To the surprise of many, in the case of LED technology heat is the greatest enemy. From the LED chip to related products such as LED lamps, modules and fixtures, all have a major issue with high temperature. It brings imminent mechanical failure or significant drop of performance.
How the LED behaves when subjected to higher operating temperatures is directly related to the LED quality. High quality LED (such as Nichia or Cree) will function within parameters and high temperatures, while low quality LEDs will break down, change their color, loose brightness or a combination.
However, most people are not aware of this fact. They generally give heat and warmth a positive connotation. A warm embrace, the warmth of the sun or a fireplace. Even technology seem to contribute to this positive image with classic light bulbs and petrol engines as prime examples. Light or mechanical energy almost always seems to have heat as a byproduct. An obvious fact, we do not think much about it or even view it in a positive way.
The misconception that heat is good comes also from the fact that the sun and the classic incandescent or halogen lamps have a warm beam. The warmth comes from the significant amount of infrared radiation in their spectrum, which can be high enough to use incandescent lamps to keep food warm at fast food restaurants. In contrast, white LED and fluorescent lamps have a cool beam due to the absence of infrared radiation.
The spectrum of LED (Blue line), Fluorescent (Green line) compared with the Sun (Orange line) and Incandescent/Halogen lamps (Purple line)
Heat and LED technolgy
As most other light sources, LEDs require electricity to function and electricity generates heat. Unlike other light sources, this heat must be channeled away from the LED into the ambient with heat transfer technologies (usually heat sinks).
Such a setup is needed because for LED technology high temperature operation must be avoided at all costs. Failure to do so while lead to at least one of the following:
- complete failure of the LED
- light output is decreased permanently (Lumen Degradation) even if the issue with high temperature is solved
- light output is decreased temporally while the LED functions at high temperature
- the color temperature of the white LED changes
I will explain each in brief:
Complete failure of the LED
Most materials that compose a LED do not resist at high temperature and when the "weakest link" reaches the breaking point, the LED will fail. This concept is very easy to understand but only a small part of the whole story...
Before a LED can reach the above "end" state, important transformations happen to the materials is made of, with effects on lifetime, performance and light color:
LED light output is decreased permanently (Lumen Degradation)
Before the "weakest link" inside a LED reaches the breaking point, it and the other materials suffer, gradual and permanent mutations which lower the light output of the LED, permanently. We can easily understand this with an analogy with the tires on our automobile. As the materials that make up a tire are subjected to wear and tear, our car will not break as effectively or have difficulty with gripping the road.
As we must replace tires while they still allow the car to move about and not wait for a catastrophic failure, the LED lamps or fixtures must be replaced when their light output has degraded by a certain amount.
Usually, LED should be replaced when their have lost 30% of their initial light output. An equivalent statement is "the light output is at 70% of the initial value" and has the acronym "L70". This is what LED lifetime actually means and not the moment the LED fails completely. In the image below, the LED to the left has 35.000 hours lifetime and the one to the right (Nichia) 100.000 hours lifetime, both at a temperature of 85 °C.
The speed at which the materials inside an LED degrade to the point we need to replace it (lifetime L70, 30% light loss) is very dependent on the operating temperature, the quality of the materials and of the design of the LED package. LEDs could lose 30% of light in a few thousands hours or in over 200.000 hours, as the two graphs show.
To the surprise of some, LED lifetime can be as little as 2000 hours in some cases.
Light output is decreased temporally while the LED functions at high temperature
Temperature does not only have long term influence on LED lifetime, it also has immediate influence on its performance. To continue the analogy with tires and automobiles, winter tires have a gradually decreasing performance (grip, breaking) as the outdoor temperature increases beyond 7°C while summer tires have more and more issues as the outdoor temperature goes below 7°C and into negative territory. The 7°C temperature is not chosen at random but a industry standard based on testing and the properties of the rubber used in summer, all season or winter tires.
LED performance has a similar relationship with temperature with one important difference: there is no standard temperature value, no "7°C threshold" . Instead, the quality of the materials and of the design of the LED package determine it, with quite extreme differences for one LED brand to another.
For example at 75°C the performance of a LED can be 5%-70% less than what we would expect from it (what is written in the datasheet).
The color temperature of the white LED changes
Besides lifetime and performance, temperature also influences the color of the white light (color temperature). Poor quality materials or deficient design can change in such a way when exposed to high temperatures that the white light of a LED can become almost blue, permanently.
With so many variables and effects temperature has on both LEDs and the products that use them, the high variation in quality and great confusion in the market among buyers that are not experts in the filed come at no surprise.
In a next post we will use put this information about temperature and LEDs to a practical use: a shopping guide to high performance LEDs, strips and fixtures.