Optical arrangement for a light scattering type smoke detector

In a light scattering type smoke detector having a test chamber, an optical transmitter for directing radiation toward said test chamber, an optical receiver disposed at an acute angle with respect to said optical transmitter and also directed toward said test chamber for receiving back-scattered radiation from smoke particles present in said test chamber, the improvement for minimizing the structural length of said detector comprising:

• a pair of optical lenses respectively disposed in front of said transmitter and said receiver for directing radiation respectively transmitted and received thereby in substantially parallel respective overlapping paths;
• a plurality of diaphragms disposed adjacent to said beam paths for intercepting only non-parallel scattered and reflected radiation;
• and
• a light trap associated with each of said transmitter and said receiver disposed at an opposite side of said test chamber for absorbing unscattered and unreflected radiation.