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How LED Lights Performance Are Affected by Heat?

lighting design indoor

The misunderstanding of the heat of the lamps comes from the warmth of the sun and classic incandescent or halogen lamps.

In fact, its warmth comes from a large amount of infrared radiation in its spectrum. That may be high enough to use incandescent lamps to keep the food in the fast-food restaurant warm.

In contrast, white LEDs and fluorescent lamps have a cold light source because they do not have infrared radiation.

Then why do LED lamps heat up?

First, what is the cold light source?

The so-called cold light source is an illuminating light source that contains almost no infrared spectrum.

Spectral comparison of LED (blue line), fluorescent (green line)
Spectral comparison of LED (blue line), fluorescent (green line) and sun (orange line) and incandescent/halogen (purple line)

An LED is a cold light source, and the light it emits is almost visible light. The visible wavelength is between 380 and 780 nm.

The electric energy of the LED light source is converted into light energy of only about 30%, and the other energy is converted into heat energy, which is transmitted by heat conduction. Therefore, LED lamps will be hot.

led bulb cold sourse

The traditional incandescent lamp is the heat source. Traditional incandescent lamps convert only 2-3% of electrical energy into light energy (visible light), and the rest is radiated in the form of infrared light.

The cold light source is characterized by almost all other energy being converted into visible light, and other wavelengths of light are few.

The thermal light source is different. In addition to visible light, there is a large amount of infrared light, and a considerable part of the energy is converted into infrared light that does not contribute to the illumination.

So will this heat affect LED lamps?

The answer is yes. And some of the effects are irreversible.

The light output will temporarily decrease  at high temperature

philips-t5-lumen-vs-ambient-temperature

Temperature not only affects the life of the LED for a long time but also has a direct impact on its performance. Just as a mobile phone is below zero degrees Celsius, the response speed of the mobile phone will slow down and the battery will run out faster. The LED output will be affected to varying degrees when the LED is too low or too high.

The figure below shows the performance of several different brands of LED chips at different temperatures. At 75 ° C, the performance of LEDs may be 5%-70% lower than we expected (in the datasheet).

performance of several different brands of LED chips at different temperatures

Permanently reduced LED light output (lumen attenuation)

SOLERIQ® S | OSRAM

SOLERIQ® S | OSRAM

Before the “weakest connection” inside the LED reaches the breakpoint, it and other materials suffer a gradual and permanent abrupt change, permanently reducing the light output of the LED.

77 degrees Fahrenheit or 25 degrees Celsius is a near-universal temperature standard, and the lighting manufacturer calculates the L70 rating based on this standard. Typically, the LED should be replaced when it loses 30% of its initial light output. The equivalent statement is “Light output is 70% of the initial value(l70)”. This is the actual meaning of LED life, not the time when the LED is completely ineffective.

LED may lose 30% of the light in thousands of hours or more than 200,000 hours

The degradation of the material inside the LED to the speed we need to replace (lifetime L70, 30% light loss) depends to a large extent on the operating temperature, the quality of the material and the design of the LED package. As shown in the two figures, the LED may lose 30% of the light in thousands of hours or more than 200,000 hours.

The color temperature of the white light changes

In addition to lifetime and performance, temperature also affects the color (color temperature) of white light. Inferior materials or inadequate design can change when exposed to high temperatures, causing the white light of the LED to permanently become almost blue.

color temperature of the white LED changes

LED is completely faulty

Most materials that make up an LED will not resist at high temperatures, and the LED will fail when the “weakest connection” reaches the point of the break. This concept is easy to understand.
70% of LED faults come from excessive temperatures, and at half the rated power, the rate of failure doubles for every 20°C rise in temperature.

Common heat dissipation methods for LED lamps

In order to provide a practical cooling solution for high-brightness LED luminaires, the cooling technology must be low-energy and suitable for different types of luminaires, with a life similar to or higher than the source.

LED lamp housing heat dissipation depends on the power level and the place of use. There are mainly the following cooling methods:

Air hydrodynamic heat dissipation

Air fluid mechanics uses the shape of the lamp envelope to create convective air, which is the lowest-cost way to enhance heat dissipation.

Thermally conductive plastic case

1. Philips Hue LED Smart Bulb

Fill the thermally conductive material during injection molding of the plastic case to increase the heat conduction and heat dissipation capability of the plastic case.

Aluminum fins

high bay light fixture

This is the most common way to dissipate heat, using aluminum fins as part of the enclosure to increase heat dissipation.

Fan cooling

cooling_high_power_LEDs_fig2

The inside of the fan lamp housing is reinforced with a long-life and high-efficiency fan. However, it is more troublesome to change the fan. This design is more common in large sports lighting fixtures, often with aluminum heat sinking scales.

Conclusion

The temperature has so many variables and effects on LEDs.  It’s no surprise that buyers in non-professional areas have huge differences in quality and huge disruptions in the market.

When we buy lamps, especially for large lamps, such as floodlights, street lamps, and high bay lights, we must carefully ask the seller for relevant parameters:

  • Chip brand (Nichia, Cree or Osram has always been trustworthy)
  • Lamp shell material (die-cast aluminum material is reliable for heat dissipation)
  • Whether there is a built-in fan. (there is a fan generally means that the product generates a large amount of heat)
  • Warranty period (high-power lamps generally have a 5-year warranty)

Of course, if we buy low-power lamps, we don’t need to consider so many factors.

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