Karthik R
Administrator
Hello Apologies for the late response on this one.
I'm not sure if you can do this. You can perhaps use expressions to specify the different black body temperatures (Please see section 14.3.8.1.2. Black Body Temperature in the link below).
https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v211/en/flu_ug/flu_ug_mods_rad_bc.html?q=radiation%20udf
However, you might not be able to use it as a function of wavelength.
Karthik