How do surface roughness or coating thickness affect VT results?

• A. They do not affect reflectivity.
• B. They improve reflectivity and contrast.
• C. They change reflectivity and contrast; rough surfaces scatter light, coatings can mask underlying defects.
• D. They only affect inspection speed.

Answer: (C)

Surface conditions change how light behaves when you inspect, so it directly affects what you see in VT. A rough surface doesn’t reflect light in a single direction; it scatters light, which lowers the amount of specular reflectivity and often reduces image contrast, making defects harder to spot or making areas look blobbed or grainy. On the other hand, coating thickness can alter the light path and intensity too. A coating can mask defects beneath it by filling in cracks or covering surface features, changing the apparent brightness and contrast of underlying flaws. It can also introduce its own variations in reflectivity where the coating isn’t uniform, which changes how defects inside or beneath the coating appear. In short, surface roughness and coating thickness change both the amount of reflected light (reflectivity) and how distinct defects appear (contrast), which is why they affect VT results.