Visual Inspection (Metrology)

23.15.
It is a non-destructive technique used to detect and examine a variety of surface flaws and surface cracks. The visual inspection is aided by borescope (which illuminates and observes internal, closed or inaccessible areas), interference microscope (which measures the depth of scratches in the finish of finely polished/lapped surfaces), image sensors (which develop permanent visual records of remote object) ; magnifying system to evaluate surface finish, shape and microstructure) and dye and fluorescent penetrants (which enchance the observation of surface cracks).
Borescope. It consists of a long tube (rigid or flexible) available in a wide variety of lengths and diameters. A light source fitted at observer end sends light signal at other end where the surface in difficult to-reach areas is to be examined. The optical connection between the viewing end and an objective lens at the tip of borescope is possible by,
(i) using a series of achromatic relay lenses in a rigid optical tube.
(ii) using a bundle of optical fibers (rigid/flexible tube) that conduct light from external source.
(Hi) using wiring to carry the image signal from a charge-coupled device (CCD) imaging sensor installed at the distal tip of a flexible tube.
The distal tip with combination of prisms and mirrors can be designed to define the direction and field of view and thus used for a variety of applications. Usually the surface to be inspected is illuminated by a fibre optic light guide.
The tube diameter of rigid borescope may range from 0.9 to 70 mm and length from 0.15 to 30 metres. Its use is limited to applications with a straight line path between the observes and surface to be observed. The field of view can range from 10 to 90°. It is possible to have orbital scan which involves the rotation of the optical shaft for scanning purpose. The illumination system incorporates features like orbitalscan, different viewing head, and adjustable focussing of the objective lens. Some variants available are miniborescope (using a single image-relaying rod or quartz fibre in the optical tube in place of conventional relay lenses), hybrid borescopes (using rod lenses combined with convex lenses to-relay the image),
extendable borescopes (enabling use of extension tubes, or rigid chamberscope (having zoom facility and used for more rapid inspection of large chambers).

Fig. 23.11. Rigid Borescope.
Flexible Fiberscopes. These are available in 1.4 to 13 mm diameter and upto 12 m length (Fig. 23.12). For distances more than 15 m, videoscopes with CCD probes are used for electronic transmission of colour or black and white images to a video monitor. A CCD chip (consisting of thousands of light-sensitive picture elements, known as pixels arranged in a pattern of rows and columns) is placed at the distal end. The objective lens focusses the image of object on the surface of the CCD chip. The light at CCD chip is converted to electrons that are stored in each pixel. A voltage proportional to the number of electrons at each pixel is determined electronically for each pixel site. This voltage is then amplified, filtered, and sent to the input of a video monitor. Generally videoscopes produce higher resolution than fiberscope and have longer working length. The display reduces eye fatigue and enables point-to-point measurement on the viewing screen using reticles. The electronic form of the image signal allows digital image enhancement and the potential for integration with automatic inspection systems.

Fig. 23.12. Flexible fibrescope.