spectacle

A spectacle is a metal frame carrying two or more coloured glass lenses, mounted on a semaphore signal post so that light from a lamp behind shines through the appropriate lens depending on the signal's position. At night, when the semaphore arm cannot be seen clearly, the spectacle projects a coloured light: typically red for danger, yellow or amber for caution, and green for clear. The frame pivots or rotates with the signal arm so that as the arm moves to different positions, a different lens comes into line with the lamp behind it.

Spectacle frames are usually cast iron or steel and hold lenses of 100 to 150 mm diameter. The lenses themselves are toughened glass, because they must withstand weather, vibration from passing trains, and the heat of the lamp filament. Red and green glass absorb much of the lamp's output, so a spectacle signal typically uses a bulb of 20 to 60 watts to produce a visible light at the required distance, usually 200 to 400 metres depending on the railway and sight lines.

Design variations and failure modes

British semaphore signals most commonly use a two-lens spectacle (red and green), while some older or continental designs use three lenses. The frame must be sealed at the rear to prevent rain ingress into the lamp chamber, but poor sealing allows condensation to form on the inside of the lenses, making them opaque at night and rendering the signal unreliable. Cracked or fogged lenses are a common maintenance issue and must be replaced promptly; a faulty spectacle can cause signal failure and block the line.

The term 'spectacle' comes from the visual resemblance to a pair of eyeglasses or spectacles, each lens sitting in its own frame aperture. The spectacle is distinct from the semaphore arm itself, which is the moving indicator seen in daylight, and from the lamp assembly, which merely provides the light source. Together, they form the complete signal head that conveys information both day and night across the railway network.