A solution for monitoring an area uses color histograms and size information for blob identified in an image of the area and model for existing object track for the area. The invention includes a switch for the electrical circuit of each heater which selects between two or more voltages to apply to the power control element and the heater with at least a high and a low voltage option. A highly homogeneous and bulk-colored flat glass may thus be manufactured by an installation having short transition times. The first number is different from the second number.