  Dark Energy studies with type Ia supernovae set very tight constraints on the
photometric calibration of the imagers used to detect the supernovae and follow
up their flux variations. Among the key challenges is the measurement of the
shape and normalization of the instrumental throughput. The DICE system was
developed by members of the Supernova Legacy Survey (SNLS), building upon the
lessons learnt working with the MegaCam imager. It consists in a very stable
light source, placed in the telescope enclosure, and generating compact,
conical beams, yielding an almost flat illumination of the imager focal plane.
The calibration light is generated by narrow spectrum LEDs selected to cover
the entire wavelength range of the imager. It is monitored in real time using
control photodiodes. A first DICE demonstrator, SnDICE has been installed at
CFHT. A second generation instrument (SkyDICE) has been installed in the
enclosure of the SkyMapper telescope. We present the main goals of the project.
We discuss the main difficulties encountered when trying to calibrate a wide
field imager, such as MegaCam (or SkyMapper) using such a calibrated light
source.
