Efficiency of Incandescent Light Bulbs Vs. Florescent Light Bulbs

Lucy Dikeou
December 11, 2014

Submitted as coursework for PH240, Stanford University, Fall 2014

How Incandescent Light Bulbs Work

Fig. 1: Schematic of incandescent light bulb. (Source: Wikimedia Commons)

Incandescent means to glow with heat. Incandescent light bulbs are made up of a glass enclosure that encompasses a tungsten wire. [1] Tungsten is used because is has the highest known melting temperature at 3680k of the pure metals while also having very low rate of evaporation. [2] The bulb is then filled with gas, such as Argon, which has a high molecular weight, to reduce evaporation of the tungsten. [1] Hindering this evaporation allows for higher temperatures to be reached. Hindering this evaporation allows for higher temperatures to be reached. Given Tungsten's vaporization pressure, incandescent bulbs are kept at a temperature below 3000°K. [2] Over time the tungsten in the filaments will evaporate and land on the inside of the bulb, resulting in a dimmer bulb. At this temperature only a fraction of the light emitted is visible. When electric current is passes through the tungsten filament, electrons move and bump into the atoms that make up the coil. These constant collisions heat up the atoms creating the heat used to emit light. [1] 90% of the energy is given off as heat while only 10% is given off as visible light. [3]

How Florescent Light Bulbs Work

Florescent bulbs are tubes filled with argon and mercury vapor. The inside of the tube is coated with phosphor. [4] Electricity is passed through the electrode, which is typically made of tungsten. Electrical charge is passed into the mercury vapor, energizing it and causing it to give off UV (ultraviolet) light. The phosphorus coating captures the UV lights making it to fluoresce, thus creating visible light. [2] Florescence is defined as taking in light of shorter wavelength and emitting light of longer wavelength. Because of this, the light we see is not actually emitting from the mercury atoms, rather from the glowing phosphorus. Florescence produces light by other means than heat alone. This allows florescence bulbs to turn much more of the energy consumed into light rather than heat. 85% of the energy consumed by florescent bulbs is turned into light with only 15% of the energy being lost to heat. [1]

Cost Benefits

Florescent lights also offer a cost benefit. Although Florescence bulbs are more expensive to buy than incandescent bulbs, they are much cheaper to own. [3] We will go through some brief calculations. An 18W florescent bulb is equivalent in light to a 75W incandescent bulb. Over a 10,000 hour lifetime (the average for florescence bulbs) the Florescent light will consume 180kW at 7.3cents per kW comes to a total of $13 for usage + $20 for buying the bulb = $33. [3] Compare this to Incandescent bulbs with an average lifetime of 750 hours. We would need to buy 13 incandescent light bulbs to have light for as long as 1 florescent light bulb. The cost of these 13 bulbs is about $7. Over 10,000 hours, an incandescent light bulb will consumes 750kw of energy at 7.3 cents per kW which comes to a total of $55 thus making the total $62. You save $29 dollars per florescent bulb over its lifetime 3.

© Lucy Dikeou. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] R. Kane and H. Sell, Revolution in Lamps: A Chronicle of 50 Years of Progress. 2nd Ed. (Fairmont Press, 2001).

[2] D. MacIsaac, G. Kanner, and G. Anderson, "Basic Physics of the Incandescent Lamp (Lightbulb)," The Physics Teacher 37, 520 (1999).

[3] "The Virginia Energy Savers Handbook," Virginia Department of Mines, Minerals, and Energy, 2008.

[4] W. M. Yen, S. Shionoya and H. Yamamoto, Practical Applications of Phosphors (CRC Press, 2006).