UPSC Prelims 2021 Q72: Sodium Lamps vs LED Street Lights — Explained
With reference to street-lighting, how do sodium lamps differ from LED lamps?
1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps.
2. As street-lights, sodium lamps have 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
Statement-wise explanation
- 1: correct — A sodium lamp is a gas-discharge lamp. Its glowing arc tube gives off light in all directions, so the fixture needs a reflector to send light down onto the road. An LED emits from a flat chip and is directional, covering roughly a 180° hemisphere. A US Department of Energy (DOE) note on outdoor lighting (2008) says LED fixtures use this directional emission to cut wasted backlight and uplight.
- 2: incorrect — The comparison runs the other way. The DOE outdoor-lighting note (2008) gives LED fixtures a life of 50,000 hours or more, against 15,000 to 35,000 hours for high-intensity discharge lamps such as high-pressure sodium. LEDs also lose brightness slowly instead of failing suddenly. That lower maintenance cost is one reason India's Street Lighting National Programme (SLNP) replaced conventional street lights with LEDs.
- 3: correct — A low-pressure sodium lamp gives out almost all its light in the sodium doublet at about 589 and 589.6 nm. The result is near-monochromatic yellow light in which colours cannot be told apart. High-pressure sodium is broader but still orange-yellow, with a colour rendering index (CRI) of about 22. White LEDs typically score 70 to 75 (DOE). This is why roads look 'orange' under sodium lamps and 'white' under LEDs.
How to eliminate
Statement 2 can be removed with basic awareness of current affairs. India's LED drives (UJALA for homes, SLNP for streets) are publicised for lower power use, longer life and less maintenance. If sodium lamps lasted longer, replacing them would make less sense. Once statement 2 is gone, only two codes remain: '3 only' and '1 and 3 only'. Statement 3 matches everyday experience, since sodium-lit streets look uniformly orange and colours are hard to see under them. So the decision rests on statement 1. Remember the fact that settles it: a discharge lamp's arc radiates in all directions (360°), while an LED chip is directional (about 180°). Directional emission is why LED street lights waste less light. The word 'almost' in statement 3 is a hedge, not an absolute. It makes the statement safer, not a trap.
Know more
Why street lighting moved from sodium to LED
Sodium lamps are gas-discharge lamps. An electric arc excites sodium vapour, and the excited atoms emit light at sodium's characteristic wavelength near 589 nm, the yellow 'D-lines'.
- Low-pressure sodium (LPS): almost purely 589 nm. It is very efficient, but the light is monochromatic and colours cannot be distinguished.
- High-pressure sodium (HPS): the higher pressure broadens the spectrum into a golden-orange light. This was the standard Indian street lamp before LEDs. Colour rendering is still poor.
An LED (light-emitting diode) is a solid-state source. Light comes from electrons and holes recombining at a semiconductor junction. A 'white' LED is usually a blue LED coated with a phosphor.
| Feature | Sodium (HPS) | LED |
|---|---|---|
| How light is made | Arc through sodium vapour | Semiconductor junction |
| Emission | All directions (360°); needs reflector | Directional (~180°) |
| Colour rendering (CRI) | ~22 | ~70–75 |
| Typical life (DOE, 2008) | 15,000–35,000 h | 50,000 h or more |
| Switch-on | Warm-up and restrike delay | Instant; dimmable |
| Hazardous content | Contains mercury/sodium | No mercury |
Key terms
- CRI (Colour Rendering Index): how faithfully a light source shows colours, on a 0–100 scale.
- Lumen maintenance: LED life is usually counted until output falls to 70% of the initial level (L70).
India's programmes
- SLNP, launched on 5 January 2015, is run by Energy Efficiency Services Limited (EESL), a joint venture of PSUs under the Ministry of Power.
- It uses a pay-as-you-save model. EESL pays the upfront cost, and municipalities repay it from energy savings under 7-year contracts that guarantee about 50% energy saving.
- By September 2021, about 1.1 crore street lights had been replaced (Ministry of Power).
- UJALA does the same for household bulbs.
In the news
UJALA and the Street Lighting National Programme completed six years in January 2021. In September 2021, the Ministry of Power reported that about 1.1 crore conventional street lights had been replaced with LEDs. Sodium-to-LED replacement was therefore a visible government achievement in the run-up to Prelims 2021.
NCERT Concepts to Revise
- Electromagnetic spectrum: Statement 3 compares the visible-light spectra of sodium and LED lamps, so know where visible wavelengths sit on the electromagnetic spectrum.