Feedback on evaluating LED lights

Back in November I posted a few words on wisdom on how I go about assessing the quality and performance of LED machine vision lights from the many vendors out in the marketplace. The point I was trying to make was that most of us lack any sophisticated test equipment, so instead we have to make do with a few more subject methods.

Well the guys at Microscan took the time to write a detailed comment to my post, but as I’m not sure how many readers will actually go back to look at that article I thought I’d take the liberty of reproducing their comments here.

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 [regarding the need for good mechanical rigidity and mounting points]. 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 [regarding the importance of LED placement uniformity]. 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.

[On how to assess uniformity] For non-diffuse, short working distance area arrays and ring lights, this is basically true. [Small LEDs are to be preferred over big ones.] 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.

[Assessing value-for-money – I suggested calculating the price per watt.] 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.

This is great feedback, although I’m not sure how many of these are ‘actionable points’ that can be applied by the machine vision end-user. I think my main point is still valid: it’s really difficult to tell what makes one light worth double the price of an identically-sized product.

Keep those comments coming!