When LEDs were first developed as an indicator light source for electronic products, they replaced miniature incandescent bulbs. Because the earliest LED illuminators produced negligible output power, they could only be used with monochromatic sensors and at short working distances in machine vision applications. However, the performance of other machine vision light sources at that time varied over their relatively short lifetimes, resulting in inaccurate imaging results and increased maintenance requirements.
While it is sufficient to flood a scene with photons in basic machine vision applications, turning up the lights in most machine vision applications has limited effects and can actually hurt performance. Since LEDs were photonically stable and could be turned on and off rapidly, which made them especially suitable for machine vision applications, Advanced illumination (Ai) developed the first commercially available LED lights for that purpose in 1993.
Ai’s pioneering design used a solid epoxy, dual inline package (DIP) molded with a diode and reflector cavity. This first LED package combined the low cost of a volume indicator lamp with the efficient light delivery provided by a molded-in lens. By using various lens shapes to produce a variety of beam angles, some with as little as a 6° Full Width Half Max (FWHM), these lights were an ideal choice for machine vision illumination applications.
When choosing LED lighting for a machine vision application, it’s good to start by considering the maximum radiant power the light can output on the target when running continuously at 100% power. To overcome the low radiant power of these early LEDs, Ai developed a patented process that concentrated the light from a number of LEDs, focusing it at a relatively small viewing area, which was appropriate for machine vision applications at the time.
During those early days of LED light development, Ai understood the ability to overdrive strobe (pulse) LED lights further improved their viability. Controlling the electrical impulses and thermal buildup of LEDs is not only essential to their performance, reliability, and lifetime, but it also helps minimize degradation and failure and ensures that the LED illumination system delivers maximum return on investment.
Low Duty Cycle Makes Overdrive Strobe Possible
Many machine vision applications in discrete manufacturing require only short exposure times for each image acquisition. This translates into a low duty cycle for the LED light and creates an opportunity to overdrive strobe the LED by 10 or more times its nominal, constant rated current. The ability to overdrive strobe those early LED lights further improved their viability in industrial machine vision applications.
With this in mind, Ai invented the first LED overdrive strobe controller in 1994 as a means of concentrating greater amounts of light. In addition to increasing LED light output beyond the LED manufacturers specified maximum, overdrive strobing has become a powerful machine vision illumination technique that has impacted the machine vision industry in many ways.
Overdrive Strobe Extends LED Life
Strobe overdrive pulse widths for LEDs typically last between one microsecond and a few milliseconds. Overdriving LED illuminators at such low duty cycles not only results in significantly higher light output during brief periods, but it also minimizes thermal buildup at LED junctions, extending LED life.
Overdrive Strobe Simplifies Lighting Designs
By dissipating heat adequately between high-current pulses, the LEDs avoid any damage or decrease in performance. This reduction in heat leads to simpler lighting designs that do not require heat sinks or require minimal heat sinks.
Overdrive Strobe Minimizes Filter Damage
Besides minimizing LED degradation and failure, overdrive strobe control also prevents continuous LED illumination from causing damage to delicate filters or polarizer films that may be used in the system.
Overdrive Strobe Allows for Faster Inspections
With shorter camera exposure times, more light is needed for image acquisition because there is less time for photons to reach the camera sensor. If constant illumination is used, a defined exposure may be able to freeze motion in some cases, but the images may suffer due to lower light intensity. By using overdrive strobe to increase light output, it is possible to maximize pixel sharpness during high-speed inspections by emitting short, high-intensity pulses of light that in combination with shorter exposure time can “freeze” motion during high-speed inspection applications.
Overdrive Strobe Increases Depth of Field
As stated previously, LEDs can be overdriven by 10 or more times their brightness rating in short pulses to eliminate motion blur in images of fast-moving objects. Since exposure time can be reduced when brightness is increased, the whole system can not only run faster but it may also be possible to reduce the aperture to achieve a greater depth of field by using a higher light output. This is useful in applications that require sharp focus across a range of working distances.
Overdrive Strobe Reduces Ambient Light Effects
Ambient light conditions frequently interfere with machine vision applications. By overdriving the system’s LEDs, the problem of ambient light impairing machine vision measurements can be overcome. For example, driving the LED at 200% doubles the light intensity, which halves the camera exposure time and reduces ambient light effects by a factor of four. This means that the camera only uses light from the LED source, not ambient light, during exposure.
Overdrive Strobe Enables Multi-Lighting Schemes, Reducing System Complexity
In many machine vision applications, LED lighting control enables camera station number reductions. In scenarios where it is possible to highlight particular features of an image using different lighting, one camera station may include several lights that pulse at different intensities and durations in a predefined sequence. Rather than using multiple camera stations, multiple measurements can be made at a single camera station. This reduces mechanical complexity and saves money. For example, a single camera could acquire images for bar code reading, surface defect inspection, and dimensioning in rapid succession by sequentially triggering three different types of lighting.
Overdrive Strobe Facilitates Computational Imaging
In addition to enabling multi-lighting schemes in imaging applications, overdrive strobe control can also be used to facilitate computational imaging. With photometric stereo, a type of computational imaging application, four different lights are fired sequentially at a component from four different directions. Combining the resultant images eliminates the effect of random reflections from the component surface and can amplify surface details when needed.
Ai has played a pioneering role in strobe control for LED-based machine vision lighting. As a result, we’re pleased to see how our work has impacted all the powerful machine vision illumination techniques that are in current use today.
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