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Intelligent lighting for the 21st century | Part 3

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Illumination

Selecting and correctly configuring the optimal illumination setup is one of the key factors when designing a vision system. Missing image information caused by incorrect illumination makes subsequent analysis much more difficult or even impossible, as no analysis algorithm can recover information that has never been captured.

Many years of experience show that this most critical element of a vision system is very often underestimated. It requires experience and extensive knowledge to select the appropriate illumination for a demanding task, as well as understanding why different types of illumination are best suited for specific applications. Even when equipped with this kind of expertise, an evaluation trial is typically a valuable step to gain confidence in the reliability of an approach. At STEMMER IMAGING, we maintain an evaluation laboratory with a wide selection of illumination equipment. This allows customers to either visit us and test illumination options with our engineers, or send samples in for us to undertake a feasibility study.

A crucial challenge when designing a lighting configuration for an application is to ensure that the variation of environmental light does not affect the inspection result. In practice, this means the design should consider all elements of the optical path, such as optical filters and lenses.

Heavy multi-stage preprocessing of image data is typically an indicator that the illumination approach should be re-evaluated. Optimising lighting – and therefore revealing properties physically – can often eliminate the requirement for costly and time consuming image preprocessing, while giving far better and more stable results.

As image sensors are not able to adapt to widely different environmental conditions like the human eye, an illumination set-up has to aid in highlighting the properties of the object considered as errors. Cameras do not actually see objects. They are recording the light reflected by objects. By properly controlling the illumination configuration, the appearance of objects seen by the camera can be controlled.

As an example, light is reflected differently from a ball bearing than from a flat white label or a printed circuit board. Therefore different illumination techniques are required for each.

In the early days of vision, illumination systems were often hand-made for each application. Nowadays, rather than trying to design and develop custom illumination systems, there are many benefits to using off-the-shelf illumination solutions:

  • Cost-effective
    saves time, money and R&D resources

  • Reliable
    reliable designs include thermal modelling to ensure a long service life.

  • Proven
    thousands of models in the field with minimal failures

  • Repeatable
    LED diode matching during manufacture ensures consistent intensity with even illumination.

  • Large variety
    many different devices and techniques are available


Important considerations

There are a number of considerations when determining which type of illumination is best suited for a particular job:

  • is it a monochrome or colour application?
  • how fast is the object moving in front of the camera?
  • what physical size (area) needs to be illuminated?
  • what is the nature of the object ‒ geometry, structure, reflectivity, colour?
  • which features are of particular interest?
  • what does the service life of the light need to be?
  • what are the mechanical constraints and environmental considerations to apply?
  • is feedback with respect to light degradation helpful to prevent unexpected system down time?

The answers to these questions determine which light solution is best for a given application.

The following sections give a brief explanation of the technologies on which common illumination products are based. Due to the outstanding technical advantages of LED and laser illumination for imaging and machine vision solutions, a focus is put on these light sources.