1. Install each pair at the opposite positions. Then there is an infrared area between these two detector parts. When someone passes the infrared region, the detector will send an alarm signal to the security alarm host (which is not included here). 2. Install Distance(Max.) in the above chart means the maximum install distance between the two detector parts. 3. It is great to be installed for security systems in schools, office buildings, hospitals, home yards, etc. 4. The detecting range can be reduced to 30% in bad weather conditions, like rain, dust, etc. 5. An English Usage Manual is included in the pack. It will give you detailed instructions. Note: Due to the light and screen setting difference, the item's color may be slightly different from the pictures. Please allow slight dimension difference due to different manual measurement. Sold by-Minnanxin