Laser light scattering from holographic sinusoidal gratings has been investigated with a view to its use in the calibration of the linearity of BRDF instruments, a task that requires a wide dynamic range in the scattered intensity. An aluminum-coated grating of an amplitude of approximately 90 nm and a spatial wavelength of 6.67 micrometer was used. Measurements and calculations were performed for an angle of incidence of 6 degrees and for light incident from a HeNe laser (lambda equals 0.6328 micrometer). Experimental results are compared with the predictions of two theories: Beckmann's scalar theory and Rayleigh's vector theory applied to sinusoidal gratings. Both theories, which apply to perfectly conducting scatterers, produce nearly identical results. However, these predictions differ significantly from some of the experimental results. The measured scattering pattern has a large background of scattered light and the higher-order peak intensities are larger by several orders of magnitude than the computed ones. The measured peak intensities are polarization dependent. The large background scattering is shown to be due to the residual surface roughness. The profile of the grating was measured using a stylus instrument with a 1-micrometer-radius tip and a 0.1-micrometer-radius tip, and it appears that the profile does not contain significant harmonics that might be responsible for the large higher-order peak intensities. Scattering from a gold-coated specimen with the same specifications was also measured and compared with that from the aluminum-coated one to determine the effect of non-topographic scatter. Possible causes of the discrepancy between the measured and the computed magnitude and polarization dependence of the higher-order peak intensities are discussed.
Proceedings Title: Proceedings of SPIE, Scattering and Surface Roughness, Zu-Han Gu, Alexei A. Maradudin, Editors
Conference Dates: July 30, 1997
Conference Location: San Diego, CA
Conference Title: Surface Roughness and Surface Defects Detection
Pub Type: Conferences
BRDF instrument, light scattering, non-topographic scatter, residual roughness, sinusoidal grating