What is a Phoropter?
A phoropter is the instrument used by an eye doctor to measure the glasses prescription. The phoropter resembles the physical appearance of a butterfly. This instrument is utilized in the classic test of “1 or 2” in which eye doctors present two options of lenses and ask which lens is more clear. Many lenses with different powers are contained in this ophthalmic diagnostic tool. A phoropter is also known as a “refractor,” and it is used to measure the eye’s refractive power during an eye exam.
- The phoropter is an instrument containing many lenses of different powers.
- The phoropter is used to determine the distance and reading glasses prescription, including the amount of nearsightedness, farsightedness, and astigmatism.
- In some cases, the phoropter may also be used to check eye coordination and the eye’s resting position.
Understanding the Phoropter
A phoropter is a device which has many different lenses inside of it. It has a unique design with a butterfly-shaped appearance. The patient is instructed to look through the two circular openings of the phoropter with their eyes. The doctor manually changes the lens power by moving wheels and turning knobs on the machine. In some cases the doctor may cover, or occlude, one of the eyes and evaluate the glasses prescription for each eye individually. This procedure is called a monocular subjective refraction.
A phoropter can also be used to objectively check the glasses prescription, without any input from the patient. This procedure is called retinoscopy. Retinoscopy can be performed with or without a phoropter. During retinoscopy, the eye doctor instructs the patient to look at a large letter on a screen. They then shine light into the eyes to evaluate the retinal reflex. The retinal reflex is neutralized with lenses. For example, a retinal reflex that moves in the same direction of the doctor’s light is neutralized with plus lenses. A retinal reflex that moves in the opposite direction is neutralized with minus lenses. Retinoscopy is a very useful technique to determine the glasses prescription for nonverbal patients. For example, children, people with cognitive impairments, or those who speak a different language. Retinoscopy is a method of obtaining the glasses prescription without the patient choosing between ‘1 or 2’.
The Steps of Refraction in Phoropters
A refraction is a procedure in which the eye doctor determines the glasses prescription. Refraction is done using a phoropter, which allows the doctor to test different lens powers. The following actions are involved in a standard phoropter refraction:
- Sphere power
The initial step is to examine the patient’s sphere power. You may have noticed that your prescription for corrective lenses has “spherical” (or “SPH”) written on it. The term “sphere” simply denotes the amount of lens power required to correct nearsightedness (-) or farsightedness (+). The doctor will ask which spherical lens power looks clearer between two options (“1 or 2”.)
- Cylindrical power
The doctor checks for the presence or absence of astigmatism by using a cylindrical lens (“CYL”). Astigmatism indicates that a patient’s lens strength is not uniform across the eye, so different areas of the eye require different lens powers. .
- Reading power
Patients who are presbyopic, approximately over the age of forty, will typically require a reading glasses prescription. The eye doctor will check the reading power (“ADD”) and determine if the patient may benefit from magnifying glasses to aid in reading small print.
- Contact lens power
Can also be determined using a phoropter. For patients with a moderate amount of farsightedness or nearsightedness, the glasses prescription and contact lens prescription are different. The contact lens prescription is typically ‘less strong’ than the glasses prescription because contact lenses sit closer to the front of the eye. The phoropter gives the spectacle prescription, which is then mathematically converted into the appropriate contact lens prescription.
Phoropters’ Key Benefits
- Quick prescription determination
Since the phoropter contains a wide range of lens powers (+16.75 to -19.00) , changing lens powers is significantly quicker than with demonstrating each trial lens individually by hand.
Patients do not feel any pressure or weight from testing high powered lenses, because the lenses are not sitting directly on their face.
- Jackson Cross-Cylinder
There are many different phoropter parts. The Jackson Cross-cylinder (JCC) is the part of the phoropter that determines astigmatism. It tests for the magnitude and direction of astigmatism. Astigmatism simply means that the eye does not have one single power since it is not perfectly round. It is a common finding in approximately half of all Americans.
- No lens smear
Because patients cannot touch the lenses that are encased in the phoropter, they remain relatively clean and free of smudges or fingerprints.
- Risley prisms
This apparatus allows eye doctors to measure the natural eye alignment (phoria). Prism works by bending light, and moving the location of a perceived image. Prism testing can determine if the eye coordination is robust and is used to evaluate symptoms of double vision, eye strain, or headaches.
Some clinics use automatic, digital phoropters, which are fast and intuitive to use, while still maintaining prescription accuracy.