Operating temperature effects on LED life & brightness

Operating temperature effects on LED life & brightness

For LED technology, from the LED chip to related products such as LED lamps, modules and fixtures, high operating temperature can result in mechanical failure and significant drop of performance.

How a LED behaves when subjected to higher operating temperatures is directly related to its quality. High quality LEDs (such as Nichia or Cree) will function within parameters at high temperatures too, while low quality LEDs will break down, change their color, loose brightness or a combination of these. 

For LED technology, we need to avoid operating at temperatures beyond those specified by the manufacturer. Failure to do so while lead to at least one of the following:
  1. complete failure of the LED
  2. light output is decreased permanently (Lumen Degradation) even if the issue with high temperature is solved
  3. light output is decreased temporally while the LED functions at high temperature
  4. the color temperature of the white LED changes

As explained next:

1. 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. Then can also be seen visually, in many cases, below a 4 chip LED with all permanently damaged.

2. LED light output is decreased permanently (Lumen Degradation)

Before the "weakest link" inside a LED reach the breaking point (as above) it, and the other materials, will suffer, gradual and permanent changes. These will lower the light output of the LED, permanently.

LED lamps or fixtures must be replaced when their light output has degraded by a certain amount, usually when the loss is 30% of the 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.

LED lifetime can be as little as 2000 hours in some cases.

3. 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.

There is no standard performance loss, the quality of the materials and of the design of the LED package determine it, with substantial 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).

4. 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.

 

An example that shows effects 2, 3 and 4:

With the above in mind we have selected the Nichia 757G LEDs as the LED of choice for most our Lumistrips. The proprietary Nichia design of the 757G has the best resistance to high temperature on the market and thus the best performance in most applications.

LumiFlex Nichia 757G Flexible LED Strips

 

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