The most commonly asked questions about microscopes apply equally to an $800 stereomicroscope as to an $80,000 compound inspection microscope—yet have little to do with microscopy. To help you evaluate your inspection needs, here are the answers to questions most often asked about using microscopes and accessories effectively.
What About Those Terms?
The field of microscopy consists of many specialized terms, including:
Chromatic aberration is an optical defect that occurs when different colors (wavelengths) of light pass through the same point of a lens and focus at different distances.
An achromatic objective will use more than one lens to correct this defect and allow the red and blue wavelengths to focus at the same point.
An apochromatic objective corrects for red, green, and blue, providing significantly higher resolution and a crisper, sharper image.
A fluorite objective uses lenses made from synthetic fluorite for semi-apochromatic correction that is greater than an achromat but not the full correction of an apochromat.
Spherical aberration is an optical defect where monochromatic light passing through different radii of a lens focuses at different distances. A plan objective corrects this problem and allows the entire field of view to focus across very flat surfaces like wafers.
Numerical aperature refers to the half angle of the cone of light gathered by the front lens of an objective multiplied by the refractive index of the space between the objective and the specimen. Simply put, it is a mathematical representation of an objective’s capability to gather light and its resolving power.
A plan achromatic microscope satisfies the minimum performance level in most industrial applications.
How Do I Illuminate My Sample?
Illumination is the key to optimizing the performance of any microscope and is extremely sample dependent. Different types of illumination and contrasting techniques may produce very different images of the same structure.
What is acceptable for one person may not work for another. An operator at one wire-bonding station may prefer a shadow-free fluorescent ring light, and another operator may demand the intensity of a fiber-optic ring light. An engineer may use dark field for looking for contamination on wafers, but use bright-field polarized light for all other inspection applications.
For highly reflective samples, polarized coaxial illumination with a quarter-wave plate can show you details no other type of illumination can reveal. The polarizer illuminates the specimen with waves of light that are aligned in a single direction. In the reflected light path, the rotatable quarter-wave plate lets light waves in two-axis pass through, giving you a variable, enhanced contrast.
How Do I Get the Image Into My Computer?
There is no best way. The most frequent question deals with video. I ask three questions in return for every one posed to me, usually beginning with “What do you want to do with the image?”.
One of the most important considerations is where the image is going to be used—digitized in an archival system or sent to an e-mail address, a video printer, Adobe® Photoshop® or a monitor at an ergonomic workstation? Digital video systems such as the Optronics LEI 750D ™ or the Polaroid DMC™ seem to be the future of inspection protocol. Where cost is the deciding factor, many video applications are easily filled with a compact, high-resolution analog charge coupled device (CCD) such as the Leica 2D™ or Cohu 2200™.
These four cameras have different performance levels, features, advantages, and disadvantages. A single-chip analog CCD video camera may have 460 TV lines of information. A three-chip analog CCD will have 750 TV lines because it uses three of the same CCD chips and an RGB prism to spread the light to the individual chips. In a three-chip camera, each chip gathers a different color—red, green, or blue—which provides for significantly higher resolution.
Operational differences between analog and digital are the ease of focusing and the capability to record temporal resorts. Analog camera systems tend to be easier to work with. You are looking at a live image displayed on a monitor while the same analog signal is at the frame-grabber board waiting to be acquired. When you see the exact image you want, one keystroke captures the image and displays it on your computer monitor.
Some digital systems repeatedly acquire images and display them one after another automatically in a low-resolution mode to give you a sensation of live video. Most, however, take a snapshot in time and display it on the monitor. If that image is anything less than perfect, you must adjust the sample, focus, or illumination and then capture another image.
A key advantage of a digital CCD system is the high resolution created by nearly 1.9 million pixels on the Polaroid DMC to gather information. An analog camera with a half-inch chip uses roughly 380,000 pixels. If you intend to use software such as Photoshop to enhance or enlarge the image, you will have better results with a digital CCD.
The digital camera usually plugs into your computer’s SCSI port; an analog camera connects to a frame-grabber board. The analog frame-grabber boards on the market have very wide ranging performance and price levels and can be quite confusing.
Archiving or e-mail requires the image to be in a digital format such as TIFF or JPEG. The caliber of that image is somewhat proportional to how much you want to spend on hardware.
A quality low-end video system starts around $2k. If you have a suitable computer and desire publishing-grade images, real-time enhancement, or other digital manipulation, you can spend from $12k to 20k.
Conclusions
Ask questions and expect answers when you are working with microscope vendors. Think of your microscopes as tools and how they can be modified or accessorized to enhance performance, production throughput, and ergonomics.
There is a huge industry based solely on making accessories for all the major microscope vendors. Custom stages, stands, illumination, handling devices, and reticules are some of the components available.
About the Author
Karl Johanson is a sales executive at Leica. He has 14 years experience in the U.S. Navy and Naval Reserve as an Airborne Avionics Technician and seven years experience as a field service engineer and sales executive.
Leica, 2362 Qume Dr., #E, San Jose, CA 95131, (800) 248-0665, www.Leica.com.
Copyright 1998 Nelson Publishing Inc.
April 1998
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