Abstract: Systems and methods are provided for determining the wideband spectrum of downhole fluids based on downhole optical measurements. In some aspects, a plurality of optical data measurements associated with a subsurface fluid can be obtained from a subsurface optical measurement device. In some cases, a comparison can be made between the plurality of optical data measurements associated with the subsurface fluid and one or more sets of optical data stored in an optical data library. In some examples, the one or more sets of optical data can correspond to a plurality of different fluid samples. In some instances, a first fluid sample from the plurality of fluid samples that corresponds to at least a portion of the subsurface fluid can be identified based on the comparison. In some aspects, an absorbance spectrum of the subsurface fluid can be determined based on the first fluid sample.