The magic behind machine vision: Clever use of optics and mirrors

Sometimes, we’re tasked with some of the most complex machine vision and automated inspection tasks. In addition to our standard off-the-shelf solutions in the medical device, pharmaceutical, and automotive markets, we often get involved with the development of new applications for machine vision inspection. Usually, this consists of adapting our standard solutions to a client’s specific application requirement. A standard machine vision camera or system cannot be applied due to either the complexity of the inspection requirements, the mechanical handling, part surface condition, cycle time needs or simply the limitation to how the part can be presented to the vision system. Something new is needed.

This complex issue is often discussed in customer and engineering meetings, and the IVS engineering team often has to think outside the box and perhaps use some “magic”! This is when the optics (machine vision lenses) need to be used in a unique way to achieve the desired result. This can be done in several ways, typically combining optical components, mirrors, and lighting.

So what optics and lenses are used in machine vision?
The primary function of any lens is to collect light scattered by an object and reproduce a picture of the object on a light-sensitive ‘sensor’ (often CCD or CMOS-based); this is the machine vision camera head. The image captured is then used for high-speed industrial image processing. When selecting optics, several parameters must be considered, including the area to be imaged (field of view), the thickness of the object or features of interest (depth of field), the lens-to-object distance (working distance), the intensity of light, the optics type (e.g. telecentric), and so on.

Therefore, the following fundamental parameters must be evaluated in optics.
Field of View (FoV) is the entire area the lens can see and image on the camera sensor.
Working distance (WD) is the distance between the object and the lens at which the image is in the sharpest focus.
Depth of Field (DoF) is the maximum range over which an object seems to be in acceptable focus.
In addition, the vision system magnification (ratio of sensor size to field-of-view) and resolution (the shortest distance between two points that can still be distinguished as separate points) must also be appraised.

But what if the direct line of the site cannot be seen due to a limitation in the presentation or space available? This is when the magic happens. Optical path planning is used to understand the camera position (or cameras) relative to the part and angle of view required. Combining a set of mirrors in the line of site makes it possible to see over a longer working distance than possible without. The same parameters apply (FoV, WD, DoF etc), but a long optical path can be folded with mirrors to fit into a more compact space. This optical path can be folded multiple times using several mirrors to provide one continuous view through a long path. Cameras can also be mounted closely with several mirrored views to offer a 360-degree view of the product. This is especially true in smaller product inspections like pill or tablet vision inspection checks. Multiple mirrors allow all sides and angles of the pill to be seen, providing a comprehensive automated inspection for surface defects and imperfections for such applications. This technique is also applied in the subtle art of needle tip inspection (in medical syringes) using vision systems, with mirror systems to allow for a complete view of the tip from all angles, sometimes providing a single image with two views of the same product. The optical paths, the effect of specific machine vision lighting and the mechanical design must all be considered to create the most robust solution.

So when our engineers say they will be using “magic” optics, I no longer look for a wand to be waved but know that a very clever application of optics and mirrors will be designed to provide a robust solution for the automated vision system task at hand.

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