The symbols of lamps differ in their styles. This is not because the inventor of the symbols could not decide for the nicest one of them, but it is because of the different kinds of lamps which can be distinguished by the differences in their symbols. The different ways to produce light became the reason for different symbols, and so there are a few symbols we'd like to introduce, for example: The SIGNAL lamp.
As a signal lamp can not illuminate more than a few square inches around it, and as someone thought that it might be important to have a different symbol for the illuminating lamp also, this is what that substitution for a bulb looks like: (here: in a disco-light-effect-application)
Lamps are often mentioned in the history of mankind. Though it was not always clear where the light came from (e.g. the light at the end of the tunnel which came from the approaching train), it was most often a simple lamp which caused little photons to die in darkness, somwhere far away from their other photon-friends. What all lamps have in common is the fact that they produce light out of energy which is fed into their inlet(s). Even YOU (i.e.: the reader) could be a lamp, but we don't want to bring up discussions about the amount of energy that would be needed for that. Probably it would depend on too many factors like: Personal alignment to Micro$oft products or the size of your towel. But looking for a raw value, it would be about 1.21 GWatts, I suppose.
Ready for takeoff !