  Ground-based whole sky cameras are extensively used for localized monitoring
of clouds nowadays. They capture hemispherical images of the sky at regular
intervals using a fisheye lens. In this paper, we propose a framework for
estimating solar irradiance from pictures taken by those imagers. Unlike
pyranometers, such sky images contain information about cloud coverage and can
be used to derive cloud movement. An accurate estimation of solar irradiance
using solely those images is thus a first step towards short-term forecasting
of solar energy generation based on cloud movement. We derive and validate our
model using pyranometers co-located with our whole sky imagers. We achieve a
better performance in estimating solar irradiance and in particular its
short-term variations as compared to other related methods using ground-based
observations.
