An explosion-proof sensor for detecting combustible gases is provided with a glass seal for establishing an electrically conductive connection with the interior of the housing. The video information can include a plurality of frames each having a plurality of blocks. The repeating pattern of grooves creates a plurality of radially and circumferentially offset rings of grooves. On receiving the inputs the method determines that the slave power supply system has malfunctioned when the received inputs substantially mismatch. The digital image pickup apparatus further reads the digital image data from the main memory in a vertical direction.