Pioneering use of ‘wasted' light
Souter was built to make use of the ‘wasted' light’ that shined landward - a pioneering design. A series of prisms directed the excess light to the room below, shining it south over Sunderland Harbour to warn of dangerous rocks to the South of the harbour. If sailors were in direct line with the rocks the light shined red; if they were clear of the rocks the light shined white.
This light is located on the Half Way Landing, six metres below the main lens. The light had a range of six miles and was divided into two sections: seaward (the white section which showed a white light) and landward (the red section which showed a red light due to a red screen being fitted to the landward section of the lens).
Ships approaching Sunderland from the south would locate the white light. Keeping the white light in view they would sail inside the white sector until they were opposite the harbour entrance when they would turn westward and sail into harbour.
If the ship strayed off course (landward) into the red section they would lose sight of the white light and the red light would be seen warning them that they were running into danger.