  We demonstrate feedback-optimized focusing of spatially coherent
polychromatic light after transmission through strongly scattering media, and
describe the relationship between optimized focus intensity and initial
far-field speckle contrast. Optimization is performed using a MEMS spatial
light modulator with camera-based or spectrometer-based feedback. We observe
that the spectral bandwidth of the optimized focus depends on characteristics
of the feedback signal. We interpret this dependence as a modification in the
number of spectral modes transmitted by the sample, and introduce a simple
model for polychromatic focus enhancement that is corroborated by experiment
with calibrated samples.
