Absorption coefficients of dental enamel in the infrared: a solution to a seemingly straightforward problem

Infrared lasers have been investigated for the modification and ablation of dental hard tissue. Understanding the laser- tissue interaction relies on accurate determination of the optical properties of the tissue. Biological tissues absorb strongly in the infrared region of the spectrum at wavelengths resonant with vibrational absorption bands. In these regions scattering is negligible, therefore only the absorption coefficient and the reflectance are necessary to describe the laser energy deposition in the tissue. Measurement of absorption coefficients is a seemingly simple problem that presents many difficulties when a brittle material such as dental enamel is highly absorbing in the spectral region of interest. Disintegration of dental enamel ensues as a result of fracture before the sample is thin enough to allow direct transmission experiments to determine absorption coefficients at highly absorbing wavelengths (9.3 and 9.6 micrometer). Time resolved radiometric measurements combined with computer modeling of heat deposition and redistribution was used to estimate the absorption coefficient of enamel at various wavelengths in the infrared. This technique allows determination of absorption coefficients in regions not accessible to direct transmission measurements. Problems with the radiometric method and solutions to those problems are presented. The absorption coefficient at 9.6 micrometer was determined to be 5000 cm-1 plus or minus 1000 cm-1.