  We describe the scientific motivation for achieving photometric precision and
accuracy below the 1% level, and we present a calibration philosophy based on
using calibrated detectors rather than celestial sources as the fundamental
metrology reference. A description of the apparatus and methodology is
presented, as well as preliminary measurements of relative system throughput
vs. wavelength for the Mosaic imager at the CTIO Blanco 4m telescope. We
measure the throughput of the optics, filter, and detector by comparing the
flux seen by the instrument to that seen by a precisely calibrated monitor
photodiode, using a tunable laser as the illumination source. This allows us to
measure the transmission properties of the system, passband by passband, with
full pupil illumination of the entire optical train. These preliminary results
are sufficiently promising that we intend to further pursue this technique,
particularly for next-generation survey projects such as PanSTARRS and LSST.
