angular alignment of the slat in order to fix the closing angle of the slats at a selected value subsequent adjustment of the slats is controlled by the sun tracking photodetector which can in turn be controlled externally by a manually operated keying mechanism the photodetector used in measurement of background brightness comprises three aligned photodiodes which are directed skyward in three different directions and which together detect the incident light over the full range of 180 u00b0 in front of the venetian blind application_number US 68935091 A description referring to fig1 there is shown a section of a building wall 10 with a window 12 and a venetian blind 14 arranged outside the window 12 the venetian blind 14 is shown for purposes of simplicity both in the raised position thereof wherein the slats 18 of blind 14 are gathered together at 14a and in the lowered light shielding or draped position at 14b in the raised position 14a venetian blind 14 is received in a housing 16 the raising and lowering of venetian blind 14 as well as the rotation or turning of the slats 18 in the lowered position of blind 14 is carried out in a known manner by means of an electric motor 20 and a turning gear as is described for example in german publication opened to public inspection 36 25 365 it will be understood that the features of the invention described hereinafter are independent of the number dimensions and.