Machine vision lighting is one of the most critical elements in automated visual inspection. Consistent lighting at the correct angle, of the correct wavelength with the right lux level allows automated inspection to be robust and reliable. But how do you select the correct light for your machine vision application?
Where the inspection takes place on the production process has to be set according to the requirements of the quality check. This part of the process may also include what is often referred to as ‘staging’. Imagine a theatre, and this is the equivalent of putting the actor centre stage in the best possible place for the audience to see. Staging in machine vision is often mechanical and is required to:
- Ensure the correct part surface is facing the camera. This may require rotation if several surfaces need inspecting
- Hold the part still for the moment that the camera or lens captures the image
- Consistently put the part in the same place within the overall image ‘scene’ to make it easy for the processor to analyse
- Fixed machine vision lighting for each inspection process
Lighting is critical because it enables the camera to see necessary details. In fact, poor lighting is one of the major causes of failure of a machine vision systems. For every application, there are common lighting goals:
- Maximising feature contrast of the part or object to be inspected
- Minimising contrast on features not of interest
- Removing distractions and variations to achieve consistency
In this respect, the positioning and type of lighting is key to maximise contrast of features being inspected and minimise everything else. The positioning matrix of machine vision lighting is shown below.
So it’s important to select the machine vision light relative to the position of the light with respect to the part being inspected.
Machine vision lighting is used in varying combinations and are key to achieving the optimal lighting solution. However, we also need to consider the immediate inspection environment. In this respect, the choice of effective lighting solutions can be compromised by access to the part or object to be inspected. Ambient lighting such as factory lights or sunlight can also have a significant impact on the quality and reliability of inspection and must be factored into the ultimate solution.
Finally, the interaction between the lighting and the object to be inspected must be considered. The object’s shape, composition, geometry, reflectivity, topography and colour will all help determine how light is reflected to the camera and the subsequent impact on image acquisition, processing, and measurement.
Due to the obvious complexities, there is often no substitute other than to test various techniques and solutions. It is imperative to get the best machine vision lighting solution in place to improve the yield, robustness and long term effectiveness of the automated inspection solution.