Accurate quantification of defects (scratches and impacts) is vital to the certification of flight hardware and other critical components. The amount of damage to a particular component contributes to the performance, reliability, and safety of a system, which ultimately affects the success or failure of a mission or test. The launch-commit criteria on a Space Shuttle Orbiter window are governed by the depth of the defects that are identified by a visual inspection. This measurement of a defect is not easy to obtain given the environment, size of the defect, and location of the window(s). The determination of depth has typically been performed by taking a mold impression and measuring the impression with an optical profiling instrument. Another method of obtaining an estimate of the depth is by using a refocus microscope. To use a refocus microscope, the surface of the glass and bottom of the defect are, in turn, brought into focus by the operator. The amount of movement between the two points corresponds to the depth of the defect. The refocus microscope requires a skilled operator and has been proven to be unreliable when used on Orbiter windows. White light interferometry was chosen as a candidate to replace the refocus microscope.

The White Light Interferometer (WLI) was developed to replace the refocus microscope as the instrument used for measuring the depth of defects in Orbiter windows. The WLI consists of a broadband illumination source, interferometer, detector, motion control, displacement sensor, mechanical housing, and support electronics. The illumination source for the WLI is typically a visible light emitting diode (LED) or a near-infrared superluminescent diode (SLD) with power levels of less than a milliwatt. The interferometer is a Michelson configuration consisting of a 1-in. (2.5-cm) cube beam splitter, a 0.5-in. (1.3-cm) optical window as a movable leg (used to closely match the return intensity of the fixed leg from the window), and a mirrored prism to fold the optics into the mechanical housing. The detector may be one of many C-mount CCD (charge-coupled device) cameras. Motion is provided by a commercial nanostepping motor with a serial interface. The displacement sensor is a custom device specifically designed for this application. The mechanical housing and support electronics were designed to integrate the various components into an instrument that could be physically handled by a technician and easily transported.

The WLI is placed over a defect using the video image from the camera. The electronic control is used to reposition the movable mirror. Interference fringes at the surface of the glass are imaged onto the camera (surface position), the mirror is then moved, and interference fringes are formed at various defect site(s). The position of each defect site can be read from the controller’s LCD (liquid crystal display). The difference in these positions from the surface determines the depth of the defect(s).

The device contains an interferometer, and alignment of the optics is critical to the operation of the instrument. Maintenance would consist of the proper alignment of the optics and calibration of the position. The measurement resolution for the instrument was expected to be better than 0.0001 in. (2.5 μm.); the unit has exhibited a resolution on the order of 2 μin. (0.05 μm). This capability is more than adequate for this application, but could be extended with different optics.

While in operation, the WLI displays a continually updated depth measurement on the integrated LCD. The LCD shows displacement information in microns and inches, provides scan speed and direction, mode information, and prompts the operator. Both manual and automatic scans are supported by the electronics; the information is also available from a serial data port. The WLI’s use of a video camera allows several people to observe and comment on the defect; consulting/collaboration while using the refocus microscope was not possible.

This work was done by Robert Youngquist, Stephen Simmons, and Robert Cox of Kennedy Space Center. For more information, download the Technical Support Package (free white paper) at under the Physical Sciences category. KSC-13417