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Old December 12th, 2010, 09:44 PM
Doug Kerr Doug Kerr is offline
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Join Date: May 2006
Location: Alamogordo, New Mexico, USA
Posts: 8,634
Default Fabulous eyewear

Outrageous eyewear is all the rage at costume balls, but here we see Carla, always in style, with something a little different:

Douglas A. Kerr: These glasses are such a trial!

She is sporting a modern optometric trial frame, a Hong Kong-made reproduction of a classic American Optical Company model, equipped with reduced diameter, corrected curve additive power trial lenses by Marco Ophthalmics. We'll get a closer look at it shortly.

The trial frame system is a traditional, but still very valuable, way of determining the optical parameters of the lenses needed to correct a person's errors of refraction, a class of vision defect that includes farsightedness (hyperopia or hypermetropia) nearsightedness (myopia), astigmatism, and various phorias and vergences (errors in the natural pointing position of the eyes).

In effect, the examiner (an ophthalmologist or optometrist) constructs an emulation of the final lens by combining calibrated lenses in an eyeglass-like trial frame. Through a strategy of A-B comparison (to the tune of, "which is better, one or [clink] two"), the optimum lens for each eye is "designed". An ophthalmic prescription then conveys the optical parameters (in a very specialized, and curious, system of notation) to the dispensing optician, who arranges to implement it in a pair of glasses.

The process is a bit tedious, and the use of the trial frame system has been greatly supplanted today by the use of the refractor, the scary mask-like instrument supported on a crane in front of the examination chair. Here we see an especially scary view of one in a U. S. Navy shipboard clinic:

Burton 7500-2 refractor
U. S. Navy Photograph (public domain)

The refractor is in effect an eyeglass simulator. A complex system of lenses in rotating turrets, controlled by knobs and wheels through a complex mechanism, allows lenses of a wide range of optical parameters to be "constructed" in front of each eye. Again, "which is better, one or [click] two" is heard as the examiner closes in on the ideal lens design for each eye.

Nevertheless, many authorities feel that an examination with a trial frame and trial lenses (often called a "trial case refraction", after the case typically used to carry the repertoire of calibrated lenses, often over 260 of them, needed to cover the waterfront with the technique) is better than measurement with a refractor. Among other things, once the proposed design has been set up in the trial frame, the patient can be asked to walk around, go up and down stairs, read his or her favorite reading material, look at the instrument panel of his car, knit and purl a little, or whatever, to be certain of the practical effect of the design.

The trial frame we saw in the first photo is what many (including myself) consider a modern design, but in fact almost this exact design (including the appearance and cosmetic details) was introduced in about 1938. In fact, the design (again including a detailed and corresponding appearance portrayal) appears in a patent applied for in 1939.

Here we get a little closer view of our "Hong Kong" model, again with four contemporary Marco trial lenses in place.

Douglas A. Kerr: Hong Kong-made trial frame (after American Optical Company design)

The rearmost lenses on each side are "spherical" lenses, having uniform refractive power in any transverse direction. These lenses correct basic near- or farsightedness (farsightedness for these particular ones, with positive powers).

The frontmost lenses are "cylindrical" lenses, having a maximum refractive power along one direction, zero refractive power along the direction at right angles to that, and intermediate values at intermediate directions (and it varies in a surprising way). These are used to correct for astigmatism.

Because the orientation of the cylindrical lens is pertinent, the turret carrying the lenses can be rotated with a small knob, and there is a scale against which an index mark on the cylindrical lens can be read. (This also rotates the spherical lenses, but of course that doesn't do anything, since they are rotationally-symmetrical.)

In some cases a third lens is added in the extreme front position, and in some cases, a fourth lens can be placed in a position on the back side of the frame.

Two small knobs move the lens units laterally to match the separation of the patent's pupils (and even to accommodate the fact that they might not be evenly spaced about the nose, from which the glasses themselves get their lateral position.

Two other knobs raise and lower the angle of the temple pieces, so as to get the proper "tilt" of the lenses (the pantoscopic angle) despite differing altitudes of the external ears.

Here we see an easier design, this one from American Optical Company itself (another unit from our personal collection).

Douglas A. Kerr: American Optical Company trial frame, ca. 1895)

We believe this one dates from about 1895. The various scales are engraved on ivory. This is shown with four trial lenses in place, these of the older design, with thin frames and "full-diameter" lenses. Trial lenses of this type are still made.

In this model, there is only one "real" lens position on the front on each side. There is provision for placing a second lens momentarily in front of the other one (as if often done during the examination procedure). But the second "real" lens on each side goes on the rear of the frame.

On the frontmost lenses (again, the cylindrical ones) we see frosted areas. The cylinder axis of these lenses passes up the center of the clear area, and is in fact shown precisely by tick marks on the lens (you can see one on the right side of the photo). The frosted areas allow the examiner to quickly see the approximate orientation of the axis. They also make it obvious which lenses are spherical and which are cylindrical. Not all manufacturers' lenses have that feature, however.

Lenses of this genre from different manufacturers exhibit a fabulous range of different schemes for showing the spherical vs. cylindrical category of the lens and the sign of its power (plus or minus) with combinations of explicit markings in different forms, shapes of the handling tabs, colors of the rims and handling tabs, and/or the presence or absence of tick marks or frosted areas (which appear only on cylindrical lenses).

Best regards,

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