Crisper images made possible by overdriving LED lighting
24 Jan 2022 | Reading time: 3 min
Sophie Steiner, Portfolio Manager for Optics and Illumination at STEMMER IMAGING
Interview with Sophie Steiner, Portfolio Manager for Optics and Illumination at STEMMER IMAGING
Selecting the right illumination is one of the key factors to optimise image quality and can also greatly simplify the subsequent image evaluation. LEDs are versatile, generate little heat and have a long service life.
Overdriving is a technique where the LED is activated beyond the continuous rating of the device for a short period of time. Our Portfolio Manager Sophie Steiner explains the benefits of overdriving and for which applications it is particularly suitable.
What does "overdriving" mean exactly?
Sophie Steiner: Overdriving is the intentional strobing of an LED light source while dramatically increasing the current flow beyond the normal continuous rating of the device at very short pulses without causing damage. The LEDs are only activated for a few microseconds and allowed to cool off for a significantly longer period.
This ensures the average current rating remains within specification and the device does not overheat. The light output can typically be increased by a factor of 2 to 5.
What are the benefits of this technique?
Sophie Steiner: At this point, I must begin with a little background information: To create a sharp, well-illuminated image, the amount of light falling on the sensor is very important. With more incident light you can have a shorter exposure time which reduces the blur in an image when the objects are moving.
The amount of incident light is also affected by the aperture of the lens. The wider the aperture, the more light reaches the sensor, however, this also decreases the depth of field.
So, having enough light available in the first place is clearly the best possible option. The faster the object moves, the shorter the exposure time required and the more light is needed in the short time period. This is exactly where I see several benefits of overdriving: Coupled with the camera via a trigger signal, the lighting is only activated when it’s really needed and switched off at other times.
It can be much brighter than in continuous operation and the result is a very crisp image. In addition, the light’s service life is considerably increased, as the strobe-like switching on and off produces only a fraction of the heat which accelerates ageing of LEDs, an effect that causes a decrease in intensity for the same input current over time.
And what are the challenges?
When using the overdrive technique, I strongly recommend shielding this area from manual workstations, no matter if you strobe with visible or non-visible light. Strobing visible light is irritating to the human eye, even at greater distances.
With non-visible wavelengths, it is even more important to shield the workplace, as the corneal reflex, also known as the blink reflex, is missing with this light. This can cause people working at the facility to unknowingly look directly into the flashlight, which can result in permanent eye damage.
It’s also very important that the strobing is kept within the specification limits not to cause permanent damage to the light. The use of automatic controllers that sense the light's specification is highly recommended.
What application areas benefit particularly from overdriving?
Sophie Steiner: Overdriving is particularly interesting for application areas where speed is paramount, for example, quality inspection of high-volume products moving at high speeds. In these areas, we often encounter further challenges such as variable object heights or vibrations, which require not only a short exposure time but also a small aperture.
Another typical application that comes to my mind is in the packaging industry: Overdriving is a highly efficient way of checking the correct imprinting of expiry dates or batch numbers on packaging.
What lighting do you recommend for overdriving?
Sophie Steiner: In general, I recommend paying attention to quality and, above all, compatibility for high-performance products. I prefer offering our customers the combination of CCS illumination with Gardasoft controllers. In my opinion, this is the most versatile solution and has already proven itself in many applications. The two manufacturers work closely together with the result that new requirements from one company are supplemented with product innovations from the other.
As an easy entry-level option, I can strongly recommend the CCS EL series of auto-strobe lights which manage the overdrive while for extra challenges, we can offer the CCS PF series, which achieves brightness levels comparable to those of a Xenon lamp.
Looking for more information on overdriving and other lighting techniques?
Download the whitepaper from CCS:
How to control an LED light source and the different forms of intensity control