Have You Ever Tried To Measure The Heat Dissipation of LED Light With Just An Illuminance Meter?

As we all know, thermal design is the most important process in LED production, as a good thermal design is essential to enabling the substantial lifespan of the LED product. To most LED lighting purchasers, even those with professional LED knowledge, it is difficult to determine if a lamp has thermal design defects without professional testing equipment to measure the LED junction temperature.

Purchasers tend to test a lamp’s heat dissipation performance by touching the surface of a lit lamp and judging the temperature by how it feels on their skin. Clearly, however, this method is unreliable. The skin’s perception of temperature differs from person to person, is also affected by various factors, so it is difficult to make an objective judgment. Equally importantly, low temperature on the lamp surface does not always equate to excellent design. On the contrary, the low temperature may even be caused by a narrow design of the heat conduction channel meaning that the internal heat of the chip cannot be conducted to the radiator surface in a timely manner, thus making the inside temperature higher (and lower outside) but leading to reduced lifespan. 

 

As mentioned above, determining the junction temperature of the LED lamp is a professional way to test the thermal design. However, the process to obtain the junction temperature is really rather complicated. So then, are there any easy but reliable ways to test if a lamp has a good thermal design?

Engineers at iLUXZ deal with many different LED products every day. In the initial stage of overall assessment, they tend to evaluate the thermal design of one particular lamp by comparing the initial luminous flux with the luminous flux when the heat of the lamp has stabilized. Experiments have proven that when the LED chip junction temperature increases, the luminous flux of the chip will decrease. When the light source has just been lit, the temperature of the chip is low and the lamp is at its top brightness. When the lamp remains turned on, the temperature of the lamp will increase and the luminous flux will decrease. However, this reduction is not an irreversible loss as the luminous flux will revert to the start state when the temperature of the chip falls back to low.

relative flux Vs LED die junction temperature

The radiator of the lamp plays a role to curb the chip temperature and preventing it continuing to rise, thus enabling the internal and external temperatures to reach equilibrium. When the internal and external temperature reaches equilibrium (usually it takes 15-30 minutes) luminous flux becomes stable. A good radiator helps the chip to achieve heat balance in a very short period of time, meaning that there is only a limited reduction in the luminous flux.

According to our experience, if the luminous flux decrease is less than 10% of the initial luminous flux, the radiator is fulfilling its function effectively. On the other hand, if the luminous flux after the thermal balance is much lower than the initial luminous flux (for example, less than 80%) then the thermal design of the lamp is very poor. 

Here I would like to share an easy method to you, which can test the change of the luminous flux with an illuminometer, by letting the probe of the illuminometer face the light emitting surface of the lamp, light up the lamp and record the initial value (Lux1) quickly. Then keep the lamp on for a period until the value is stable and record the illuminance value (Lux2). If Lux2/Lux1>90%, then the thermal design of the lamp is quite good; if not, it is poor. The closer the value of Lux2 and Lux1, the better the thermal design.

However, as an important reminder, this method need to be conducted in the environment with stable illumination, better in a dark room. You also have to limit the air convection to the minimum level. The reason is that the brightness change of the environment will affect the value (for example, clouds float over and block the sunlight out of the window), and air convection will enhance the heat exchange around the LED lamp which also lead to an inaccurate assessment. So it is better to get windows closed and air conditioning/heating/and or fans turned off.

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