Whether it’s a machine vision system inspecting cans of food at high speeds or a label inspection or verification system on a fast-moving conveyor, the effects of strobing lights are felt by people who work near the system and see these lights on a regular basis. Smart Vision Lights explains how its Hidden Strobe technology for industrial applications, including machine vision, minimises the impact while allowing the vision benefits of strobe lighting.
Machine vision systems operate at increasingly high speeds, which means the lighting deployed in such systems must be capable of helping to capture the right images. Strobing (or pulsing) an LED light helps freeze images of fast-moving objects in applications, including logistics, consumer goods, and pharmaceutical manufacturing. From an imaging standpoint, LED strobe offers several benefits, but doing so also causes unintended environment hazards to nearby personnel, as these flashing lights can be disorienting and cause issues for employees. A new hardware feature, however, can minimise the negative impacts of strobing LEDs while allowing manufacturers to get the most out of their machine vision systems.
Even the fastest camera shutter speeds may be too slow to freeze the motion of fast-moving objects for high-speed machine vision applications. Even if the shutter speeds are up to the task, a light must be strobed for very short durations to minimise movement during exposure.
Certain applications, however, require more intense light. Performing optical character recognition (OCR) through plastic wrap, for example, presents a set of challenges. Plastic is highly reflective, which results in hot spots in the captured images that render the software incapable of reading a barcode underneath. Polarisers help reduce reflections but eliminate a significant amount of available light.
Running a light in ‘overdrive’ mode – where the LED is overdriven past a manufacturer’s rating, producing more light but also more heat – helps solve these problems. In fact, traditional overdrive mode can deliver light that is five times more intense than the same light running in continuous mode, while a new ‘dual overdrive’ mode offers ten times the light of a continuous mode. For this to work, the LEDs must be running in strobed mode.
The strobed mode can cause issues in unshielded machine vision systems for nearby operators and factory personnel. These lights can be disorienting and can annoy and distract employees. To reap the benefits of strobed lights in overdrive mode requires the use of shields around the machine vision system to protect operators, else the manufacturer will need to forego the benefits and use its LEDs in continuous mode. But opting for this method requires the use of five to ten times the number of total lights. A new Hidden Strobe feature solves all these problems and allows companies to take advantage of LEDs in their most powerful modes while still offering the convenience, ease of use, and safety of a continuous light source.
LED lights equipped with the new Hidden Strobe feature internally self-trigger thousands of times per second. The short triggered pulses occur continuously and follow the duty cycle of the individual light
upon which the feature is implemented. Since the human eye perceives light that flashes faster than 60 to 100 times per second as continuous, this feature allows human operators working near the system to see the light as continuous. When the light is triggered in sync with the camera to capture an image, the Hidden Strobe pulses stop, and the programming pulse for the camera is executed. After image capture, the light returns to continuously generated Hidden Strobe pulses to deliver a ‘continuously on’ appearance to anyone nearby.
Traditional overdrive pulses intended for camera exposure can have a separate duration than the faster Hidden Strobe pulses in the background. Nearby personnel will only notice a slight flicker as the light switches between the camera exposure pulse and the Hidden Strobe pulses in the background. This flicker is barely noticeable when compared to the same exposure pulse that would be used in the standard strobed operation.
Lights operating in standard strobe operation cause humans to adjust their vision to the darker light level present before a strobe flashes. The human body uses light for information. When the strobe flashes, however, it overpowers a person’s vision before their eyes can react. When presented with a quick burst of light, the human body attempts to quickly adjust it to gain information, but the process occurs so quickly that this is impossible. When a person is near a machine vision system where strobed flashes occur at the same repetition rate, the human body is continuously adjusting between the light level changes, which can be disorienting and distracting.
With Hidden Strobe, the opposite effect takes place. Human vision normalises to the brighter, near-continuous Hidden Strobe pulses. When a camera exposure pulse and Hidden Strobe pulse take place at the same time, a slight flicker may occur, especially if the exposure pulse is longer and has a longer duty cycle. In this case, the Hidden Strobe feature minimises the physical impacts on personnel and allows companies of all types to maximise the effectiveness of their machine vision lights without having to shield the light itself.
Available lights that offer the Hidden Strobe feature include Smart Vision Lights’ Lightgistics series lights, which feature dual overdrive technology – delivering ten times the brightness of continuous lighting.