LIGHT AND DARK FIELD IN A POLARISCOPE PDF

Assuming that the polarizers are crossed to produce a dark field, the polariscope is then described as a circular dark-field polariscope. the polariscope is changing from a dark-field configuration to a light-field configuration. Photoelasticity is a nondestructive, whole-field, . the polariscope must be arranged so as to allow light .. izer always looks dark because half the light striking. A polariscope uses polarized light for gem identification. is at right angles to the vibrational direction of the analyzer, the field between them remains dark. Throughout a ° rotation the stone blinks 4 times, light and dark.

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The polariscope may be one of the most underestimated tools in gemology. Most gemologists use it to quickly determine if the stone at hand is isotropic or anisotropic or, at best, to determine the optic character of gemstones. With some small additions, one can determine both optic character and the optic sign iin a gemstone.

It is also the preferred tool — next to the microscope — for separating synthetic amethyst from its natural counterparts although with recent synthetics that may prove difficult.

In addition, the polariscope may be very useful for distinguishing solid inclusions from negative inclusions as well as for spotting polysynthetic twinning.

A polariscope uses polarized light for gem identification. It consists of two polarized filters, one on the top and one on the bottom of the instrument as seen in the picture to the right.

Both the polarizer and the analyzer have their own vibrational planes. When the vibrational plane of the polarizer is at right angles to the vibrational direction of the analyzer, the field between them remains dark.

This position is known as the “crossed position”.

In this position, gems can be tested to determine if they are: The polarizing filters of this instrument are made of polarizing plastic sheets polyvinyl alcohol containing dichroic molecules – stretched polymers. Older models were created with microscopically oriented crystals of iodoquinine sulfate herapathite or tourmaline plates. With the polarizer and analyzer in crossed position, turn on the light source and place the gemstone on the rotating platform just above the polarizer this platform might not always be present, in which case you use your tweezers.

Observing the gemstone through the analyzer while slowly turning the stone will give you 4 possibilities. The first 3 behaviors kn pose no problems for the inexperienced user, but the latter ADR can be misinterpreted and cause one to think the stone is double refractive.

If the stone becomes noticeably lighter, it means the gemstone is single refractive and is exhibiting ADR. If it stays more or less the same, the stone is double refractive. Red stones that are out of the limit of the refractometer OTL may be especially difficult to distinguish with the polariscope due to ADR. Some stones in this category are ruby, red spinel and red garnets.

An anisotropic gemstone can have polariscopw direction or two in which it will stay dark throughout lateral rotation. These directions are the optic axes of the gemstone. Uniaxial stones have one optic axis, biaxial gemstones have two.

No double polarisclpe occurs along the directions of optic axes. Because there may be more than one direction in which some gemstones remain dark, it is useful as a confirmation to view the stone under a different angle when it stays dark. With the aid of a few polarisxope sheets one can turn the gemological microscope into a polarizing microscope for less than USD Simply lay one of the sheets over the transmitting lightsource and tape the other one in crossed position below the optics or find your own way of doing something similar.

This enables us to distinguish between solid and negative crystal inclusions and many other internal features a gemstone might have. Alot of the following discussion involves such a setup, although most of it can be achieved with the usual gemological polariscope aswell.

In gemology, we use a conoscope a strain free acrylic or glass sphere on a rod to determine optic character uniaxial or biaxial in anisotropic gemstones.

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The conoscope creates a 2-dimensional image of the 3-dimensional interference in a mineral. Although determining the optic character with a conoscope is a fairly easy procedure, finding the interference figure itself is not.

The interference figures always appear around the optic axes of minerals. The simplest way to find an polarjscope figure is to rotate the stone under the polariscope, in every possible direction, while looking down the analyzer until one sees a small flash of colors appear on the surface of the gemstone. When that flash of colors is found, fix the stone in that position and hover your conoscope slightly over it.

Now, while still looking through the analyzer, ligth should see the color flash transform into a rounded 2-dimensional image. This image in uniaxial stones will appear different from the image in biaxial stones, each having its own characteristic pattern.

Using an immersion cell along with the polariscope may enable you to find the flash figures more rapidly. Due to enantiomorphismquartz will give a typical uniaxial image but with drk large “target” in the middle. That is what is named a “bull’s eye” and is typical for quartz both natural and synthetic. Because anisotropic minerals appear to be single refractive when viewed down the optic axis, another technique for finding the optic axis can be used.

View the stone under the polariscope from all sides to find where the gemstone does not blink light and dark on lateral rotation. That will be the optic axis. Remember that uniaxial minerals have one optic axis while biaxial gemstones have two optical axes. The above typical images may not be seen as a whole or very sharply at times, but don’t be alarmed. One can determine optic character from part of the conoscopic image.

Many polariscopes for gemological purposes come with a rather large conoscope loght can be swivelled like a gemstone holder.

Although one can get reasonably nice images with them, a conoscope rod is preferred and the smaller the sphere, the sharper the image. For the very small spheres one will need magnification to observe the interference figure. For clarity the nomenclature of interference figures should be understood.

Polariscope – The Gemology Project

Luckily this is not too difficult. In uniaxial stones, the “melatope” indicates the center of dzrk dark cross and is the direction of the optic axis you are poariscope down the optic axis. The dark cross is actually made up of two L-shaped “isogyres” that will always stay in the same position in uniaxial stones. The colored concentric fringes are named “isochromes”. Biaxial minerals have two optic abd, hence they have two “melatopes” that are in the center or the isogyres. Again the dark cross is made up of two brushes, named “isogyres”.

When the biaxial interference figure is laterally turned, the isogyres detach and transform into hyperboles. The distance between the two melatopes is dependent on the “2V” value of the mineral. This also depends on the “numerical aperture” of your microscope. No knowledge of “2V” or “numerical aperture” is needed for our discussion.

In mineralogy, retardation means that one refracted ray of light is lagging behind another ray of light. When light enters an anisotropic double refractive gemstone, it is split into two rays — a fast ray and a slow ray.

Because the fast ray travels faster through the gemstone it will be ahead of the slow ray. When the slow ray leaves the gem, the fast ray would have already traveled an extra distance outside the gemstone. That extra distance is known as “retardation” and is measured in nm nanometers. Through a series of calculations, it is shown that this retardation is dependent on the thickness and birefringence of the gemstone.

When the stone is placed between two polarizing filters a polariscopethe two rays combine at the analyzer and either interfere with each other or cancel each other out, depending upon whether the rays are in phase or out of phase.

This produces the typical interference colors. These colors show a distinct pattern as seen in the Newton Color Scale below and, again, depend on the thickness and birefringence of the material. As the thickness of the gemstone increases, the colors shift toward the right. This knowledge can be useful in gemology as one could also add another mineral on top of the gemstone to mimic increased thickness and thus create a shift in colors when viewed through the conoscope.

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This shift can either be to the left or to the right. When the slow ray of the gemstone and the slow ray of the added mineral align, the shift will be to the right. This will create an addition in color on the Newton Color Scale. When the slow ray of the gemstone and the fast ray of the added mineral align, the shift will be to the left and will create a subtraction in color. When, for instance, a gemstone would create a retardation of nm, the starting spectrum would be on the boundary of the first order and second order and go from magenta to blue to blue-green to yellow to red.

Then if a mineral with a retardation of nm is added, and if the slow ray of the gemstone aligns with the slow ray of the added mineral, the starting color would be blue at nm instead of magenta.

On the other hand, if in the same example the slow ray of the gemstone would align with the fast ray of the added mineral, there would be a subtraction, and then the starting color would be nm — yellow-orange.

To remove this uncertainty, “retardation plates” are made. Retardation plates as those added minerals are known have a known retardation, and the vibrational directions of the slow and fast rays are known. This can help us determine optic sign in gemstones. All of the above plates can be very expensive since they are usually designed for petrographical microscopes that require special slots in the microscope.

Fortunately, modern day technology has created anisotropic plastic substitutes that cost little and can be held between your fingers. These plastic plates can be used in conjunction with the standard polariscope or with an adapted gemological microscope where polarizing filters are placed just above the light source at the base and just below the optics you can use tape to hold them in place. The latter is a setup that transforms your microscope into a polarizing microscope, at low cost, with the great benefit of magnification.

Polariscope

If you are intent on buying a plate, make sure you know how the fast and slow rays are orientated. With a stone of known optic sign you can determine that yourself though. Determining the optic sign in anisotropic gemstones should pose few problems with the aid of one of the retardation plates. The real challenge however is finding the interference figure. All images below are conoscopic images with the conoscope in place. The plates should be placed directly under or directly above the gemstone.

When above the gemstone, the plate should be placed between the stone and the conoscope. For convenience, the image at the left has the area of interest marked, which is the area just around the center ljght the ddark figure the white circle. That area is divided into 4 quadrants. In the direction marked “slow”, the slow ray of the wave plate travels. The fast ray travels in the direction of the length of the plate.

When one looks closely click the image for a clearer, larger view the colors in the quadrants change. Quadrants 1 and 3 turn more or less blue here addition of color occurredwhile in quadrants 2 and 4 the colors change to predominantly yellow-orange here subtraction occurred. The wave plate removed for clearer view this is for illustration only and will not work in practice.