Optics: Lens Types
Fixed focus lenses
For the vast majority of industrial applications with a fixed object size and a fixed working distance, the most common type of lens used is a fixed focus model. Because they are designed for a fixed set-up, they offer the best optimised image quality. Adjustment on these lenses is usually limited to the focus and/or iris.
Zoom lenses change their focal length by changing the position of the lens elements in relation to each other. This allows various magnifications to be achieved, making them ideal for use in dynamic environments. For precise measurements or applications with high repeatability they tend not to be ideal due to their flexible construction and difficulty in repeating exact settings.
Some zoom lenses can come with presets which are resistors which provide feedback to monitor the position. They aid repeated set-ups but are not precise, while some precision motorised macro lenses use stepper motors to deliver a precise setting of the magnification.
One feature of a zoom lens when set-up correctly is that they stay in focus as the zoom (magnification) is changed. You may also see the term varifocal lenses. These are similar to zoom lenses but change focus when changing zoom. This makes them suitable for fixed applications where the actual focal length needed might not be known before installation, but will not change after installation. As this is not normally the case in machine vision they tend to be rarely used.
Lenses for multi-chip cameras
Multi-chip lenses are specially designed for colour cameras with 2, 3, 4 or more sensors where light is transmitted onto the sensors through a prism, which require lenses that correct for the optical effects of the prism. In order to avoid mechanical damage of the prism, rear protrusion of the lens should be kept very low.
The output from the standard lens shows distinct chromatic aberration, which appears as coloured fringes around the image due to the difference in the way the R, G and B components are transmitted through the lens elements. The output from the colour corrected lens does not suffer from these problems, making it superior for use in colour vision applications where accuracy and colour fidelity is important.
Standard resolution lenses
The most widely used lens type for resolutions less than about a megapixel. Generally, available in standard fixed focal lengths from 4.5 - 100 mm, these lenses are optimised for focusing to infinity and are typically rated at an MTF of 70-90 lp/mm with low distortion and vignetting. Lenses with shorter focal lengths usually produce images with 'fisheye' distortion
High resolution lenses
Precision or high resolution lenses offer improved imaging performance over standard lenses. Typically they are available up to 75 mm, with an MTF in excess of 120 lp/mm and very low distortion (<0.1 %). They are especially suited for cameras with a small pixel size or for precise measurement applications.
These are specifically designed for small fields of view (FOV), which are approximately equal to the camera's sensor size. Macro lenses are specified in terms of their magnification relative to the camera sensor and are optimised for 'closeup' focusing. Very good MTF characteristics and negligible distortion make them ideal for many vision applications. However, they lack flexibility, because it is not possible to change the iris or working distance. Special 'reverse rings' can be used on some standard or high resolution lenses, allowing them to be used as a macro lens.
Large format lenses
Large format lenses are required when a camera's sensor dimension exceeds that which can be accommodated with C-mount lenses. Typically they are connected to the camera using Nikon F-bayonet, M42x1, M58x0.75 or M72x1. Large format lenses are often modular in construction, requiring several separate components to function, such as focusing adapters, helical mounts and spacers. Large format lenses are most commonly used in line scan applications.
Telecentric lenses are designed specifically for use in specialist measurement applications where perspective projections and incorrect image scaling can cause errors. They are particularly suited to measuring 3D objects where scaling due to working distance differences in standard lenses will introduce measurement errors.
These lenses do not suffer from distortion problems, as they collimate the light that enters the lens. This results in equal magnification, independent of object distance without perspective distortion. As a consequence of collimating the light, the front aperture of the lens needs to be as a minimum, the same size as the FOV. Therefore lenses for large fields of view, need to be quite big and are relatively expensive. For the most demanding measurement applications double-sided telecentric lenses are used where both the object-side of the lens and the sensor-side of the lens are telecentric. This helps to maintain accurate measurements, even when the image starts to move out of focus, providing even lower distortion.
Some CCD or CMOS cameras can only be used with image-side telecentric lenses due to microlens arrays in front of the sensors. These require a perpendicular incidence of light to avoid shading effects. This is also true for cameras with beam splitter prisms, which seperate colours to severals sensors. A telecentric light incidence should be favoured for multi-chip cameras.
This image shows an electrical assembly that needs to be inspected for damage. As can be clearly seen, one of the pins is bent, and the imaging system needs to locate this fault. The use of a standard "endocentric" lens will provide an image with perspective distortion, making the job of detecting the problem difficult.
The next two images show how telecentric lenses overcome the problems associated with perspective distortion.
This image shows the pins apparently 'fanning out' from the central axis of the lens. Under these circumstances, the bent pin appears very similar to the good pins and presents a much harder challenge to the vision software.
The image shows the same component when viewed through a telecentric lens. All the components except the bent pin now appear perpendicular to the lens, with no perspective distortion. The damaged pin is now revealed very clearly,
Another application where telecentric lenses have proved very useful is in the inspection of drawn wire. The gauge of the wire must be checked very accurately as it leaves the die. However, due to the nature of the process, a resonance often occurs in the wire which causes its position to fluctuate and this makes conventional lensing insufficiently accurate.
If a standard lens is used, the distance from the wire to the lens is constantly changing and hence the apparent width or gauge of the wire. An important feature of telecentric imaging is that, as the target moves closer or further away, the size of the image projected onto the sensor remains the same. This means that no matter where the wire is in relation to the lens, the width remains the same. In this application, using a telecentric backlight will increase the accuracy of the inspection.
In general, the ideal configuration for optimum results is to adopt both a telecentric lens and a telecentric illuminator. Using such a setup, any light is parrallel and removes errors due to light diffraction.
The image on the left shows the wire, both without blurring and also correctly sized, making it easy for the image analysis to gauge the diameter accurately.
The two images show the same wire in different positions when viewed through a standard, non telecentric lens.
There is an obvious size difference because of the change in working distance and also a corresponding focusing problem, as the wire moves up and down in the field of view.
The next diagram illustrates the reason why telecentric lenses are often so large. Using the same component as a target for both lens types, the "endocentric" lens collects the light rays from the object in an angle. The telecentric lens, however, only collects parallel or collimated rays that originate from the surface of the target and so the front aperture must be at least the size of the object. As lenses can only be produced up to a certain size, there are limitations for applications where larger objects have to be imaged.
Liquid lenses - electrically tunable lenses
Where there is a need to rapidly change the focus of a lens, for example imaging on different high boxes, the use of lenses made from elastic polymer-based materials might be a solution. Within milliseconds, the focus of the lens can be adjusted by applying a control current to a diaphragm that changes the shape of the lens. This unique principle enables the design of faster and more compact optical systems without complex mechanics and deliver a long working life as there are minimal moving parts.
Electrically tuneable lenses enable vision systems to focus (within milliseconds) over a large working range maintaining high optical quality. They are normally mounted either between the lens and camera or on the end of the lens depending on the required working distance. By applying pressure to a ring on the outer part of lens, the liquid changes shape and adjusts the focus.
This technology delivers a number of key application advantages including the ability to image across very large working distances, accurately focus images under computer control, all with response times of between 1.4 and 15 ms with an impressive MTBF in excess of 1 billion movements.
Applications include optical surface inspection, code reading on random distance packages, surveillance and 3D microscopy.
The lens shape is adjusted by applying a current to an electromechanical actuator which changes the shape of the liquid lens. Telecentric lenses require special lens designs to work with liquid lenses. SILL and Optotune have developed a full series of such lenses to be available off the shelf
Telecentric lenses with tunable working distance
Telecentric lenses are indispensable when measuring objects with different heights. Unfortunately, there are limitations with respect to the depth of field.
In order to achieve a significantly greater variation of the working distance while maintaining the necessary resolution, variable focusing is required.
These new products with temporally variable working distance allow telecentric measurements with a frame rate of about 40 fps. The working distance depends almost linearly on the refractive power of the variable focus lens.
Because of the influence of the focal length, the magnification of the lens is not constant. However since this behaviour is linear, it can be corrected by calibrating the setup.
Due to the telecentricity condition, the aperture diaphragm is projected to infinity on the telecentric side. This means that for an object-side telecentric image, the variable-focus lens must be positioned behind the the aperture diaphragm. With such a configuration, it is however no longer possible to achieve image-side telecentricity.
Liquid lenses and computational imaging
Fast stepping through varying focal lengths with a liquid lens while subsequently acquiring images opens up many interesting applications. This technique provides image data that can be merged afterwards, in order to create extended focus images or 3D data (depth from focus).