Apparatus and method to measure optical absorption spectra with spatial resolution on the micron scale. An exemplary setup combines a continuous white light excitation beam in transmission geometry with a GRIN-based detection path in place of a typical confocal microscope. The apparatus and method enables the investigation of spatial variations in the optical density of small samples on the micron scale and the study of biological assemblies at the single cell level, leading to applications in optical diagnostics, microfluidics, cytology, and other areas.