10x Loupe

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Revision as of 10:42, 22 July 2006 by Doos (talk | contribs) (Testing of the loupe)
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After the eye and the brain, the 10x Loupe (or "hand lens") is the most used instrument by gemologists.
It is used to inspect the interior end exterior of a gemstone and it is the primary instrument used for clarity grading of Diamonds. A good 10x Loupe can be bought for under USD 50.00 and a gemologist should carry it with her/him at all times.

Gemological loupes come in different magnifications (from 3x up to 30x) and with different field of view. For day to day use a 10x triplet lens with a large field of view (about 20mm) is your best choice. The higher magnification ones usually have a smaller working distance (the distance between the loupe and the gemstone).
The standard 10x loupe has a working distance of about 2.5 cm, whilst a 20x loupe will have a 1.25 cm working distance.

About lenses

Lenses used in optics are usually made of crown and flint glass.
Crown glass has a refractive index between 1.5 and 1.6 and is low dispersive, while flint glass may have higher refraction indices and is highly dispersive. Both types work complementary and are widely used in gemological loupes.

Lenses are cut in different shapes (convex, concave and combinations) following Coddington Shape factors for best form. This is to eliminate spherical and chromatic aberrations, which would distort the image.
The best forms however do not eliminate the aberrations completey, therefor several lenses (two or three) are cemented together for optimum performance. The "doublet" loupes have 2 lenses whilst the "triplet" loupes have 3 lenses of various shapes cemented together.

The 10x means that the loupe will magnify an image 10 times, so if a 1mm2 square will appear to be 1cm2.

Chromatic aberration

Chromaticaberration.jpg


Chromatic aberration refers to color fringing of the image due to dispersion inside the glass lens. The outer edges of the lens refract the most and hence more dispersion will occur from light rays at those points.

To overcome this, two lenses (one bi-concave and one bi-convex) of different dispersive powers (crown and flint glass) are placed after eachother.
Loupes that are corrected for chromatic aberration are named "achromatic".


Spherical aberration

Sphericalaberration.jpg


Lenses also suffer from spherical distortion, also caused by differences in refraction on the outer edges from the lens and the rays which travel through the center. This results in a hazy and out of focus view.

Correction can be done by placing a lens with different curves in the loupe. A spherical corrected loupe is called "aplanic".
When the loupe is both aplanic and achromatic, we name it "apochromatic".


Doublet loupe

Two lenses "doublet"


Doublet loupes consist of two lenses (hence the name "doublet"). The lenses are either corrected for chromatic aberration (achromatic) or spherical aberration (aplanic). In the image to the left two lenses (one bi-concave and one bi-convex) are placed close after eachother and cemented together, which will correct the chromatic aberration (achromatic).
When two lenses are used that each have a different curvature (concavo-convex or plano-convex) it is an aplanic or othoscopic loupe.

These loupes are relatively inexpensive (under USD 10.00) and should better not be used in gemology for reasons outlined earlier.


Triplet loupe

Three lenses "triplet"


The triplet loupe concists of three lenses cemented close together. In the image on the right you see three lenses that make up a triplet loupe (other configurations are possible). From left to right they are:

  • A plano-convex lens to eliminate the spherical aberration
  • A bi-concave lens
  • A bi-convex lens

The latter two overcome the chromatic aberration. All three together they form an apochromatic loupe. This is the loupe you will need as a gemologist.



Testing of the loupe

You can test your loupe for chromatic aberration by looking at a white lightsource through your loupe. If the image remains white (also on the outeredge) it will be achromatic.
Spherical aberration can be tested on mm squared drawing paper. The squares should remain square in the total view of your loupe.

Proper use of the 10x loupe