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.
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.