The RIF50 is no longer available and has been replaced by the SENSIVA range
http://www.remotecontrolgates.co.uk/contents/en-uk/p125_Sensiva_XS_Infra-red__Safety_Beam_Set.html
The set consists of a transmitter and receiver creating an invisible safety beam that when broken will prevent your gates closing. Power is usually taken from the gate control panel and this can be 12v or 24v AC or DC
The receiver has both a normally open and a normally closed connection so an additional set can be used to trigger the gate opening if you wish.