The source of radiation in remote measurements is often an environmental source, such as sunlight or the thermal radiation from a warm object, and this makes it necessary to distinguish spectral absorption from changes in the source spectrum.
远程测量的辐射源往往是一种环境源，例如太阳光或来自温暖物体的热辐射，这就很有必要将光谱的吸收从源光谱的变化中区分出来。