A guide to the different lenses used in industrial vision systems

Optical lenses are used to view objects in imaging systems. Vision system lenses are imaging components used in image processing systems to focus an image of an examined object onto a camera sensor. Depending on the lens, they can be used to remove parallax or perspective error or provide adjustable magnifications, field of views, or focal lengths. Imaging lenses allow an object to be viewed in several ways to illustrate certain features or characteristics that may be desirable in certain applications. Alongside lighting, lens selection is paramount to creating the highest level of contrast between the features of interest and the surrounding background. This is fundamental to achieving the greatest possible image quality when working with vision systems.

In this respect the lens plays an important role in gathering light from the part being inspected and projecting an image onto the camera sensor, referred to as primary magnification (PMAG). The amount of light gathered is controlled by the aperture, which is open or closed to allow more or less light into the lens and the exposure time, which determines how long the image is imposed onto the sensor.

The precision of the image depends on the relationship between the field of view (FOV) and working distance (WD) of the lens, and the number of physical pixels in the camera’s sensor.

FOV is the size of the area you want to capture.

WD is the approximate distance from the front of the camera to the part being inspected with a more exact definition taking into account the lens structure.

The focal length, a common way to specify lenses, is determined by understanding these measurements and the camera/sensor specifications.

The performance of a lens is analysed in reference to its modulation transfer function (MTF) which evaluates resolution and contrast performance at a variety of frequencies.

Types of lens

In order to achieve the highest resolution from any lens/ sensor combination the choice of the former has become an increasingly important factor in machine vision. This is because of the trend to increasingly incorporate smaller pixels on larger sensor sizes.

An ideal situation would see the creation of an individual lens working for each specific field of view, working distance and sensor combination but this highly custom approach is impractical from a cost perspective.

As a result there are a multitude of optical/lens configurations available with some of the key varieties shown below:

Lens Type	Characteristics	Applications
Standard resolution lenses	sensor resolution of less than a megapixel fixed focal lengths from 4.5 – 100 mm MTF of 70 – 90 lp/mm low distortion	most widely used
High resolution lenses	focal lengths up to 75 mm MTF in excess of 120 lp/mm very low distortion (<0.1%)	cameras with a small pixel size precise measurement
Macro lenses	small fields of view approximately equal to camera’s sensor size very good MTF characteristics negligible distortion lack flexibility – not possible to change iris or working distance	optimised for ‘close-up’ focusing
Large format lenses	required when camera sensor exceeds that which can be accommodated with C-mount lenses often modular in construction including adapters, helical mounts and spacers	most commonly used in line scan applications
Telecentric lenses	collimated light eliminates dimensional and geometric image variations no distortion images with constant magnification and without perspective distortion whatever the object distance to enable collimation front aperture of lens needs to be at least as large as the field of view meaning lenses for large fields of view are comparatively expensive	specialist metrology
Liquid lenses	Change shape within milliseconds enables design of faster and more compact systems without complex mechanics MTBF in excess of 1 billion movements Longer working life due to minimal moving parts	where there is a need to rapidly change lens focus due to object size or distance changes
360˚	view every surface of object with as few cameras as possible	complex image shapes
360˚ – pericentric lenses	specific path of light rays through the lens means that a single image can show detail from the top and sides of an object simultaneously	cylindrical objects
360˚ – Catadioptric lenses	sides of inspected object observed over wide viewing angle, up to 45° at max., making it possible to inspect complex geometries under convenient perspective	small objects down to 7.5 mm diameter
360˚ – Hole inspection optics	a large viewing angle >82° compatible with a wide range of object diameters and thicknesses	viewing objects containing holes, cavities and containers imaging both the bottom of a hole and its vertical walls
360˚ – Polyview lenses	provide eight different views of side and top surfaces of object	enables inspection of object features that would otherwise be impossible to acquire with a single camera

You need enough resolution to see the defect or measure to an appropriate tolerance. Modern vision systems use sub-pixel interpolation, but nothing is better than having whole pixels available for the measurement required. Match the right lens to the right camera. If in doubt, over specify the camera resolution, most machine vision systems can cut down the image size from the sensor anyway.