which would tend to increase the temperature of the coating and perhaps increase the likelihood that the surface could deteriorate or that the instrument could become hot to the touch or cause tissue to adhere to the instrument the surface must be thin enough to permit most of the energy from a particular laser beam to be conducted through the surface to the high thermal conductivity substrate before the next laser beam application impinges upon the surface although a full solution to this problem involves the solution of a nonsteady state heat conduction equation that is fourier 39 s second law of heat conduction a satisfactory approximation may be obtained from the much simpler einstein relationship the maximum thickness is thus proportional to the square root of the thermal diffusivity of the surface material multiplied by the time the laser energy impinges on the surface a proportionality constant of 0 10 is selected to ensure that approximately ninety percent 90 of the laser beam energy is transferred to the high thermal conductivity substrate before the next application of the laser beam fig9 shows a generalized graph of time versus thickness of surface material the time is the time of one laser beam application most lasers are.