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basics of led strobing

WHEN SHOULD WE STROBE AN LED LIGHT?

We typically need to strobe the LED lighting when the scene is of a moving object. It is critical to limit the amount of pixel-blur in the captured image to less than 1 pixel. Otherwise, the features we are dependent upon for alignment or inspection may not be properly and consistently presented. Any calibration may become invalid.

The faster the object is moving, the shorter the strobe light pulse needs to be to maintain motion blur below 1 pixel in the captured image.

CALCULATING THE MAXIMUM STROBE PULSE WIDTH TO MAINTAIN <1 PIXEL OF MOTION BLUR IN THE CAPTURED IMAGE

A simple calculation of the maximum strobe light on-time required to maintain <1 pixel of motion blur is:

While reducing the camera’s exposure time or strobing the light will minimize pixel blur due to motion, we’re also reducing the amount of light available to the camera to create an image. It may become necessary to strobe in an overdrive mode to achieve sufficient light “intensity” to capture a high-quality image.

WHEN ARE OVERDRIVE STROBE LIGHTS NEEDED?

Sometimes the light intensity required exceeds the ability of the lighting configuration to deliver it. This “light starvation” is often the result of:

    • Imaging an object traveling at a higher rate of speed, which requires a shorter strobe pulse and higher light output to stop motion artifacts from being captured in the image.
    • Using a strobing light, but with the same intensity as a continuous use light.
    • Using a high-magnification or other specialty lens system that does not transmit light as efficiently to the camera sensor.
    • Using polarizers or other filters on the lighting and lens, reducing total light intensity available.
    • Reducing the size of the lens aperture to increase depth-of-field.

HOW DOES THE LED OVERDRIVE STROBE FUNCTION INCREASE LIGHT INTENSITY 3X-10X?

With basic strobing, the light’s intensity will not exceed the maximum “always-on” intensity of the light, but the light will “flash” for the specified period, effectively “stopping motion” in the captured image. If a short enough duration strobe pulse can be used to stop the motion and if the light intensity is sufficient to capture the image, basic strobing may be used.

The concept of overdriving LEDs is to send a higher-than-normal current to the LEDs for a shorter period, increasing the light’s intensity by 3X-10X during the shortened strobe output pulse without damaging the LEDs.

OBSERVING THE MAXIMUM DUTY CYCLE PREVENTS DAMAGE DUE TO EXCESS HEAT BUILDUP WHEN USING OVERDRIVE

Overdriving the LEDs increases the amount of heat created, so the maximum duty cycle, or percentage of on-time to off-time, will be specified by the manufacturer to maintain a safe operating temperature that won’t damage the LEDs. If the lights reach a dangerous temperature, there are built-in safety functions, on some lights, to shut down or attenuate the intensity so as not to damage the LEDs. This may affect the vision system’s performance and uptime, so all the lighting parameters need to be considered in advance.

When overdriving the LEDs, the maximum duty cycle is typically 3%-5% to minimize heat buildup on the LED. The duty cycle is typically calculated for 1 second of light on-off activity, as the target strobe rate is often provided in Hz or kHz.

Duty Cycle Calculation:

For special high-speed imaging situations, custom driver software may be developed by our vendors to push the limits of strobe rate and duty-cycle when needed, while maintaining safe operation.

WHAT TRIGGERING METHODS ARE USED WITH LED LIGHTING?

The trigger signal for the lights may come from the camera, a vision controller, or a PLC. There are two primary methods of triggering the LEDs, based on the light controller’s configuration:

1. The Strobe Output Pulse Width Follows the Trigger Input Pulse Width

    •  This method is typically used with lights offering built-in or inline strobe drivers.
    •  The camera’s output or a PLC will initiate the camera and light strobe pulse, dictating the width of the strobe output pulse based on the width of the trigger input pulse.

2. The detection of the leading edge of the trigger signal initiates the strobe output pulse, regardless of the trigger input pulse width.

    •  This method is used with most external light controllers.
    •  The strobe output timing can be configured independently of the trigger signal width, including for strobe output pulse width, any required trigger delay, and intensity control.

Additional information on LED Light Controllers is available on our Light Controllers page.

Not all LED lights can be overdriven or strobed. We encourage a conversation to discuss the application details.