Tuesday, July 12, 2016

Let There Be Light

Are high pressure sodium vapour lamps more efficient than LEDs?

The key to answering this question is "Luminous Efficacy", a measure of how well a light source a Watt of electricity into Lumens, a measure of light intensity visible to the human eye.

Today, while clearing some litter at a nearby park, I came across this box -

It was lying on the grass, right below a high-mast lamp. Even though I knew that these sodium vapor lamps consumed a lot of electricity, what struck me was the quantum of light produced by these bulbs. A 400W bulb produces more than 44,000 lumens!

If this makes little sense, just compare it to the standard LED lamps being distributed by the government, in its efforts to being down household electricity  consumption -

In terms of luminous efficacy, one would have thought that the LED lamps are a lot more efficient that any incandescent, power-guzzling bulb. However, in terms of light output (lumens), the sodium vapour lamp produces 110 lumens/W while the LED lights prouduce 114 lumens/W, which is hardly any difference.

If there is not much difference in the efficiency of LED bulbs and sodium-vapour lamps, is there a big difference in cost? The sodium-vapour lamp costs INR 675 per unit while an (unsubsidized) LED lamp costs anywhere between INR 150 and 250. This means that the cost of luminous efficacy is INR 0.16 lm/W/INR for the sodium-vapour lamp and 0.57 lm/W/INR for LEDs.  

This simply means that you are getting 3.5 times more bang for your buck with LEDs, compared to sodium-vapour lamps. So it does make sense to switch to LEDs.

Now, what is the difference is cost between LEDs meantand SVs for high-mast lamps? That would berhaps perhaps be a better comparison of oranges with oranges!


* Energy efficiency of LEDs - http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/led_energy_efficiency.pdf

* https://en.wikipedia.org/wiki/Luminous_efficacy

* 2009 - Toshiba releases 93 lm/W bulbs - http://ledsreview.com/news/367/

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