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Technology / Re: What is this Strange Optical Device on the London Underground Platforms?
« on: 28/11/2015 12:03:43 »
Hello all, sorry to drag up such an old thread but I couldn't find any other more recent discussions on the topic and I wanted to answer this one.
I've been wondering the same thing for several years now and I see them on most of the stations I use (Victoria, Euston and Archway mainly but also on Green Park and others in the area) and I had a search and found this post.
I'd completely disagree with it being smoke detectors and the 'paired device' is actually a prism which reflects the light into another prism and bounces it around the platforms to a receiver somewhere else. I would think it uses either some form of invisible or safe laser beam, or that it is only made active when the station is closed overnight for example.
(remove spaces from links below, I couldn't work out how to verify my account to be able to post them properly)
I've had another search and found reference to an accident report: http : / /content.tfl.gov.uk/16-safety-alert-monitoring-prisms-in-passenger-areas-oct-2012.pdf where it was called a "monitoring prism" and from there, that led me to find that they are for surveying and monitoring measurements over a long period of time, there is some good information about them on this website: http : / / www .berntsen.com/Surveying/Survey-Prisms/Mini-Prisms but presumably they are to make sure that the station is not slowly sinking or collapsing over time and to monitor the structural integrity of the building. Normally they seem to have 3 or more on the ceiling above escalators going down towards the platforms, normally there is a transmitter near the bottom of escalators (the same as the one imatfaal pictured above as well but I'm unsure where the receivers are or how the system works. I would guess a laser point is projected on the centre of the receiver, and if the building were to move at all, it would cause the point to move slightly on the sensor and having several prisms reflecting off of each other allows it to give a much more accurate reading, because if even one of them moved by a less than a millimeter it could cause the end dot to move on the receiver a lot more and indicate a problem.
Mystery solved!
I've been wondering the same thing for several years now and I see them on most of the stations I use (Victoria, Euston and Archway mainly but also on Green Park and others in the area) and I had a search and found this post.
I'd completely disagree with it being smoke detectors and the 'paired device' is actually a prism which reflects the light into another prism and bounces it around the platforms to a receiver somewhere else. I would think it uses either some form of invisible or safe laser beam, or that it is only made active when the station is closed overnight for example.
(remove spaces from links below, I couldn't work out how to verify my account to be able to post them properly)
I've had another search and found reference to an accident report: http : / /content.tfl.gov.uk/16-safety-alert-monitoring-prisms-in-passenger-areas-oct-2012.pdf where it was called a "monitoring prism" and from there, that led me to find that they are for surveying and monitoring measurements over a long period of time, there is some good information about them on this website: http : / / www .berntsen.com/Surveying/Survey-Prisms/Mini-Prisms but presumably they are to make sure that the station is not slowly sinking or collapsing over time and to monitor the structural integrity of the building. Normally they seem to have 3 or more on the ceiling above escalators going down towards the platforms, normally there is a transmitter near the bottom of escalators (the same as the one imatfaal pictured above as well but I'm unsure where the receivers are or how the system works. I would guess a laser point is projected on the centre of the receiver, and if the building were to move at all, it would cause the point to move slightly on the sensor and having several prisms reflecting off of each other allows it to give a much more accurate reading, because if even one of them moved by a less than a millimeter it could cause the end dot to move on the receiver a lot more and indicate a problem.
Mystery solved!
The following users thanked this post: chris