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STROBING LED LIGHTING TO STOP MOTION AND INCREASE LED OUTPUT

WHEN DO WE TYPICALLY NEED TO STROBE AN LED LIGHT?

We typically need to strobe an LED light when the scene is of a moving object, as the pixel-blur in the captured image must be less than 1 pixel. Otherwise, the features we are dependent upon being in the image may not be properly and consistently presented and may appear at multiple points in the image. Any calibration will become invalid.

HOW TO CALCULATE THE MAXIMUM STROBE PULSE WIDTH TO MAINTAIN <1 PIXEL OF MOTION BLUR

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

Whether by reducing the camera’s exposure time or by strobing the light, we will minimize pixel blur due to motion, but 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

BASIC LIGHT TRIGGERING & STROBING

· When motion is involved, many internal and inline controllers can provide a basic strobe capability to freeze the motion of a slow-moving object, reducing blurring in the captured image.

· With most internal and inline drivers, the width of the light’s strobe output pulse corresponds to the width of the input trigger pulse.

· 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”.

· If a short enough duration trigger pulse can be used to stop the motion and if the light intensity is still sufficient to capture the image, basic strobing may be used.

WHEN ARE OVERDRIVE STROBE LIGHTS NEEDED?

In some applications, the amount of light intensity required exceeds the ability of the lighting configuration to deliver it. This “light starvation” is often the result of:

· Imaging an object in motion by reducing the camera’s exposure time.

· Strobing the light to minimize motion, but only at the standard maximum intensity.

· Using a high-magnification or other specialty lens system that does not transmit as much light to the camera sensor.

· Using polarizers or other filters on the lighting and lens, reducing total light intensity.

· Reducing the size of the lens aperture to increase depth-of-field.

LED OVERDRIVE STROBE FUNCTION INCREASES LIGHT INTENSITY 3X-10X

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

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 light. This will affect vision system performance, so all the lighting parameters need to be considered in advance.

Typical Maximum Duty Cycle is 3% to 5%, when overdriving the LEDs, 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 OPTIONS ARE AVAILABLE FOR LED LIGHT TRIGGERING?

There are two types of triggers to strobe the LEDs. The triggers will typically be provided by the camera or machine vision system to ensure proper timing with the image capture:

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 (link to page). Some controllers include strobe overdrive modes and the means to monitor current and light head feedback so that no damage is done to the light head.

An example of this is with Advanced Illumination’s proprietary SignaTech® (Signature Technology). A microprocessor-based controller in the Light Controller reads a SignaTech® memory chip embedded in and specifically programmed for each illuminator, to establish maximum operating parameters that cannot be exceeded through programming. The memory chip is preprogrammed with details about the illuminator’s circuit design as well as specific LED characteristics.

PROVIDING HIGHLY CONTROLLED, VERY SHORT AND BRIGHT STROBE LIGHTING PULSES WILL MAKE A NOTICEABLE DIFFERENCE IN THE QUALITY OF CAPTURED IMAGES OF A MOVING OBJECT.

There are many things to consider when putting together a lighting configuration. This web page is intended as a summary only and we encourage a conversation to discuss the application details.

Not all LED lights can be overdriven or strobed. Please contact R.J. Wilson, Inc. to discuss your application.