Impact of surface finish and target size for testing and monitoring using cameras

Measurement and analysis using camera based techniques is becoming increasingly widespread across a wide range of industries from motion capture to materials analysis, number-plate recognition, surveying and quality control systems. Within the fields of Material Testing, Non Destructive Testing (NDT) and Condition Monitoring (CM), the advantages of camera based systems (notably being non-contact and keeping a visual record) are being recognised and starting to be exploited(1). This is particularly true of the composites industry(2), but includes many materials and structures. The techniques have not been widely adopted at present, in part due to concerns about ease of use, resolution, time taken to generate measurements and the need for patterns on specimens. This paper seeks to test a system that has been designed to address these concerns(3). In particular, two areas of the technology are looked at which are relevant to adoption within NDT and CM. Firstly, an investigation is carried out to determine whether measurements can be taken without the need for any pattern to be applied to a material or structure. Stainless steel, mild steel, brick, stone, concrete and carbon-fibre composite are all assessed. Secondly, the impact of the size of the 'virtual target' that the software uses for analysis is examined. This has a direct bearing on spatial resolution that can be achieved when trying to analyse an object under test and the imaging locations required. The paper should enable the end user to determine whether for the type of testing they are seeking to undertake, an image based technique would be worth considering. Results demonstrate that measurements are achievable on a wide variety of surfaces where no target has been applied, to resolutions of over 1/100th pixel. Surface pattern is shown to be key in achieving high resolution, but in all cases considered, suitable resolutions for some applications are achieved. Optimum virtual target sizes vary depending on the nature of the pattern on the material under investigation and the experimental setup, but measurement resolution broadly increases with target size.