Controlling machine vision light output intensity

10 May 2018

Jools Hudson explains the importance of lighting control in machine vision applications.

Lighting control is vital for machine vision systems. The intensity of illumination influences how an object appears to the camera, and will affect all subsequent measurements and inspections based on the image. 

Dedicated lighting controllers can provide the control needed for consistent illumination, and can offer enhanced capabilities which can facilitate applications that might otherwise be impossible or reduce the cost of ownership of a vision system. 

Precision control 
For many machine vision applications, the challenge is delivering enough light to the object to allow the acquisition of a reliably, measurable image, especially in high speed inspection applications. However, there are numerous requirements where very low illumination levels are needed to avoid saturating the camera sensor. These typically occur in backlit applications where work pieces are being imaged in silhouette and the background illumination is bright; for brightfield illumination of highly reflective objects; transmission inspection of transparent objects; and test equipment for high-sensitivity sensors.?

Accurate control of LEDs is achieved by regulating the current applied, rather than the voltage, because the light output from an LED is directly proportional to drive current, not the voltage. Indeed, a small change in voltage will cause a much larger change in the current through the LED, resulting in a large change in brightness. Even when using current control, regulating very low light output is a challenge because LEDs respond to very tiny currents. 

Just 10µA of current can cause an LED to illuminate and create a proportionally very large change in output intensity when low intensities are needed. It is therefore important to control all light leakages and prevent the LED from illuminating at the wrong time. 

Control systems must have very low noise and achieve an output that is stable within a few µA. This performance is orders of magnitude better than most electronics can achieve and gets close to the limits of physics. In addition, the controller must have all of the functionality of a traditional lighting controller, with multiple output channels, fast accurate pulsing and safety features. 

The S106 low power lighting controller from Gardasoft, for example, is designed to minimise the effect of noise, errors and crosstalk and achieves accurate ultra-low light control in continuous or pulse modes. It offers 16 independent constant-current output channels which each have two current ranges that are software-configurable. The low current range operates from 7 µA to 10 mA in increments of 3.5 µA and the high current range operates from 25 µA to 90 mA in increments of 25 µA. 

Control in action
One application involved inspecting batches of components with very large variations in surface finish. Shiny parts required very low lighting to prevent the camera from saturating, while parts with a dark matt finish required much brighter lighting. The lighting controller was configured in the low range to supply a stable drive of only 90 µA for inspection of the reflective parts, and to operate at 10 mA in the high range for inspection of the matt parts.

Variations in light intensity can have an impact on machine vision measurement repeatability. Both internal factors, such as the age and temperature of the LED, and external factors, such as dust and dirt, can affect LED intensity. Even with a simple caliper measurement, a 10% reduction in light level can cause a change of up to 0.5% in the measured values. 

So, when developing machine vision solutions which require very low and precise lighting levels do consider the use of lighting controllers designed specifically for the job.

Jools Hudson is marketing manager at Gardasoft Vision

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