Skip to content





Take an ordinary metal house key, bend it in half, and then straighten it out. Then bend it in half again and keep bending it back and forth, back and forth. What eventually happens? The metal key breaks into two pieces.

Why? It’s called Metal Fatigue. Micro cracks form on the metal surface when it is deformed by bending. With each bend more cracks appear and existing racks deepen until it breaks into two pieces.


Most metals expand when heated and contract when cooled; they have what is called a coefficient of thermal expansion. When an incandescent light bulb (ILB) is turned on, the thin tungsten filament coiled up inside rapidly heats up and glows white hot filling a room with light. Getting very hot (2,500 Cº or 1,400 Fº) the filament expands rapidly, when turned off the filament cools and contracts. Each time the light is turned on or off the expansion and contraction causes metal fatigue until the filament breaks. When that happens we say the bulb ‘burned out’.


Think about it, don't ILBs always seem to burn out precisely when they are turned on? You may see a blue flash or hear an audible clink when it goes. This is the last time the filament was expanding like when the metal house key that was bent the last time and broke.


That's not the only thing that causes ILBs to burn out. Better ILBs are either evacuated or filled with a relatively inert gas like nitrogen. The filament then does not come in contact with oxygen. Hot metal oxidizes in the presence of oxygen gas, and oxidation in ILBs produces tungsten oxide which vastly shortens the life of the filament.


Ever notice an old used ILB that looks darkened inside? That dark material on the inside surface of the glass is material from the metal filament itself. Tungsten atoms constantly evaporate from the filament, when turned on, and are deposited on the inside of the glass envelope. The already thin filament continues to become thinner and thinner until at some point it is too thin and breaks the conductor, ruining the ILB.


Most ILBs, if you read on the package, tell you how long it’s intended life expectancy is. The cheapest last 750 to 1,000 hours, less than a year of normal usage. A long life span ILB can be up to 1,500 to 2,000 hours, but they cost a lot more. Even these 'long-life' ILBs last only about 2 years with normal use.


      CFL Lights


Fluorescent lights and compact fluorescent lights (CFLs) have a similar burn out scenario as ILBs already discussed.


If you have ever replaced old burned out 4 foot glass fluorescent tubes

(T8s or T12s) you would have noticed a blackening at either or both ends of the old tube. Although CFLs do not have filaments that burn out like

ILBs do, the same enemy, oxidation, is at work along with evaporation of active electrical components inside the ends of the fluorescent tubes. In some cases either the starter (if it has one) or the ballast has gone bad but the fluorescent tube itself will eventually burn out.


Manufacturers have been steadily improving the life spans of fluorescent and CFL lighting. The average rated life span of a CFL is between 6 and 15 times typical ILBs. CFLs nominally have a rated life expectancy of 6,000 to 15,000 hours (q.v. Wikipedia 'Compact fluorescent lamp').


      LED Lights


LEDs (Light Emitting Diodes) are altogether a horse of a different color, they NEVER burn out! LEDs have no filaments, no metal fatigue, no oxidation and no evaporation of electrical components. Both ILBs and CFLs are fragile and have to be handled like eggs. On the other hand LEDs are robust, very rugged, solid state devices with no glass to break. However, LEDs will stop working if placed next to an atomic explosion.


Seriously, though, if LEDs never burn out, why do they even have a stated life expectancy rating at all, even if very high like 30,000 or 50,000 or even 100,000 hours?


Generally, the life expectancy rating of all light bulbs is based on a year of 1,100 hours usage because 365 days per year times an average of 3 hours a day usage comes out to exactly 1,095 hours.


             Hours           Years          End of

   Lamp      of Life         of Life        Life span

   Type      Expectancy      Expectancy     Result


   LED       100,000         91             70% of original brightness

   LED        50,000         46             70% of original brightness

   LED        30,000         27             70% of original brightness


   CFL        15,000         14             Burn out

   CFL         6,000          6             Burn out


   ILB         2,000          2             Burn out

   ILB         1,000          1             Burn out


Though LEDs don't burn out, their brightness slowly decreases over their long life spans. LEDs might last for a thousand years but they will be operating at 70% brightness when they have reached the full term of their expected life.


So the question isn't "Why Do LED Lights Last So Long?", rather the question should be "Why do ILBs and CFLs burn out so soon?”.


Next article History of LEDs