Author: Ing. Carlos Álvarez, May 11, 2022.
Visual inspection is a simple test compared to other non-destructive testing methods; however, since the main tool for carrying out this activity is the human eye, various factors can affect the sense of sight and distort the observation of surface imperfections. Therefore, these factors must be understood and taken into account in order to obtain reliable results during the inspection.
Below are a number of conditions that can affect visual perception.
Lighting: It is one of the most important factors to consider, if it is insufficient or otherwise excessive, it will be evident that the imperfections sought on the surface of the material will not be correctly appreciated, even if magnification is used, it will not to improve the image.
The attributes of the material or component to be inspected represent a group of factors that also alter the appreciation of the visual data. Each of them is detailed below:
*-Cleanliness: obtaining visual data is impossible through layers of opaque dirt, unless the objective of the examination is precisely the cleanliness of the component.
*-Color changes on the surface of the material: it is another attribute to consider, characteristics such as corrosion or oxidation of metals or the deterioration of organic materials is frequently accompanied by a change in color that affects the appreciation of the detail to be displayed .
*-The illuminance or brilliance: an excessive brightness of the surface of the specimen causes an unpleasant sensation which we call “glow”, which affects the ability of a clear vision, and with it a critical observation with good judgment.
*-Surface condition: conditions such as polishing, milling, lapping, chemical attack, sand cleaning, shot blasting, among others; as well as turned shapes, all of which can hinder the ability to appreciate details correctly.
*-Shape of the specimen: details in its construction or design such as holes, protruding parts, slots, force to determine the best viewing angle of any surface or the use of instruments such as mirrors, prisms, videoscope, for a better observation of the surface.
*-The size: if the object is larger than the light beam, it is evident that multiple passes, or greater lighting conditions, will be required.
*-The temperature of the specimen: if the heat emanating from the specimen and or around it is excessive, it can distort the display of the image of the surface, making it difficult to examine.
*-Texture and reflection: these variables affect the perception of detail and easier ways to ensure adequate lighting, placing the light source and the eye as close to the surface to be examined as the focal length allows, will help mitigate this condition.
Environmental factors: refers to the reflections and shadows on the surface of walls, ceiling, various components and parts, which must be considered.
Effects of fatigue: the efficiency of the observer to make an exact interpretation of the visual detail that is being observed can be reduced if the observer presents some type of fatigue, which can be emotional, physical, mental, among others. For example, prolonged viewing with poor lighting, a tense position due to lack of ergonomics, and heat stress should be avoided, these conditions affect detailed observation and good judgment.
Observer health: it can also affect the activity of a visual inspection, for example, in the presence of some anatomical conditions (glaucoma, hyperopia, retinopathy, among others), a direct or indirect alteration of the individual’s ability to watch.
Psychological factors: refers to the psychological state, tensions and emotions of the examiner, which can influence the appreciation of the visibility or details of objects and that also influence the execution of visual or observation tasks, especially when these are performed under many conditions.
By taking into account the factors described, the conditions and instrumentation necessary to carry out a correct visual inspection and obtain reliable results can be planned.
Bibliographic references.
- Visual Inspection Levels I and II, NP-123-456-304, 2006 Edition.
- Course “Visual Inspection. LLOG SA de CV.
- Manual for Visual Testing at Level II. International Atomic Energy Agency, IAEA. 2013.
- Course “Visual Inspection Methods and Penetrating Liquids”, Ing. Carlos Álvarez (Diploma “Non-Destructive Testing, uses and applications in the oil and gas sector”, Inspenet 2022).
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