Intersecting passages can be viewed using a rigid borescope or semi-rigid borescope with the appropriate direction of view. If coaxial lighting is needed to view into a very small passage, a mirror tube may be needed. However, if the passage is large enough it can be illuminated using separate prisms. If intersecting passages are to be viewed, a mirror tube may be the best way to achieve the direction of view. A mirror tube is a close fitting tube that slips over the OD of the scope to position a mirror at the tip of the scope to reflect the image and illumination at 90°. While mirror tubes are very susceptible to dirt, they offer a coaxial lighting effect that helps to view deeply into a passage at right angles to the scope axis. When the passage is at an angle other than 90°, a mirror tube may not be the appropriate solution.
A flexible borescope may be needed to enter the intersecting passage, and a tool of some kind can be used to help the scope around the bend. Normal practice has been to use articulating borescopes for this, but they are not necessary. Standard borescope products will do a good job if they can be guided into the passages. They can also be customized for special requirements.