Table 3 Qualitative comparison of the three measurement methods: digital holography, uni-axial DIC and deflectometry.
From: Comparison of three full-field optical measurement techniques applied to vibration analysis
Parameter | Method | ||
|---|---|---|---|
Holography | Uni-axial DIC | Deflectometry | |
Measured quantity | Displacement (out-of-plane) between two consecutive instants, usually converted to velocity. | Displacement (out-of-plane). | Bending slope (out-of-plane). |
Time resolution | \(1/f_{Cam}\), where \(f_{Cam}\) is the frame rate of the camera. | ||
Max acquisition time | Depends on image size and on-board camera memory; typically a few seconds of 1 Mpix images at 10 kHz. Low-speed (\(\sim\)1 kHz) applications can use off-board storage for much longer acquisitions. | ||
Minimum measurable quantity | \(\approx 2 \pi /160\) for the Doppler phase between two consecutive instants; for conversion to velocity refer to Eq. (4); about 1 nm in this study (after conversion to displacement). | \(\approx 5\times 10^{-4} L_{s} / (N_{p} sin(\theta ))\), where \(\theta\) is the angle between optical and displacement axes, \(L_{s}\) the size of the measured surface, \(N_{p}\) the number of available pixels; about 100 nm in this study. | \(\approx p/4\pi L\), where p is the grid pitch and L is the grid-sample distance; about 10 nm in this study (after conversion to displacement). |
Parameters influencing the noise floor | Speckle decorrelation; depends on local phase jump density and orientation, and sensor dimensions. | Quality and contrast of projected pattern, number of pixels, size of the measured surface, photon noise. | Quality of reflection and the number of pixel per grid period in the recorded image. |
Dynamic range | From few nm to about 10 μm (depending on spatial sampling of phase jumps; at least 4 pixels per phase jump in the Doppler phase are required). | From few hundreds of nm to few cm, as long as the surface is imaged by the cameras and the angle (optical axis/normal of the surface) is not too large. | From few tens of nm to a few mm, provided the bending curvature is not too large. |
Spatial resolution | Depends on the structure-to-sensor distance, pixel pitch, number of pixels of the sensor and wavelength of light; about 0.3 mm in this study. | Depends on the pattern and sensor resolution; optimal configuration, \(\approx\) 1 point every 1–3 pixels; about 1 mm in this study. | Depends on the number of pixels per grid period in the image; about 0.3 mm in this study. |
Sample dimensions | From mm\(^2\) to 500 cm\(^2\). | From few cm\(^2\) to several m\(^2\), with adapted calibration procedure. | From cm\(^2\) up to about 1 m\(^2\), provided the observed grid pattern is large enough. |
Sample surface preparation | Non-depolarizing diffuse reflection. | Speckle/random pattern. | Specular reflection (defined by Rayleigh criterion). |
Setup installation time | \(\approx\) 4 h, negligible surface preparation. | \(\approx\) 1–2 h with surface preparation. | \(\approx\) 30 mins, not including surface preparation (depends on material). |
Acquisition time | Depends on the studied phenomenon and regime (stationary or transient); typically a few seconds. | ||
Image download time | Depends on the camera and the storage media (HDD, SSD); typically 5 minutes for 10,000 images. | ||
Processing time | \(\approx\) 12 h for 10,000 images. | A few mins for 10,000 images. | \(\approx\) 1 h for 10,000 images. |
