The output coupler usually has reflectance in the range 40% to 70%. In order to optimize power and spatial profile of the laser beam, the output coupler and rear mirror have optical surfaces with well-defined radii of curvature. The output coupler is typically composed of ZnSe in order to minimize absorption and thermal distortion while transmitting a high-power laser beam. The end mirror usually has reflectance 99.5% which means that the transmitted laser beam has low power. However, because absorption in the coating is higher than in the output coupler, substrate materials, such as Germanium or Gallium Arsenide, with high thermal conductivity are used in most lasers.