Flexible borescopes are used for remote viewing inspection. They are particularly useful for areas around corners, behind other materials, or within tight areas where a rigid borescope cannot easily maneuver in for inspection. These borescopes have the same basic components as rigid borescopes: an objective lens, optical relay system, source of illumination, and image lens with magnification. In a flexible borescope however, optical fibers are used to transmit both the light, and image being viewed. These fibers provide the flexibility. High-end flexible borescopes also have the capability for their objective lenses (distal end) to articulate when the user moves a joystick or similar control device.
Below, the parameters affecting these borescopes are elaborated on.
Resolution- Number of Optical Fibers/Pixels
Flexible Borescopes can contain from 10,000 to 30,000 tiny optical glass fibers to transmit images. Each fiber can transmit a pixel, meaning the number of pixels and therefore resolution of the borescope corresponds to the number of fibers.To prevent cross-talk, which occurs when light from one fiber interferes with another, the fibers are acid-leached.
The fibers are housed in a plastic frame surrounded by either a stainless steel or tungsten sheath to provide strength and durability. In some designs, a polyurethane sheath can be used as well. The choice of material for the borescope housing will be a function of the severity of the environment, with severe environments including exposure to corrosive gases or fluids, and/or high temperatures. Tungsten is best for high temperature conditions.
The articulation is provided in either a two-way or a four-way plane. The choice of articulating type should be made considering the complexity of the part being viewed and the need for flexibility during an inspection.
There are a few viewing parameters that must be understood for optimal selection and use of a flexible articulating borescope. These include the of direction of view (DOV) and field of view (FOV). Direction of view is the angle with respect to the axis of the borescope through which the lens views the object. DOV is usually either straight ahead (00) or perpendicular (900), but can be 650 or 1100 as well. The field of view can be thought of as a cone that extends from the distal end outwards. The wider this angle, the greater the area of view, but the less sharp the image. FOV angles can range from from 35 to 120 degrees.
Type of Illumination
In modern borescopes, the illumination source is transmitted through the scope using optical fiber. Some available light sources are: the flashlight type, metal halide arc lamps, high wattage xenon lamps and ultraviolet light. The latter is useful in forensic investigations, for example. The intensity of light required is based on the size of the cavity or area being viewed, and sensitivity required during investigation.