11.02: 10x Loupe

After the eye and the brain, the 10x Loupe (or "hand lens") is the instrument most used by gemologists.
It is used to inspect the interior and exterior of a gemstone, and 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.

It is recommended that you buy a loupe with a matte black casing around the lenses to prevent reflections. Golden and chromed casings may look nice, but they can interfere with colors.

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 completely, therefore several lenses (two or three) are cemented together for optimum performance. These are named compound lenses. The "doublet" loupes have 2 lenses while the "triplet" loupes have 3 lenses of various shapes cemented together.

Magnification

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

Gemological loupes come in different magnifications (from 3x up to 30x) and with different fields of view. For everyday use, a 10x triplet lens with a large field of view (25.4mm) 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, while a 20x loupe will have a 1.25 cm working distance.

You can measure what the working distance for you is by the following formula:

\[Working\ distance = \frac{least\ distance\ of\ focus}{10}\]

The "least distance of focus" is different for every person. You can measure it by holding a ruler next to your eye (pointing away from you and the zero position lined up with your eye), then place the nail of your thumb (from the other hand) close in front of your eye. Close your other eye.
You will notice that the nail is out of focus. Now slowly slide your thumb along the ruler until the nail of your thumb is in sharp focus. Read the value of the ruler where the focus is full and that is your personal "least distance of focus". For me, the original author of this section, that was at 17cm, so my working distance is 1.7 cm.

Although one might expect that a 20x loupe will give a better view of the item being examined, that is only the case for experienced users and only then after having first examined the object with a 10x lens. The reason for this is that the higher the magnification, the lesser your field of view and the lesser the margin of focus.
With a 10x lens you will have an average working distance of about 2.5 cm with some margin below and before. Although the focus outside your working distance is not optimal, you can still see the area reasonably well.
For a 20x loupe, those margins are all half and it will be extremely difficult to see "the big picture", causing you to miss an inclusion in a stone completely. Therefore use the 20x hand lens only in addition to the 10x loupe for a closer inspection.

Another good reason for using a 10x loupe is that it is the standard in clarity grading for Diamonds.

Chromatic aberration

Figure \(\PageIndex{1}\)

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 side by side.
Loupes that are corrected for chromatic aberration are named "achromatic".

Spherical aberration

Figure \(\PageIndex{2}\)

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 achieved by adding a lens with different curves (like a plano-convex lens) in the loupe. A spherical corrected loupe is called "aplanic".
When the loupe is both aplanic and achromatic, we name it "apochromatic".

Doublet loupe

Figure \(\PageIndex{3}\): 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 each other and cemented together, which will correct the chromatic aberration (achromatic).
When two lenses are used where each lens has 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 best not be used in gemology for reasons outlined earlier.

Triplet loupe

Figure \(\PageIndex{4}\): Three lenses "triplet"

The triplet loupe consists of three lenses cemented close together. In the image above, 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 light source through your loupe. If the image remains white (also on the outer edge) 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

In order to be successful working with the hand lens, one must practice with it first.
Unfold the loupe and hold the lens casing between index finger and thumb. Place the lens just before your eye (or just before your spectacles), letting the back of your thumb rest against the side of your nose to keep the loupe steady.
The three remaining fingers should be parallel with or just below your index finger.

With your other hand, hold an object and bring it in front of the lens. To steady the object, let the thumb of the hand that holds the object rest against the middle finger of the hand that holds the loupe. Manipulate the object in a wiggly motion while keeping the thumb and middle finger in contact. Practice this with several objects until you get a feel for the loupe.
From there on you can exercise with gemstones held in tweezers to explore external and internal features in gemstones. When you find yourself on hands and knees searching for stones that popped out of the tweezers, don't be alarmed. All gemologists have red knees.

To keep your eyes from strain, try to keep both eyes open, the unused eye staring blank.

Related topics

• Microscope

Sources

• Gemmology 3rd edition (2005) - Peter Read
• Gem-A Foundation notes
• Gem Identification Made Easy 3th edition (2006) - A. C. Bonanno, Antoinette Matlins