- Definition of Low Pressure Sodium Vapour Lamp: An LPSV lamp is defined as a gas discharge lamp that uses sodium to produce light, combining characteristics of HID and fluorescent lamps.
- Working Principle: The lamp works by creating an arc through a gas mixture, initially emitting pink light and eventually producing a bright orange light as sodium vaporizes.
- Construction: The lamp has a borosilicate glass outer envelope with an indium oxide coating and a U-shaped arc tube containing sodium and inert gases.
- Photometric Parameters: LPSV lamps have a luminous efficacy of 150-200 Lumens/Watt, a poor CRI, a CCT below 2000K, and take 5-10 minutes to reach full brightness.
- Applications: LPSV lamps are used in road and security lighting, especially effective in foggy conditions due to their monochromatic light.
A low-pressure sodium vapour lamp, also called an LPSV or SOX lamp, is a gas-discharge lamp. Its discharge operates in sodium vapour at low pressure and produces a narrow yellow-orange spectrum.
The lamp was widely used for road, tunnel, industrial and security lighting where high lamp efficacy mattered more than accurate colour recognition. LED systems have now replaced it in many installations.
The main construction features of a traditional LPSV lamp are:
- A clear, evacuated outer glass envelope surrounds the discharge tube. A transparent heat-reflecting coating, historically based on indium oxide, returns infrared energy towards the tube while allowing much of the sodium light to pass. This thermal insulation helps keep the sodium at its working temperature.
- A long discharge tube is bent into a U shape so it fits inside the outer envelope. It contains metallic sodium and a low-pressure starting-gas mixture, commonly neon with a small amount of argon. Electrodes are sealed into the ends, and external control gear limits the current.
Starting and warming an LPSV lamp is a staged gas-discharge process. The exact electrical circuit depends on the lamp and its control gear, but the light output develops as follows:
- The control gear applies a starting voltage and then limits the lamp current.
- The initial discharge passes through the neon-argon starting gas, so the lamp first has a reddish-pink appearance.
- Energy from the discharge heats the tube and the sodium deposits within it.
- More sodium enters the vapour phase as the temperature rises.
- The sodium discharge becomes dominant and produces the familiar yellow-orange light. Its strongest visible emissions are the sodium D lines at about 589.0 nm and 589.6 nm, not 489.6 nm.
The lamp is designed to maintain the low sodium-vapour pressure and tube temperature needed for efficient resonance-line emission. Values vary by lamp design, so a single pressure or temperature range should not be used as a universal operating specification.
Photometric Parameters
The luminous efficacy depends on lamp wattage and control gear. Published Philips SOX examples range from 100 lm/W for an 18 W lamp to about 176 lm/W for a 180 W lamp, while some SOX-E models were rated higher. Its CRI is effectively zero because the narrow sodium spectrum cannot reveal most object colours. Its CCT is commonly listed as 1800 K, although CCT gives limited information for an almost monochromatic source. Representative products were rated for 18,000 hours to 50% failures and required several minutes to reach useful output; these are product-specific values, not universal ratings.
Applications of LPSV Lamps
LPSV lamps were used for roads, tunnels, ports, industrial areas and security lighting where colour discrimination was not important. Their narrow spectrum once made them attractive for efficient lighting and some astronomy-sensitive sites. However, it does not make them automatically superior in fog, and poor colour rendering can hinder recognition by people and cameras. Modern road-lighting designs generally favour controllable white LEDs with better colour information and optical control.
