GEMLAB REPORT:
March 1998 - Issue #9
By: Ted Themelis
themelis@ganoksin.com
In This Edition:
- Gemological Spectroscopes
http://www.ganoksin.com/borisat/nenam/gemlab9.htm
Gemological Spectroscopes
ABSTRACT.
The spectroscope, microscope, and refractometer make up the
basic “triad” of gemological instrumentation. Prism and
diffraction grating type spectroscopes are the two common choices
for gemological investigations. This report is a practical guide
-based on the author’s extensive experience in the subject
matter- for determining the most practical way to perform,
observe, and record accurately spectroscopic analysis as part of
the gem identification procedures. Classical hand-spectroscope
made by Beck (England), Zeis-aus Jena (German), OPL (England),
and of unknown Japanese maker were examined and discussed.
INTRODUCTION
Traditionally, the typical gemologist approaches the prism
spectroscope with some degree of fear and uncertainty, due to
insufficient gemological training and experience. The prism
spectroscopes in use today were actually developed for simple
emission analysis spectra, such as the Bunsen flame for alkaline
and alkaline earth metals, the spark gap, etc. Prism
spectroscopes are also used for examining coloured liquids and
solids of homogeneous shaped substances, like flat surfaces, or
solids without facets. On the other hand, the diffraction grating
type spectroscopes are relatively new for gemological
investigations, although its principles are known for a long time
and used on a variety of applications. Hand-spectroscopes have a
variety of applications in many sciences, where absolute accuracy
is not necessary. These hand spectroscopes must be used in
conjunction with a light source, most commonly a fibber optic
unit. The hand spectroscope is fixed in various ways over the
light source, which may provide transmitted and/or reflected
light. The mechanical fastening of the hand-spectroscope to the
fibber-optic should accommodate all types of adjustment required
for spectroscopic observations.
FEATURES, CLASSIFICATION, and TYPES of HAND-HELD SPECTROSCOPES
Based on the configuration, operating principle, and other
characteristics, all spectroscopes may be classified into
various categories and types.
Spectrum Range.
In both types, the spectrum range is from about 400nm to 700nm
(visible spectrum). Some spectroscopic observations are possible
from about 370nm to 775nm by attaching a closed circuit
television camera, replacing the human eye. That is ideal for
group viewing, spectroscope training, etc.
Spectrum measurement.
Spectrum is measured in nanometer (nm) units. The old designated
units expressed in Armstrong (A) units have long been abolished.
One nanometer is equal to 10 Armstrong units.
Spectrum Field of View.
The prism spectroscope’s field of view is located horizontally
to the eye tube and it can be seen as once. Magnification of
absorption lines and/or bands is not possible. The field of view
of the scanning spectroscope may be located vertically to the eye
tube and is 10 times larger than that of the prism device. An
adjustable 10X wide field eyepiece magnifies the image, making
the absorption lines and/or bands more prominent and easier to
read. However, the entire field of view of the scanning
spectroscope cannot be seen at once. In order to compensate for
that, however, those who prefer to see the entire spectrum at
once should use a 5X eyepiece instead of the 10X. This requires a
simple adjustment of the digital display, since the length of the
spectrum will otherwise be reduced; the range of the spectrum,
then, remains unchanged at 360nm-775nm.
Spectrum observation position.
Most spectroscopes have fixed observation position and the
visible spectrum can be viewed at once. In some other
spectroscopes, when special train of prisms (in the prism type)
or large grating (in the diffraction type) are used, the visible
spectrum cannot be viewed at once; in this case, appropriate
mechanism is employed to move the spectrum vertically or
horizontally, to the observation field of view.
Spectrum Linearity
In the prism-type spectroscope, the spectrum linearity is a
“curve”, meaning that the spectrum is unequally spaced in the
observation field, causing larger spacing towards the violet
portion, while in the red portion is considerably narrower. That
will provide better observations at the blue and violet portion
of the spectrum (like observing the 415nm line on certain type of
diamonds), while the observations towards the red portion of the
spectrum will be more difficult.
In the diffraction grating type spectroscope, the spectrum is
equally spaced throughout the field of view causing equal
observations at the violet as well as at the red portion of the
spectrum Wavelength scale. Both types of spectroscopes may be
equipped with a wavelength scale.
In the prism type, the wavelength scale divisions are unequally
spaced and superimposed over the spectrum. A drawtube is required
for focusing and it may be illuminated with an external light
source (usually with adjustable light intensity). Calibration of
the wavelength scale is performed using an adjustable knob screw.
In the diffraction grating type, an equally spaced wavelength
scale may be installed and superimposed over the spectrum, or an
electronic mechanism may be fitted to indicate the relative
position on the spectrum. Calibration of the wavelength scale is
performed using an adjustable mechanism. Slit. Light path entered
in the observation tube may be controlled using an adjustable
slit. In some spectroscopes the slit is attached at the end of
the observation tube, other spectroscopes do not have slit.
Resolution.
To most spectroscopes are equipped with adjustable observation
tube that increase the resolution and sharpness of the spectrum
observations.
Problems encountered in gem spectroscopy
Centralisation of Spectrum
On the prism type spectroscope, the pupil of the human eye must
be centralised exactly at the middle of the spectrum, which is
seen as the field of view. Reference centralisation co-ordinates
are not available for hand-held type spectroscopes. A slight
horizontal movement of the pupil, though it is still enabling the
viewer to see any absorption lines and/or bands, will result in
false readings on the wavelength scale superimposing the
spectrum. Constant manipulation of the focusable drawtube is
required.
On the scanning diffraction grating type spectroscope, the pupil
observes a predetermined reference point usually a horizontal
thin cross-hair reticule. By moving the scanning mechanism, any
absorption bands and/or lines observed are aligned with the
horizontal line of the cross-hair reticule and theoretically
should be always in focus. Movement of the pupil will not alter
the position of the cross-hair reticule. Inaccurate Spectrum
Linearity Linearity is the relative position, in the form of a
scale, in which the wavelengths are spaced out across the
spectrum.
The linearity of the prism type is variable at any given point
of the spectrum length. Constant refocusing of the absorption
bands and/or lines from red to the violet portions is required.
With the scanning spectroscope, the spectrum linearity is
theoretically almost perfect, a result of the diffraction
grating system employed. However, a harmless, minute distortion
was observed, and this requires a slight refocusing of the
spectrum, which can be achieved without affecting the wavelength
scale measurements. Ghost images of the absorption band(s) and/or
line(s) Often, the prism spectroscope creates a “ghost” image
that could be mistaken as faint or very faint absorption band or
line, especially at the 610-620nm (red emerges with orange),
550nm-560nm (green emerges with yellow) and 460nm-470nm (blue
emerges with green) spectral regions. This is due to the
wavelength scale superimposed over the spectrum and to the
incorrect position of the reflected and/or unresolved rays
entered to the Amici prism. To eliminate the “ghost” problem, the
spectroscope may employ an external mechanism which can move the
entire spectrum by scanning, while the position of any absorption
lines and/or bands is calculated automatically.
Wavelength scale Inaccurate Readings
The wavelength scale in the prism spectroscope cannot produce
accurate readings. Firstly, there is inadequate resolution of the
wavelength scale. Secondly, the wavelength divisions are very
limited with a large estimated tolerance factor. However, for
approximately spectroscopic readings in gem materials, the prism
type can be considered satisfactory.
The scanning spectroscope does not use a superimposed wavelength
scale, since all readings are calculated automatically and
displayed electronically during the spectrum scanning. Accurate
measurements are within 1mn tolerance, utilising almost perfect
linearity (-+0.25%). However, accurate readings will be displayed
at all times within the 360-775nm range. Inadequate Light source
Both spectroscope types may use a fibre optic illumination
system, producing transmitted and/or reflected illumination,
However, the scanning model requires more light (about 25%-30%)
that the identical prim model in order to produce the same
illumination, measured in lumens per candle-foot. Both types use
virtually the same spectroscope base.
Expandability
One of the highlights and strongest points in favour of the
scanning spectroscope is its expandability. Attachments are
available for both closed-circuit- television (CCTV) and 35m
cameras. The CCTV system will reveal any lines and/or bands
located at the near infrared (700-775nm) position of the
spectrum. Furthermore, it will enhance the absorption bands
and/or lines at the visible region (400nm-700nm) of the spectrum.
This is due to the high sensitivity and perception of the
camera’s “vidicon” tube as compared with the human eye. Lastly,
the entire system can be used for group viewing and for teaching
spectroscopy. . The prism spectroscope cannot perform this task
without major modifications to its optical system. However, the
scanning spectroscope can easily be integrated with a CCTV
camera system without any modifications to produce a sharper
image of the absorption lines and/or bands at the grey scale from
356nm (long wave) to 775nm. Experiments using colour CCTV
produced spectacular results at much higher costs. . By means of
a special adapter, prism and diffraction grating spectroscopes
can be converted into a small spectrograph. The superiority and
performance of the 35mm camera adapter adapted to the diffraction
grating over the prism spectroscope is phenomenal. With the
adapted scanning spectroscope, spectroscopic data may be
photographed with any 35mm camera back, something not previously
possible.
Conclusions
The prism spectroscope satisfies most common gemological
applications and it should be used with great care and knowledge.
However, the gemologist/spectroscopist must be well versed and
thoroughly familiarised with the internal operation of the
instrument, since constant manipulation of the various
adjustments is required. Experienced gemologists agree that
easier readings are obtained using reflected illumination with or
without double surface mirror attached. Wavelength readings will
be approximately but acceptable in most gemological applications.
The scanning diffraction grating spectroscope is very easy to
operate and its will satisfy any gem spectroscopic task without
elaborate adjustments. It is highly recommended for gem
laboratories where speed, confidence, and accuracy are musts.
Wavelength readings are accurate at all times using the
electronic digital display readout. Confirmation of published
data on absorption lines and/or bands can be checked throughout
within 1nm tolerance. The unit can be expanded from 400-700nm to
365-775nm with the addition of a CCTV camera attachment for
several purposes: gem spectroscopy training, group observation,
increasing resolving power, infrared and ultraviolet
observations. It is also worth observing that published drawings
resembling photographic images on gem spectroscopy subjects are
actually drawings for wishful thinking; they do not represent
what the spectroscopist really sees using any type of
spectroscope. However, the most prominent lines and/or bands can
be seen at much lower intensity and all approximately readings,
if obtainable, are perfectly accepted in the gemological
community. All four-digit wavelengths readings published m on gem
spectroscope subjects are copies of the appropriate agent-element
spectra measure via single or dual beam spectrophotometer. As an
example, the 415.5nm line of the yellow diamond cannot be
indicated accurately using any prism spectroscope, but it can be
seen as a very narrow band somewhere between 410nm and 420nm.
With the scanning spectroscope, it seems that the 4155A line is
actually a narrow band measured at 414nm-416nm. I another
important point is that most published data on gem spectroscopy
do not correctly defined absorption bands, but use the term
“narrow [or broad] band at xxx, or centred at YYY.” This does
not tell where the bands begin neither it ends. The length of the
band depends on the amount of light entering through the slit.
The intensity of the gem’s colouring agent together with the
position of the gem on the observation stage are the most
critical factors for intense and verifiable spectroscopic
analysis in the hands of the experienced gemologist.
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