At first glance, it seems improbable to many people that the digital image correlation method is able to resolve motions well below 1/1000 pixel. This probably stems from an oft-repeated myth that the resolution of DIC is around 1/100 pixel. This value has been given in the literature and has been repeated for almost three decades. Unfortunately, no clear distinction between resolution, accuracy and noise has been made. The resolution of DIC, i.e., the smallest motion that will produce a measurable change in the displacement signal, is theoretically only limited by the information content of the input data. This means that there is no inherent resolution limit. The accuracy of DIC is limited by a number of factors, e.g., noise-induced bias and interpolation bias. Both types of bias have been virtually eliminated in CSI's software through the use of advanced optimized interpolation filters. The noise in DIC data, of course, is highly dependent on the noise in the input signal, i.e., the images themselves. The reason why 1/100 pixel "accuracy/resolution/noise" for DIC is often assumed is that with typical camera hardware and a typical speckle pattern of decent quality, experimenters typically see a noise floor in their data around 1/100 pixel. This noise floor is then incorrectly labeled as "resolution" or "accuracy".
What does this mean for vibration measurements with DIC? For a vibration measurement, it is typical to acquire an image sequence with a few thousand images. After analysis with DIC, we obtain a displacement signal that contains (Gaussian) noise and potentially very small vibrations at a number of frequencies. Since Gaussian noise is uniformly spread over the entire frequency range, we obtain a constant noise floor in the FFT. As this noise energy is "smeared" over the entire spectrum, each frequency bin only contains a small fraction of it, resulting in a low noise floor over the entire spectrum. The more images are taken, the smaller this noise floor will be. The signal itself (i.e., the vibration) is typically contained in very few neighboring bins in the FFT, with possibly multiple such localized "peaks". Because of this localization in the frequency domain, the signal we are seeking to measure does not experience the same reduction in amplitude as the noise level. Thus, a signal with smaller amplitude than the standard deviation of the background noise level can be separated from the noise and easily detected in the frequency domain.
In order to make vibration analysis with digital image correlation practical, the analysis code used has to have certain properties. First, the large data volume requires efficient processing in order to provide data within a reasonable time frame. Since digital image correlation is a full-field technique, it is not uncommon to require the analysis of tens of thousands of data points per image. To process an image sequence of several thousand frames within a reasonable time frame, it is clear that an extremely efficient processing code is required. Vic-3D's correlation engine is highly optimized to analyze dense data sets within a minimal amount of time and can typically process several thousand images per hour.
Second, many vibration measurement applications have the property that all motions are very small fractions of a pixel, i.e., the motion in the images cannot even be seen by eye. Unfortunately, the digital image correlation method has the highest amount of noise induced bias in cases where the motion is close to zero. While the choice of camera can greatly influence the amount of noise in the images, and thus the amount of bias in the vibration data, all cameras have some amount of noise and residual bias can be expected if the implementation of the digital image correlation algorithm does not specifically address this problem. Through the use of interpolation filters developed using non-linear filter optimization techniques, the Vic-3D correlation engine has been optimized to suppress this type of bias to a far greater extent than is possible using traditional interpolation techniques such as B-splines. This allows the accurate analysis of minuscule motions without the contaminating bias that results from image noise.
In summary, the notion that the resolution of DIC is limited to 1/100 pixel is completely wrong, even though it is unfortunately pretty common. In reality, much higher resolution is available. By acquiring a large number of images with high speed cameras, it is possible to measure vibrations with minuscule amplitudes, as their localization in the frequency domain effectively separates them from the background noise.
Hubert W. Schreier, Ph.D.
Correlated Solutions, Inc.