LED lighting is pretty much ubiquitous in machine vision these days, and for good reason: LED’s last forever and they’re totally stable.
Well that’s what we’ve been told, but it’s not completely correct. Yes, LED lighting is superior to fluorescent in that it’s a monochromatic source that shows very little intensity reduction with age, but it is vulnerable to the effects of heat.
Perhaps the biggest issue with LED’s is the risk of thermal runaway – they draw more power as they get hotter until they go bang (well actually they just die, but you get my point.) This is why it’s so important to use a current driver with LED lighting, and it’s also advisable to provide a big heat sink. But there are a couple of other issues too.
As noted in LED’s Magazine, light output reduces as temperature rises – another good reason for a big-assed heat sink – but what I find really interesting is that this effect is related to the wavelength of the LED itself. Specifically, red LED’s are much more temperature sensitive than are the blue ones. An argument for using blue or green lighting rather than red?
A second issue, noted in the LEDs Magazine article and reiterated on The Bergquist Company website, is that the wavelength of the emitted light is influenced by temperature. In fact LEDs Magazine notes an increase of as much as 0.13nm per degree C. Okay, in a factory environment that might mean a shift of only 2nm, but that could be enough to reduce the number of photons getting through a bandpass filter to your CCD.
So what to do? Well don’t give up on LED lighting: it’s the best we’ve got. Just be conscious that temperature can be a problem. I suggest good heat sinks and airflow are the way to build robustness into your vision system.
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1 comment:
Good post! I knew of output power derating versus temperature, but I hadn't much considered wavelength shift before. I'll bet the issue of compatibility with bandpass filters has fooled some people before. "We're losing light -- the LED must be dying!"
With that in mind, I went to Lot-Oriel's site. An interesting find (http://www.lot-oriel.com/files/downloads/andover/en/standardbandpassfilters.pdf) is that bandpass filters have temperature dependent wavelength shifts that are typically smaller magnitude than LEDs, but there is some overlap with certain types! Thus, if you're careful, you may be able to match the two up and live happily with reasonable temperature variations. We leave the details as an excersise. Meaning, of course, I'm too lazy to do it myself. :-)
Spencer
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