A phoropter is a common name for an ophthalmic testing device, also called a refractor. It is commonly used by eye care professionals during an eye examination, and contains different lenses used for refraction of the eye during sight testing, to measure an individual's refractive error and determine his or her eyeglass prescription. It also is used to measure the patients' phorias and ductions, which are characteristics of binocularity.
Typically, the patient sits behind the phoropter, and looks through it at an eye chart placed at optical infinity (20 feet or 6 metres), then at near (16 inches or 40 centimetres) for individuals needing reading glasses. The eye care professional then changes lenses and other settings, while asking the patient for subjective feedback on which settings gave the best vision. Sometimes the habitual glasses Rx or an automated refractor is used to provide initial settings for the phoropter, and sometimes a retinoscope is used through the phoropter to measure the vision without the patient having to speak, which is useful for babies and people who don't speak the language of the practitioner.
The major components of the phoropter are the battery of spherical and cylindrical lenses, auxiliary devices such as Maddox rods, filtered lenses, prisms, and the JCC (Jackson Cross-Cylinder) used for astigmatism measurement. The prismatic lenses are used to analyze binocular vision and treat orthoptic problems.
From the measurements taken, the specialist will write an eyeglass prescription that contains at least 3 numerical specifications for each eye: sphere, cylinder, and axis, as well as pupillary distance (distance between eyes, and, rarely, prism for one or both eyes.
The lenses within a phoropter refract light in order to focus images on the patient's retina. The optical power of these lenses is measured in 0.25 diopter increments. By changing these lenses, the examiner is able to determine the spherical power, cylindrical power, and cylindrical axis necessary to correct a person's refractive error. The presence of cylindrical power indicates the presence of astigmatism, which has an axis measured from 0 to 180 degrees away from being aligned horizontally.
Phoropters are made with either plus or minus cylinders. Traditionally, ophthalmologists and orthoptists use plus cylinder phoropters and optometrists use minus cylinder phoropters. One can mathematically convert figures obtained from either type of phoropter to the other.
Trademark and origin of the term
Phoroptor is a registered trademark currently owned by Reichert Technologies, filed Apr 25, 1921 by DeZeng Standard of New Jersey, with the USPTO, serial number 71146698. The word was coined at that time for the newest version of their phoro-optometer. DeZeng was purchased in 1925 by American Optical of Massachusetts, who continued to market the product, but the term, often spelled phoropter, has become a genericised trademark for all brands of modern vision testers, especially since AO's main competitor, Bausch and Lomb, stopped making their Greens' Refractor in 1970s. Reichert bought AO's refracting equipment division in 1980s, and their current version is named "Ultramatic Rx Master Phoroptor".
The history of the phoropter, as a binocular refracting device which can measure phorias, ductions, and other traits of binocularity, as distinct from the monocular optometer, which cannot, starts in 1917, with the introduction of the Ski-optometer by Michael Woolf and the Phoro-optometer by Henry DeZeng. These two inventions, as they continued to improve, were accompanied by a third device, the Greens' Refractor, which entered the market in 1934. European manufacturers were working on similar devices as well.
Introduced in 1917, the Woolf Ski-optometer, so named for its usefulness in doing skiascopy, was a device produced in New York City which contained a battery of convex lenses for each eye, ranging in power from +0.25 to +6.00, as well as a battery of cylindrical lenses for each eye, ranging from -0.25 to -2.00, combined with auxiliary lenses which gave the device a total power range of +12.00 to -12.00 sphere, as well as the cylinders. To extend the range, there were clips on both the front and back of each eye hole for the insertion of hand held trial lenses, the front ones having a mechanism to rotate the axis with the thumb. There were Risley prisms for each eye, and a Steven's phorometer. Maddox rods were optional. The patent dates were 1910 and 1917. Around 1922 the patents and rights were transferred to General Optical Company of Mount Vernon, NY, which had been making a much larger, heavier and more solidly encased instrument, called the Genothalmic Refractor, since around 1920. This instrument had a range of +17.75 to - 22.50, and up to -3.75 cylinder, Maddox rods, Risley prisms, and a Steven's phorometer. The clips on the backs for holding trial lenses were dropped, and replaced with rubber eye cups, and the front ones were replaced with simple slots without the thumb mechanisms. It weighed 7 pounds 5 ounces, and unlike all earlier devices of this kind, it hung from a horizontal mounting bar instead of being supported from the bottom. Like the Woolf, it had no Jackson cross-cylinders at first, so a separate hand-held one was required. Later models of the Genothalmic were fitted with JCC's. General Optical sold out to Shuron Optical of Geneva, NY in 1927, which sold the refractor until the 1930s. A refined and improved version of the Genothalmic Refrator was manufactured in London starting in 1932, and sold in the UK by S. R. Stearman, S. Pulzer & Son Ltd., and others, as the British Refracting Unit (B.R.U.).
Also introduced in 1917, and possibly earlier, the DeZeng Phoro-Optometer was a device produced in Camden, New Jersey, which contained a battery of convex lenses for each eye, a battery of concave lenses for each eye, and auxiliary lenses which gave it a total power range of +15.75 to -19.75, as well as a Maddox rod and Risley prism for each eye, and a Steven's phorometer. There were no cylindrical lenses, so testing for astigmatism required the use of manual trial-lenses, for which there were rotating holders on the front of each eye hole, and there were stationary ones on the backs as well. Jackson cross-cylinders were optional, but they did not flip, they rotated in the same plane. The patent date was 1909. It weighed 3 lb. 2 oz. Around 1919 an improved version was given model No. 574, reduced in size but with the same range (lenses reduced from 1 inch to ¾ inch), the forehead rest was removed, and the rear trial lens clips were replaced with rubber eye guards. It weighed 2 lbs. 12 oz. In 1922, DeZeng replaced No. 574 with No. 584, and shortened the name to Phoroptor. This device became so popular that its name became genericized, though often spelled phoropter. The Phoroptor was smaller (lenses reduced again, to 9/16 inch diameter) and more precisely made than the 574, but with a similar power range, and the front clips for hand-held trial lenses were removed and replaced with batteries of cylinder lenses ranging from -0.25 to -4.75. The Steven's phorometer was dropped, and there were no Jackson cross cylinders. It weighed 2 lb. 8 oz. In 1925, American Optical Co. bought DeZeng, and in 1927 introduced No. 588, the AO Wellsworth DeZeng Phoroptor, which was slightly larger; the lenses were increased to 11/16 inch and it weighed 3 lb. 2 oz. This was the first in the DeZeng/AO line to hang from a horizontal mounting bar, the earlier ones were supported from a bar below it. In 1934, AO introduced No. 589, the Additive Effective Power Phoroptor, once again enlarged and improved. The lenses were increased to ¾ inch, the permanent size, and the unit was much more massive, with a weight of 7 lb. 9 oz., and with a range of +16.87 to -19.12 and -6.00 cylinder, with auxiliary lenses to increase these to +18.87/-21.12/-8.00. All these models resembled the original DeZeng model in design, but No. 590 of 1948 was a completely re-designed device, much larger and heavier, and more modern. It weighed 10 lbs. 7 oz. This was followed by another complete re-design in 1956, the RxMaster, which became the prototype of all modern phoropters, and was updated to the Ultramatic RxMaster in 1967, which is the current model. AO sold their phoroptor division to Reichert in 1982, who still makes the Ultramatic.
At the turn of the 20th century, ophthalmologists A. S. Green, L. D. Green, and M. I. Green, of San Francisco, CA, designed an optometer, which they developed slowly over many years. Drs. Green teamed up with inventor Clyde L. Hunsicker of San Francisco, who applied for a patent on October 25, 1926. The title of their invention was simply an "optometrist instrument", and the text described it as an optometer. Patent 1,804,690 was granted to the Greens and Hunsicker in 1931, and sold to Bausch & Lomb, who had them redesign it (patent 1,873,356, granted 1932). B&L trademarked it as "Greens' Refractor" and introduced it in 1934. It was far more advanced than the competition in many ways. The power could be read right off the dial without having to do mental calculations, the range was far higher, from +19.75 to -28.00 and with cylinders up to -7.50, the battery of cylinders was much more intuitive and easy to use, and it had Jackson cross cylinders affixed, which the competing lines didn't. (The first AO Phoroptors with JCC's were late models of the Additive). It weighed 13 lbs. 1 oz. The Greens' Refractor soon became the gold standard among eyecare professionals. It helped put the Woolf/Genothalmic/Shuron line out of the market and forced AO to completely redesign their phoroptor, not once, but twice (the 590 failed to compete). The Greens' Refractor remained unchanged for over 4 decades, but sales slipped when AO introduced the Ultramatic RxMaster with its revolutionary yoked JCC in 1967, and production finally ended in the late 1970s. In 1978 B&L introduced the Greens II refractor with yoked JCC, but was immediately forced to pull it due to patent disputes with AO. In spite of the fact that production stopped decades ago, thousands of Greens' Refractors are still being used today, as they are virtually indestructible, and have a devoted rank of optometrists who swear by them.
|Wikimedia Commons has media related to Phoropter.|
- Dictionary.com, Definition of "phoropter", American Heritage Stedman's Medical Dictionary. Retrieved 10-10-10.
- "Phoroptors, Early American Instruments of Refraction, and Those Who Used Them", Gary L. Campbell O.D., 2008, Wheaton, Illinois.
- Meyrowitz Catalogue of Ophthalmic Equipment, 6th edition, New York City, 1920
- Bauch and Lomb Greens' Refractor Reference Manual, date uncertain