Thermal damage threshold at 633 nm of tympanic membrane of pig

Doppler vibrometers are used by many research groups to monitor the motion of the tympanic membrane (TM) and of middle ear ossicles for in vivo and in vitro studies. Power densities in these applications reach 80 W/cm(2). To determine the safe limit of exposure, a cw dye laser at a wavelength of 633 nm was used to investigate the threshold of thermal damage of TM of pigs under exposure times of 60 s. To determine the applied power density accurately, the spot size of the laser beam was monitored by an objective lens and a CCD camera. Twenty-six laser exposed samples of TM were stained by haematoxylin and eosin stain and the semi-thin sections were examined microscopically. In none of the sections was any laser induced damage observed with power densities below 7.1 kW/cm(2), whereas serious damage occurred showing coagulation, carbonisation and perforation in all cases with laser powers above 8.2 kW/cm(2). The threshold for damage and the conical shape of the damage zone is explained by photon propagation and absorption in the tissue especially by the increase of the scattering factor at higher tissue temperature. The thermal damage threshold of 8 kW/cm(2) is compared to the maximum permissible exposure given in laser safety standards for skin.