Series 65 ionization smoke detector :
 
The sensing part of the detector consists of two chambers - an open, outer chamber and a semi - sealed reference chamber within.
      Mounted in the reference chamber is a low activity radioactive foil of Americium 241 which enables current to flow between the inner and outer chambers when the detector is powered up.
      As smoke enters the detector, it causes a reduction of the current flow in the outer chamber and hence an increase in the voltage measured at the junction between the two chambers.  The voltage increase is monitored by the electronic circuitry which triggers the detector into the alarm state at a preset threshold.  An externally visible LED will light up when the detector changes to alarm state.

Series 65 Optical Smoke Detector :
 
Optical smoke detectors incorporate a pulsing LED located in labyrinth within the housing of the detector.  The labyrinth is designed to exclude light from any external source.  At an angle to the LED is photo-diode which normally does not register the column of light emitted by the LED.
    In the event of smoke from a fire entering the labyrinth the light pulse from the LED will be scattered and hence registered by the photo - diode.  If the photo - diode " sees " smoke on the two following pulses, the detector changes to the alarm state when the indicator LED will light up.  The detector housing is identical to that of ionization smoke detector.  It is distinguished from this type by having an indicator LED which is clear in quiescent state but produces red light in alarm.
     The optical detectors are easy to disassemble and clean without the necessity of returning them to the factory.