  Quantum-intensity-correlated twin beams of light can be used to measure
absorption with precision beyond the classical shot-noise limit. The degree to
which this can be achieved with a given estimator is defined by the quality of
the twin-beam intensity correlations, which is quantified by the noise
reduction factor. We derive an analytical model of twin-beam experiments,
incorporating experimental parameters such as the relative detection efficiency
of the beams, uncorrelated optical noise, and uncorrelated detector noise. We
show that for twin beams without excessive noise, measured correlations can be
improved by increasing the detection efficiency of each beam, notwithstanding
this may unbalance detection efficiency. However, for beams with excess
intensity or other experimental noise, one should balance detection efficiency,
even at the cost of reducing detection efficiency -- we specifically define
these noise conditions and verify our results with statistical simulation. This
has application in design and optimization of absorption spectroscopy and
imaging experiments.
