What blade defects can a ground-level optical inspection reasonably establish?

The short answer

A ground-level optical inspection can establish visible surface or shape features that are adequately resolved from the available view. It cannot certify every blade surface or its internal structure. Define the defect question first, then assess whether distance, viewing angle and image quality allow that question to be answered rather than merely confirming that the blade is present.

Compare feature size with practical visibility

Magnification alone does not define useful detail. Atmospheric haze, lighting, focus, vibration and the angle to a curved surface can limit what is actually resolved. A small edge defect against a clear sky may be visible while a similarly sized line on a shaded face is not. Do not assign a universal minimum detectable crack from a binocular label or camera pixel count without validated inspection evidence.

Account for unobserved regions

An image of one face does not establish the other face, root region or hidden edge. A complete-looking silhouette can conceal coating loss or subsurface damage. DOE distinguishes visible optical inspection from methods aimed at hidden blade damage. That distinction does not make optical screening useless; it means its conclusion should remain tied to the surfaces and feature types it can genuinely examine.

A clear image can positively establish a visible missing region while still being unable to exclude a much smaller line elsewhere. Detection of one conspicuous defect should therefore not be used to claim equal sensitivity to every feature across the whole blade.

Choose follow-up from the unresolved question

Ask whether the concern needs another approved viewpoint, closer qualified access or a nondestructive method suited to the actual blade. A second equally distant image may not solve a resolution problem. Do not approach a restricted turbine or change its position merely to improve the view. No MARSTON optical acceptance criterion or photo-only structural release is established here; method suitability and operating disposition require the actual equipment context.