Validity Limits of High-Speed Laser-Speckle Modal Measurement on a Rotating Shaft: A Pre-Impact Falsification Test

Laser-speckle tracking enables non-contact vibration measurement, but modal identification during rotation remains challenging. This study examines whether resonance-like frequency response functions and high coherence provide sufficient evidence of modal identification. A steel shaft was tested at 0, 100, and 200 rpm using impact excitation at four positions. The assessment combined stationary cross-validation against an accelerometer, matched pre-impact comparisons, and measurement of optical degradation. At 0 rpm, optical frequencies differed from contact-reference values by 1.06% at P2 and 2.60% at P3. The two rotating speeds and four excitation positions produced eight measurement conditions. Two yielded no candidate within the decision band. Six conditions yielded optical candidates. In three of these conditions, coherence at the selected frequency was higher before impact than after impact. Removing a linear phase trend eliminated one additional candidate; however, this outcome varied in leave-one-out tests. Two tracking methods produced candidates 12.21 Hz apart at 200 rpm, but the cause remained unresolved. Rotation increased the optical noise floor by 35–46 dB and reduced speckle correlation from 0.999 to 0.708. Under these conditions, high coherence alone did not validate modal identification. Matched pre-impact comparisons provide an additional check, but passing this check does not confirm a structural mode.