The Space Shuttle Orbiter windows were damaged both by micrometeor impacts and by handling, and required careful inspection before they could be reused. The launch commit criteria required that no defect be deeper than a critical depth. The shuttle program used a refocus microscope to perform a quick pass/fail determination, and then followed up with mold impressions to better quantify any defect. However, the refocus microscope is slow and tedious to use due to its limited field of view, only focusing on one small area of glass at a time. Additionally, the unit is bulky and unable to be used in areas with tight access, such as defects near the window frame or on the glass inside the Orbiter due to interference with the dashboard. Bulky camera equipment was needed to acquire images for later processing and storage. The long depth of field of the refocus microscope provided crisp images of the defect, but didn’t provide the user with a feel for depth of the defect since all parts of the image appear in focus.
The surface inspection tool is a low-profile handheld instrument that provides two digital video images on a computer for monitoring surface defects. The first image is a wide-angle view to assist the user in locating defects. The second provides an enlarged view of a defect centered in the window of the first image. The focus is adjustable for each of the images. However, the enlarged view was designed to have a focal plane with a short depth. This allows the user to get a feel for the depth of different parts of the defect under inspection as the focus control is varied. A light source is also provided to illuminate the defect, precluding the need for separate lighting tools. The software provides many controls to adjust image quality, along with the ability to zoom digitally the images and to capture and store them for later processing.
Two LED light sources are included for improved illumination, allowing the user to work without an external light source. The optics enable the two cameras to be mounted in a compact manner and allow them to focus on the same image. Software provided from the camera manufacturer provides the users the capability to view the two images simultaneously to facilitate rapid defect detection. The user may use a digital zoom to enlarge smaller details, if needed. Full-resolution digital images and limited-resolution video can be captured and stored for later processing.
The surface inspection tool addresses many of the limitations of the existing refocus microscope. It is smaller and provides a live video output on a laptop computer that allows the user to locate defects more rapidly. The camera with the microscope objective has a depth of focus of approximately 0.00014 in. (≈4 μm) and a user-varied focus. This allows the user to gain a better understanding of the depth and character of the defect under inspection. Likewise, lower-resolution video capture is also an available feature not present with the refocus microscope.
The surface inspection tool consists of components that are more expensive than the refocus microscope. However, the inclusion of the wide-angle camera allows for inspection of a larger area at a time, making it quicker to scan and locate defects in large surfaces. This time savings, combined with the added features, may make it interesting enough to potential users to justify the added initial cost.
This work was done by Mark Nurge, Robert Youngquist, and Dustin Dyer of Kennedy Space Center. KSC-13580