What is the difference between a high-pressure and a low-pressure sodium vapour lamp?
P.K. Varadarajan, Hyderabad ANSWER: There are two types of Sodium Lamps Low-Pressure (SOX) and High-Pressure (SON). Sodium lamp performance is directly related to the gas pressure inside the arc cylinder. Increasing arc tube pressure improves the colour appearance of reds, blues, and greens. Low-pressure sodium lamps are the most efficacious light sources. The low pressure lamps give light which is virtually monochromatic, that is, yellow light at only one wavelength. Yellow and white objects look yellow, all other colours appear as various shades of grey and black. In low-pressure lamps the light-producing arc tube contains a small amount of argon and neon to aid in starting the lamp. The pressure inside the arc tube is around a few milli-Torr (the unit of pressure measured inside the discharge column of the tube, 1 Torr = 10{+-}{+3} mm of mercury) while this is around 20 Torr in the case of high-pressure sodium lamps. The enclosing space between the arc tube and the outer enclosure i
