  The scientific yield of current and future optical surveys is increasingly
limited by systematic uncertainties in the flux calibration. This is the case
for Type Ia supernova (SN Ia) cosmology programs, where an improved calibration
directly translates into improved cosmological constraints. Current methodology
rests on models of stars. Here we aim to obtain flux calibration that is
traceable to state-of-the-art detector-based calibration. We present the SNIFS
Calibration Apparatus (SCALA), a color (relative) flux calibration system
developed for the SuperNova Integral Field Spectrograph (SNIFS), operating at
the University of Hawaii 2.2 m (UH 88) telescope. By comparing the color trend
of the illumination generated by SCALA during two commissioning runs, and to
previous laboratory measurements, we show that we can determine the light
emitted by SCALA with a long-term repeatability better than 1%. We describe the
calibration procedure necessary to control for system aging. We present
measurements of the SNIFS throughput as estimated by SCALA observations. The
SCALA calibration unit is now fully deployed at the UH\,88 telescope, and with
it color-calibration between 4000 {\AA} and 9000 {\AA} is stable at the percent
level over a one-year baseline.
