Does LED size matter?

The machine vision industry is pretty small, so when it comes to developing new technology we tend to hang on to the coattails of the big boys. Lighting is a good example. I think I’m right in saying that no one is developing LEDs for machine vision. Instead, the vision lighting companies have to utilize what’s being churned out for the higher volume applications, like industrial and residential lighting.

These days it seems LED manufacturers are working on ever bigger LEDs, and many are finding their way in to machine vision lights, like the Monster series from Spectrum Illumination. The problem I have with these is that they are prone to creating hot spots. Interestingly, judging by the lens optics diagrams on their website, smartvisionlights has recognized this problem and tries to shape the light to reduce the problem.

Some engineers I work with take the view that a sufficiently thick diffuser is all that’s needed to smooth out the light, but in my view that wastes photons by reducing the total output. I would rather see a higher density of small LEDs, which is the approach taken by CCS in most of their lights. You might notice though that even they’ve jumped on the high intensity – big LED bandwagon.

Now I appreciate that mounting LEDs takes time, which it probably why CCS’s lights are among the more expensive, but I’ve also found the light they put out to be some of the most uniform. I guess you get what you pay for, and I’m willing to pay extra to get more even illumination from a higher quantity of lower output LEDs.

Protecting LED lights

Lighting vendors have dialed-back their claims for LED life. A decade ago they were all telling me I’d get up to 100,000 hours but now 50,000 to 60,000 is a more typical claim. In my experience though, even that is rather bold.

The issue is heat. LED’s generate quite a bit of it, and life falls as the LED junction temperature rises. (There’s a chart on page 8 of this Phillips Lumiled paper showing how a rise from 120 to 150^oC can cut life by 50,000 hours. Noribachi also publish an excellent paper: “LED Junction Temperature and Lifetime.”) So the key to long life is good thermal management.

I have to leave the internal design to the manufacturers, but I do try to ensure LED lights are mounted on aluminum heat-sinks. However, I have tended to overlook the interface between the two. Recently, I’ve realized that a thermal interface material could dramatically improve heat transfer, so that’s what I shall be including from this point on.

If you’re interested in learning more about thermal interface materials, I found a good tutorial on the Dow Corning website, and a selection of “cool” products at Indium Corporation. And yes, that was a very feeble pun.

Directional light

Ah yes, lighting. The bane of our machine vision lives. One of the challenges we face is that what works for item A on our conveyor may not work so well for item B, even though they come off the same machines and travel down the same conveyor. So when designing a system we spend hours … make that days … trying to find an optimal solution.

How about just using different lights? When space allows, this can be an effective solution, but sometimes it’s not possible. In desperation I have even resorted to placing a mask over regions of my lights, thus Mask A for item A and so on.

But wouldn’t it be easier to have control over each LED?

The RL28Q and RL16Q from Oregon-based Orled (ORegon–LED?) go some way towards this illumination nirvana. These ring lights provide independent control over four quadrants, so you can chose to cast a shadow in a particular direction, for example. (Something that might help with detecting topographical defects, for example.

Now my impression is that these lights are really intended for use with microscopes, where a person viewing would switch quadrant as necessary. But I’m pretty sure that, with a little ingenuity, segment-switching could be automated for a machine vision application.

And that might make lighting just a tiddly bit easier.

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 4^oC.
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.

Intriguing lighting configuration

A marketing email from camera-maker AVT included a link to German machine vision integrator Ziemann & Urban, so I followed the advice of Yogi Berra, (“When you come to a fork in the road, take it,) and clicked through to their website.

Good move, if I do say so myself. They have a ton of interesting application stories and videos to read and watch – clearly they know machine vision – but one I found especially interesting related to wheel inspection.

I’m not going to share the whole story – you’ll need to click through to “Automatic adjustment of wheel rims” for that – but I do want to share the photos of the lighting set up. (I don’t own the copyright but I’m hoping that Z&U will appreciate the free publicity I’m giving them.)

Now wheels are complicated things to image. They tend to be reflective and they have lots of complex geometry. I’ve seen massive cloudy day illuminators used for this kind of task but they tend to have a huge footprint. So let’s look at Z&U’s approach.

This first picture shows the camera surrounded by an array of linear LED lights. Separating the lights are aluminum baffles. I have to wonder if the baffles help create a kind of cloudy day effect.

The next picture shows the LED’s powered up. Interesting effect, don’t you think?

I’m not sure I can explain what’s going on, but if you look at the screen shots on Z&U’s website, it seems to work.

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!

LED lighting – not as stable as we’re told?

It’s probably ten years since LED’s started to make real inroads in the machine vision world. Up until that point we’d all used fluorescent and halogen lighting, but LED’s, so we were assured, were stable and long-lasting.

Turns out, that’s not strictly accurate. Yes, LED’s make a great light source, but they are not perfect. More specifically, heat is their great enemy. As an LED warms up its output drops. Not a lot, but perhaps enough to alter a critical measurement.

What’s the answer? Well CCS has a few tips on their website, under the heading of “Skillful use of LED lights.” Take a look and pick up a few ideas for your next project.

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!

Laser lines without the laser

Over the last few weeks there’s been quite a bit of discussion on these pages about how to deal with speckle. (And thank you to all those who have taken the time to comment.)

“Speckle” is an issue with laser lines, since it complicates the business of finding the line center. If you’re doing laser triangulation-based 3D machine vision, you’ll know just what I mean.

Several commenters have reminded me that an alternative to the laser does exist. Smart Vision Lights have a family of LED Structured Light Pattern Projectors, as indeed do Opto-Engineering. I’ve not tried the projector from Smart Vision Lights, but I did have an opportunity recently to get “hands-on” with the Opto product. This did a fine job of projecting a uniform line, but I have to say that it lacked the intensity of a laser. In a 3D application that might mean making a trade-off between speed and precision, but it’s definitely something worth exploring.

These projectors have been around for a while, but Opto-Engineering has now taken them further with the addition of Schiempflug optics. These “straighten out” the distortion caused by projecting a pattern of lines from an angle, and if you’re interested in learning more, there’s a great video on Opto’s YouTube channel.

The influence of heat on LED lighting

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.

White light for machine vision?

One of the current trends among manufacturers of LED lighting seems to be white light. Everyone seems very excited that they can offer white LED’s, and are rushing to fill the perceived gaps in their product lines.

Well today I’m going to tell you not to buy those white lights.

OK, there is an exception. If you’re doing color machine vision you probably need white light, BUT … as there’s no such thing as a white LED (LED emission is inherently monochromatic,) be sure you understand how the spectral distribution will affect the way colors appear.

So with that caveat out of the way, let’s talk about why I think white lighting should be such a no-no.

From my experience, there are two problems: chromatic aberration and wavelength-surface interaction.

Chromatic aberration refers to the fact that refraction is related to wavelength. So, as light passes through a lens, different wavelengths refract by differing amounts. Of course, the lens manufacturers know about this and try to minimize its impact, but the laws of physics are not easy to evade. Monochromatic light avoids this problem, so why not make life easier for yourself?

Wavelength-surface interactions are more complex. You know that IR light can pass through some materials, don’t you? Well rather than there being a simple transmit-reflect threshold there’s a gradual drop-off in transmission. If near IR light is transmitted through a material, then a fraction of red light will also transmit rather than reflect off the surface. This means that, depending on what you’re illuminating, a shorter wavelength may do a better job of actually revealing the true surface.

To reinforce this point, I recall reading about inspection of CD cases. The task was to find very small cracks, and the developers found that shorter wavelengths did a better job of creating good contrast. (If anyone has a link to the article, please share it with us.)

So the messages here are: don’t use white light unless the application needs white light, and don’t just assume that red light is the best tool for your particular job. Experiment to see what works best.

Now, what about white LED lighting for the chandelier in my hallway that I can’t reach without a tall ladder? That would be interesting.

Microscan on YouTube

I don’t want it seem like I’m plugging Microscan – they’re paying me nothing for this publicity – but building on my previous post about their lighting products, I’d like to draw your attention to their YouTube presence.

As an inherently visual technology, I’m surprised machine vision companies don’t make more use of YouTube for marketing and education. The Microscan movies don’t exactly take my breath away – I’d really like to see more “How to…” information - but they’re a start. Here’s an example of what I mean, and this relates to my previous post on their “Hi-Brite” lighting family.



If you look through a list of their movies – more than 50 – you’ll see the views are very low. That’s a pity because this is such a great way of getting the word out. Join me in encouraging other machine vision vendors to make more use of YouTube.

Thoughts on LED lighting

Few machine vision people will argue that LED lighting is the way to go. Longevity and stability make LED the most robust solution for the majority of applications, the exception being when you need great intensity, such as in web inspection. But even that is changing as LEDs get ever brighter.

One consequence of this evolution however is that thermal management is becoming more important. Let your LED light get too hot and it’s life could become as short as those halogen bulbs you’re trying to avoid using.

Obviously, machine vision users of LED lighting have a responsibility to limit the temperatures around their installations – don’t forget that camera hate heat too – but the manufacturers are also working on smarter lighting products. There’s a great overview of technology development in “LED Drivers Get Smart,” published in Design News June 22nd, 2010.

One issue that concerns me though is the way in which LED manufacturers will achieve this thermal control. The approaches discussed in the Design News piece seem to boil down to restricting the current and thus dimming the output. That’s all well and good in a domestic situation but it could play havoc with a vision system. And one problem we face is that our industry is not big enough to have any real clout with the LED makers.

So what do we do? I suggest two actions: watch out for such “smarts” being incorporated into your LED lights, and manage the temperature in the environment around your vision system so as to avoid getting into thermal issues in the first place.

Keep your (lights) cool

The advantages of LED lighting in machine vision are well known: stable output over a long lifetime. But did you realize that LEDs themselves are vulnerable to high temperatures? In fact a single spike over 100 degC in the junction temperature can dramatically shorten the lifetime of an LED lamp.

So what do you do? I suggest designing your system with thermal management in mind. For example:

• Mount LED lights securely to large aluminum rails where they can act as heat sinks.
• Keep the lights away from heat sources such as gas jets or induction heaters.
• Ensure that shrouding around the vision system has sufficient ventilation to maintain airflow.
• Run the lights at less than maximum power. If possible, strobe them to reduce the total duty cycle.
• When shopping for lights, don’t buy the cheapest you can find. Quiz the manufacturer about his design handles high temperatures.

Lighting manufacturers quote some very long lifetimes, but you won’t see these if you let your lights get too hot. Increase the reliability of your system by take a few minutes to consider how you’re going to keep the temperatures down.

Pricing LED lighting for machine vision

Staying with my previous plea for prices to be published, while browsing the LDDlight website I found that they make everything public. In fact they offer a complete online store, making it possible to cut out the distributor completely. However, having saved that 30% markup I’m somewhat surprised that their products aren’t a little less expensive.

I think I said previously that LDDlight seem to be cloning the high quality products from CCS, and that impression was reinforced when I stumbled across the line of thin dome lights. I thought CCS had this locked up with a patent or two covering their “Flat Dome” lights, so I imagine LDD have found a way through the claims.

Lighting tricks with wavelength

You know that lighting is the single most important part of any machine vision application. You also know that it makes sense to use LED lighting, thanks to the long life and relatively stable output, But how much thought do you give to the wavelength?

There’s no law that says you have to use light at 660 nanometers (a.k.a. “red light.”) In fact sometimes there are advantages from going out into the UV and IR regions of the spectrum, as discussed by Brent Evanger of Banner Engineering, in “Lights, Camera and Special Effects,” (Vision & Sensors magazine, November 30th, 2009.)

Brent gives a good summary of the benefits, although he does miss one point. In discussing the use of UV lighting, he covers only its use as a means of exciting visible wavelength fluorescence in the subject part. As some materials are naturally fluorescent that can be a neat way of doing presence/absence tricks, but there is another use too.

A consequence of the short wavelength is that diffraction effects are reduced. This means that if you need to do high precision gauging, and every pixel counts, UV light rather than red will give you a small but significant advantage.

But back to the magazine article … after exploring the value of wavelength, Brent also covers polarization. Now this is a really interesting topic because polarization, if properly understood and applied, can be a very powerful tool for solving “impossible” lighting problems.

I’m not going to steal Brent’s thunder by telling all here: use the link and read the source material: “Lights, Camera and Special Effects.”

New kid in town

When I got word of new lighting company LDDlight.com I will admit that my first thought was, "Great, another LED lighting company. Just what the world needs.” But now that I’ve perused their web site I’m willing to give them the benefit of the doubt.

First, some background: as best I can tell, LDDlight is a joint venture between IMAC and Symco of Japan and three ex-NERLITE employees. They launched on-line in October 2009 and appear to offer a full range of machine vision LED lighting.

Their unique selling proposition seems to be that they provide an on-line store with transparent pricing. In other words, there’s no calling up your local distributor to haggle over price, something that always leaves me wondering if I’ve been screwed (in the nicest way possible, of course!) So if you’re pricing up a system and need to include the cost of lighting, LDDlight make that very easy for you. On the downside, I have to say that cheap they are not. Pricing is getting in to CCS territory, but having not tested the product, I can’t say if the quality is comparable.

Also on their web site are a number of useful technical notes on machine vision lighting. For example, there’s a “Light Selection Checklist” (not so much a checklist as a diagram really,) and this interesting note on “Color Considerations.” A feature of the latter that caught my attention was a table relating scatter to wavelength: apparently, “The shorter wavelength and higher scattering rate of a blue light results in more apparent surface defects than that of a red light. “ That’s something I hadn’t thought about before.

LDDlight. You heard it here first.

So many lighting vendors

If you went to the Robots & Vision show determined to find a source for your LED lighting needs you’ll have been spoiled for choice. I noted CCS, AI, and Spectrum among the vendors competing for attention. All offer a range to meet 95% of applications; DOALs, rings, low angle, backlights, spotlights and so on. So how is a buyer supposed to make an informed decision based on technical criteria?

Or to put it more bluntly, aren’t most of us just buying on price?

If the machine vision lighting world wants to avoid cutting its own throat, the sales, marketing and engineering people have got to come up with some differentiation. It might help to appear more data-driven (after all, most engineers prefer numbers to opinions.) For example, how about some performance measures such as:

• Lumens per watt
• Lumens per square millimeter (at a ‘standard’ working distance)
• Homogeneity, in terms of lumens per millimeter variation over the working area

Pick fault in these if you like; I won’t pretend to have thought them through. I just want to make the point that without feature or performance differentiation all you’ve got to fall back on is price.

Even more on color

Don’t make the mistake of thinking you need a color camera to inspect a colored target. It’s often the case that you can create appropriate contrast by matching the color of the light to that of the target.

Last month, in “More on color,” I mentioned that you can lighten a surface by illuminating it with light of the same color, while an opposite color (refer back to the color wheel to find the opposite,) will darken the target.

Well, courtesy of LED lighting specialists CCS Inc., here’s an illustration of what I mean.

And when you’ve finished browsing that particular page I would encourage you to look at the test of their Technical Guide. It has some good information about using LED lighting.

Changing footprint?

For the uninitiated, StockerYale (SY) make machine vision lighting. While probably best known for their lasers, they also offer LED illumination, (I’ve been impressed with their COBRA line scan illumination product,) and a range of fluorescent lights. In previous years they’ve earned most of their money in the North American industrial market, but according to their Q1 2009 financial report, that may be changing.

CEO Mark Blodgett commented that they are seeing growth in defense and medical markets, that while not compensating for the decline in automated inspection business, is helping to maintain margins.

Diversification is good, but my suspicion is that this growth is happening within the US. In other words, SY are not taking advantage of opportunities in other parts of the world. The structured lighting marketplace is quite crowded and I can’t help thinking that it’s easier to fall back on domestic defense business than it is to go head-to-head with some of the European players such as Global Laser and Z-Laser. Fortunately, SY have avoided taking the axe to R&D, so perhaps they have some innovations up their sleeve.