Video goniometry was used to study the angular dependence of scattering from tissues and test materials. Tissues and standard roughness samples (sandpaper) were placed vertically in front of a 543 nm He-Ne laser with the tissue surface normal at 45° from the incident beam. The scattered light patterns projected onto a screen that was photographed by a digital camera. The scatter pattern showed a specular peak centered at -45° which was described by a Henyey-Greenstein function. The pattern also presented a diffuse Lambertian pattern at 0° (normal to the tissue). The line between the peak specular and the peak Lambertian identified the scattering plane, despite any slight misalignment of the tissue. The analysis utilized a coordinate transform based on mathematics for mapping between a flat Mercator map and a spherical planetary surface. The system was used to study the surface roughness of muscle tissue samples (bovine striated muscle and chicken cardiac muscle).
A difficulty with using an OPO laser to reshape the cornea by ablation is the tendency for ablation to stop prematurely. We report on using an OPO laser (2.94 um wavelength, 5 ns pulse duration, 7 mJ pulse energy, approximately 0.5-mm 1/e<SUP>2</SUP>-radius Gaussian beam) to ablate a 20% acrylamide gel as a model for the cornea. Experiments demonstrated that ablation proceeds at an average rate of ablation of 3-4 μm/pulse then stops at about 1 mm depth. A computer model was developed to simulate the ablation and desiccation processes. Using a range of operating parameters, the model could achieve ablation rates of 2.8-3.5 μm/pulse and cessation of ablation after 0.25-2.1 mm. A key factor is the absorption coefficient of desiccated gel which was measured experimentally to be about 1700 cm<SUP>-1</SUP>. In conclusion, desiccation from residual heat after an ablative pulse creates a dried layer that attenuates subsequent pulses. If the threshold energy density required for ablation is too high, then too much residual energy remains after each pulse and the consequent dried layer halts the ablation process.