martin2345uk
Established Member
Apparently the signals at St Pancras are being changed from fibre optic to LED ones… how does a fibre optic signal work in terms of showing the aspects?
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How does a fibre optic signal work?
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RailUK Forums
Looking at https://railway-news.com/wp-content/uploads/2016/12/signalling-at-the-st-pancrasctrl-terminal.pdf , the fibre optic signals have halogen bulbs for all the aspects abd indicators in a single module at the bottom of the signal head, and fibre optics inside the head point the light out of the right hole in the front.
High power reliable white LEDs were a few years off still when st Pancras international was done, so this would have been the height of progressive modular signalling tech.
Presumably they're just taking off the entire head and replacing it with a nice sensible ordinary LED head now, like we use everywhere else.
High power reliable white LEDs were a few years off still when st Pancras international was done, so this would have been the height of progressive modular signalling tech.
Presumably they're just taking off the entire head and replacing it with a nice sensible ordinary LED head now, like we use everywhere else.
zwk500
Veteran Member
That's a really interesting setup, with the bulbs in a single unit at the bottom of the signal and fibre optics to the actual arpetures. Is it used anywhere else in the UK?
ISTR Paddington approaches had something similar on the big gantries in the 1990s resignalling. In that case the light boxes were up on the gantry but easier to access than the the head units themselves.
The 'OFF' and 'CD/RA' (Close Doors/Right Away) platform indicators used to help dispatch trains were also fibre-optic before the wide-spread use of LEDs. We have samples of each at St Albans South on display. An interior view of the 'OFF' indicator:
This shows the 55w halogen bulb, a vented housing with a heat filter and the bunches of fibre-optic 'cables' going off to the face of the unit.
For demonstration purposes, we've replaced the halogen bulb with a similar 3.5w LED bulb.
(There is a temporary connection direct from a power supply to the lamp visible to save mucking about with a 100vAC signal supply.)
This shows the 55w halogen bulb, a vented housing with a heat filter and the bunches of fibre-optic 'cables' going off to the face of the unit.
For demonstration purposes, we've replaced the halogen bulb with a similar 3.5w LED bulb.
(There is a temporary connection direct from a power supply to the lamp visible to save mucking about with a 100vAC signal supply.)
zwk500
Veteran Member
Where you have fibre-optic route indicators that display multiple indications within the same case, is there a bulb for each indicator or would there be a shutter of some kind to select the appropriate group of fibre-optics?
The same technique was used for alphanumeric route indicators for many years, both the miniature 'stencil' size and the larger standard 'theatre' type. The bundle of fibres for each indication is arranged in the face plate matrix to depict the required characters. A miniature could display up to three separate indications, while a standard could handle up to ten. Banners and Preliminary Route Indicators were also supplied using similar fibre optic units.
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A lamp for each separate indication. The rear of the unit is divided into several light-proof compartments, each containing a lamp for a particular indication and a bunch of fibres going to the face plate matrix.
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There are separate lamp units - our 'CD/RA' unit has two, one for each indication, but I don't appear to have any photos of that one's interior.