  Patient-specific therapies require that cells be manufactured in multiple
batches of small volumes, making it a challenge for conventional modes of
quality control. The added complexity of inherent variability (even within
batches) necessitates constant monitoring to ensure comparable end products.
Hence, it is critical that new non-destructive modalities of cell monitoring be
developed. Here, we study, for the first time, the use of optical spectroscopy
in the determination of cell confluency. We exploit the autofluorescence
properties of molecules found natively within cells. By applying spectral
decomposition on the acquired autofluorescence spectra, we are able to further
discern the relative contributions of the different molecules, namely flavin
adenine dinucleotide (FAD) and reduced nicotinamide adenine dinucleotide
(NADH). This is then quantifiable as redox ratios (RR) that represent the
extent of oxidation to reduction based upon the optically measured quantities
of FAD and NADH. Results show that RR is significantly higher for lower
confluencies ($\leq$50$\%$), which we attribute to the different metabolic
requirements as cells switch from individual survival to concerted
proliferation. We validate this relationship through bio-chemical assays and
autofluorescence imaging, and further confirmed through a live-dead study that
our measurement process had negligible effects on cell viability.
