A light scattering type smoke detector minimized in size and protected from erroneous operation and from internal accumulation of water drops is provided. The optical comprises light emitting and receiving elements and light shielding means which are compactly mounted on a base plate at different heights...http://www.google.com.au/patents/US3985453?utm_source=gb-gplus-sharePatent US3985453 - Light scattering type smoke detector
