Laser light source limited uncertainty of speckle-based roughness measurements

Surface microtopography measurement (e.g., form, waviness, roughness) is a precondition to assess the surface quality of technical components with regard to their applications. Laser speckle-based roughness measurement is an optical scattered light measuring technique that provides field of view dimensions of some square millimeters and measuring frequencies in the kilohertz domain enabling in-process roughness characterization of even moving part surfaces. However, camera exposure times of microseconds or less and a high detector pixel density mean less light energy per pixel. This affects the minimal achievable measurement uncertainty, which has not been clarified yet for almost plain sample shapes. For this reason, the measurement uncertainty limit of the surface roughness parameter Sa due to fundamental, inevitable noise sources such as laser shot noise and detector noise is analytically estimated and compared to experimental data. The results show a mainly shot-noise-limited measurement uncertainty contribution of less than 0.033 nm. In addition, a significant influence of laser beam profile variations on the achievable roughness measurement uncertainty is identified for the current experimental setup, which is generally below 0.3 nm and can be improved in future setups. The already achieved low measurement uncertainty offers ideal preconditions for in-process roughness measurements on samples with a similar surface structure in industrial environments.