Low Pressure and High Pressure Sodium Vapour Lamp

May 22, 2013 November 22, 2013

There are two types of Sodium Lamps —

Low-Pressure (LPS)
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 one of the most efficacious light sources.

Low-pressure sodium (LPS) lamps have a borosilicate glass gas discharge tube (arc tube) containing solid sodium, a small amount of neon, and argon gas in a Penning mixture to start the gas discharge. The discharge tube may be linear (SLI lamp) [2] or U-shaped. When the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright yellow as the sodium metal vaporizes. These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength. As a result, the colors of illuminated objects are not easily distinguished because they are seen almost entirely by their reflection of this narrow bandwidth yellow light.

Thus, 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.

The high-pressure sodium lamps have excellent colour rendition with a white colour appearance. High-pressure sodium (HPS) lamps are smaller and contain additional elements such as mercury, and produce a dark pink glow when first struck, and an intense pinkish orange light when warmed. Some bulbs also briefly produce a pure to bluish white light in between if the mercury achieves its high pressure arc discharge characteristic before the sodium is completely warmed.

The light is still biased towards the yellow, but is very acceptable in general-purpose applications and allows colours to be readily distinguished. In high-pressure lamps the light-producing element is an arc tube that is small in diameter to maintain a high operating temperature. Because of the small diameter, there is no starting electrode inside the arc tube.

The arc tube is made of a ceramic material due to corrosive effect of sodium on ordinary glass or quartz, operating at a slight pressure and high temperature.