MERIS is a passive optical pushbroom wide-field instrument (CCD technology) intended to measure reflected radiation the Earth's surface, the ocean, and from clouds with high spectral resolution (high radiometric resolution in the VIS range). MERIS features individual gain settings to optimize the dynamic range of each of its fifteen bands (ability to reprogram the position and width of the spectral bands in flight). More details here.
To help users familiarise themselves with MERIS and start using MERIS data products, the MERIS Product Handbook is available. Also available are the Frequently Asked Questions and a set of technical notes providing updated information to accompany the handbook.