Instead of developing new technologies, manufacturers are refining existing technology to make installation easier, to improve detection, and to reduce false alarms. In addition, increased competition is resulting in lower prices for end users. Passive infrared sensors (PIR) are the most common indoor space sensors; ultrasonic and microwave sensors are also widely used. Typical dual-technology sensors combine PIRs with one of the other approaches, with each compensating for the weakness of the other. Methods of protection range from shrubs and low fences to electronic sensors and detectors. Checklists of criteria for indoor and outdoor security systems, list of products, and photographs.