Eye Care Terms

Eye Care Terms/Glossary

We have found that many doctors discourage their patients from “surfing” for medical information.  We, however, encourage you to become as informed as possible about issues that you are concerned about.  A well-informed patient is a more compliant patient and ultimately a happier patient about their care. Our concern for you is that you do not act abruptly about your concerns or problems without proper consultation with doctors that can properly instruct you about your issues. Contact us if you have additional questions about specific problems.

Many of these words you may have just heard during your exam and you would like to learn more information about it.  This glossary isn’t intended to give you a medical definition of the term, but rather a brief explanation of the term. Please send us any suggestions that you may have regarding this material.  Words that have been omitted will be gladly added if you offer them as suggestions. However, we have mostly left off proprietary names of products simply due to the extensive number of products in our field.

Add power  

This is frequently just called the “add”.  This is the number that indicates the power of the reading portion of a bifocal or trifocal lens.  This number represents the difference in power between the distance portion of the lens and the reading portion.  Some lensometers will record the actual add power some will record the total power of the reading portion, which is the distance and near powers added together.

A/R coating

Antireflection coatings are coatings on both surfaces of a lens, designed to reduce the total light reflected from each surface.   An A/R coating cannot reduce the reflection equally of each color of light (wavelength) that is reflected.  Therefore a reflection from the surface will have a slight color, depending upon the type of coating that is used.  This does not cause the lens to have a color, just the reflection. A/R coatings cause an ophthalmic lens to be less noticeable, to the wearer and to others, thus reducing eye fatigue and strain caused by glare and reflections.

Amblyopia

Amblyopia has many causes, but is the result of improper development of the neural connection between the eye and the visual cortex of the brain.  As a result, the visual acuity of that eye is worse than that of the other eye. However, the eye itself is perfectly healthy.

Anisometropia

This describes a condition when the refractive error (glasses prescription) of one eye is significantly different than the other eye.

Aspheric lenses

Aspheric lenses have surfaces that are not the same curve in all places.  A spherical lens has the same surface curve at any distance from the center of the lens.  Aspheric lens surfaces gradually flatten towards the edge of the lens. The advantages of aspheric lenses are: lenses are made thinner, lighter, and more cosmetically attractive.  They also help to eliminate some noticeable distortions in the peripheral portion of the lens.

Astigmatism

Astigmatism is a condition of the eye that causes the eye to not focus all the light entering it into one focus point.  Astigmatism can be combined with nearsightedness or farsightedness or may be by itself. As a result, the lens that is used to correct astigmatism will have an “out of round” surface. Often an attempt to describe astigmatism will include a comparison of the eye to the side of a football.  The long curve of the side of the football would not focus light the same as the short curve.

Autorefractor

Refraction is the process of deciding the glasses or contact lens prescription of an eye.  An autorefractor will estimate that prescription without

Autokeratometer

A keratometer measures the curvature of the front of the eye (cornea).  It does so by measuring the size of a light reflection off the cornea. An autokeratometer measures the same curve with less observation and subjective input from the operator.  This measurement is most valuable for the management of certain eye diseases and for contact lens fitting and management.

Axis

Axis is one component of a prescription for an ophthalmic lens or contact lens.  It is part of the astigmatism prescription. The axis indicates how the astigmatism power is oriented across the front of the eye or the lens.   If astigmatism were compared to the side of a football, the axis would indicate how the football (or lens) is oriented. (Lying on its side, standing on end, or tilted.

Base Curve

The base curve is used to help define the design of an ophthalmic lens or a contact lens.  A range of ophthalmic lens may be made from the same base curve semi finished lens. In general, the front surface of an ophthalmic lens is considered the base curve.  Grinding the appropriate curve on the back of the lens will create the patient’s specific hyperopic, myopic, or astigmatic prescription. Contact lenses also have a base curve, which is on the back of the lens.  This curve helps determine how a particular lens will fit on the patient’s eye.  The specific power of the contact lens is defined by changing the curve on the front of the lens.

Bifocal lenses

Bifocal lens refers to any lens that has two different powers.  Traditionally, that would be one power in the top for distance vision and one lower power for near work.  Although technically they are not bifocals, the newer progressive lenses are frequently referred to as “bifocals”.

Bridge size

Frames have several different measurements. The distance between the nasal edges of the fram for each lens is the bridge size. It is sometimes also called the distance between the lenses (DBL).

Cataracts

A cataract is any opacity or discoloration of the lens that is inside the eye.  Normally, this lens is very clear. The list of possible causes for cataracts can be very long.  The most common causes are aging, smoking, systemic disease, ocular disease, trauma, and exposure to light. Currently, the only treatment for cataracts is to surgically remove them and replace the lens with a plastic lens.  Some patients may ask if the cataract can be removed with a laser – this is not possible now.

Chief Complaint (CC)

The CC is the main reason that the patient is in your office today.  Most major medical insurances will require a CC. The most notable, Medicare, always requires a CC in order to pay a submitted claim. Medicare uses a system for recording the exam and history of the patient encounter called Documentation Guidelines (DG).  The DG requires a CC for claims payment.

CLT contact lenses

These lenses are marketed and called several different names.  A more generic name would be overnight orthokeratology rigid contact lenses.  Highly oxygen permeable rigid materials make a lens designed to be worn overnight.  In the morning the patient can remove the lenses and see well all day without wearing the lenses.  This is a great advantage for very active patients or conditions difficult to wear soft lenses or glasses.  Good examples would include; football, baseball or softball catcher, swimming, wrestling, water skiing, hostile working environments (high fumes, smoke, spray painting).

Color blindness

In most cases, color blindness is a misnomer.  People that have color vision problems are more correctly called color deficient. There are three types of color deficient person. Most have a problem correctly identifying differences in shades of red and green. Most of these people can see some degree on red and green, but the shades give them significant problems. Most color deficient people are men.  The gene that causes color deficiency is on the chromosome that dictates the sex on an individual and therefore carries over to usually only the men.

CR-39

This is the original plastic ophthalmic lens.  This plastic has excellent optical properties. However, it has a low ability to change the direction of light, which causes this lens to be thicker and heavier than some of the newer plastic lenses.  At this time CR-39 is still very widely used however, it is slowly losing some of its market to other plastics.

Cylinder power

This is the number, in the lens prescription, that indicates the amount or strength of the astigmatism.  The axis describes the orientation of the cylinder power. The prescription for astigmatism is not complete without both the cylinder power and the axis.

DBL

Frames have several different measurements. The distance between the lenses is the DBL. It is sometimes also called the bridge size.

Decentration

Decentration is the amount that the center of the lens is decentered from the center of the frame in order to put the lens center in front of the eye of the patient that is going to wear the frame. Several items cause us to use decentration.  

  1. The PD of the patient must be measured either for distance or near depending upon the use of the glasses.
  2. The PD of the frame must be measured.
  3. If the frame PD and the patient PD are the same, no decentration is necessary.
  4. If the frame PD is wider than the patient PD, the lens centers must be moved inward from the center of the frame to put the lens centers in front of the eyes.
  5. A simple example would be Frame PD = 70 mm, Patient PD = 60 mm. 70 mm – 60 mm = 10 mm, therefore, each lens must be moved inward 5 mm (5+5=10).

Documentation Guidelines

The documentation guidelines (DGs) were developed to help doctors and insurers communicate what had been done during an examination.  This was to help each judge if the code used to describe the patient encounter to the insurance company was an accurate description of the work that had been done. In every office, the doctor(s) and the billing staff should be knowledgeable about how the DGs were designed to work and how they affect your practice.  Medicare and some other insurers will use the DGs to grade your claims and pay your office for your work.

EHR/EMR

Both are terms to describe the software used in medical practices to record the results of the examination.  There are many versions on the market and it is currently changing fast. Electronic Health (Medical) Records should integrate with the business office software but may not and are considered separate components used in the office.  In 2011 our office started using Practice Director.

Esophoria / Esotropia

Phoria is a term we use to describe eyes trying to misalign, but not actually misaligned.  Tropia describes eyes when they actually misalign. Eso means turning in. Prism may be prescribed for either.  An esophoria means the eyes may try to turn in (or cross). Esotropia means the eyes do actually cross.

Exophoria / Exophoria  

Exo is used to describe turning out.  Exophoria would be eyes trying to turn out from straight, but not actually turned out. Exotropia would mean eyes turning out. Prism may be prescribed for either.

Eye size

This is the common way of describing the size of a frame across the lens surface.  This is measured horizontally across the widest part of the frame. It is usually measured in millimeters.

Farsighted

Far sightedness (hyperopia) is a condition where the components of the eye do not have enough focusing power to make light from far away focus clearly on the back of the eye without additional help.  Trying to see closer objects is even more difficult for the uncorrected hyperopic patient. Nearsightedness means the eye is focused too much to make light from far away focus clearly. Farsighted patients may see clearly far away or near uncorrected if they have enough additional focusing power.

Flashes and Floaters

Floaters are small pieces of tissue that have come loose in the inside of the eye.  They float in the semi-fluid contents of the eye. As they move about they create a shadow that the patient may see.  Floaters themselves are not a danger to the eye, although they can disturb vision if they are large enough or numerous.  Floaters may be completely normal or they may be an indicator of serious ocular problems. Only by having a doctor look inside the eye will a patient know if the floaters they have are an indication of a serious problem. Flashes are small flashes of perceived light that are caused by changes in either the eye or the visual cortex.  They may be implicated with various diseases, including retinal detachment and migraine headaches.

Floaters

Floaters are small pieces of tissue that have come loose in the inside of the eye.  They float in the semi-fluid contents of the eye. As they move about they create a shadow that the patient may see.  Floaters themselves are not a danger to the eye, although they can disturb vision if they are large enough or numerous.  Floaters may be completely normal or they may be an indicator of serious ocular problems. Only by having a doctor look inside the eye will a patient know if the floaters they have are an indication of a serious problem.

Frame P.D.

The frame P.D. is the distance between the geometric centers of the two lens openings in the frame.  Since it is difficult to determine where the unmarked center of the opening is, and thus difficult to measure the distance between the two centers there are two accepted ways of determining the Frame P.D.  

  1. Measure from the right side of on lens opening to the right side of the other (or left to left).  
  2. Add the eye size and bridge sizes together.  Example of a 52/17 frame would have a 69 frame P.D.

Fundus Photography

The fundus is the back of the eye, also called the retina.  A fundus camera in specially designed to focus through the pupil, some through an undilated pupil and some only through a dilated pupil.  This may be used to document eye disease or used to document a healthy retina for later reference.

Glaucoma

Glaucoma is a disease that historically was characterized by the presence of  “higher than normal pressure” inside the eye. This is still frequently true. However, it is now known that often patients may have glaucoma and either never have abnormal pressures or may, only at times during the day, have higher than normal pressures.  Now glaucoma is thought of, as when a patient has pressures that are higher than their eye can tolerate without damage. Unfortunately, there is no magic number to tell the doctor when the pressure is too high or not.

Hi-Index Glass

Standard glass ophthalmic lenses have an Index of Refraction of 1.56.  This was the original glasses material. Ophthalmic glass has many good qualities but also has some less than desirable qualities.  Hi-Index glass is any ophthalmic glass with an Index of Refraction higher than 1.56. These materials are used mostly to make the lenses thinner and lighter weight than standard glass lenses

Hi- Index Plastic

Standard plastic ophthalmic lenses have an Index of Refraction of 1.50.  The lenses are also called CR-39. This was the original plastic material for ophthalmic lenses.  Ophthalmic plastic has many good qualities but also has some less than desirable qualities. Hi-Index plastic is any ophthalmic plastic with an Index of Refraction higher than 1.39.  These lenses may now come in 1.56, 1.60, 1.66, 1.74 index and possibly others in the future. These materials are used mostly to make the lenses thinner and lighter weight than standard plastic lenses

History of Present Illness

HPI is part of the documentation guidelines, and also part of the case history that is asked of the patient.  This helps the doctor get a clear idea of what problems have initiated the patient encounter and how it has been affecting the patient.  The DGs specify 8 parts to the HPI. In every office, the doctor(s) and the billing staff should be knowledgeable about how the DGs were designed to work and how they affect your practice.  Medicare and some other insurers will use the DGs to grade your claims and pay your office for your work. The documentation guidelines (DGs) were developed to help doctors and insurers communicate what has been done during an examination.  This was to help each judge if the code used to describe the patient encounter to the insurance company was an accurate description of the work that had been done.

Hyperopia

Hyperopia (farsightedness) is a condition where the components of the eye do not have enough focusing power to make light from far away focus clearly on the back of the eye without additional help.  Trying to see closer objects is even more difficult for the uncorrected hyperopic patient. Nearsightedness means the eye is focused too much to make light from far away focus clearly. Farsighted patients may see clearly far away or near uncorrected if they have enough additional focusing power.

Impact resistance

This is a measure of how resistant to breakage an ophthalmic lens will be.  Glass lenses are required to be individually tested with a standard (drop ball) test before being delivered to the patient.  Plastic lenses are generally more impact resistant than glass, Hi-index plastic even more and polycarbonate lenses are considered the most impact resistant.  Plastic lenses do not have to be individually tested. Industrial safety lenses have higher impact standards that dress safety lenses.

Index of Refraction

This is the measure of how quickly a lens material changes the direction of light.  If two lenses to have the same power, the lens with a low Index will be thicker compared to a lens with a higher Index.

Industrial safety lens vs. Dress safety lenses

OSHA has established criteria for the breakability of ophthalmic lenses.  Industrial safety and dress lenses have different standards for each type of lens.  The lenses may be made with exactly the same materials and treated the same way. However, industrial safety lenses will always be thicker and thus less breakable than dress lenses.  Industrial safety lenses must have a logo engraved in the upper temporal corner of each lens and may only have this logo if placed in an OSHA approved industrial safety frame. A completed pair of glasses is considered industrial safety only if all three of these criteria are fulfilled.

IOP

The pressure inside the eye is called intraocular pressure.  This has traditionally been a concern for patients suspected of having or with glaucoma.  IOP is measured with a tonometer, of which there are several types. This procedure is called tonometry.

Keratometer

A keratometer measures the curvature of the front of the eye (cornea).  It does so by measuring the size of a light reflection off the cornea. This measurement is most valuable for the management of certain eye diseases and for contact lens fitting and management.

Keratometry

Using a keratometer to measure the shape of the front of the cornea is called keratometry.

LASIK

This is an acronym for Laser in situ Keratomileusis.  Currently, LASIK is the method of choice for most refractive surgery cases.  It involves a laser to reshape the cornea after a thin layer of the front of the cornea has been partially removed and then is replaced over the area the laser has treated.  In general, this is a very accurate and safe procedure for correction of refractive errors. It is approved for nearsightedness, farsightedness, and astigmatism.

Lensometer

A lensometer is a device to measure the power of a lens.  It may be completely manual, completely automatic or a combination of both.  The lensometer should be able to read nearsightedness, farsightedness, astigmatism, prism, and the bifocal power.

Lensometry

Lensometry is the act of measuring the prescription of the lenses of a pair of glasses.  There are a number of different types of lensometers.

Myopia

Myopia (nearsightedness) is a condition where the components of the eye have to much power to correctly focus incoming light, from far away, on the back of the eye without additional help.  Trying to clearly see, objects that are closer, is less difficult. Farsightedness means the eye is not focused enough and close objects are more difficult.

NCT

The Non-contact tonometer is a device to measure the IOP (intraocular pressure) of an eye.  It measures the amount of time required for a small puff of air to flatten the front of the eye by detecting a beam of light reflected off the front of the eye.  From this, the tonometer determines the IOP. NCT also is frequently used to indicate the act of measuring the IOP with a non-contact tonometer.

Nearsighted

Myopia (nearsightedness) is a condition where the components of the eye have to much power to correctly focus incoming light, from far away, on the back of the eye without additional help.  Trying to clearly see, objects that are closer is less difficult. Farsightedness means the eye is not focused enough and close objects are more difficult.

OCT

An instrument used to give the doctor microscopic view of different structures.  Although the patient often thinks it was an x-ray, it uses visible light. This is very valuable for diagnosing and treating glaucoma, macular degeneration, diabetic retinopathy and other diseases.

Ophthalmoscope

The ophthalmoscope is a flashlight like instrument intended primarily to be used to view the back of the eye.  There are several different types that your doctor may use. Ophthalmoscopy is the examination of the back of the eye.

Orthokeratology

This involves fitting a rigid gas permeable contact lens onto the eye with the purpose of reshaping the cornea to cause a predicted change in the prescription of the eye.  The purpose is to allow a limited amount of time when the patient may see clearly without wearing either glasses or contact lenses.

Panoscopic tilt

Ophthalmic lenses are designed to tip slightly away from the face at the top.  This tip is called panoscopic tilt. The amount is supposed to be about 5 degrees, it looks like just a small tilt.

P.D.

This is the acronym used for the distance between the two eyes of the patient (papillary distance or intrapupillary distance).  More correctly, it is the distance between the centers of the pupil for each eye.

P.D. ruler

Most ophthalmic dispensaries will have a number of small rulers lying around.  Often they are called “P.D. rulers.” Some will have a metric scale on one edge and an inch scale on the other edge.  Some may have only metric scales. P.D. rulers were the original way of measuring the patient’s P.D.  Often, now more accurate instruments are used. In most cases, it is considered standard to use millimeters for this measurement.

Past Personal and Family History

Past Personal and Family History (PPFH) is part of the documentation guidelines, and also part of the case history that is asked of the patient.  This helps the doctor get a clear idea of what past problems, family history, social or behavioral patterns affect the patient’s current problems.  The DGs specify 3 parts to the PPFH. In every office, the doctor(s) and the billing staff should be knowledgeable about how the DGs were designed to work and how they affect your practice.  Medicare and some other insurers will use the DGs to grade your claims and pay your office for your work. The documentation guidelines (DGs) were developed to help doctors and insurers communicate what has been done during an examination.  This was to help each judge if the code used to describe the patient encounter to the insurance company was an accurate description of the work that had been done.

Photogray Extra

Photochromatic lenses change color and darkness when exposed to ultraviolet light.  These lenses may be worn as clear lenses when inside or at night and as sunglasses when in outdoor daytime conditions.

Presbyopia

Presbyopia is the aging process that causes the lens in the eye to lose its flexibility.  This causes a decrease in the ability of the eye to change its focal distance. This causes problems with reading and the need for bifocals.

Polarized lenses

Polarized lenses are created by stretching a sheet of cellophane and fixing it between two pieces of an ophthalmic lens.  The stretched cellophane selectively absorbs (polarizes) some light. This is useful because light that is reflected from a flat surface is also partially polarized.  By aligning the stretched sheet correctly, the greatest amount of light can be selectively absorbed. This creates a lens that is very comfortable and is very useful for driving and people that are outside around water or snow.

Polycarbonate

Polycarbonate is a newer plastic in the world of ophthalmic lenses.  It has some noticeable advantages and some disadvantages. Polycarbonate lens is the least breakable ophthalmic lenses.  They naturally filter all ultraviolet light striking them.

Prism

Prism is put into ophthalmic lenses to help align the light coming in the eyes.  This is necessary for patients that have difficulty with eye alignment or with eye stain.  In small amounts, prism is not easily noticed, but as the amount increases there will be a noticeable thickening of the lens on one side or the top or bottom

Progressive Lenses

Progressive lenses are also called no-line bifocals or no-line multifocals.  These lenses are created by slowly changing the power of the lens from the distance to the near portion.  These lenses have gradually gained popularity and are now the predominant form of multifocal lens for patients getting their first pair multifocal lenses and many patients who originally wore line type lenses have converted to progressive lenses and are very satisfied.

Pupilometer

Several years ago, devices were developed to aid in the accurate measurement of the patient’s P.D.  This was developed about the time dispensers were realizing the need for very accurate measurements for the newer progressive (no-line) bifocal lenses. Although it was and still is important to have an accurate P.D. for every patient, regardless of the type of lens they will use, the need became especially apparent at this time. The pupilometer is now just about a standard piece of equipment in every dispensary and is considered to be a more accurate and reliable way of measuring the P.D.  There are experienced staff and doctors that do this very well with the old standby P.D. ruler.

Radial Keratotomy

RK is a type of refractive surgery that has nearly been replaced by the newer laser surgery techniques.  With RK the surgeon would make anywhere from 4 to as many as 32 small incisions in the front of the cornea.  These incisions would create the appearance of “spokes on a bicycle wheel”. RK is still possibly indicated for low power myopic corrections.

Refraction

The refraction is the power of glasses that a patient needs in order to see clearly.  A refraction is the act of determining the power of glasses that a patient needs.

Refractive surgery

Refractive surgery is any surgery intended to change the glasses prescription that a patient needs.  Examples would be RK, LASIK, PRK, and even cataract surgery, although changing the prescription is a secondary consideration to getting the cloudy cataract out of the eye.

Review of Systems

The Review of Systems (ROS) is part of the documentation guidelines, and also part of the case history that is asked of the patient. This helps the doctor get a clear idea of what is occurring in the other organ systems and how this affects the patient’s current problems.  The DGs specify 14 organ systems of the ROS. In every office, the doctor(s) and the billing staff should be knowledgeable about how the DGs were designed to work and how they affect your practice.  Medicare and some other insurers will use the DGs to grade your claims and pay your office for your work. The documentation guidelines (DGs) were developed to help doctors and insurers communicate what has been done during an examination.  This was to help each judge if the code used to describe the patient encounter to the insurance company was an accurate description of the work that had been done.

Retina

The retina is the sensory portion of the eye.  It is a direct extension of the brain. The rods and cones of the retina absorb light and convert it into an electrical signal that is sent to the brain.  The retina is the inside layer of the back of the eye.

Retinal Detachment

The retina is the inside layer of the back of the eye.  It is attached to deeper layers and occasionally can become detached from those tissues.  If this is not detected and repaired there is a significant chance that the retina may become completely detached and cause complete and possibly irreversible blindness of that eye.  Generally, ophthalmoscopy is used to diagnose retinal detachment.

Seg. Height

This term refers to the height of the reading portion of a bifocal, trifocal or progressive lens in the frame the patient has chosen.  It is generally measured in millimeters from the lowest portion of the frame.

Slab off prism

Occasionally, a patient’s eyes do not align well vertically.  The amount that the eyes misalign is different looking straight ahead and looking down.  As a result, a different amount of vertical prism is needed in the straight-ahead portion of the lens compared to the lower portion of the lens.   Slab off prism is a way of putting different amounts of prism in these two parts of the lens.  This prism is put in only one lens. At the junction of the two different parts of the lens, there will be a slightly visible line.  Generally, the order to the lab is just to indicate slab off and allow the ophthalmic lens lab to determine how much and in which lens.

Sphere power

Sphere power refers to the amount of myopia or hyperopia in a prescription.  This is normally the first power written in the complete prescription.

Temple length

The temple is the piece of the frame that runs from the front of the frame and touches the top of the ear.  This measurement may be in millimeters or inches depending upon the units the manufacturer uses. This is the total length of the temple.

Titanium frames

Several different materials may be used to manufacture a metal frame.  Titanium is a relatively new material to this use. It has several advantages, including being very strong, very lightweight, and very hypoallergenic.

Tonometer

The pressure inside the eye is called intraocular pressure.  This has traditionally been a concern for patients suspected of having or with glaucoma.  IOP is measured with a tonometer, of which there are several types. This procedure is called tonometry.

Tonometry

The pressure inside the eye is called intraocular pressure.  This has traditionally been a concern for patients suspected of having or with glaucoma.  IOP is measured with a tonometer, of which there are several types. This procedure is called tonometry.

Topography

A topographer is a device that is able to create a map of the shape of the surface of the front of the eye.  It also combines a measurement of the actual curvature of the cornea similar to the measurement that a keratometer.  The computer display and printed report is in color that helps to describe the actual shape. Topography is useful for the doctor for fitting contact lenses, refractive surgery both pre-op and post-op and diagnosing and monitoring corneal disease.

Transitions 6

Transitions 6 is a tint that can be added to any type of plastic lens.  The tint is nearly clear inside or in dim or dark light. When the lens is illuminated with ultraviolet light (either sunlight or artificial UV) the lens becomes darker.  Transactions 6 is lighter than Transactions Extra Active both in the light and dark mode. This lens is not considered a dark sunglass and it doesn’t tint inside a car. Car manufacturers started putting UV tinting in windows about 1985 causing these lenses to no longer activate in the car.

Transitions Drivewear

A darker lens which has significant tint even inside a car or at night.  It would be considered a light sunglass in the car, but a dark sunglass outside.  The tinting is polarized giving it better driving characteristics than standard Transitions.

Transitions Extra Active

When the lens is illuminated with ultraviolet light (either sunlight or artificial UV) the lens becomes darker.  Transactions Extra Active is darker than Transactions 6 both in the light and dark mode.

Transitions Vantage

Another polarized changing lens.

Trifocal lenses

Trifocal lenses have three distinct lens powers each separated, from the other portions of the lens, by a visible line.  The lens is designed with the largest top portion intended for distance vision, a smaller bottom portion intended for reading, and the smallest portion between the two intended for seeing approximately arm’s length distances.  There are other lens designs with three powers that are not called trifocals.

Vertex distance

The vertex distance is the distance from the eye to the back surface of the lens.  For glasses, with high prescription, the vertex distance may alter the effective power of the lenses.  It is the change in vertex distance that causes the glasses prescription to be altered in order to prescribe contact lenses.

Visual Acuity

This is the measure of how well a patient can see detail.  The standard chart is called the Snellen chart. The notation to indicate acuity is a fraction like 20/20.  In this fraction, the top number is supposed to indicate the distance from the patient to the letters being read.  The bottom number is supposed to indicate the relative size of the letter read. A 20/40 letter should be approximately twice as big as a 20/20 letter.  Sometimes a Snellen equivalent is used (for instance 20/20 when the test distance is 16 inches for reading) for the sake of familiarity. There are other measurement formats that your doctor may choose.

Visual Fields

Visual fields refer to how well the eye sees light across the entire surface of the retina.  A perimeter is a device used to measure the visual field. The modern perimeters can do a quick screening field to establish relatively normal or abnormal field.  They can also do a much more complex threshold field to measure how much reduction is present in a particular part of the field.  The most common causes of field loss are glaucoma, retinal disease, and neurological disease.

Zyl frames

Zyl was one of the original plastic frame materials.  Zyl has properties that are good for use as an ophthalmic frame.  It softens when heated to a relatively low temperature, has good coloration properties.  Now there are a number of materials that are used in place of the original zyl.