control device 26 can here also be used for start up and or connection of the additional power plant fig2 a shows fisheye optics 50 which collimates onto the optics 54 of a digital camera 56 solar radiation 52 incident from all directions these devices are known in the art the camera 56 has an electrical output 58 which is connected to the input 60 of a comparator 62 the comparator 62 compares in particular consecutive images from the camera 56 which may be accomplished for example by evaluating the incident luminous intensity pixel by pixel as illustrated schematically in fig2 b the temporal spacing of the individual consecutive images may preferably be identical fig2 b shows a small segment 64 of the pixel set from three digital images which were sequentially produced at the times t n t n 1 and t n 2 in form of a matrix in the uppermost row of the matrix on the left five adjacent pixels 66 with associated luminous intensity values 5 6 8 10 9 at the time t n are illustrated the row underneath has the luminous intensity values 6 7 7 8 9 a large luminous intensity value corresponds here to a large intensity in the subsequent image taken at the time t n 1 the cloud cover has moved somewhat with the.