Q. With reference to street lighting, how do sodium lamps differ from LED lamps?

  1. Sodium lamps produce light at 360 degrees but it is not so in the case of LED lamps.
  2. As street lights, sodium lamps have a longer life span than LED lamps.
  3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street lighting.

Select the correct answer using the code given below.

  • 3 only
  • 2 only
  • 1 and 3 only
  • 1, 2 and 3

Answer: (c) 1 and 3 only

Sodium Lamps
  • A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm.
  • The spectrum of visible light from sodium lamps is almost monochromatic as they are known for their warm yellow glow. 
  • As street lights, sodium lamps have on an average shorter lifespan than LED lamps.
  • Two varieties of such lamps exist: low pressure and high pressure.
  • Low-pressure sodium lamps
    • Low-pressure sodium lamps are highly efficient electrical light sources, but their yellow light restricts applications to outdoor lighting, such as street lamps, where they are widely used.
    • Low-pressure sodium lamps only give monochromatic yellow light and so inhibit color vision at night.
    • The bulb principally contains solid sodium metal inside a borosilicate glass tube that vaporizes once the lamp is turned on. During start (while the sodium is still in solid form) the lamp emits a dim reddish/pink glow. Once the metal is vaporized the emissions become the characteristic bright yellow associated with sodium vapor lamps.
    • The spectrum of visible emissions from an LPS light is actually very close together (589 and 589.6 nm, virtually monochromatic) resulting in the colors of illuminated objects being nearly indistinguishable.
    • Used in outdoor lighting of some parking areas and bridge underpasses.
  • High-pressure sodium lamps
    • High pressure sodium vapor (HPS) lights, similar to LPS lights, are a specific type of gas- discharge light (also known as a high intensity discharge, HID or arc light).
    • The principal difference between low and high pressure sodium lights is the operating pressure inside the lamp. As indicated by the name, “high” pressure sodium vapor lights operate at a higher internal pressure.
    • The arc tube is made of aluminum oxide and the sodium metal is combined with several other elements like mercury which counter-balances the yellow glow with some white to light blue emissions.
    • High-pressure sodium lamps emit a broader spectrum of light than the low-pressure lamps, but they still have poorer color rendering than other types of lamps.
    • Bulb Failure: Prone to flickering and activation failure. At the end of the bulbs’ lives, they are more susceptible to turning on and off without input until they inevitably fail entirely.
    • Lumen: Maintain their luminance for 90% of their life span, and still maintain 80% emission at the end of their life.
    • Lifespan: High Pressure Sodium lights tend to have a lifespan averaging 24,000 hours. 
    • Bulb Angle: 360-degree bulb angle
    • Efficacy: High efficacy that is easily comparable to LED lights.
LED Lamps
  • LED stands for light emitting diode. A diode is an electrical device or component with two electrodes (an anode and a cathode) through which electricity flows – characteristically in only one direction (in through the anode and out through the cathode).
  • Diodes are generally made from semi-conductive materials such as silicon or selenium – solid state substances that conduct electricity in some circumstances and not in others (e.g. at certain voltages, current levels, or light intensities). When current passes through the semiconductor material the device emits visible light. It is very much the opposite of a photovoltaic cell (a device that converts visible light into electrical current).
  • LED lights do not contain mercury, and therefore it has a very low environmental impact than incandescent bulbs. LEDs are increasingly common in street lights, parking garage lighting, walkway, and outdoor area lighting, refrigerated case lighting, modular lighting, and task lighting.
  • Color Temperature: Variety of color temperature options providing better lighting solutions including cooler lights than High Pressure Sodium counterpart.
  • CRI: Higher Color Rendering Index (CRIs) for better color vision at night.
  • On/Off: Responds instantaneously to on/off input without any delays and can produce a steady non-flicker light throughout their entire lifespan.
  • Warm-Up: LEDs have faster switching (no warm-up or cool-down period).
  • Size: LEDs can be much smaller than other lights.
  • Bulb Failure: LED lights slowly dim over time instead of flickering or turning on/off sporadically.
  • Lifespan: LEDs have an extremely long lifespan relative to every other lighting technology. New LEDs can last 50,000 to 100,000 hours or more.
    • The typical lifespan for a fluorescent bulb, by comparison, is 10-25% as long at best (roughly 10,000 hours).
  • Bulb Angle: 180-degree bulb angle to preserve light efficiency and allow for target lighting over areas.
  • Efficacy: The most efficiency lights on the market with high efficacy averaging about 50 lumens/watt.
Lamps