Observations of the wall surfaces of small-diameter holes that are inaccessible to a borescope must be conducted from outside the hole.
Consequently, observations can only be performed to a depth approximately equal to the diameter of the hole itself.
We attempted observations of a wall surface with a diameter of 2 mm and a depth of 2 mm using three different methods.
(1) Oblique-viewing microscope for hole observation
The object is observed at a 45-degree angle.
By rotating the object, the entire circumference can be examined.
Observation is feasible if the diameter and depth of the hole are identical.
(2) Inspection hole lens
This is a lens specifically designed for internal wall verification using a fisheye lens.
It is suitable for observing the inner walls of holes with diameters of 10 mm or larger.
It is not appropriate for smaller diameters.
For reference, it was tested with a 2 mm diameter hole.
(3) Wide-angle borescope
Utilizes a borescope with a diameter of 2.7 mm and a variable magnification camera adapter lens.
Typically employed by inserting it into a hole,
the wide-angle design permits a certain degree of observation even from the outside.
