NR TI 151 ISSUE 1 PDF

Purpose

Although the optical performance of LED main signals is generally very good in difficult sighting conditions, a particular problem can arise with the top yellow aspect of 4-aspect signals when the sun is very low in the sky and sunlight falls on the un-illuminated aspect. This can cause an effect which may be confused with a legitimate signal (a "phantom" top yellow aspect). The same reflections can lead to swamping of the legitimate aspect on a three aspect LED head. The effect is strongly directional and is significantly reduced if the sunlight strikes the signal "off-axis". It is therefore especially important that potentially affected signals are accurately aligned on their respective alignment points and no higher. Unlike SL35-lamp signals, LED types have a conical beam with wide vertical spread, so have good distance readability even when the Alignment Point is low.