lighting the path

I traced a few key changes in the types of bulbs used for street lights.  At the same time comparing the different reasons for change.  I found that the usual reason for change was the improvement of the technologies. The thought of lighting being used outside was brought by Benjamin Franklin  when he took the candle outside and lit the streets.  

The invention of gas power for heating, lighting and cooking brought a better source for lighting outside.  

Though it requires a lot of maintenance the arc lamp gives off a larger area of light.  

Mercury vapor expanded the range but the color was not pleasant.  

Thus the Sodium Vapor lamp became popular and is widely used today.  

The changing of the type of light has expanded the use of street lights and the efficiency.  Street lights serve an important purpose.  They are now much more dependable.  There must have been a shift that brought not only dependability but an esthetic factor.  As dependability became higher so did the expectations.