In a machine vision system, the timing of the lighting often determines whether an inspection is sharp, reproducible and reliable. Many LED lights can be connected directly to 24 V DC, and for simple or slow applications, continuous lighting may be sufficient. However, in many industrial inspections this is not the optimal way to use light.
The camera only needs light during the effective exposure time of the sensor. Outside that short moment, the lighting does not contribute to the image, while the LEDs still consume power and produce heat. This heat affects the lifetime of the LEDs and can make the light output less stable. That is why machine vision lighting is triggered or strobed in many applications.
Why the camera should usually trigger the lighting controller
An important point in the system architecture is that the lighting controller is usually not triggered directly by the PLC or product sensor. The sensor or PLC can start the image acquisition, but the lighting is preferably switched by the camera.
In many vision systems, the correct trigger chain looks like this:
- sensor, PLC or encoder to camera trigger input
- camera exposure output to lighting controller
- lighting controller to LED lighting
The reason is that the camera knows exactly when the sensor is actually exposing. A PLC or product sensor only knows that a product position has been reached, but not when the camera internally starts the exposure. Between an external trigger signal and the actual exposure of the sensor, delays can occur due to trigger delay, exposure settings, sensor timing and internal camera processing.
PLC and sensor remain important, but not as the direct strobe source
If the PLC triggers the camera and the lighting at the same time, the light pulse may occur too early or too late. With long exposure times this may go unnoticed, but with short strobes in the microsecond range it can quickly lead to variable brightness, dark images or unstable measurements.
In practice, the PLC or product sensor monitors the machine position, while the camera determines the lighting timing. The lighting controller follows the exposure output of the camera and not the machine cycle directly. This is especially important with short exposure times, fast conveyor belts, overdrive strobing and applications with global shutter cameras where every image must be captured under the same lighting conditions.
Why continuous lighting is technically limited
Continuous lighting is simple. The LED lighting is connected to a power supply and remains on as long as the machine is running. The disadvantage is that the average power remains high, even when the camera only captures images during a small part of the time.
This has two consequences. First, energy is wasted. More importantly, the LEDs are continuously thermally loaded. LEDs lose light output as they age, and a higher operating temperature accelerates that process. With compact ring lights, dome lights or powerful line lights, heat dissipation can become the limiting factor.
Triggered lighting reduces the average load. The LEDs only emit light during image acquisition. As a result, the lighting stays cooler, lifetime is extended and light output remains more stable. This stability is important in image processing because thresholds, edge detection, OCR and contrast measurements depend on reproducible grey values.
Motion blur determines the maximum exposure time
With moving products, the exposure time cannot be chosen freely. A product on a conveyor belt moves during the exposure. If that displacement becomes too large compared to the resolution on the object plane, motion blur occurs. In machine vision, this is often referred to as pixel smear.
The practical relationship is:
motion blur in pixels = product speed × exposure time / object resolution per pixel
This results in:
maximum exposure time = allowable blur in pixels × object resolution per pixel / product speed
Suppose a camera sees 0.1 mm per pixel on the product and a maximum of 0.5 pixel motion blur is acceptable. At a product speed of 1 m/s, or 1000 mm/s, the maximum exposure time becomes:
0.5 × 0.1 mm / 1000 mm/s = 0.00005 s = 50 µs
This means that the camera must receive enough light in only 50 microseconds. A continuously operating light is often not powerful enough in that situation, unless the aperture is opened wide or the camera gain is increased significantly. Both solutions have disadvantages. A larger aperture reduces depth of field, while more gain adds noise and reduces measurement reliability.
Lighting, camera and lens must be selected together
The example shows why lighting, camera and lens must always be evaluated as one system. Higher resolution often means smaller pixels or a lens with higher magnification. This increases the need for light. A shorter exposure time prevents pixel smear, but requires more light in a shorter time window.
The aperture of the lens also plays an important role. A smaller aperture improves depth of field, but transmits less light. As a result, powerful lighting or a strobe solution becomes necessary more quickly. The right choice therefore does not start with maximum light intensity, but with motion, resolution, exposure time and required depth of field.
Strobing makes short exposure times practically achievable
With strobing, the LED lighting is switched on for a short pulse. The pulse is synchronized by the lighting controller with the exposure output of the camera. As a result, the light falls exactly within the time window in which the sensor collects photons.
The advantage is that the light energy is concentrated in the image acquisition. Instead of providing continuous light, the lighting delivers short and reproducible pulses. This reduces average power and makes it easier to capture sharp images of moving objects.
The timing remains critical. The light pulse must be long enough to cover the full exposure, but not unnecessarily long. A pulse that is too short causes underexposure or variation between images. A pulse that is too long increases the duty cycle and limits the ability to safely drive more current through the LEDs.
The lighting controller becomes part of the vision system
In fast applications, the lighting controller is not just an accessory, but a functional part of the vision system. The controller determines when the lighting switches on, how long the pulse lasts, with which current the LEDs are driven and within which thermal limits the system remains safe.
The choice of lighting controller therefore directly affects image sharpness, LED lifetime and system stability. This is especially true for applications with high line speeds, short exposure times or highly reflective products where small light variations are immediately visible in the image.
Overdrive strobing for more light from the same LED lighting
In addition to standard trigger controllers, overdrive strobe controllers are also available. These drive more current through the LEDs for a short period than would be allowed during continuous operation. As a result, the lighting temporarily produces more light. In practice, this can often provide approximately two to three times more light, depending on the lighting, controller, pulse duration and duty cycle.
Overdrive strobing is especially useful when the maximum exposure time is very short. Instead of extending the exposure and accepting motion blur, the light intensity is increased during the short exposure. The image remains sharp, while the camera requires less gain and the lens does not need to be opened unnecessarily far.
Overdrive strobing requires safe limitation
Overdrive strobing has clear limits. Not every LED light is suitable for increased peak current. The maximum current, maximum pulse duration and maximum duty cycle must be taken from the lighting documentation. If too much current flows through the LEDs for too long, the lighting can fail or degrade faster.
A common mistake is to look only at peak current. However, the thermal load is determined by current, pulse duration and repetition rate together. A 100 µs pulse at one image per second is thermally very different from the same pulse at 500 images per second. That is why an overdrive strobe system must always be evaluated at application level and not only at component level.
Why lighting and controller should preferably match
Many modern controllers can measure or recognize the connected lighting during startup. Based on this, they limit the current over time. This makes overdrive strobing safer because the controller prevents the user from operating outside the safe limits of the LED lighting.
This is an important reason to preferably choose the controller and lighting from the same brand or system. The manufacturer knows the electrical and thermal limits of the lighting and can match the controller accordingly. With separate combinations, the engineer must verify that current, pulse duration, duty cycle, connectors and protection functions are correctly matched.
In practice, a matched combination reduces the risk of damage and shortens commissioning. This is especially relevant in machines where recipes change, line speeds vary or operators adjust settings later.
DCM iBlue lighting with integrated strobe control
With DCM lighting using iBlue drive, the lighting controller is integrated into the lighting. This simplifies the system design because no separate controller is needed between the power supply, camera and lighting. The camera can control the lighting directly via a trigger or strobe signal, while the internal electronics monitor the LED load.
The automatic mode is especially valuable when maximum light output is needed without requiring the user to manually calculate all current and duty cycle settings. The iBlue drive adjusts the control to the trigger cycle of the application and aims to deliver as much light as possible within safe limits.
For technical beginners, this reduces the risk of incorrect settings. For experienced users, the main advantage is that the lighting can be set up more quickly and reproducibly. Via software, automatic functions can be disabled and exposure time and current can be set manually when the application requires it.
TMS lighting with external lighting controllers
TMS lighting is often combined with external lighting controllers. This provides more freedom in system design, especially when multiple lights, higher power levels or specific strobe settings are required. The disadvantage is that the engineer has more responsibility for selecting the right combination of controller and lighting.
The controller must be able to supply enough current, but must not drive the lighting outside its safe limits. It must also match the required pulse duration, trigger frequency and duty cycle. A controller suitable for a small ring light is not automatically suitable for a powerful line light. Conversely, a powerful controller can be dangerous when connected to a smaller light without proper limitation.
The advantage of TMS with an external controller is mainly flexibility. In fast applications, a short and powerful pulse can be used to prevent motion blur. In inspections with multiple lighting angles, different channels can be controlled separately. In more complex setups, the controller can become part of a lighting strategy in which every image is captured under exactly the same lighting conditions.
Multi channel controllers for colour and segment control
In addition to simple trigger and strobe controllers, multi channel controllers are also available. They are used when different LED colours or different segments of a light need to be controlled separately. This is technically relevant because not every defect becomes visible with the same wavelength or lighting direction.
A scratch on a glossy surface may become more visible with angled segment lighting. A colour difference may respond more strongly to red, blue or infrared light. A relief defect may become visible by deliberately creating shadow from one direction. The controller then makes it possible to use light not only as an on and off function, but as an active measurement tool.
The limitation is that multi channel control adds extra timing complexity. When multiple colours or segments are strobed one after another, the camera must capture multiple images or know exactly which channel belongs to which image. At high transport speeds, this can limit the maximum inspection speed. The additional contrast information must therefore be balanced against cycle time, data volume and software complexity.
A good lighting choice starts with the application
The correct design sequence starts with motion and the required resolution, not with the nominal light intensity of the lighting. First, it must be clear how fast the product is moving, how many millimetres per pixel are required and how much motion blur is acceptable. This determines the maximum exposure time. Only then can it be determined how much light is needed and whether continuous lighting, triggering or overdrive strobing is required.
This sequence prevents incorrect component choices. A camera with higher resolution may seem attractive, but it may have smaller pixels and therefore require more light. A lens with greater depth of field often requires a smaller aperture and therefore stronger lighting. A shorter exposure prevents pixel smear, but makes a strobe controller or overdrive lighting more likely.
In practice, this leads to better vision systems and better hardware choices. Not because the most expensive lighting is automatically the best, but because lighting, controller, camera and optics are selected as one system. This prevents an application from failing during testing because of blur, noise, thermal drift or insufficient light reserve.
Conclusion: lighting is part of the timing architecture
Triggering and strobing make machine vision lighting a precise part of image acquisition. The PLC or product sensor usually determines when a product is present, but in most systems the camera should determine when the lighting actually switches on. Only then does the light pulse reliably coincide with the sensor exposure.
Continuous lighting can work in simple applications, but with moving products, short exposure times and high inspection speeds, a triggered or strobed solution is often necessary. Overdrive strobing makes short exposure times practically achievable, but only when current, pulse duration and duty cycle are safely controlled.
DCM iBlue lighting offers an integrated solution in which the strobe control is built into the lighting. TMS lighting with external controllers provides more flexibility for systems with specific power levels, channels or lighting strategies. In both cases, the same technical principle applies: the lighting must match the camera, the lens, the product motion and the timing of the application.
A reliable machine vision system is not created by simply adding more light. It is created by using light at exactly the right moment, with the right intensity and within safe electrical limits.
