A novel method for determining absorbance is described that is especially applicable to remote environmental measurements. Fluorescent dyes (Stilbene 420 and 3-hydroxyflavone) placed before and after the sample provide an internally-referenced signal proportional to transmission that is independent of the source light intensity. Concentrations as low as 50 ppb (w/w) of biphenyl in ethanol were detectable using this technique. This detection limit is comparable to that of standard laboratory absorption spectrometer measurements.