Patent ID: 7075445

Claim:
A rapidly responding, false detection immune smoke detector of a light obscuration type, comprising: a light source from which a light beam propagates; a light detector having a light receiving surface with a light detecting area and producing a signal in response to light incident on the light receiving surface; a light reflective imaging assembly in optical association with the light source and the light detector, the imaging assembly including first and second spaced-apart optical components having respective first and second opposed light reflecting surfaces forming between them a spatial region that smoke particles can occupy, the first and second optical components having light directing properties that cooperate to reflect the light beam between the first and second light reflecting surfaces and to direct the light beam toward the light detector for incidence on its light receiving surface, and the first and second optical components controlling the light beam by providing it with a beam width that is sufficiently wide to render insignificant contributions of anomalous light reflections but that converges to illuminate the light receiving surface within the confines of the light detecting area and thereby cause the light detector to produce a signal corresponding to a concentration of the smoke particles occupying the spatial region; a rate of change measurement detector operatively associated with the light detector to measure a rate of change of the concentration of smoke particles occupying the spatial region, the rate of change measurement detector responding to the signal produced by the light detector to determine an elapsed time between changes in concentrations of smoke particles between first and second smoke concentration threshold levels, wherein the first smoke concentration threshold level is less than the second smoke concentration threshold level and the rate of change measurement detector produces a signal indicating that a rate of rise of the concentration of smoke particles from the first smoke concentration threshold level to the second smoke concentration threshold level has exceeded a first predetermined threshold rate and the concentration of smoke particles persists above the second smoke concentration threshold level for a predetermined time; and an alarm threshold circuit to which an alarm smoke concentration threshold level is set, and in which the first smoke concentration threshold level is less than the alarm smoke concentration threshold level.