Exit sign light bulbs such as 15T6, 20T6.5, fluorescent and LED light sources.

For most older facilities that have not retrofitted to LED technology, there are still the incandescent variety which includes the 15T6 and the 20T6.5 lamps which have a tubular shape.  They were used for many decades as the lamp of choice in exit signs but much of that changed once more efficient light sources came about.  They are tubular in shape and hence the designation with the letter T.  They have a long filament inside which does make them vulnerable to things like vibration which is another reason people have been switching from those to fluorescent or LED sources.  We still have them available though and there are many people that still use them.  For those of you who may have these, you may want to consider going to a LED retrofit or to a fluorescent source and in some cases, if the sign is just really old, you may want to consider changing those out.  Another light source that was mentioned as one of the exit sign light bulbs used today are the Dulux or PL lamps that are a fluorescent light and operates on a ballast, but it too has largely been replaced by LED light source.  As with all fluorescent bulbs, they rely on ballasts inside of the fixtures or units to make them work. The ballast is important because it is specific for each type of lamp.  You can't change from one type of fluorescent to another without changing the ballast in most cases.