To summarize (with the simplifying assumptions of zero non-Planck feedbacks, a perfect blackbody surface for LW, and that all solar heating is at the surface, or at least beneath the tropopause — which is not to say that none of this applies to all other cases): (realclimate.org)
In a perfect blackbody — the frequency would make no difference. (judithcurry.com)
To the extent that something is not actually acting as a perfect blackbody, yes, some measurements are needed. (wattsupwiththat.com)