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determine which provides the best contrast for the work being done. Heat may be a concern, too, when working
with task lighting for prolonged periods. Halogen lights heat up more quickly than fluorescent lights and LEDs, and have a shorter lifespan. LEDs can last for 50,000 hours or more, and do not generate much heat — fluorescent lights fall somewhere in between. Regardless, it is important to choose lighting that is comfortable on the eyes to both reduce fatigue and to improve visibility of the component being observed or inspected, even when using tools.
Stereo Microscopes Where higher magnification is needed,
stereo microscopes (“stereos” for short) are a logical choice. Magnifiers have one optical path, and operators still benefit from binocu- lar vision to preserve a sense of depth of the sample. Stereo microscopes have two optical paths, one for each eye, and they are designed to deliver a 3D experience similar to magni- fiers, even at higher magnifications. Stereo microscopes come with two general
types of magnification: fixed and zoom. Where a single magnification is needed and sufficient, a fixed magnification stereo microscope (“fixed mag stereo” for short) can deliver both excep- tional performance and consistency. Fixed stereos typically have two or three magnifica- tions on the objective from which to choose. The eyepieces are often available in different magni- fications, e.g. 10, 15 or 20x, thus giving greater flexibility, while allowing the operator some cus- tomization for different tasks. Since the eyepiece is part of the optical path,
www.us-tech.com What’s Next for Visual and Optical Inspection?
a zoom stereo microscope (“zoom stereo” for short) may be a good choice. Magnification is changed simply by rotating a knob on the stereo body. Keep in mind the magnification range does not always start at 1x. Just as with fixed stereos, the magnifi- cation of the eyepieces contributes to the overall magnification produced by the zoom stereo. Zoom stereos also have a zoom ratio and a
magnification range. The zoom ratio is just a way of communicating the possible change of magnifi- cation. A 6.3:1 zoom stereo can continuously
Adding to the flexibility or customizability of a
zoom stereo, auxiliary or supplemental lenses can be attached to the bottom of the objective to either decrease the magnification range (a lens with mag- nification factor less than 1.0, sometimes referred to as a reducing lens) or increase the magnification range (a lens with magnification factor greater than 1.0). So, adding a 0.5x auxiliary lens to a stereo with a 0.7 to 4.5x zoom range results in a new zoom range of 0.35 to 2.25x. As an added benefit, the working distance also doubles in this scenario —the zoom ratio of 6.3:1 never changes. Stereo microscopes use one of two optical
construction approaches. The Greenough design has separate optical paths for each eye- piece. Lenses are color-corrected to deliver a pleasing image and good resolution. For envi- ronments where static electricity poses a threat to the sample, Greenough microscopes may be finished with an electrostatic discharge (ESD) coating. Although a much older design, Greenough
stereo microscopes are still the most widely used stereos today, and can deliver excellent results with sharp images. This is exemplified by the System 273 stereo microscopes from LUXO Microscopes by UNITRON.
Common Main Objectives The second optical approach is referred
LUXO Microscopes by UNITRON System 373 traditional, ESD-safe true trinocular stereo microscope.
do not forget to multiply the magnification of the stereo microscope by that of the eyepiece to get the total magnification of the sample as seen through the eyepieces. For example, a 2x objective with 10x eyepieces gives a total magnification of 20x. On the other hand, if the work would benefit from observation at multiple magnifications, then
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change the magnification from 1 to 6.3 times the original (1x) magnification. The magnification range is represented by the markings on the zoom- changing knob on the stereo body. For example, a zoom stereo having a zoom ratio of 1:6.3x typically has a magnification range of 0.7 to 4.5x. Factor in the eyepiece magnification to get the total magni- fication range afforded by that microscope. With 10x eyepieces, the range would be 7 to 45x.
to as Galilean or, more commonly, common main objectives (CMOs). CMOs generally have a larger/taller main body, a large CMO attached to the bottom of the main body, and a distinct observation head attached to the top. Objectives and lenses used with CMOs
are more highly corrected for color and for a flat- ter, more consistent view across the entire sample. These lenses are referred to as plan achromatic or plan apochromatic. With the more complicated CMO design
comes higher resolution (needed for observation at higher magnification), greater flexibility with attaching accessories, and a heftier price tag over
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May, 2018
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