UPSC Prelims 2021 Question:

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 a longer lifespan 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.

Parameter

Sodium Lamp

Led Lamp

Starting time

Sodium Lights have a long warm-up time, usually around 15-20 minutes.

LED fittings are instantly at full output.

Directionality

Sodium lights are omnidirectional, i.e, they produce light in 360 degrees. 

LED lamps produce light in 180 degrees. This is typically an advantage because light is usually desired over a target area rather than all 360 degrees around the bulb. So, statement 1 is correct.

Lifetime

Sodium Lamps  have good life spans – around 14,000 hours – though this is significantly less than LEDs.

LEDs' lifespan is  typically 40,000 - 60,000 Hours. So, statement 2 is not correct.

Colour and its associated wavelength

Lights have a very narrow color spectrum (monochromatic), limited to warm deep yellow light.

LEDs are available in a wide range of colour temperatures (ranging from 'warm white' to 'daylight'). So, statement 3 is correct.