can be used as the phosphor excitation source though such a source requires use of a blue emissive phosphor a disadvantage of a u v excitation sources is that it can lead to a degradation the display surface when it is made of a plastics material and special care needs to be taken to prevent u v light escaping which can be harmful to an observer a further advantage of the use of blue light excitation is that it is relatively safe to an observer compared to u v and consequently the sign can be lit in many different ways such as for example front lit with a blue flood lighting as illustrated in fig1 the phosphor and or phosphors can be provided on the underside of the substrate 5 as one or more respective layers with a binder material alternatively the phosphors can be provided as a mixture in a single layer moreover the phosphor layer can be provided on the outer surface of the substrate 5 or incorporated within the substrate material during manufacture referring to fig2 a and 2 b there are respectively shown an exploded perspective view of a backlit light emitting u201c exit u201d sign 12 in accordance with the invention and a cross sectional view through the line u2018 aa u2019 of the sign of fig2 a throughout the description the same reference numerals are used to denote like parts in the embodiment illustrated in fig2 a and 2 b the light box 2 and light emitting display surface 3 are rectangular in shape like the sign of fig1 the.