| Techniques for the non-contact determination of large scale surface roughness, on the order of millimeters, are greatly needed in the field of Nondestructive Evaluation. The ability to detect metallic surface irregularities is an important tool for both corrosion location and on-line process control. Traditional methods such as mechanical profilometers are typically time consuming and impractical in cases where the surfaces are covered and/or can not be touched.; Microwave speckle contrast measurements can be used to perform remote determination of macroscopic roughness on conductive surfaces. This procedure is non-contact and can be accomplished through windows of certain dielectric materials. The technique is wavelength unspecific and can be used over a large range of surface roughnesses. The frequency of the electromagnetic irradiation is typically chosen so that its wavelength is roughly four times the maximum root-mean-squared (RMS) surface variation. The microwaves are focused onto a surface and are scattered upon reflection. A receiver, placed at some distance in the diffuse reflection, is used to make the necessary measurements of speckle contrast or sharpness.; In this work 60 GHz ({dollar}lambda{dollar} = 5 mm) microwaves are used in a non-imaging speckle configuration to examine the surface roughness of conductively coated sandpaper. A variety of different grit sandpapers, 80 to 16, are painted with silver electrode paint and are used as roughness standards. The speckle contrast measured by the receiving horn is well predicted by equations developed in this work. Using these equations and the measurement of the real and imaginary parts of the speckle field, the RMS surface roughness (out-of-plane dimension) and the surface roughness correlation length (in-plan dimension) may be calculated. |
