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Advances in Optics Sharpen Focus
by James F. Manji - AIA Posted 11/16/2000
Telecentric lenses represent one of the most significant technology advances in optical elements that now render machine vision systems much more accurate and versatile. The rapid growth of electronic image processing for critical manufacturing and process applications has resulted in a tremendous demand for non-contact optical metrology solutions requiring higher degrees of accuracy than ever before.
Xenoplan Bilateral Telecentric Lenses from Schneider Optics Inc., Hauppauge, NY, are said to provide greater dimensional accuracy than ever thought possible. The company's Bilateral Optical Design ensures that this family of lenses is telecentric in both object and image space.
'The accuracy of non-contact optical metrology is effected by the telecentric rays, not only on the object side but also on the image side of the measurement lens,' explains Stuart Singer, technical director for Schneider Optics. 'The Xenoplan Bilateral Telecentric Lenses are designed to work with standard 2/3 in. CCD cameras and have adjustable iris and focus controls. All have front filter accessory threads. The lenses also have a rotating support mount for increased systems stability.'
Telecentric lenses incorporate certain advantages over ordinary lenses. 'When you use a conventional lens to look through a hole to make a measurement, and you looked at that hole on a TV monitor through a CCD camera, that hole would be shaped like a funnel,' says Singer. 'It wouldn't be like a pure circle, because the light rays are further down in the hole, so there's a change in magnification, so that what you get essentially is a funnel-shaped hole. This makes it very difficult to do gaging and precision measurement.
'A telecentric lens reproduces the hole image and represents it just as the human eye sees it-as a straight cylinder. This would allow you to do metrology,' Singer continues. 'If you were measuring pins off a chip, as is done in the semiconductor industry, the measurement people would want to ensure that none of the pins are bent. When you look at the pins through a conventional lens, the pins would all look trapezoidal. But in a telecentric lens, the pins would all be straight.'
Yet another advantage of telecentric lenses is that you can gain a constant perspective in viewing objects, which is also critical for gaging applications, according to Spencer Luster of Light Works, Toledo, OH. 'Because telecentric lenses only accept parallel rays of light, this makes it very appropriate for detecting defects in transparent materials as well as in plastics,' explains Luster. 'There isn't inherently any more in-depth field in a telecentric lens than in a normal lens at the same aperture.' This property of a telecentric lens also avoids the effects of vibrations that cause changes in magnification when using conventional lenses, which in turn, results in errors in measurements.
Another important variable to be considered is a value called field of view, which is basically a value of how much of your parts you need to see, the working distance, and how far away or how close the lens needs to be, along with a consideration of the resolution, according to Jeremy Govier, senior applications engineer at Edmund Industrial Optics, Barrington, N.J.
'With regard to resolution, you need to consider Modulation Transfer Function (MTF), which is the contrast plotted on a graph,' explains Govier. 'The graph helps you consider contrast, where you're trying to do processing with enough contrast so that your metrology systems runs through algorithms quicker. So, the three factors to consider are resolution, working distance, and field of view. If the part you are trying to measure has a certain depth, you need to consider two factors-the first is depth of field or how much can stay in focus at one time, and the second is telecentricity.'
Telecentric lenses have been known to optical engineers for a long time, but their use in machine vision has started to build momentum over the last two years. 'Manufacturers and users are learning what they can do and the greater accuracy they can gain from these lenses,' opines Stuart Singer. 'I see telecentric lenses as one of the most significant issues in machine vision.'
Another advance in machine vision is a lens that is corrected for the visible spectrum. This type of lens is only designed to correct and make a picture of the visible wave lengths that the human eye can see. 'If you were to send ultraviolet light through the lens, it wouldn't form a good image,' continues Singer, or, if you were to send infrared light, it wouldn't form a good image because the lens is only corrected for visible light. That was very common in the military world. It is also still common when they use CCDs as the imaging medium. A silicon wafer is sensitive all the way out to almost 1200 nm or 1.2 microns. The visible light to the human eye is 400 to 700 nm. So, with a CCD, it sees a lot further into the near IR than the human eye. The CCDs have a peak sensitivity at about 780 nm, or close to 800 nm. But, because the human eye can't see the image at this range, the lens now is given a broad band lens design and is also simultaneously color-corrected, from about 400 to 1100 nm.
'What you can do when you use the silicon CCD is to pull out the near IR blocking filters, which is starting to become common, allowing you to avail yourself of all this light,' Singer continues. 'All these vision systems are almost always light starved, and, by employing these broad band designs (that are common in the military in photo reconnaissance), you can double your light.'
James F. Manji is a free-lance writer specializing in manufacturing automation topics. He is based in Brunswick, OH.
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