Why The Street Light is Yellow

Street Lamps


Street lights are bright yellow in color most of places, the reason behind this is the type of lamps used in streets, Sodium Vapour Lamps


How Sodium Vapour Lamps work

Sodium Vapour lamps are made of a glass tube filled with inert gas like helium and argon and a small amount of sodium. Two electrodes are placed on the ends of the tube. When a voltage is applied across the electrodes, the inert gas inside the tube ionizes and current starts flowing through it. It creates a pinkish red glow. Because of the flow of current the temperature increases and this caused the sodium to vaporise. The sodium vapor then stabilizes the electric arc and sodium vapor creates a monochromatic yellow light.


Advantages & Use


Human eyes can detect light from wavelength 390 - 700 nm , and sodium vapor generates light of around 600nm wavelength which is quite useful zone of visibility. That means most of the energy coming out this lamps is in the form of visible light. This is the reason Sodium vapor lamps generates upto 200 lumen/watt of electric power consumed. Which is higher than any other lamp. The single color yellow can travel a larger distance and can penetrate dust effectively. That's the reason sodium vapor lamps are used to light open space like streets, yards, parking lots etc. Most importantly this lamps are cheap and easily available in market.



Limitations


In Spite of these advantages one big problem is color, bright yellow color can not be used everywhere. Many claims that yellow light may cause accident on the streets. This lamps takes time to glow as vaporization of sodium is time taking that's another limitation.
Now efficient LEDs are in the market with comparative longer life and almost same energy output. Thus street lights are now getting replaced by LEDs.


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