Borescopes are inspection devices with a rigid or flexible optical tube for insertion into bores and cavities for visualization. They are used in a wide range of industrial quality control and inspection applications. A borescope consists of an objective lens positioned in front of a long relay tube containing an image relay and an illumination relay. The relay tube can be rigid or flexible and is followed by a handpiece containing an eyepiece or direct video optics. Rigid borescopes are preferred over flexible borescopes when possible as they have better image quality at a lower cost. When you cannot access what you need to inspect with a rigid borescope, then a flexible borescope offers a great alternative.
Important specifications for borescopes include rigid or flexible image relay, working length, field of view (FOV), direction of view (DOV) and minimum focus distance. The image relay diameter is the diameter of probe for insertion into cavity to be inspected. This diameter typically determines the minimum size hole the scope can be used within. The working length is the length of the probe. This effectively determines depth of inspection capability. The field of view is the viewing cone included in the viewing angle. It is a measure of how much of the visual field is taken up by the image. The direction of view is the direction the axis of the viewing cone points, where zero degrees is straight ahead and 90 degrees is looking to the side. The minimum focus distance for the borescope is the minimum effective focusing distance.
Tips of borescopes have several specifications that differentiate them. The direction of view or viewing angle for the tip can be direct, forward oblique, side and retro. A direct viewing angle tip affords direct straight-on viewing. A forward oblique viewing angle is between 0° and 90° for combination of forward navigation and side visibility. A side viewing allows for greater visual detail on bore or cylinder walls. A retro viewing angle has retrospective or backward oblique viewing. The tip may come equipped with a swing prism that allows the user to change the viewing angle without changing scope position by pivoting a prism front and back. Interchangeable objectives allow tips with different viewing angles to be readily interchanged. A rotatable tip allows the tip to rotate for viewing around interior walls. Flexible borescopes tips may be bent back and forth using a control in the handpiece, which allows changes to view angle or navigation through paths that are not straight. A two way articulated tip can be manipulated or "curled back". Two-way implies single plane of articulation, as in only right-left or up-down. A four way articulated tip can be manipulated or "curled back" in two plane articulation, as in both right-left and up-down. Specifications for articulation range give the maximum angular articulation possible, measured from 0° as straight-ahead. Articulations greater than 90° correlate to retrospective views. These can be specified as +/- 120° or by giving the total range of 240°. The minimum sheath bend radius is the minimum radius around which sheath or flexible tube can bend. Repeated bending of a flexible borescope below the minimum bend radius will result in broken image and/or illumination fibers.
Borescopes have one of four common relay lens types:
- Achromatic doublet
- Gradient index
- Fused quartz fibers
- Rod lens
An achromatic doublet lens is a widely used construction for medium to larger diameter borescopes. It has good color correction, but is limited to medium size (approximately 5 mm) and above in diameter. A gradient index lens relay is a relatively newer technology in which the index of refraction profile from center to edge is precisely controlled during manufacture instead of grinding, polishing, coating and cementing individual lens elements. Gradient lens relays are typically used in small to medium diameter scopes and provide very high image quality using a simpler and reliable technology. Fused quartz fiber relays are used for very small diameter industrial borescopes and medical endoscopes. Construction may permit limited flexibility. A rod lens relay, originally conceived by H. H. Hopkins, provides a higher numerical aperture relay than an achromatic doublet relay and therefore brighter images for a given diameter. The field or view, color correction and image quality of a borescope does not depend upon which relay technology is used, but the overall optical design, quality of materials and manufacturing tolerances.
Features common to borescopes include integral illumination (usually using illumination fibers) and the ability to attach a video camera for image display and capture. Video imaging is used to augment or replace direct visual inspection. Cameras can be C-mount, CS-mount, S-mount, F-mount, and K-mount.
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