Users are likely to quote different specifications as critical because their needs vary depending on what they'll use a microscope to examine. Semiconductor wafer inspectors constantly push the limits of resolution, so they think first about numerical aperture. Inspectors of bare circuit boards, however, care more about magnification, which determines how easy it is to spot what they want to find. Table 1 lists the most useful common optical specifications for a microscope, along with a concise description of what each specifies. Many of these specifications, such as magnification and numerical aperture, are taken directly from the optical concepts introduced in previous articles.
Magnification is the first thing most people think about when they consider buying a microscope.
After all, magnification lets them see a sample as it would look if it were much larger than it really is. What users really want, however, is better visibility. They want to examine something and see what it is. If you need to read the label on the top of an IC package, you must see the characters well enough to identify them. The trick, therefore, is to understand what characteristics of a microscope define the visibility necessary for your particular application.