Based on the principle of illuminating the object by light in the visible region of the electro magnetic wave spectrum.
- Eye – The sensor;
- Light – The probing medium.
- Testing of the vision of the inspector, measurement with a light meter of the light falling on the specimen and establishing the actual ability to see the area being inspected.
- Refer the code for the specifications.
Calibration of Eye:
- Near Vision Acuity;
- Colour blindness.
Near Vision Acuity Chart:
- Clean the test object sufficient enough to observe the details.
- Keep it in a well illuminated space – generally 800 – 1000 lux.
- Inspect the object by eye or by light sensitive devices such as photocells.
Aids for visual testing:
- Mechanical aids;
- Optical aids.
- Measuring rules & tapes;
- Calipers & micrometers;
- Squares and angle measuring devices;
- Thread, pitch and thickness gages;
- Level gages (liquid & laser) and plumb lines;
- A variety of weld gages.
- Mirrors (especially small, angled mirrors);
- Magnifying glasses, eye loupes, multilens magnifiers, measuring magnifiers;
- Microscopes (optical and electron);
- Optical flats (for surface flatness measurement);
- Borescopes and fiber optic borescopes;
- Optical comparators;
- Photographic records;
- Closed circuit television (CCTV) systems (alone and coupled to borescopes/microscopes);
- Machine vision systems;
- Positioning and transport systems (often used with CCTV systems);
- Image enhancement (computer analysis and enhancement).
Enhancement Techniques in visual testing:
- 1. Microscope.
- 2. Borescope.
- 3. Endoscope.
- 4. Flexible Fibre-Optic Borescope (Flexiscope).
- 5. Telescope.
- 6. Holography.
Application in visual testing:
- Everywhere and anywhere.
- Also before conducting any NDT.
- Least expensive of all NDT methods.
- Permanent records are possible with the help of camera.
Advantages of visual testing:
- Low cost;
- Minimal training;
- Minimal equipment requirements;
- Ability to be performed while the specimen is being used or processed.
- In many cases will eliminate the need for the need for more sophisticated NDT.
Limitation of visual testing:
- Can be applied to surface discontinuities only.
- Least sensitive of all NDT methods.
- The poor and variable resolution of the eye, fatigue of the inspector, distractions and hence heavily dependant on the inspector.
- In some cases cost of the visual aids (equipments) can be very high.