Wednesday, September 19, 2012

Heat is the enemy

When LED lighting arrived on the machine vision scene, maybe a dozen years ago, the salesmen made all sorts of claims. LED lights, they told us, would last for 60,000 hours or more, their output would never drop, and they’d even make you, the buyer, more attractive to the opposite sex.

Well with the benefit of experience, we now know none of that is accurate. I can’t think of any LED installations that have lasted 60,000 hours, (that’s almost 7 years of 365 x 24 service.) I have found that light output diminishes – something especially evident when replacing one of a matched pair. (Note to self: always replace both lights.) And the only time an LED light increases my attractiveness is when it goes out and I’m plunged into darkness.

What’s gone wrong?

Drawing on my experience, I think the problem lies in our factory installations. I suspect that on the test bench the claims for LED lights are broadly true, but those conditions are far from what the lights see in a factory. Vibration and dust are both problems but I’m of the opinion that heat is the big killer.

As LEDs Magazine noted back in 2005, “…LED performance is measured yb [sic] the manufacturers under laboratory conditions, and is usually specified at a junction temperature of 25 °C, even though this is pretty much guaranteed to never occur in real situations.” And they added, “Most significantly, the junction temperature affects the lifetime of the LED.”

You may have noticed that some light manufacturers add pretty dramatic fins to their products in an effort to get rid of the excess heat, and I recall that a few years back a UK company introduced water cooled lights. Other producers use fans to improve airflow. But what they can’t control is how you install the light.

Five LED installation tips

  1. Use the biggest heatsink you can fit in the space available. I measured what this could do and found a reduction in temperature of about 4oC.
  2. Consider airflow: orient the fins of the heatsink so air can flow over them. That means vertically rather than horizontally.
  3. Build airflow into your vision enclosures. This doesn’t have to mean fans, just provide vents for the hot air to escape out the top.
  4. If in doubt, measure the ambient temperature around your lights. You may well be surprised how warm they can get at the end of a summer’s day.
  5. In extreme cases you might want to resort to some kind of chiller. Yes there’s a cost, but replacing LED lights is expensive too.

The bottom line

The tired old cliché tells us that lighting is the most important part of machine vision. Sure that means getting the geometry and wavelength right, but it also means keeping it stable and consistent. Paying attention to thermal issues can help you do that.

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