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206 Gas Turbine Handbook: Principles and Practices
for inspecting the combustor. Their use is further enhanced with
articulating, side viewing mirrors. However, the most versatile boroscopes
are the flexible fiber optic scopes (Figure 13-2). These are
available in diameters ranging from 0.3 millimeters to 13 millimeters
and working lengths of approximately 250 millimeters to 6,000 millimeters.
Boroscopes are also available with either two-way (up and
down) and four-way (up, down, left, and right) articulation.
Special adapters are also available for the attachment of still
or video cameras. Much like a camera, a boroscope is an optical instrument
consisting of an object lens, a series of relay lenses, and an
eyepiece lens 1 . In addition, an illumination system is also provided.
Care should be taken in selecting the light source. For visual inspection
only, the less expensive 150 watt light sources are adequate. For
video use, a 300 watt white light source is recommended. The size of
Figure 13-1. Courtesy of Olympus America Inc. The rigid boroscope is
best suited for use in areas where the insertion distance and viewing
direction is well defined. Rigid boroscopes are available in a range
of diameters and working lengths.