Small diameter micro borescopes, about the width of three human hairs, can benefit a range of applications.
Small diameter micro borescopes are able to inspect some of the smallest passageways when precision matters most. Today, micro borescopes can be manufactured to be smaller than 0.35 millimeter (0.013 inch), or about the diameter of three human hairs. Small diameter borescopes may be the only choice for some inspections and have a variety of unique capabilities.
Close Fitting Inner Diameter (ID) Micro Borescope Inspection
When a borescope is being used to inspect the inside wall of a bore the following considerations are important. If a view directly at the wall is desired, and the scope is a close fit in the bore, the focus and illumination must be optimized for this condition. A direction of view between 110 degrees and 70 degrees can be used depending on the condition of the wall. The direction of view chosen will depend on what you are trying to inspect, and the reflectivity of the wall.
A highly polished or reflective bore is very difficult to view. The polished surface reflects the light in a way that minimal light gets reflected back into the scope. When looking at welds that have a flatter finish, a borescope will be able to inspect the area of the weld; however, the polished area may be difficult to view.
A forward, or 0-degree direction of view, may be used to view the walls of a close fitting bore. A very wide field of view may be used to optimize the view of the sidewall and minimize the view down the center of a bore. A 0-degree direction of view borescope can be used to view the entire ID at once without needing to scan the wall with a side viewing borescope. A forward view borescope is the quicker method.
Intersecting passages can be viewed using a small diameter rigid or semi-rigid borescope with the appropriate direction of view. 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 with a mirror at the distal end that slips over the outside diameter of a borescope to change the direction of view from zero to 90 degrees.
A flexible borescope is another option for inspecting an intersecting passage. Articulating borescopes may be used for this, but they are not necessary. Standard non-articulating flexible micro borescopes will do a good job if they can be guided into the passages. Flexible borescopes may also be customized for special requirements.
Orbital Welds in Piping
Inspecting welded in very small diameter piping requires a flexible borescope with a 90-degree direction view. Inspection of orbital welds in sanitary piping or other high purity piping requires a flexible borescope that can negotiate twist, turns and sharp bends. The small diameter flexible borescope will need to view the sidewall and rotate 360 degrees to inspect entire welds. A forward view with a wide field can be used; however, the detail will most likely not be sufficient. A forward view borescope image may show some of the weld in shadow depending on the shape of the weld bead. Centering tools can be fixed to the borescope’s distal end to maintain alignment in a pipe and optimal focal distance to the weld.
Infrared Applications (IR)
IR illumination can be useful to take advantage of a charge-coupled device (CCD) camera’s high sensitivity in the infrared spectrum. Most cameras have filters that eliminate the IR for color balance reasons. When this filter is removed, IR sensitivity is increased along with some corresponding loss of color balance. Most light sources generate sufficient infrared; however, it is typically filtered out to prevent burning of fibers. IR filters may need to be removed. Illumination fiber in a borescope will be heat damaged if too much infrared is present, or cooling of light guide tip is not adequate. Scopes built with fused quartz image fiber may lose image quality in the IR spectrum because the fiber is optimized for use in the visible light spectrum. Leached image bundles are recommended for IR use, or verification that fused quartz will perform well as applied is advised. Fused quartz fiber being used in radioactive environments will degrade more slowly in the infrared region.
Ultraviolet (UV) Illumination Applications
UV illumination can be delivered through a borescope by using UV-transmitting illumination fiber. Typically this fiber has a lower acceptance angle and requires a narrow field of view in the image optics to match the illumination pattern. UV probes may be built for borescopes with continuous light guides. This construction has a continuous illumination fiber bundle from the light source to the probe tip, maximizing UV transmission.
The sheathing a borescope’s working length is constructed of may be a very important consideration. The sheathing serves to provide a proper balance of protection and flexibility for a borescope. Some sheathing may pose a risk of damage to the object being inspected or provide too much friction to make sharp turns easily. Materials typically used in the construction of micro borescopes include: polyimide, nylon, Teflon, stainless steel, nylon coated with Kevlar braid and urethane coated with steel braid.
Flexible high temperature un-cooled probes have been tested to 600 F. Small diameter high temperature borescopes for applications that require insertion through difficult paths and static uses have been successfully constructed. All high temperature borescopes work better the less they are cycled and all will have some finite life. This varies by application, and can sometimes vary unpredictably. The fiber optic bundle has a chemical treatment applied that enhances flexibility, and depending on the manufacturer, these will bake off over time. If the borescope is static, it is not usually a problem. These borescopes can be built up to approximately three meters in length.
Rigid high temperature borescopes have more options. Cooling is easier to apply, and they can be constructed using fiber optics without regard to bending issues. Rigid borescopes can be built with conventional relay lens system for ultimate resolution, but if un-cooled, they must be built without any optical cement joints if working above approximately 400 F. This will make the images more likely to be flawed by incoming debris from the operating environment.
Shaped / Shape Memory Probes
Semi-rigid probes can be built with a special metal alloy that will hold its shape. The shape is put into the borescope tube using a heating process. The borescope can be straightened with an outer sheath, and then deployed by retracting the sheath, or it can be used without the sheath.
This type of scope is not advisable for general use where the shape memory properties are not needed because the tubing is more expensive and has a thicker wall that limits illumination possibilities. A scope of this type can replace a much more expensive and fragile articulating borescope. Conventional semi-rigid borescopes can also be permanently bent into shapes specified by the customer. These scopes will remain bent and will not straighten or deflect like shape memory probes.