  Hyperspectral imaging shows promise for surgical applications to
non-invasively provide spatially-resolved, spectral information. For
calibration purposes, a white reference image of a highly-reflective Lambertian
surface should be obtained under the same imaging conditions. Standard white
references are not sterilizable, and so are unsuitable for surgical
environments. We demonstrate the necessity for in situ white references and
address this by proposing a novel, sterile, synthetic reference construction
algorithm. The use of references obtained at different distances and lighting
conditions to the subject were examined. Spectral and color reconstructions
were compared with standard measurements qualitatively and quantitatively,
using $\Delta E$ and normalised RMSE respectively. The algorithm forms a
composite image from a video of a standard sterile ruler, whose imperfect
reflectivity is compensated for. The reference is modelled as the product of
independent spatial and spectral components, and a scalar factor accounting for
gain, exposure, and light intensity. Evaluation of synthetic references against
ideal but non-sterile references is performed using the same metrics alongside
pixel-by-pixel errors. Finally, intraoperative integration is assessed though
cadaveric experiments. Improper white balancing leads to increases in all
quantitative and qualitative errors. Synthetic references achieve median
pixel-by-pixel errors lower than 6.5% and produce similar reconstructions and
errors to an ideal reference. The algorithm integrated well into surgical
workflow, achieving median pixel-by-pixel errors of 4.77%, while maintaining
good spectral and color reconstruction.
