Shining a light on lenses Product focus
Stemmer Imaging’s series of technology days included talks from various lens manufacturers. Here, we round up some of what was discussed at the event
Mike Giznik, of Midwest Optical Systems, advises on how to choose a machine vision filter
Optical filters are critical components for machine vision applications. Tey’re used to maximise contrast, improve colour, enhance subject recognition and control the light that’s reflected from the object being inspected. When selecting an optical filter for a machine vision system, there are a few key features to look for: stability, passband performance and anti-reflection coating. Stability: Because various angles are commonly used when imaging, having a stable filter is essential. Short-shiſting can occur when the angle of light passing through a filter increases. Tis is most commonly seen when the filter is placed in front of a lens with a focal length of 12mm or less (lenses with greater than 50 (+/-25) degree angular fields of view). Tis accounts for almost 60 per cent of
all lenses used today – a number that continues to grow as the demand for space forces inspection footprints to shrink. Choose a filter that is designed to work at variable angles and a wide field of view. Passband performance: Now that the filter is stable, it is also important to make sure that as much ambient light is blocked as possible. Te most common type of illumination used in machine vision systems is LED light. To ensure a machine vision filter is effective, the position, height and width of the passband should emulate the bell-shaped spectral output curve of the LED illumination being used. Selecting a filter that is too broad can allow more unwanted ambient light to pass through the filter, while selecting a bandwidth that is too narrow can result in a loss of the desired illumination and overall darkening of the image, both of which can sharply reduce contrast. Anti-reflection coating: When a ray of light passes through a glass surface, a portion of the light is reflected, resulting in a 4 per cent transmission loss per surface. Tis means only 92 per cent of the light
actually passes through the glass, and the rest is reflected from the surface, known as reflection loss. Look for a filter that uses anti-reflection coating to reduce surface reflection and increase transmission. Tis improves the efficiency of the vision system by allowing more of the intended wavelength to enter and by eliminating ghost images. A vision system’s stability, repeatability and performance is dependent on the information that enters the system. Tis begins with a proper machine vision filter.
www.midopt.com
Andreas Platz, of Sill Optics, on telecentric lenses
Sill Optics now offers a series of telecentric lenses with variable working distances. Te range covers lenses with magnifications from 0.13x to 0.66x for sensors up to 16mm diagonal length, and magnifications from 1x to 3x for sensor diagonals of 35mm. Sill Optics designed the telecentric lenses in
cooperation with Optotune. Tey are able to focus without any mechanical movement to give good performance over a given tuning range. Telecentric lenses are used for measuring
objects of different heights. Unfortunately, users quickly hit the limitations of the exploitable depth of field. Tese new products allow telecentric measurements with temporally adjustable
working distances up to image capture speeds of 40fps. Te working distance is linearly proportional to the dioptic power (reciprocal of the effective focal length) of the focus tuneable lens element. Te magnification of the lens is not constant
because of the retroactive effect of the focal length. With calibration, this small difference can be
34 Imaging and Machine Vision Europe • December 2017/January 2018
corrected for consistent accuracy thanks to the linear behaviour. Te lenses also offer an approach for measuring an object in 3D via a z-scan. Te first prototypes have been launched,
and Sill Optics gave a presentation on the potential uses of the lenses at Stemmer Imaging’s technology day in the UK on 16 November. Sill Optics also provides a modular telecentric
lens set-up, which is able to adapt the standard series (polarised coaxial coupling) to meet the requirements of the measurement configuration. First of all, the beam splitter can be replaced by a non-polarised version, and furthermore the illumination homogeneity can be improved by a retardation plate. A smart mechanical solution avoids the necessity of an expensive external retardation plate for large object fields greater than 60mm.
www.silloptics.de
@imveurope
www.imveurope.com
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