Tips for lighting vision inspection solutions
Along with camera placement, lighting is one of the most important aspects in a vision inspection system. So what makes for good lighting? Here are some tips from our experience.
There are numerous ways to light a vision inspection system. Choosing the right lighting for your vision system really will make a difference — even more so than choosing how to light your office or factory to give the best ambient light for productivity. That’s because this lighting style will impact on both the accuracy of the inspection and the time taken to inspect each item. Considering reliability and repeatability — not to mention speed — are key to automated machine vision inspection solutions, then getting the lighting right is fundamental.
The first thing to note about vision inspection solutions is that they create images by analysing the light reflected from an item — not by analysing the actual item. So that means you really need to understand just how light will reflect from what you want to inspect. Factors to think about here are the substrate (including finish), size and shape. Your application and goods produced will obviously have an impact on these. All these influence how the object will reflect light. Despite a vision solution being monochromatic, colour is also important because different colours reflect differently — even, for example, pink and red objects.
On the upside, it’s good to note that when you do take all these factors in combination, selecting the right light type of lighting can actually simplify what may start out as a difficult application.
Picking up the earlier comment about lighting style impacting inspection accuracy and speed, this is because the better the lighting type for your application, the easier it is for computer analysis to produce a workable image. Just as when you’re taking a photo yourself, the right light and a focused shot result in a good image; conversely, bad lighting and bad focus mean a bad shot. In the case of vision inspection, “bad lighting” can include uneven illumination and poor contrast, so the imaging computer has to do a heck of a lot more work. This will take more time and potentially reduce reliability and repeatability, likely resulting in a greater number of incorrect image analyses. Hardly the outcome you want for your automated machine vision inspection solution.
Typically, vision inspection systems focus on the differences between dark and light areas to form conclusions about what’s being inspected — the pre-set parameters — which could be edges, textures, markings and so on.
Images are created when light reflects from an object. Light striking the object and reflecting towards the camera is perceived as “bright”. Light that the object absorbs, or is reflected away from the camera, is “dark”.
Light wavelength comes into play because certain colours are more easily separated by certain illumination colours (that is, they’ll seem dark in the image). Enhancing date codes on jar lids is a great example; bruises on fresh produce are contrasted very well by infrared light; while ink or glue fluoresces well with ultra violet light.
Aligned, or “collimated” light rays produce sharp images. These can even be shone through frosted glass to avoid glare and shadows.
Two other things to take into consideration with lighting are how quickly the inspected objects are moving and the camera’s exposure time. Both these will impact what you choose as the light source.
So what are some different techniques of lighting? Each has its own positives and negatives, so it comes back to what object you are inspecting, and what you are inspecting for. The five basic techniques are:
- back lighting
- direct lighting
- structured lighting
- on-axis diffuse lighting
- off-axis diffuse lighting
Lighting your inspection system is fundamental to its success, and clearly there are numerous factors to consider. However, it all starts with accurately defining what you want the system to inspect. Once you’ve identified the machine vision’s aim, can the rest of the solution can be built — including choosing the right way to light it, so the “what” (be that features or defects) are captured properly in images.
The right lighting means the vision solution should handle images easily, with strong reliability and repeatability.
This area is complex, so the above is not an exhaustive explanation. For more detail, or a machine vision inspection solution that lights up your needs, please contact us via email or call 1300 IQVISION (1300 478 474). We have years of experience customising applications from the simple to the complex.
iQVision also has a host of information in our resource library, including case studies, whitepapers, FAQ’s, videos, our blog and brochures. They’re all free to download.