Generally, the junction temperature of the LED rises and the luminous flux decreases. Then, as long as we measure the illuminance change of the luminaire in the same position, we can reverse the change of junction temperature.

“Half-hour illumination method” indirectly measures junction temperature changes

First, choose a place that is not subject to external light and illuminate the fixture.

After lighting,take a illuminometer  immediately to measure it, for example we have 1000 lx here.

Then we keep the position of the luminaire and illuminance meter unchanged. After half an hour, take the illuminance meter and measure it.

If the measurement is 500 lx, indicating that the luminous flux has dropped by 50%. The insdie is hot enough. When your hand touch it is not hot, indicating that the heat is not coming out. It’s a bad heat sink.

If the measurement is 900 lx, the illumination is only reduced by 10%, which means that it is in a normal range, and the heat dissipation is very good.

The scope of application of the “half hour illumination method”

We will use Osram led chips example for  “Luminous Flux vs. Junction Temperature” curve of several commonly used chips.

From this curve, we can see how much lumen loss of the luminous flux can indirectly know how many degrees Celsius the junction temperature has risen.

Example 1

♠OSRAM S5 (3030) chip, the luminous flux is 20% lower than that at 25 C, and the junction temperature has exceeded 120 C.

Example 2

♠OSRAM S8 (50 50) chip, luminous flux is 20% lower than 25 ° C, junction temperature has exceeded 120 ° C

Example 3

♠OSRAM E5 (56 30) chip, luminous flux is 20% lower than 25 ° C, junction temperature has exceeded 140 ° C

Example 4

♠OSRAM SSL 90 white chip, luminous flux is 15% lower than 25°C, junction temperature has exceeded 120°C

As Seen from the above figures, if the thermal illuminance after half an hour is 20% lower than the cold state, we say that the junction temperature has exceeded the tolerance range of the chip. Basically, it can be judged that the heat dissipation system is unqualified.

For most LEDs, we can use the half-hour illumination method and the 20% drop to judge whether it is good or not.

Now have you get it? Next time when you make the selection of lamps don’t just look at the appearance of the lamps. Use our methods to check them and make the right decison.


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