  For the development and optimization of novel light emitting materials, such
as fluorescent proteins, dyes and semiconductor quantum dots (QDs), a reliable
and robust analysis of the emission efficiency is crucial. The emission
efficiency is typically quantified by the photoluminescence quantum yield (QY),
defined by the ratio of emitted to absorbed photons. We show that this
methodology suffers from a flaw that leads to underestimated QY values,
presenting as a 'parasitic absorption'. This effect has not been described
and/or corrected for in literature and is present already under common
experimental conditions, therefore, it is highly relevant for a number of
published studies. To correct for this effect, we propose a modification to the
methodology and a correction procedure.
