Abstract: A photosensitive surface is enclosed behind a wall formed with a vertically open slit which forms with the surface a vertical angle of 80.degree.. This slit is open about 3.degree. and is displaced horizontally past the surface through at least 150.degree. so as to let parallel rays of sunlight fall onto this surface and suppress diffuse nonparallel ground light. The wall can be carried on a support also carrying the photosensitive surface, e.g. a photoresistor, photocell, or photodiode, which is used to control an awning, roller blinds, or the like.