The phrase "incident radiation" refers to the radiation that reaches or falls upon an object or surface. It is the incoming radiation that comes from a source and interacts with the target material. Full definition
The proportion of incident radiation reflected by a substance is called its albedo. (sjsu.edu)
You would think it fairly easy to set up multiple containers with different gas concentrations and similar incident radiation. (dotearth.blogs.nytimes.com)
That would seem to represent the heat accumulation when there was no ice to counter the difference between incident radiation and exiting radiation. (realclimate.org)