Here are some common terms used in the Microscopy field.
We have listed terms that are more common to Microscopes used in the Semiconductor and related High Technology industry.
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High Power Microscopes:
Bright Field – This is the standard inspection mode. Light rays from the vertical illuminator strike a beam splitter. These rays are directed towards the object plane. The light rays strike the object plane and reflect back through the objective and back to the observer. Bright field gives you distinct image and true specimen color.
Dark Field – Same light flow as the Bright Field system however in the Dark Field Mode , the light does not pass through the central portion of the lens. Instead the rays pass through the sides of the objective and strike the sample at an oblique angle. Reflected rays now pass back up through the objective. Any featureless specimen will appear black. Features with diffracts, refract or causes the scattering of light rays will stand out clearly.
Field and Aperture Diaphragms – The Field Diaphragm collimates the light so that the maximum amount of light is directed through the numerical aperture of the objective. The Aperture Diaphragm regulates the depth of field and contrast. Generally it is recommend to close the Aperture Diaphragm as a percentage of the NA on the lens. For example a 100X with a NA of .90 would require closing the AD down to 90%. This will maximise the resolving power of the lens. This technique is important on higher mag lenses such as 50X on up.
Nomarski – Sort of a 3D image technique – here is the technical verbiage: Light rays entering a Wollaston Prism are split into two vertically polarize rays, 90 degrees to one another. If the point of divergence is where the Wollaston Prism is of equal thickness, the rays will be in phase. If the point of divergence is lateral to the isothickness point , a phase shift will occur. This phase shift renders the familiar three dimensional image associated with Nomarski. This technique is used for: Staking faults on silicon wafers, surface analysis , structural analysis, verifying concave or convex structures and/or defects.
Numerical Aperture (NA) – A lens with a large NA will accept a wide angle of light. A high NA , say a 100X lens with a NA of .90, will resolve very well but will have minimum working distance and depth of field. A lower NA , such as a .30 would have a narrower angel of light, will not resolve as well as the 100X but will give you better depth of field and working distance.
Polarized Light – Disorganized light wave bundles strike the Polarizser. The Polarizer orients random light waves into a common vibrational direction. The emitted light waves are said to be “Plane Polarized” since their vibrational direction or orientation are the same. The plane polarized light waves are again deflected towards the sample via the beam splitter. A the samples surface, the plane polarized waves will generally encounter areas of differing refractive indices. As the Plan Polarized waves strike these areas, the amplitude of each wave front is modified. A vibrational change may occur as well.
As the reflected wave fronts pass through the bean splitter, they will encounter another polarizer termed the “Analyzer”. The Analyzer, when rotated in reference to the polarizer, reorients all wave fronts into a common vibrational direction. Since all wave fronts are propagating in the same direction, and interference phenomenon will occur due to varying wave amplitudes. Rotation of the analyzer will reveal areas of the sample composed of differing refractive indices. ( e.g. cracks in substrate, grain boundaries.).
Transmitted Light – Super simple… Light from the bottom of the Microscope stand. This technique is used with clear, semi-clear with some sort of opaque pattern substrates. With Transmitted light, shadows, outlines and edges of clear and opaque substrates are generally observed well with Transmitted light.
Stereo Zoom Microscopes:
Two separate compound microscope systems situated at a specified angle to the object plans with each system sharing a common objective or maintaining individual main objectives in order to attain specified numerical apertures. Due to this unusual design, the Stereo Zoom Microscope has a very long working distance, wide field of observation and a very well defined three dimensional image.
a. Greenough Design – Composed of Matching objectives ( Most common type of Stereo Zoom construction). A less costly Stereo Zoom Microscope.
b. Common Main Objective – An infinity corrected system that allows for a wider selection of accessories that would not be attainable with the Greenough design.