sign in sign up
<<
Home , Hyperfocal distance

Integration Insights

Looking at liquid

Liquid lenses can be used to maximize imaging system flexibility and functionality in machine vision systems

Nicholas Sischka and Mark Ventura

The landscape of the machine vision world Ring with voice coil is one of rapid changes, with new technol- ogies emerging constantly, and new tools Membrane coming to market incredibly fast to make tackling automation problems easier. In the past decade alone, the machine vision market has seen the introduction of more advanced Fluid sensors in terms of both smaller and Container larger sensors, software platforms which con- Figure 1: Working principle of a typical liquid . A voice coil actuator applies a force to a tinue to become more accurate, and light- membrane, forcing fluid into the clear of the lens, changing its curvature and increas- ing which is growing brighter and becom- ing optical power ing more efficient. Additionally, the which are responsible for relaying the infor- ing on the order of milliseconds, these (or the object) or using a lens with mation from the object under inspection to lenses cannot be underestimated in today’s a large , which often means the focal plane array also continue to evolve world of rapid automation, where speed and increasing the f/# and thus making compro- to produce higher contrast and better mea- throughput are directly proportional to effi- mises in resolution or choosing a lens with a surement accuracy. ciency and cash flow. small and obtaining a less than One of the most exciting optical devel- Liquid lenses offer several advantages in optimal angular field of view. opments that has matured over the last few machine vision applications that could previ- Furthermore, unlike most mechanical years has been the introduction of liquid ously only be achieved with motorized lenses systems, liquid lenses can perform billions lenses. Capable of focusing and refocus- - if they were solvable at all. Liquid lenses of cycles without any wear and tear. Figure allow the user to change the plane of 1 demonstrates the operating principles of a of the lens system in milliseconds, which is typical liquid lens, where a voice coil actua- Nicholas Sischka, Optical Engineer, useful for applications that require focusing tor applies a force to a membrane, imparting Edmund Optics (Barrington, NJ, USA; at multiple distances, such as barcode read- curvature (optical power) into the lens. www.edmundoptics.com), and Mark ing, where the objects under inspection are Ventura, Vice President Sales and often different sizes or at different distances Retrofitting lenses away from the lens. One of the ways that liquid lenses can be Marketing, Optotune (Dietikon, Switzer- Before liquid lenses, the only options for integrated into the optics of machine vision land; www.optotune.com) solving applications such as these were either systems is through retrofitting existing fixed bulky motorized lenses, moving around the focal length lenses. While this modification

www.vision-systems.com VISION SYSTEMS DESIGN February 2015 27

1502VSD_27 27 1/27/15 1:12 PM Integration Insights

may take a little more trial and error to “tune in” to the best configuration as compared to using an objective designed with an inte- grated liquid lens, it is a practical solution demonstrating the flexibility of this innova- tive lens design. To achieve the ideal solution from such a modification and obtain the optimal image quality, several different considerations need to be taken into account, including place- ment of the liquid lens and focal length of the objective. Figure 2: Two different systems retrofitted When retrofitting an off-the-shelf objec- with liquid lenses. The system on the left is tive, the only two possible places to insert optimized with the liquid lens in front of the objective while the system on the right, co- the liquid lens is either in front (closest to developed by Edmund Optics and Optotune, the object) or behind (closest to the sensor) the Dynamic Focus VZM Lens, has the liquid the objective. Liquid lenses that are available lens integrated behind the optics to maxi- today are still somewhat limited with respect mize performance. to their diameter; most common of electrically tunable lenses are currently lim- “in focus” all the way down to half of the hyper- ited to 10 mm or smaller. focal distance. When comparing the hyperfo- This works well when liquid lenses are cal distances of two objectives of different focal positioned in front of micro-video (M12 lengths, the shorter focal length will have a threaded) lenses but most machine vision of an objective is determined by both the focal shorter working distance where the hyperfocal lenses’ front optic diameter is larger than 10 length of the objective and the sensor size of condition is met when compared to the longer mm, which means that (blocking the camera that it is being used with, assuming focal length objective. of rays through the optical system) can occur. a small percentage of distortion. The longer What this means is that using liquid lenses The large diameter of the front lens element the focal length of the lens (and the smaller with short focal length objectives can often make it appear as though it is advantageous the sensor), the smaller the angular field of not buy much in terms of the ability to sec- to place the liquid lens behind the objective. view. Longer focal length objectives are more tion through different working distances, However, this is often not the case, as placing advantageous when used with liquid lenses whereas longer focal length objectives give the liquid lens behind the objective disrupts not only because their smaller fields of view the user the ability to sharply focus on objects the back flange distance such that the lens make it easier for the light rays to pass through at a variety of working distances away from may be unable to focus. the entire lens system without vignetting, but the lens, while blurring unessential portions Good results without vignetting can be due to the hyperfocal condition. of the field of view. achieved, however, when the back focal length While retrofitting objectives with liquid is significantly longer than the distance intro- Hyperfocal distance lenses can offer wide-focusing ranges from duced by the liquid lens between camera and All fixed focal length objectives have a hyper- very close working distances to infinity, it is objective. In such a configuration, the liquid focal distance associated with them when nevertheless important to optimize the work- lens works like a distance ring that brings the the lens is effectively focused at infinity. The ing range of the liquid lens. Liquid lenses are focus plane closer to the camera. An example hyperfocal distance of a lens is the plane of good at imparting a range of optical power, of each system is shown in Figure 2, with the focus beyond which all objects are within from acting as windows (effectively zero opti- 25 mm lens best optimized for use with the focus to some acceptable blur criteria. Gen- cal power) to bending into powerful focal liquid lens in front of it and the high magnifi- erally it is associated with a resolution and lengths on the order of 50 mm (or 20 diop- cation objective best optimized with the liquid contrast value important to the application ters). In liquid lenses, tas with standard lenses, lens behind the optics. where the lens is being used. the smaller the focal length of the lens, the There are two additional considerations If a lens is focused at its hyperfocal distance, larger the curvature that is imparted onto the to take into account when placing the liquid the depth of field can be considered to stretch membrane to allow the light to focus. lens on the front of an objective: the angular from that position out to optical infinity in the field of view of the objective and the objec- direction away from the lens. Paraxially, when Aberrations and curvature tive’s aperture (f/#). The angular field of view moving toward the lens, the lens is said to be Where the optical design is not optimized,

28 February 2015 VISION SYSTEMS DESIGN www.vision-systems.com

1502VSD_28 28 1/27/15 1:12 PM Integration Insights

cern, but fortunately, there are liquid lenses available with very low dispersion (Abbe number > 100) which can be combined with off-the-shelf optics without having to worry about correction. Lastly, it is impor- tant to consider the aperture (f/#) at which the objective is running to further mitigate vignetting and to provide greater control over the depth of field. Liquid lenses are an incredibly power- ful tool to have available when facing chal- lenging imaging applications, and have pro- gressed far in the relatively brief time that Figure 3: A properly optimized liquid lens/objective combination can produce high resolution and contrast images. Both images were taken with a 25 mm objective and a liquid lens on the they have been available. This technology front. The image on the left has the object 200 mm away from the system while the object is will only continue to grow more exciting over and 350 mm away on the right. The only refocusing was performed using the liquid lens. time as larger aperture lenses become avail- able to enable a wider range of imaging appli- higher amounts of curvature can introduce tances required is paramount to achieving cations. Understanding how best to use liquid spherical aberration and field curvature. optimal image quality (Figure 3). lenses not only on their own, but in combi- When retrofitting a liquid lens to a fixed focal Minimizing the necessary curvature will nation with objectives, becomes key to offer- length objective, minimizing the amount of usually also optimize the power consumption ing state-of-the art solutions in in R&D, life necessary curvature imparted on the liquid of the liquid lens at the same time. Introduc- sciences, high magnification inspection and lens while still obtaining the range of dis- ing chromatic aberrations can also be a con- automation applications.

get Structured lighting solutions wwithith Intercon 1 STREAMLINE LASER A Division of Nortech Systems OSELA IS SPECIALIZED IN DEVELOPING, CUSTOMIZING AND MANUFACTURING LASER PATTERN GENERATORS FOR MACHINE VISION APPLICATIONS.

Camera Link and Mini Camera Link

Industrial Rugged Camera Link Camera Link HS

GigE USB 3.0 FEATURES Violet 405nm Blue 450nm Superior beam shaping Custom-ready Green 520nm COME VISIT US AT Rugged and reliable design Up to 200mW Red 640-690nm booth #453 Stable output power Easy focusing IR 785-830nm

DEVELOPING, CUSTOMIZING AND MANUFACTURING Intercon 1 off ers a wideid selection INNOVATIVE BEAM SHAPING SOLUTIONS of cable assembly options. Intercon 1 nd Call us to see how we can help 1860 32 Avenue with your custom application. A Division of Nortech Systems Lachine, QC, H8T 3J7, Canada (218) 828-3157 Precision Cable Assemblies for the Vision Industry [email protected] 514 631.2227 [email protected] www.intercon-1.com osela.com

www.vision-systems.com VISION SYSTEMS DESIGN February 2015 29

1502VSD_29 29 1/27/15 1:12 PM