Beam detectors

A method for detecting smoke obscuration of an optical beam in a reflective-type optical beam smoke detector that is configured to adjust alignment of a projected beam with a reflector apparatus if an intensity is below a threshold, the method comprising:

• (a) projecting a first linearly polarised beam of light of first linear polarisation across a volume to be monitored;
• (b) receiving the first linearly polarised beam of light at a spherical retroreflector, an elliptical retroreflector, or a corner-cube retroreflector across said volume to be monitored and either:
• i) modifying the first linearly polarised beam of light into a circularly polarised or elliptically polarised beam of light and then reflecting the circularly polarised or elliptically polarised beam of light across the volume to be monitored or a further volume to be monitored;
• OR
• ii) reflecting and then modifying the first linearly polarised beam of light into a circularly polarised or elliptically polarised beam of light before the beam of light traverses the volume to be monitored or the further volume to be monitored;
• (c) receiving the circularly polarised or elliptically polarised beam of light across the volume to be monitored or the further volume to be monitored, and modifying the received circularly polarised or elliptically polarised beam of light into a second linearly polarised beam of light, wherein the first and second linearly polarised beams of light have different polarisation;
• and
• (d) detecting smoke obscuration if an intensity of the second linearly polarised beam of light is below a threshold.

A method for commissioning a reflective-type optical beam smoke detector apparatus that is configured to adjust alignment of a projected beam with a reflector apparatus if an intensity is below a threshold, the method comprising:

• (a) projecting a first linearly polarised beam of light of first linear polarisation across a volume to be monitored;
• (b) receiving the first linearly polarised beam of light at a spherical retroreflector, an elliptical retroreflector, or a corner-cube retroreflector across said volume to be monitored and either:
• i) modifying the first linearly polarised beam of light into a circularly polarised or elliptically polarised beam of light and then reflecting the circularly polarised or elliptically polarised beam of light across the volume to be monitored or a further volume to be monitored;
• OR
• ii) reflecting and then modifying the first linearly polarised beam of light into a circularly polarised or elliptically polarised beam of light before the beam of light traverses the volume to be monitored or the further volume to be monitored;
• (c) receiving the circularly polarised or elliptically polarised beam of light across the volume to be monitored or the further volume to be monitored, and modifying the received circularly polarised or elliptically polarised beam of light into a second linearly polarised beam of light, in which the first and second linearly polarised beams of light have different polarisation;
• and
• (d) detecting and comparing with a commissioning threshold the intensity of the second linearly polarised beam of light, and adjusting alignment of the projected beam with a reflector apparatus if the intensity is below the commissioning threshold.