11.03: Spectroscope

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.

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.

Figure \(\PageIndex{1}\): Absorption of all wavelengths except red

Figure \(\PageIndex{2}\): 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.

Figure \(\PageIndex{3}\): Diffraction grating spectrum	Figure \(\PageIndex{5}\): Inside the diffraction grating spectroscope
Figure \(\PageIndex{4}\): 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.

Figure \(\PageIndex{6}\):Prism spectrum	Figure \(\PageIndex{8}\): Inside the prism spectroscope
Figure \(\PageIndex {7}\): 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.

Figure \(\PageIndex{9}\): 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 45º 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

• Fraunhofer
• Dispersion
• Diffraction

Sources

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