Visual Testing Method Fundamentals and Theory
The Eye as a Sensor
Visual testing (VT) is the oldest and most widely used NDT method, but it is governed by physics and physiology. The examiner must understand how light, the eye, and the brain interact to detect discontinuities.
Key concepts:
- Light and contrast: Detection requires sufficient illumination and contrast between the discontinuity and its background. The minimum detectable contrast is about 1–2% for the unaided eye under ideal conditions, but higher in practice.
- Resolution and acuity: Visual acuity is the ability to distinguish fine detail. Normal acuity (20/20) corresponds to resolving about 1 arc minute. Acuity decreases with age and low light.
- Illumination: The eye adapts to ambient light; sudden changes require adaptation time. For critical VT, minimum illumination levels are specified (e.g., 100 fc or 1000 lux for general, higher for critical).
- Color vision: Defects may be indicated by color differences; color blindness can impair detection.
- Direct vs. indirect: Direct viewing (eye close to surface) is preferred; indirect uses mirrors, borescopes, cameras, which reduce resolution and contrast.
Traps in questions:
- Confusing illumination (amount of light) with contrast (difference in light).
- Assuming more light always improves detection; excessive glare can reduce contrast.
- Forgetting that the eye's accommodation and adaptation affect inspection reliability.
- Misunderstanding that magnification does not always improve detection; it can reduce field of view and depth of field.
Rule of thumb: For critical visual inspection, use at least 1000 lux (about 100 foot-candles) at the inspection surface, and ensure the angle of viewing is at least 30 degrees to the surface to maximize contrast.
Worked example: If a specification requires 100 fc, and a light meter reads 800 lux, is it acceptable? 100 fc ≈ 1076 lux, so 800 lux is below requirement. Convert: 1 fc = 10.76 lux. Thus 800 lux ≈ 74 fc, which is insufficient.