Visual inspection
Visual inspection (also: visual inspection ) is the optical inspection of a product or a product part for defects as a regular production step at the end or after the production for quality assurance . Visual inspections are also carried out on parts that are in operation.
General
The visual inspection as a non-destructive test method has been standardized in DIN EN 13018 (general principles of visual inspection). This standard also distinguishes between:
- Direct visual inspection without tools (i.e. viewing the test area with the naked eye)
- Direct visual inspection with aids (optical aids such as magnifying glasses, endoscopes, mirrors)
- Indirect visual inspection (with camera, videoscope, etc.)
The distinction between direct and indirect visual inspection is defined in the standard as follows;
- Direct visual inspection = visual inspection without interrupting the beam path between the test surface and the observer's eye.
- Indirect visual inspection = visual inspection with interruption of the beam path between the test surface and the observer's eye, d. H. Conversion of the light beam into another form of energy, i.e. into electrical information (camera)
The usual abbreviation for visual testing, standardized in ISO 9712, is VT (visual testing).
A special form of visual inspection is the automatic optical inspection (AOI), in which every component is photographed with the help of a computer and compared with the computer's own component library. This means that an assembly can be checked very quickly for the presence or absence, or correct or incorrect polarity of the components. This method can also be applied to each individual solder joint of the relevant assembly.
Other special forms are; Triangulation, line scan camera technology, photogrammetry, phase delay measurement, interferometry . (These special forms of visual inspection will be explained at a later point in time).
Typical defects that should be discovered by visual inspection
Mechanical damage (scratches, cracks), dirt deposits, assembly errors, burr formation, end crater cracks and undercuts. In addition, color changes (annealing colors in austenitic steels, for example), surface roughness and attachment faults can be easily recognized by visual inspection.
application areas
- Manufacture of electronic circuit boards
- electronic and mechanical (surface quality) assembly production
- in food production: cleaning of returnable bottles and egg inspection, also for
- non-destructive testing of all kinds
- Combination with other test methods to clarify the surface imperfections (e.g. automotive industry, energy industry, aviation ...)
Tools
- optical magnification (magnifying glass, microscope, endoscope )
- Cameras
- special lighting (different light colors, directed light or diffuse light, ...)
- automatic optical inspection
Disadvantages of visual inspection
Since the visual inspection is mainly carried out as an activity by humans, it is therefore subject to a poor degree of efficiency compared to automated tests. Reasons for this are e.g. B. concentration fluctuations, pressure to perform, fatigue, environmental influences, etc. This results in a "slip through" (undetected faults), which can be between 5% and 30% depending on the complexity of the test object and the corresponding working conditions. Even several similar visual checks one after the other only improve the effectiveness moderately.
Individual evidence
- ↑ Crack testing techniques and crack testing variants , (Scholz Rissprüftechnik GmbH).