Set the ice cloud optical thickness at 550nm. Other wavelengths are scaled accordingly.  

ic_set_tau550 value

 
The optical thickness defined here is the integral from the surface at the user-defined altitude to TOA (top of atmosphere). Note that this option requires for technical reasons that the wavelength interval defined by wavelength does contain 550nm. If you use this option in combination with the ice cloud properties by Fu1996, please make sure that you understand the explanation of ic_fu_tau.