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[Gemlab Report] Gemstone Inclusions Photomicrography Definitions


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GEMLAB REPORT:

                   March 1998 - Issue #10

                       By: Ted Themelis
                    themelis@ganoksin.com

In This Edition:

Gemstone Inclusions Photomicrography Definitions.
http://www.ganoksin.com/borisat/nenam/gemlab10.htm

The process of photographing various objects through a microscope
is called photomicrography. Microphotography, on the other hand,
refers to a very small photo, one that requires enlargement to
bring out its details. These terms should not be confused.
Photomicrography opens new horizons in gemology by providing the
gemologist a means of studying and recording important gemstone
inclusions. Photographing the inclusions is a skill that requires
talent, experience, and considerable patience. The most important
prerequisites for any gemologist attempting to photograph gemstone
inclusions through a microscope are to understand fully a) the
concepts of microscope optics b) the photographic apparatus being
used and c) the behavior of the light used.

Apparatus.

Although numerous sophisticated types of professional photographic
equipment are currently in the market, relatively simple and
inexpensive apparatus may be used to obtain reasonably good
quality photos of various gemstone inclusions. The following
equipment are essential for photographing gemstone inclusions:

35mm camera back.

Any good quality 35mm single lens reflex (SLR) camera back
equipped with an automatic exposure feature is suitable for
photomicrography. Lenses are not needed, but appropriate exposure
compensation dial feature is highly desirable. Nikon, Olympus,
Cannon, Minolta, and other major brand-name cameras all have these
required features. Highly recommended is the Nikon FE-2 or FE. A
shutter release cable is highly desirable, since it provides
vibration-free shutter-release operation.

Gemological microscope.

The nature and type of gemstone inclusions determine the
microscope technique used. In some cases, dry dark field
illumination is required, while in other cases immersion microscopy
technique is more desirable. In both cases, any modular microscope
optical system is suitable for photomicrography. Such microscopes
are classified according to their ocular configuration (monocular,
binocular, or trinocular) and to the magnification system
(continuous zoom, fixed, step-magnification). For most applications
magnifications up to 30X are commonly used; magnifications between
30X and 60X are desirable and used in certain applications;
magnifications over 60X are rarely used and are not generally
recommended. Binocular, stereoscopic zoom microscopes have great
flexibility in various gemological applications and provide
satisfactory quality photos. Binocular, stereoscopic microscopes
with step-magnification changer (such as Zeiss DV-4, Wild M-8, and
others) are probably a better choice, when available. Trinocular
(and trinocular-converted) microscopes are available in the market,
but are usually high priced. Their main advantage is that the photo
can be taken while the microscopist is simultaneously observing the
specimen.

Basically, there are two types of gemological microscope
configuration systems used in photomicrography:

a) The immersion microscopy technique developed by Professor K.
Schlossmacher. Gemstone examination is achieved here by
positioning the microscope pod horizontally. The gem is immersed in
a suitable liquid placed in a specially designed mechanical x-y
stage for easy manipulation. Spotlight or fiber-optic illumination
may be used. Polarized light may or may not be used. The immersion
method is particularly suitable for observing various gemstone
features such as structural growth, color zoning, lamellae
polysynthetic twinning, curved-striae, the Plato method, and more.
Diffused transmitted illumination is usually employed from various
angles. Some immersionscope systems are equipped with trinocular
pod; dual fiber-optic light can then provide unlimited positioning.

b) The dry, dark-field microscopy technique developed by R. M.
Shipley Jr. In this case the gemstone is observed dry in a vertical
position. Dark field or light field illumination may be used with
or without polarizers. This is the most common method used that
satisfies most gemological applications. The relatively inexpensive
type of gemological microscope system, Fiberscope-2, designed by
the author and manufactured by Gemlab, Inc. in the late 1970’s,
incorporated all types of illumination systems - dark-field,
light-field/transmitted, and pinpoint/oblique - by means of a
fiber-optic light guide. This microscope system was equipped with
one single light source using a tungsten halogen lamp that produced
intense bright light with a color temperature of 3100?K.

Sometimes the surface of the stone is scratched when gemologists
take photomicrographs using light/dark-field illumination, a
situation that prevents getting a clear image of the inclusion. In
cases like these, total immersion is not necessary. A droplet of a
liquid whose refractive index is close to the stone’s own
refractive index is merely required over the scratched surface.

Light source.

Illumination is one of the most important aspects of taking
photomicrographs. The following types of illumination have been
adapted in various gemological microscopes:

a) Dark-field illumination: It is used for routine observation
of gemstone inclusions. In this case, illuminating the stone from
the sides against a dark background provides indirect light. As a
result, all gemstone inclusions are seen brightly against dark
background. Most gemological inclusions are photographed using the
dark-field illumination system.

b) Light-field or transmitted illumination: In this case, the
light (which is often diffused) is transmitted directly behind the
gemstone. Used in both dry and immersion microscopy, the
light-field illumination method is suitable particularly for
observing large fluid inclusions.

c) Polarized illumination: Any microscope system may produce
polarized light, using two polarized plates. One plate is placed
between the light source and the gemstone, while the other is
placed between the gemstone and the observer’s eyes (or microscope
objective). Polarized light is used in photomicrographing strain,
twinning, included double-refractive guest crystals, and other
gemstone features. A conoscopic interference figure also may be
photographed. Polarized illumination may be used in both dry or
immersion microscopy.

d) Oblique/pinpoint illumination: This type of illumination is
usually incorporated with condensed, pinpoint fiber-optic
illumination, which is used to photomicrograph transparent
gemstone inclusions from a variety of angles. Oblique/pinpoint
illumination is used to photomicrograph surface or reflected
features, fluid inclusions of any size, fractures, interfaced
included guest crystal, and other gemstone features. With
successful manipulation of the pinpoint-illumination angle, gas
bubbles and guest crystals become easily visible. Very often,
oblique/pinpoint illumination is used in conjunction with
dark-field illumination to emphasize a particularly difficult
feature seen through the microscope. Oblique/pinpoint illumination
is used mainly in dry microscopy. It is necessary for
photomicrography of phenomenal gemstones (asteriated gems,
cat’s-eyes, and so on) as well as for gems (such as opal) that
exhibit a play of color.

Adapters. Basically, two adapters are required. One adapter (known
as T-adapter) is fixed to the 35mm camera back, in lieu of the
lens. The other adapter is a sleeve fabricated to specific
dimensions that connects the T-adapter to the microscope’s
eyepiece.

Film

For general photomicrography, Kodak’s Tungsten-160 Ektachrome
professional slide magazine (color temperature = 3300?K) is
recommended. Also, Kodak Tungsten-64 Ektachrome slide film is
used.

How to take photomicrographs using any microscope system

  1. Load the camera back with a Kodak-160 Tungsten film cartridge

  2. Set ASA/ISO film-speed ring to 160

  3. Set shutter-speed dial to “A” (for automatic operation).

  4. Set compensation dial to “O.”

  5. Install the shutter cable release cable on the camera back.

  6. Without removing the eyepiece, install the 35-mm camera adapter
    on the microscope’s eye tube.

  7. Locate the gemstone’s inclusion utilizing any type of
    illumination already mentioned. Use proper magnification and obtain
    a sharp, clear image, showing the desirable feature. Then, looking
    through the viewfinder and using the microscope’s rack-and-pinion
    mechanism focus on the main subject. Utilize the central
    split-image range finder for the sharpest image obtainable. What
    you see is probably what you are going to get.

  8. Take the photo using the shutter cable release.

  9. Advance frame

  10. Set dial compensator to +1 position

  11. Take the frame using the shutter release cable.

  12. Set dial compensator to -1 position

  13. Take frame using the shutter release cable.

Using the dial compensator. When a gemstone inclusion contrasts
sharply with the background (dark-field or light-field
illumination), overexposure or underexposure may occur. To prevent
this, a series of small adjustments are to be made provided that
the camera back used is equipped with an “exposure compensation
dial” feature. This dial also may be used to create special light
effects – for example, to emphasize a guest crystal inclusion
against a light or dark background.

After setting the camera to automatic exposure and moving the dial
compensator to a given “click” position, shift the shutter speed so
that it corresponds to the compensated amount. The dial ranges from
+2 to -2 in one-third increments, with a total of 13 positions
(referred to as “clicks”) possible. The following conditions
determine which setting of the dial to use:

+2: white background occupying the entire viewing area
+1: white background occupying half the viewing area
0: no exposure compensation needed
-1: black background occupying half of the viewing area
-2: black background occupying entire viewing area.

Dial compensator operating procedures are similar in most cameras
tested: simply unlock the appropriate button, turn the dial to the
desired compensation value, take the picture, and then return the
dial to 0 for subsequent shots.

A Few Tips

Select the desired microscope system (either dry or immersion
configuration) Determine the type(s) of illumination needed. (Both
are basic procedures.) Make sure that all microscope lenses are
free from dust. Secure a vibration-free position. Clean the surface
of the gemstone thoroughly. If immersion liquids are used, make
sure that various impurities that “float” are removed. Manipulate
the stone in every possible direction in relatively low
magnification. Spend time to obtain the best possible observation
angle of the subject matter. Try various illuminations from
different angles until the inclusion appears at its best. Admire
the results! You�ve mastered a technique.

All rights reserved. Copyright Ted Themelis Users are permitted to
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Any other use or reproduction of this document (text or graphics)
without the express written consent of Ted Themelis [
themelis@ganoksin.com ] is strictly prohibited.