Difference between revisions of "Spectroscope"

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The spectroscope is a tool for examining which parts of white light are absorbed by a gemstone (as well as in other materials).<br />
+
The spectroscope is a tool for examining which parts of white light are absorbed by a gemstone (as well as by other materials).<br />
Materials can absorb parts of the electromagnetic spectrum and when the absorbed parts fall within the visible range, that absorbed part will influence the color of the material.<br />
+
Materials can absorb parts of the electromagnetic spectrum, and when the absorbed parts fall within the visible range, that absorbed part will influence the color of the material.<br />
 
When a gemstone is observed with a spectroscope, the absorbed parts show as dark lines and/or bands in the spectroscope image.
 
When a gemstone is observed with a spectroscope, the absorbed parts show as dark lines and/or bands in the spectroscope image.
  
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Color as perceived by the human eye consists of the 7 colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo and Violet. All these colors travel at different speeds and have their own wavelengths. When all the above colors combine, we see it as white light.<br />
 
Color as perceived by the human eye consists of the 7 colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo and Violet. All these colors travel at different speeds and have their own wavelengths. When all the above colors combine, we see it as white light.<br />
  
When white light reaches a substance, part of the light components may be absorbed by the substance. The other light components (residue) form the color of that substance. For instance, if a gemstone would absorb all the colors of the rainbow except red, only the red part of the original white light will be visible and the gemstone will therefore be red.<br />
+
When white light reaches a substance, part of the light components may be absorbed by the substance. The other light components (residue) form the color of that substance. For instance, if a gemstone absorbs all the colors of the rainbow except red, only the red part of the original white light will be visible, and the gemstone will therefore be red.<br />
When viewed through a spectroscope, the absorbed parts of that gemstone will disappear from the spectrum image and only red will be visible in the prism of the spectroscope.<br />
+
When viewed through a spectroscope, the absorbed parts of light by that gemstone will disappear from the spectrum image and only red will be visible in the prism of the spectroscope.<br />
 
Likewise if all colors except red and blue are absorbed by a gemstone, the residual colors (red and blue) will give rise to a purple gemstone.
 
Likewise if all colors except red and blue are absorbed by a gemstone, the residual colors (red and blue) will give rise to a purple gemstone.
  
The pictures below give a crude example of both above mentioned situations.
+
The pictures below give a crude example of both above-mentioned situations.
  
[[image:absorption_red.jpg|framed|left|absorption of all wavelengths except red]]
+
[[image:absorption_red.jpg|framed|left|Absorption of all wavelengths except red]]
[[image:absorption_purple.jpg|framed|left|absorption of all wavelengths except red and blue]]
+
[[image:absorption_purple.jpg|framed|left|Absorption of all wavelengths except red and blue]]
 
<br clear="all" />
 
<br clear="all" />
  
Of course, in real life, the spectrum images are much more sophisticated with small lines and bands indicating specific absorption parts of white light.<br />
+
Of course, in real life, the spectrum images are much more sophisticated, with small lines and bands indicating specific absorption parts of white light.<br />
 
The energy from the absorbed colors (or better, "wavelengths") is transformed inside the gemstone into other types of energy, mostly heat.
 
The energy from the absorbed colors (or better, "wavelengths") is transformed inside the gemstone into other types of energy, mostly heat.
  
One should think of color as a form of energy traveling at a specific wavelength.
+
One should consider color as a form of energy traveling at a specific wavelength.
  
 
===Types of spectroscopes===
 
===Types of spectroscopes===
  
In gemology we make use of two different types of spectroscopes, each with their own characteristics.
+
In gemology, we make use of two different types of spectroscopes, each with its own characteristics.
  
 
====1. Diffraction grating spectroscope====
 
====1. Diffraction grating spectroscope====
Line 38: Line 38:
  
  
The Diffraction grating spectroscope is based on the principle of [[diffraction]]. Maybe the best known brand is OPL, which is produced in the UK by Colin Winter.<br />
+
The diffraction grating spectroscope is based on the principle of [[diffraction]]. Maybe the best known brand is OPL, which is produced in the UK by Colin Winter.<br />
 
Light enters through a narrow slit and is then diffracted by a thin film of diffraction grating material. This produces a linear spectrum image with a generally larger view of the red part than a prism spectroscope.<br />
 
Light enters through a narrow slit and is then diffracted by a thin film of diffraction grating material. This produces a linear spectrum image with a generally larger view of the red part than a prism spectroscope.<br />
These spectroscopes do not have a built in scale.
+
These spectroscopes do not have a built-in scale.
  
 
{|
 
{|
 
|-
 
|-
| [[image:spectrum1.jpg|framed|left|diffraction grating spectrum]]
+
| [[image:Diffraction spectrum.jpg|thumb|300px|left|Diffraction grating spectrum]]
 
|rowspan="2"| [[image:diffr_spectroscope.jpg|framed|right|Inside the diffraction grating spectroscope]]
 
|rowspan="2"| [[image:diffr_spectroscope.jpg|framed|right|Inside the diffraction grating spectroscope]]
 
|-
 
|-
| [[image:spectrum2.jpg|framed|left|diffraction grating spectrum with scale in nm]]
+
| [[image:Diffraction spectrum scale.jpg|thumb|300px|left|Diffraction grating spectrum with scale in nm]]
 
|}
 
|}
  
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The prism spectroscope is based on [[dispersion]]. The light enters through a narrow slit (some models allow you to adjust the width of the slit) and is then dispersed through a series of prisms. Some models have an attachment with a built-in scale. These models are generally more expensive than their diffraction type cousins.<br />
 
The prism spectroscope is based on [[dispersion]]. The light enters through a narrow slit (some models allow you to adjust the width of the slit) and is then dispersed through a series of prisms. Some models have an attachment with a built-in scale. These models are generally more expensive than their diffraction type cousins.<br />
Because prism spectrosocopes are based on dispersion, the blue area of the spectrum is more spread out and the red parts are more condensed than the diffraction grating types.
+
Because prism spectroscopes are based on dispersion, the blue area of the spectrum is more spread out and the red parts are more condensed than the diffraction grating types.
  
 
{|
 
{|
 
|-
 
|-
| [[image:Prismspecmatrix4photoshop_copy.jpg|framed|left|prism spectrum]]
+
| [[image:Prismspecmatrix4photoshop_copy.jpg|framed|left|Prism spectrum]]
 
| rowspan="2" | [[image:Prism_spectroscope.jpg|framed|right|Inside the prism spectroscope]]
 
| rowspan="2" | [[image:Prism_spectroscope.jpg|framed|right|Inside the prism spectroscope]]
 
|-
 
|-
| [[image:Prism_spectroscope_scale.jpg|framed|left|prism spectrum with scale in nm]]
+
| [[image:Prism_spectroscope_scale.jpg|framed|left|Prism spectrum with scale in nm]]
 
|}
 
|}
  
 
===Use of the spectroscope===
 
===Use of the spectroscope===
  
Using the spectroscope poses many problems for those who are not familiar with the instrument. Therefore, before attempting to determine the absorption spectra of gems, it is best to hold the spectroscope against some different sources of illumination, such as a fluorescent light bulb, a computer monitor etc. This will show you very clear absorption bands in most cases.
+
Using the spectroscope poses many problems for those who are not familiar with the instrument. Therefore, before attempting to determine the absorption spectra of gems, it is best to hold the spectroscope against some different sources of illumination, such as a fluorescent light bulb, a computer monitor, etc. This will show you very clear absorption bands in most cases.
  
[[image:spectroscope_use.jpg|left|thumb|350px|use of the spectroscope with reflected light]]
+
[[image:spectroscope_use.jpg|left|thumb|350px|Use of the spectroscope with reflected light]]
  
  
Proper use of the spectroscope and lightning is vital when wanting to see good spectra of gemstones.<br />
+
Proper use of the spectroscope and lighting is vital when wanting to see good spectra of gemstones.<br />
The most widely used technique is to make use of reflected light. Light enters the pavilion of a gemstone at a 45° angle and the spectroscope should be placed at the same angle on the other side.<br />
+
The most widely used technique is to make use of reflected light. Light enters the pavilion of a gemstone at a 45̊ angle and the spectroscope should be placed at the same angle on the other side.<br />
 
The light will travel its longest possible path in this way, picking up the most color.
 
The light will travel its longest possible path in this way, picking up the most color.
  
Line 80: Line 80:
  
 
There are nice spectroscope stands (some with built-in illumination) on the market, but gaining some experience eliminates the need for them. <br />
 
There are nice spectroscope stands (some with built-in illumination) on the market, but gaining some experience eliminates the need for them. <br />
For the new user, it is recommended to start with a gemstone that produces a clear absorption spectrum, such as synthetic Ruby.
+
For the new user, it is recommended to start with a gemstone that produces a clear absorption spectrum, such as synthetic ruby.
  
 
<br clear="all" />
 
<br clear="all" />
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==Related topics==
 
==Related topics==
  
* [[Absorption]]
 
 
* [[Fraunhofer]]
 
* [[Fraunhofer]]
 
* [[Dispersion]]
 
* [[Dispersion]]
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==Acknowledgments==
 
==Acknowledgments==
  
* [http://galleryvoltaire.com Gallery Voltaire] for the blank diffraction images
 
 
* [http://geminterest.com GemInterest] for the blank prism templates
 
* [http://geminterest.com GemInterest] for the blank prism templates
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<br />
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<br />
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'''Next:[[Refractometer| Refractometer]]'''
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<br /><br />
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'''[[Table_Of_Contents| Return to the Table of Contents]]'''

Latest revision as of 11:45, 30 May 2009

The spectroscope is a tool for examining which parts of white light are absorbed by a gemstone (as well as by other materials).
Materials can absorb parts of the electromagnetic spectrum, and when the absorbed parts fall within the visible range, that absorbed part will influence the color of the material.
When a gemstone is observed with a spectroscope, the absorbed parts show as dark lines and/or bands in the spectroscope image.

OPL teaching diffraction grating spectroscope on stand (left) and prism spectroscope with adjustable slit (right)

There are two types of spectroscopes used in gemology:

  1. Diffraction grating spectroscopes (based on diffraction)
  2. Prism spectroscopes (based on dispersion)

Basic

Absorption

Color as perceived by the human eye consists of the 7 colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo and Violet. All these colors travel at different speeds and have their own wavelengths. When all the above colors combine, we see it as white light.

When white light reaches a substance, part of the light components may be absorbed by the substance. The other light components (residue) form the color of that substance. For instance, if a gemstone absorbs all the colors of the rainbow except red, only the red part of the original white light will be visible, and the gemstone will therefore be red.
When viewed through a spectroscope, the absorbed parts of light by that gemstone will disappear from the spectrum image and only red will be visible in the prism of the spectroscope.
Likewise if all colors except red and blue are absorbed by a gemstone, the residual colors (red and blue) will give rise to a purple gemstone.

The pictures below give a crude example of both above-mentioned situations.

Absorption of all wavelengths except red
Absorption of all wavelengths except red and blue


Of course, in real life, the spectrum images are much more sophisticated, with small lines and bands indicating specific absorption parts of white light.
The energy from the absorbed colors (or better, "wavelengths") is transformed inside the gemstone into other types of energy, mostly heat.

One should consider color as a form of energy traveling at a specific wavelength.

Types of spectroscopes

In gemology, we make use of two different types of spectroscopes, each with its own characteristics.

1. Diffraction grating spectroscope

The diffraction grating spectroscope is based on the principle of diffraction. Maybe the best known brand is OPL, which is produced in the UK by Colin Winter.
Light enters through a narrow slit and is then diffracted by a thin film of diffraction grating material. This produces a linear spectrum image with a generally larger view of the red part than a prism spectroscope.
These spectroscopes do not have a built-in scale.

Diffraction grating spectrum
Inside the diffraction grating spectroscope
Diffraction grating spectrum with scale in nm

2. Prism spectroscope

The prism spectroscope is based on dispersion. The light enters through a narrow slit (some models allow you to adjust the width of the slit) and is then dispersed through a series of prisms. Some models have an attachment with a built-in scale. These models are generally more expensive than their diffraction type cousins.
Because prism spectroscopes are based on dispersion, the blue area of the spectrum is more spread out and the red parts are more condensed than the diffraction grating types.

Prism spectrum
Inside the prism spectroscope
Prism spectrum with scale in nm

Use of the spectroscope

Using the spectroscope poses many problems for those who are not familiar with the instrument. Therefore, before attempting to determine the absorption spectra of gems, it is best to hold the spectroscope against some different sources of illumination, such as a fluorescent light bulb, a computer monitor, etc. This will show you very clear absorption bands in most cases.

Use of the spectroscope with reflected light


Proper use of the spectroscope and lighting is vital when wanting to see good spectra of gemstones.
The most widely used technique is to make use of reflected light. Light enters the pavilion of a gemstone at a 45̊ angle and the spectroscope should be placed at the same angle on the other side.
The light will travel its longest possible path in this way, picking up the most color.

To prevent the background on which the stone lays from causing false readings, one should use a black non-reflective underground, such as a small piece of black velvet.

Another technique is to position the gemstone and the light source (penlight) in one hand in such a way that the light source illuminates the gem from behind, thus viewing the gemstone in transmitted light.

There are nice spectroscope stands (some with built-in illumination) on the market, but gaining some experience eliminates the need for them.
For the new user, it is recommended to start with a gemstone that produces a clear absorption spectrum, such as synthetic ruby.


Related topics

Sources

  • A Students' Guide to Spectroscopy (2003) - Colin Winter FGA, DGA

Acknowledgments



Next: Refractometer

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