The petrographic microscope, or also known as polarizing microscope, is a equipment designed to be used in geology laboratories, which allows to determine the optical properties, characterization of minerals, analysis of textures and relationships between minerals and rock classification.

This equipment consists of convergent eyepiece systems for the study of the samples, so that it forms a real image of the same, located at a shorter distance from the lens than the focal length of the lens, so that it allows to show an even more enlarged virtual image, in a position below the microscope deck.

Likewise, the equipment is composed of a light polarization system; the sample plate is rotating; the compensators (lambda and quartz wedge) and the Bertrand lens. In addition, it has two polarizing filters (or nicoles), placed below the sample, in the lighting system, and the second between the sample and the eye.

Petrographic Microscope Lighting System

It is a set of mechanisms that direct light towards the object of study, to improve the perception of the sample. It is located at the bottom of the stadium. This system is mobile, so that it allows to move closer or away from the deck for better control of lighting.

It is composed of:

• Lamp: Halogen type, located at the rear of the stand.
• Field Diaphragm: Controls the amount of light that reaches the condenser system and is placed at the base of the stator.
• Blue filter: it is blue colored glass and improves the dominant yellow light of the lamp.
• Polarizer—Controls the preferred light vibration orientation, oriented E-O (left-right). It stands in the way of light and can be turned to adjust it to its correct position.
• Diaphragm: controls the amount of light, varying the diameter of the beam, so that closing it gives greater contrast to the sample image.
• 1st condenser, fixed condensing lenses (lower). They are located above the polarizer and concentrate the light from the lamp on the sample.
• 2nd condenser, top condensing lenses. It is located on top of the lighting system. It is very powerful, can be removed or placed at will and the light forms a conical beam (convergent or conoscopic light), which is used in cases where a reinforcement of illumination is needed (with high magnification objectives), or to determine optical signs.

According to the above, the illumination should be measured in a way that is neither excessive nor very low, as this would cause changes in the observed colors, and cause problems of fatigue in the eyes.

Types of use of Petrographic Microscope

The type of use made of the polarizer, leads to three different methods of analysis, among them mineralogy and petrography, are the two sciences that most use this type of tools. In these areas, in fact, it is possible to frame some important properties of minerals, so the polarizing microscope remains a privileged means to have results, which would otherwise be achieved with more complex analyzes.

Currently, while mineralogy uses the polarizing microscope with low frequency (due to the diffusion of more powerful, accurate and fast means of analysis), in petrography this type of study remains basic, and allows achieving excellent results. Only clear minerals (or samples in general) can be studied, that is, they are allowed to pass through light. Otherwise, the opaque minerals would create a completely black figure due to the impossibility of being crossed by light.

Kalstein brand microscope

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