英语翻译three variables describe the radiation that is received:

英语翻译
three variables describe the radiation that is received:its intensity,its wavelength and its time dependence.怎么翻?its time dependence这个是什么意思
For thermal emission,the intensity (or radiance) is determined by the temperature and the emissivity of the background,and by the atmospheric species along the path to the
instrument.还有这句，background怎么翻？