A distributed conferencing system has a plurality of conferencing nodes to connect groups of participants to a conference. One or more embodiments relate to optimal mixing. Also shown is an apparatus including such optical assembly and a method for carrying out two- or more-dimensional optical spectroscopy using the assembly. A filling is accommodated in the depression. If there are m different driver transistor groups present, 2 different gradations result, thereby achieving good scalability. The number of LED loads carrying current is increased as the momentary amplitude of the rectified mains voltage increases, and is decreased as the momentary amplitude of the rectified mains voltage decreases.