Observation Techniques

Reflected Light Stereo microscope Biological

 Reflected light techniques for the compound microscope

Bright field (reflected) - This is the most common technique for parts inspection. The light passes through the center of the objective, is reflected off the sample and transmitted back through the objective where it can be observed. Make sure your objectives are made for reflected light, and are in good condition, as some do not have the correct coatings, or have had the coatings worn off. This, or dirt on the lens, will cause the image to appear washed out.
Dark field (reflected) - This technique is great for detecting surface contamination, scratches on wafers or edge definitions. In this technique the light is blocked at the illuminator from passing through the center lenses and passes through a portion around these lenses. The light strikes the sample at a low angle. Light may be reflected by a particle or edge causing it to travel through the center lenses where it can be observed.

 

DIC (reflected)- Differential Interference Contrast or Nomarski illumination is a technique that uses interference to bring out surface detail. There are three things required for Nomarski: a polarizer,an analyzer and a modified Wolliston prism. Polarized light from the illuminator passes through the modified Wolliston prism where it is divided into two rays that are very close together. These two rays reflect off the sample and back through the Wolliston prism where they are rejoined. The light then passes thorough an analyzer before the eye pieces. This allows small differences in the optical path, due to height or angle of reflection, to be observed as a change in color or intensity. One of the better techniques for telling small changes in height or slope

 Techniques for the stereo microscope

Fluorescent ring light - The ring light provides an even shadow-free illumination of the sample. The main difference among different models is in the color temperature of the lamp. Most lamps come in varying temperatures from a blue-white color to a yellow-white color. Polarized and yellow filters are available for photo resist coated components.

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Fiber Optic ring light - Like fluorescencent ring lights these provide a shadow free form of illumination. They usually use a halogen lamp as a light source so the color is warmer than the fluorescencent lamps. great for assembly but poor for finding solder flaws. some come with a filter holder so you can control the color easily.

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Oblique - This is one of the most basic ways to illuminate objects under the stereo microscope. It consists of a lamp or fiber optic bundle that is placed off the optical axis of the microscope. You will get shadows, but sometimes they can be a good thing. Besides, it's cheap.

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Co axial - This is similar to the compound Bright Field technique. The one draw back is that at powers lower than 1.0x the illumination spot is smaller than the field of view. Thus the center spot is illuminated while as much as 70% of the surrounding field is dark. The angle of the stereo microscope needs to be perpendicular to the work surface in order to get the most out of this type of illumination.

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Transmitted - Only used on transparent samples. This technique is great for through-hole boards or masks.

 Biological illumination techniques

  Transmitted bright field - Most commonly used in the biological field, this technique is the most basic for the compound microscope. Light travels through a sub stage collecting lens and sample before it gets to the objective and eye pieces.
  Phase contrast - Requires two parts; the phase ring in the illumination path (one for each power objective) and phase contrast objectives. The light passes through the ring where the center of the light is blocked. The remaining light travels through the sample as a hollow cone. The light that is undeviated as it passes though the sample passes through a thin ring in the objective while the light that is diffracted passes through a thicker portion of the objective. The result is that the diffracted light and the undeviated light are now 1/2 wave length out of phase as they approach the image plane. At the image plane they form destructive interference so that parts of the sample appear dark against a lighter background.