[Optical properties of human sclera and their significance for trans-scleral laser use]

The spectral dependence of the optical properties of human sclera adjacent to the limbus was investigated and related to the potentials of transscleral photocoagulation. The total transmission, absorption and reflection and the angular distribution of the transmitted and reflected light were measured at five laser wavelengths (442 nm, 514 nm, 633 nm, 804 nm, and 1064 nm) for both noncontact and contact applications. The scleral transmission is only 6% at 442 nm, but increases to 35% at 804 nm and 53% at 1064 nm. The absorption is high at short wavelengths, with 40% at 442 nm, but it is only 6% at 804 nm and 1064 nm. The reflection is generally higher than 40% and shows little wavelength dependence. The transmitted light is scattered diffusely at short wavelengths, but at 804 nm and 1064 nm it exhibits a fairly narrow angular distribution in the forward direction. Fiber contact leads to an increase in transmission, with a factor of 3.5 at 442 nm, of 2.0 at 804 nm, and of 1.5 at 1064 nm. Our results indicate that the diode laser (804 nm) and the Nd:YAG laser (1064 nm) are best suited for transscleral photocoagulation and that contact delivery leads to a reduction of the energy required for cyclophotocoagulation.