magifying glass/loupe

hi

i have a question about loupes to examine diamonds.
loupes are small.
is a normal 10x magnifying glass ok to use for grading diamonds, or must
someone use only a loupe?

On Mon, 02 May 2005 07:31:18 -0700, in ¸õ "Lawrence" wrote:

hi

i have a question about loupes to examine diamonds.
loupes are small.
is a normal 10x magnifying glass ok to use for grading diamonds, or must
someone use only a loupe?


The "loupe" form of magnifier is really only the casing and mounting of the lens used,
either in the little folding case that protects the lens, or a type one can conveniently
hold in place, somehow, in front of the eye. There are a number of types. And these
are just mounting or holding methods for the lens, and of little consequence.

What IS im portant is the lens itself. A simple glass lens, especially at
magnifications as high as ten power (which means it has a one inch focal length, by the
way, since normal unmagnified vision is assumed to be a distance of about ten inches) is
subject to several types of distortion. One, chromatic distortion is caused by slightly
different bending of light at one end of the color spectrum compared to the other, so
edges of things appear not as a sharp edge, but as a slightly fuzzy multicolored/color
fringe. The other main type os spherical distortion, so that straight lines appear
curved or bowed out.

Accuratly seeing what's in a diamond is difficult to do accuratly when the lens is
subject to these distortions.

So the standard for grading diamonds says that the lens or magnifier used must properly
correct these types of distortion.

In general, this requires either a complex system, such as an actual microscope design,
or in the case of the classic diamond loupe, what is called a triplet lens, which is a
lens made of three layers of glass, each with different refractive index. The
multilayer lens corrects the chrmoatic distortion, and most of the spherical distortion.
The so-called "Hastings triplet" type of this lens is generally considered the best
type.

There are a number of types of simple/less costly lens designs that correct some, but
less, of the distorion. typically doublet lenses are such, and can cost a small
fraction of the cost of a good triplet lens. They can be useful for informal checking,
but are not considered up to the standard for any serious diamond or gem clarity
grading.

Yes, the lenses are physically small. But this is mostly a limitation when people use
them incorrectly. Used by holding the lens close to the stone some distance from the
face, they are hard to use. Correctly, you hold the lens very close to the eye, about
the same distance as a normal eyeglass lens, and bring the stone close enough to then be
in focus. In this manner, you'll find you've actually got a reasonably large field of
view. If you're dealing with any diamond large enough that you cannot see the whole
thing with such a setup, and a simple standard 18 mm triplet lens, then I'd suggest you
can well afford a larger diameter triplet, or even a microscope (grin).

Peter

Lawrence wrote:
hi

i have a question about loupes to examine diamonds.
loupes are small.
is a normal 10x magnifying glass ok to use for grading diamonds, or must
someone use only a loupe?



The short answer is: no, you need a 10x lens (preferably a triplet), that is
both achromatic and aplanatic.

Now it's Peter's turn. He'll tell you the same thing in much more eloquent prose.



--
Abrasha
http://www.abrasha.com