a module within ASNT’s TC1A Level-III testing and certification process.

However, significant advances have been made recently in improving the accuracy, repeatability and ease-of-use of video borescopes with the development of 3D Phase Measurement that is now available with GE’s XLG3 VideoProbe inspection system..

3D Phase Measurement is based on an existing optical metrology technique. It involves projecting line patterns onto a surface, capturing the patterns in a video camera with high quality viewing optics and processing the images using proprietary algorithms to produce a point cloud, 3D map of the entire surface. This is then used in conjunction with measurement to obtain more precise information of the defect or object being viewed. Measurement itself simply involves the placing of cursors on the full-screen image, with no need for the point matching, shadow identification or dot selection steps that can be challenging with other techniques.

An innovative feature of this new measurement system is that the 3D scan can be rotated and zoomed to provide enhanced indication of the indication’s size and shape. Further assistance in assessing an indication’s size and shape is provided by the system’s Profile View feature. This is achieved when the user positions cursors on either side of an area of interest and the 3D Phase Measurement system draws a line between them. Profile View is then selected and a cross section of the part along that line is displayed, helping to visualize the shape of a pit or crack or corrosion area. At the same time, Profile View can also be used to measure depth at points along the cross-section.

An important application of the 3D Phase Measurement technology is the measurement of aircraft engine tip to shroud clearance. Aircraft engines, and other axial flow turbomachinery, are typically designed to minimize the radial gaps between the blade tips and the blade housing or shroud. Gaps between tips and shrouds can reduce efficiency by allowing gas or air to leak into the downstream stages. Consequently, it is very important to check this clearance, both during manufacturing and also during service as the gap changes during engine operation. (High operational rotating speeds and high temperatures can cause radial elastic growth of blades, as well as thermal expansion of the shroud.)

Historically, one method of measuring tip/shroud clearance has involved inserting a thin metal rod into an axially drilled bolt and attaching this assembly to the fan case so that the end of the rod is positioned where the blade tips should be. After the engine has been operated, the amount of wear on the rod is measured. Obviously, this is not a high accuracy technique and its execution often generates problems such as the liberation of metal from the rod, which can cause damage to the engine.

Phase measurement now offers a simple, non-contact and high accuracy technique for measuring tip to shroud clearance.