Visual Inspection

Visual testing is the only nondestructive testing technique that can be performed without the aid of other equipment. Because the physical means of visual testing is available to almost everyone and because the process seems so intuitive, some people may assume that the technique itself is not very complex.

Visual testing is distinguished from the other methods on nondestructive testing in that it uses the visible light portion of the electromagnetic spectrum. Visible light can only be detected by a sense organ, in other words the eye.

Visual testing was probable the first method of nondestructive testing. It has developed from its ancient origins into many complex and elaborate optical investigation techniques. Some visual tests are based on the simple laws of geometric optics. Others depend on the properties of the light such as its wave nature. A unique advantage of many visual tests is that they can yield quantitative data more readily than other nondestructive tests.

For a variety of objects visual inspections can be used to determine size, shape, quantity, surface finish, fit, function and the presence of surface discontinuities.

In many cases visual testing and inspection is used in conjunction with an applicable code or specification to determine if the structure or part meets certain specific requirements necessary to function as it was intended.

Visible inspection techniques can be enhanced with the use of optical equipment specifically designed to be used as a tool for enhancing the human eye. Such equipment as borescopes (both rigid and flexible), magnifying devices, video and still photos and other machine vision technologies are commonly used to perform these inspections.

Visual criteria are specified for magnetic particle tests and liquid penetrant tests. Light levels, indication sizes, viewing angles, color sensitivity and many other phenomena pertaining to human vision are strictly controlled in order to achieve reliable accuracy in visual tests as well as other nondestructive techniques.

In liquid penetrant tests, visual techniques are used to compare sensitivities of penetrant systems, to detect discontinuities and to verify the cleanliness of the testing materials. In magnetic particle tests, visual acuity of the inspector and the visibility of the test results are as important as they are for liquid penetrant testing.

 While the eyes have no match for scanning and evaluating small objects are large structures, trying to recall details such as the exact size, location and orientation after a visual test is difficult. Digital photographs can also be used as a permanent record for visual testing and can be a part of the permanent record of all the inspection data.