inspect illumination changes within 30 minutes. (An illumination measurement in 30 minutes)
To judge whether heat sink performance of a LED lamp is good or not, the most direct method is to measure the temperature of led chips when the lamp is on whose technical term is “junction temperature”, and obviously it is only a theory.
Maybe someone says advanced technology for heat sink performance analysis through numerical technique is a good method. But for the junction temperature is hidden when the finished lamp was made. That is why we have to collect so many measurements to help judge the heatsink quality of lamps.
As we cannot directly measure the junction temperature, is there an indirect methods which can tell the junction temperature?Fortunately, when LED junction temperature rise, the luminous flux will decline.So, if we can measure the flux of the lamp in the same location, we can know the change of the junction temperature.
The particular instruction is:
- Choose a place which without external light interference, had better be in the evening, turn off the other lights.
- Measure the illumination lux in a location, write down the reading as the “cold illumination”.
- Keep the position of lamps and illuminometer unchanged, lamps keep on working too.
- Write down the data of the illuminometer after half an hour, as the “Hot illumination”
- If the two numbers are near, in a range about 10 ~ 15%, then the heat sink of lamp is good.
- If the two Numbers are far greater than 20%, then the heat sink of this lamp should be improved.
But what is the reasonable range of an illumination decline? That could only be depends on the performance of the LED chips.
A scope of application in “An illumination measurement in 30 minute.”
We enumerate the “luminous flux vs junction temperature” change curve of several commonly used chip, and from the curve, we can know how many lumen flux decline, and indirectly know junction temperature rose to how many degrees Celsius.
From the pictures above, it can tel, if the Luminous Flux decrease by 20%, the junction temperature was already over the tolerate range. And the heatsink system is definitely unqualified.
OSLON Square 1-5w chip [PDF], when the junction temperature was raised from 25℃ to 45℃, the illumination flux was raised. And then reached to 120℃, the heatsink performance only declined about 10%. So to this chip, when adopt this measurement way, the change rate should be only 5~8%.
Summary for performance judgement of the heatsink of led lamps:
- Thermal conductivity, heat dissipation are very important to LED
- The key point is junction temperature
- Suggest to “An illumination measurement in 30 minute.”
- The reasonable change scope is based on the temperature curve of the chip