A conference paper co-authored by Nathir Rawashdeh (CC/MERET), has been accepted for presentation and publication at the 5th International Conference on Advances in Mechanical Engineering, December 17-19, 2019, in Istanbul, Turkey.
The paper is entitled, “Effect of Camera’s Focal Plane Array Fill Factor on Digital Image Correlation Measurement Accuracy.” Co-authors are Ala L. Hijazi of German Jordanian University, and Christian J. Kähler of Universität der Bundeswehr München.
Abstract: The digital image correlation (DIC) method is one of the most widely used non-invasive full-field methods for deformation and strain measurements. It is currently being used in a very wide variety of applications including mechanical engineering, aerospace engineering, structural engineering, manufacturing engineering, material science, non-destructive testing, biomedical and life sciences. There are many factors that affect the DIC measurement accuracy where that includes; the selection of the correlation algorithm and parameters, the camera, the lens, the type and quality of the speckle pattern, the lightening conditions and surrounding environment. Several studies have addressed the different factors influencing the accuracy of DIC measurements and the sources of error. The camera’s focal plane array (FPA) fill factor is one of the parameters for digital cameras, though it is not widely known and usually not reported in specs sheets. The fill factor of an imaging sensor is defined as the ratio of a pixel’s light sensitive area to its total theoretical area. For some types of imaging sensors, the fill factor can theoretically reach 100%. However, for the types of imaging sensors typically used in most digital cameras used in DIC measurements, such as the “interline” charge coupled device CCD and the complementary metal oxide semiconductor (CMOS) imaging sensors, the fill factor is much less than 100%. It is generally believed that the lower fill factor may reduce the accuracy of photogrammetric measurements. But nevertheless, there are no studies addressing the effect of the imaging sensor’s fill factor on DIC measurement accuracy. We report on research aiming to quantify the effect of fill factor on DIC measurements accuracy in terms of displacement error and strain error. We use rigid-body-translation experiments then numerically modify the recorded images to synthesize three different types of images with 1/4 of the original resolution. Each type of the synthesized images has different value of the fill factor; namely 100%, 50% and 25%. By performing DIC analysis with the same parameters on the three different types of synthesized images, the effect of fill factor on measurement accuracy may be realized. Our results show that the FPA’s fill factor can have a significant effect on the accuracy of DIC measurements. This effect is clearly dependent on the type and characteristics of the speckle pattern. The fill factor has a clear effect on measurement error for low contrast speckle patterns and for high contrast speckle patterns (black dots on white background) with small dot size (3 pixels dot diameter). However, when the dot size is large enough (about 7 pixels dot diameter), the fill factor has very minor effect on measurement error. In addition, the results also show that the effect of the fill factor is also dependent on the magnitude of translation between images. For instance, the increase in measurement error resulting from low fill factor can be more significant for subpixel translations than large translations of several pixels.