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 L

1. A. 3 only
2. B. 2 only
3. C. 1 and 3 only
4. D. 1, 2 and 3

Answer: C

Explanation

Sodium vapor lights are omnidirectional. Omnidirectional lights produce light in 360 degrees. This is a large system inefficiency because at least half of the light needs to be reflected and redirected to the desired area being illuminated. Whereas, LEDs are extremely energy efficient relative to every other commercially available lighting technology. There are several reasons for this to include the fact they waste very little energy in the form of infrared radiation (much different than most conventional lights to include fluorescent lights), and they emit light directionally (over 180 degrees versus 360 degrees which means there are far fewer losses from the need to redirect or reflect light). Hence, statement 1 is correct. In some areas (e.g. lifespan) sodium lamps are drastically inferior to LEDs. The extremely low maintenance and replacement costs with LEDs is actually a major cost benefit over the long term. LED lifespan can be greater than 100,000 hours, i.e more than 4 times than sodium lamps. Hence, statement 2 is not correct. Sodium-based lamps produce visible light directly but also in a very narrow frequency range (yellow-orange) so that objects of other colors appears as shades of grey. Such light is referred to as monochromatic. Sodium lamps are coated with indium tin oxide which permits visible light to exit but infrared radiation (heat) to be reflected back. Whereas a very broad spectrum of color rendering index (CRI) is available with LEDs ranging generally from 65-95. Hence, statement 3 is correct.