Tuesday, November 22, 2011

How to evaluate machine vision lights

Go to a vision show and you’ll find any number of vendors showing off LED lighting. They’ll have red lights, blue lights, green lights, (not so many IR lights though – I wonder why?) and all sorts of geometries: backlights, darkfield ring lights, line lights and so on. Then, as you get into discussion with the sales guy you’ll learn that prices range from the somewhat expensive to the Oh-My-God-that’s-more-than-the-profit-we-reported-last-year. So how do you decide what represents good value?

The main criteria should be how well the light illuminates the object or area your vision system will be looking at. The problem I have though, is that without specialized equipment it’s virtually impossible to quantify uniformity of light distribution, or indeed, the spectral distribution.

So here are my guidelines:

  • Hold the light in your hand, (with it turned on – duh!) If it feels really hot, that’s a bad sign. Heat is the enemy of LED’s.
  • Examine the mechanical assembly. Is it rigid? Does it have good mounting points? Will it endure years of vibration? Can it be cleaned easily?
  • Study the LED placement. How uniform is the pitch? The height? Are any pointing off-axis?
  • While it’s difficult to assess uniformity, I prefer to see more small LED’s rather than a few honkin’ big ones. This tends to reduce the incidence of hot-spots.
  • And finally, value. While it’s a somewhat sweeping assumption, I tend to believe power consumption correlates with light output. So work out the price per watt. What this might show is that the differential between the cheap and the expensive light is not as great as it appears at first.
Obviously, none of this is as good as objective measurement of light output. But unless you’re going to buy a whole lab of test gear, I suggest these checks will help in determining which LED lights offer the best value for money.

Last, a quick note to my readers outside the US: this Thursday is Thanksgiving, so I'll be taking a break for a few days. Check back after the weekend!

1 comment:

Microscan said...

Great blog post! I asked our NERLITE veteran engineers for their perspective; here is their reply:
While it is a fact that the cooler a particular LED is the longer it will last, this is much too broad of a generalization. Different LEDs have a very wide range of maximum operating temperatures. If the criteria is “hot to the touch”, some LEDs may already be past their limits while others might not be even close. This used to be a good "rule of thumb" in the early days of LED illumination, but devices have significantly evolved and heat does not always mean a bad design. For example, Microscan Nerlite’s new Smart Series DOALs run hotter than our previous design and are warmer to the touch, but the LED's are robust enough that we were able to increase intensity, improve uniformity, eliminate cooling fins and operate the unit at a 10 degree C greater maximum ambient temperature.
We agree. For example with the Smart Series product line, we were able to take into consideration feedback from application engineers and customers to incorporate more common mounting points while maintaining backward mounting compatibility to prior product. Each design was subjected to shock and vibration testing to ensure mechanical stability in the field and we also took into consideration IP ratings that are common to each product family. In terms of cleaning, it is important to note all units with beam splitters, regardless of manufacturer, have special cleaning requirements. This is due to the delicate nature of the coatings used on the beam splitters.
We agree. If the LEDs are not positioned uniformly, the output will not be uniform either. With non-diffuse, focused LED arrays, the trueness of LED placement makes a big difference and dictates uniformity of light coverage. For example when Nerlite products require specific LED alignment, we either incorporate the alignment feature into the design, or specifically design alignment tools to be used at the time of assembly.
For non-diffuse, short working distance area arrays and ring lights, this is basically true. It does not take into account the output angle of the LEDs’ optics. Even packed tightly together, an LED with a very narrow output angle can still be spotty. LEDs with a wide output pattern can be more uniform. This is true when considering the uniformity of non-diffuse, focused LED arrays, but when it comes down to having a truly diffuse light source, seeing "any" type of LED is not a good sign. Nerlite has applied many techniques, often patent protected, to achieve superior uniformity regardless of LED size or quantity. One final note is that for area arrays designed for long working distances (Hi-Brites), the overlap that occurs over the long distance makes LED density less of an issue.
This may be true when comparing "apples to apples", but Machine Vision Illumination has entered a new era where more technical features are designed into the product itself. This simplistic approach also does not take into account the quality and integrity of construction, IP rating, optical design, efficiency of the electronics and optics, ease of use (mounting options, accessories, etc.), warranty, and so on. Even if all other factors are equal, some LEDs are more efficient than others. For example, the new smart rings use about the same amount of power as their predecessors, yet they contain 25-33% more LEDs and have 3-5X the light output depending on the model. Since the price and power consumption is about the same, by this criteria, one would judge the old and new units to be equal. This is, of course, totally false. Whether it's just simple circuit protection or the comprehensive control features built into Smart Series Illuminators, there are vast differences and sometimes you truly do get what you pay for.