Basic Optics, Glasses, and Contact Lenses

     
  This page is part of the Front Desk / Receptionist Basic Training Course.
 

 

Basic Optics

 

The human eye is a compound lens system consisting of the cornea and lens. The cornea provides most of the focusing power of the eye.  The cornea is the clear "window" of the eye (red structure on the left side of the drawing).  The lens can change shape and provide variable focusing (red structure on the right).

compoundeye.gif

   
 

Spherical Refractive Errors

The human eye functions in a similar manner to a camera. In fact, the camera analogy is useful in explaining eye problems to the patient. Light is focused by a compound lens system (cornea and lens), through an aperture that regulates the amount of light (pupil, iris), onto a light sensitive medium (retina).

   
  Hereditary factors and natural variations in the development of the human body produce imperfect "camera parts". Not everyone can see 20/20 in the distance without a visual aid such as a glasses correction. These eyes with imperfect optics are said to have a refractive error.
   
 

A myopic (nearsighted) refractive error results when the cornea is too steep, or the axial length of the eye is too long, or a combination of the two. In this case parallel light is focused in front of the retina. If the patient accommodates (focuses with the lens), the vision is made worse. The myopic eye is corrected optically by using a minus lens to move the focal point back to the retina.

The next three illustrations are animations meant to be viewed on a computer screen.  If this information is printed, these illustrations will not be effective.

myope_ani.gif

   
  A hyperopic (farsighted) refractive error results from a cornea that is too flat, or an axial length that is too short, or a combination of the two. In this case parallel light is focused behind the retina. The focal point can be moved onto the retina either by using a plus lens in front of the eye (glasses or contacts) or by accommodative effort (focusing of the lens) by the patient.
 

hyperope_ani.gif

accom_ani.gif

   
 

Astigmatism and cylindrical corrections

Corneal astigmatism is created when the cornea is not a perfect sphere. The astigmatic cornea is curved more in one meridian than it is in the other. These meridians are usually 90 degrees apart (regular astigmatism).  

Astigmatism is corrected optically with a cylindrical lens.  A combination of a spherical lens and a cylindrical lens (spherocylindrical lens) is used to correct a spherical error with an astigmatic error.

Instead of a focal point, the spherocylindrical lens creates two focal lines perpendicular to one another and at different focal distances depending upon the particular curvatures.

 

Glasses

Lens power notations

A spherocylindrical lens power is written in the following manner:

-2.00-2.00x180 or

-4.00+2.00x90

The first number represents the sphere power. The second number is the cylinder power notation in plus or minus cylinder. The number after the x is the axis notation. Conversion from plus to minus cylinder notation is accomplished by transposing. The sphere and cylinder values are added together algebraically, the cylinder sign is reversed, and the axis is rotated 90 degrees. 

   
 

Near vision and bifocals

The lens in the eye gets fatter and thinner (accommodation) in order for the eye to focus on near objects (computer and reading distance).  As we age, the lens gets stiffer and the focusing ability declines.  Around age 45, most people need an adjustment to their glasses prescription in order to read.  This is called "presbyopia".  The adjustment has to be gradually made stronger until about age 65 when most of the focusing ability of the lens has disappeared.  The adjustment to the prescription is called an "add" power, or "reading" power.  At age 45, the add power is usually +1.00 or +1.25.  At age 65, the add power is usually +2.50.  Sometimes you will see add powers stronger than +2.50.  The stronger add powers are generally used for patients with poor vision who need to hold reading material close in order to see the print enlarged.

   
 

The glasses prescription

As mentioned, a glasses prescription can be in either "plus cylinder notation", or "minus cylinder notation".  Many ophthalmologists use plus cylinder notation.  Most optometrists use minus cylinder notation.  Here are examples:

   
 

-300 - 1.00 x 90      The first number (-3.00) is the sphere power.  The minus sign tells us that this is a nearsighted correction.  The second number is the "cylinder" correction.  A cylinder correction tells us that the eye has astigmatism.  The minus sign in front of the cylinder correction tells us that the prescription is written in minus cylinder notation.  The last number (90) is always preceded by an "X".  This tells us that this is the axis (orientation) of the cylinder correction.

Here is the same prescription written in plus cylinder notation: -4.00 +1.00 x 180

If the prescription has an add power, there is a notation following the lens prescription, like this:  -4.00 + 1.00 x 180  add +1.25    Sometimes the "add" is left off and you just see the "+1.25".  An add power of +1.25 tells us that the patient is probably in the 43 - 58 age group.  From this prescription, the optician can make distance only glasses, bifocals, or prescription readers.  To make prescription readers, the add power is combined with the sphere power to give this result: -2.75 + 1.00 x 180.

A prescription for a far sighted eye would look like this example, in both plus and minus cylinder:  +3.00 - 1.50 x 45    and    +1.50 +1.50 x 135    The plus sign in front of the spherical correction (+3.00 or +1.50)  tells us that the eye is far sighted.

If the eye does not have astigmatism, then there is no cylindrical correction.  The spherical correction is then followed by the notation "Sph", as follows:  -3.00 Sph is a correction for a nearsighted eye, and +2.00 Sph would be an example of a correction for a farsighted eye.

   
 

Bifocals, Trifocals, and Progressives

Although some patients with presbyopia have separate distance and reading glasses, most patients wear bifocals, trifocals. or progressive lenses.  Schematics of each type are picture below, with advantages and disadvantages

   
 

Bifocals

Advantage: Wide field of vision.

Disadvantages:  Image jump when moving from one section to another.  Obvious appearance of a bifocal. Only two focal distances: infinity and the normal 14-16 inches for the reading distance.

bl1.gif

 

Trifocals

Advantages: Three focal distances instead of two.  Wide field of vision.

Disadvantages: Image jump when moving from one section to another.  Obvious appearance of a trifocal.  Limited to three focal distances.

tf1.gif

 

Progressive Lenses

Advantages: Looks like a single vision lens.  Gradual focus change from distance to near.  Theoretically infinite number of focal distances.  They work particularly well for intermediate vision.

Disadvantages: Clear vision is only in a narrow corridor in the center of the lens.  It is sometime difficult to find the "sweet spot" that provides the best focus. There is image distortion in the periphery of the lenses.

pl1gif.gif

   
 

Contact Lenses

The front desk is often the "pickup station" for contact lenses.  As such, this is the last stop for quality control before the contact lenses are given to the patient.  If this is part of your job, you should compare the contact lens parameters on the box or on the lens package to the parameters that are listed on the order sheet, to make sure that they match.  Also, it is not uncommon for front desk personnel to be cross-trained in contact lens insertion and removal instruction.

Soft contact lens parameters

There are three basic types of soft contact lenses that are dispensed:

Spherical lenses, for distance correction only

Toric lenses, for the correction of astigmatism

Bifocal contact lenses, for the correction of distance and near vision

 

All soft contact lenses will have the following parameters:

 

  • Power: This is similar to the glasses correction, but most often it will be slightly different from the patients glasses correction.  If the power is spherical, there will be a single number such as -3.00 or +2.00.  If the lenses are toric, then the power will look very similar to a glasses correction for astigmatism, such as -2.00 - 1.00 x 180.  If the lenses are bifocals, then the power will include an add power, such as +2.00.
  • Base curve:  This is the curvature of the contact lens, which is prescribed to match the shape of the cornea.  For soft contact lenses, the base curve will generally be around 8, such as 8.4.
  • Diameter:  Soft contact lens diameters are generally in the neighborhood of 14 millimeters.
   
  Soft contact lenses that are frequent replacement usually come in a box.  This box has the contact lens specs marked on the end panel, with power (PWR), base curve (BC), and diameter (DIA).  Inside the box are individual packets similar to the one below.
   
  Soft contact lenses may come in a foil sealed container. This is a toric lens, with sphere (SPH), cylinder (CYL), and axis (AX) marked, along with base curve (BC) and diameter (DIA).  Notice that the lens has been marked with a sticker for the patient as being for the left eye (L).
   
  Soft lenses may also come in a bottle as pictured here.
   
 

Hard contact lens parameters

Hard contact lenses are also called "gas permeables" or "rigid gas permeables" or just "RGP" lenses.  They are usually dispensed as a single set or a single lens, typically in a flat plastic case with markings "R" and "L" for right and left lenses.

Parameters:

 

  • Power:  Most RGP lenses are dispensed in spherical powers, although there can be toric, bi-toric, and bifocal presciptions.
  • Base curve:  The base curve may be listed in "radius of curvature" notation, such as 8.4, or in diopter notation, such as 44.5.
  • Diameter:  RGP diameters usually range from 8 to 10 mm.
   
  Pictured here is a plastic case with a hard contact lens.  The lens specifications may be marked on a sticker on the other side of the case.  Sometimes the specs are on an accompanying printout and not on the case.
   
 

Soft contact lens insertion and removal (I&R)

This routine is usually only for the new contact lens wearer, although some experienced patients can use the review.  This duty can be drudgery, because it is always the same thing, over, and over, and over again.  To minimize the drudgery, have the patient watch a videotape of I&R procedures.  This procedure mainly has to do with the patient getting over the fear of sticking her finger in her eye.  Once that is overcome, it is usually smooth sailing.

Soft contact lenses can be flipped inside-out. They are usually most comfortable, and vision is usually the best, when the lens is inserted correct-side out.  Although with thin lenses, it doesn't seem to make much difference.  The "taco test" is used to determine which way is the correct way.

   
  The lens is slightly pinched between the thumb and forefinger, to make it look like a taco. It helps if the lens is slightly dry. The edge of the lens is observed.  If the edges slope inward, then the lens is in the correct position.
   
  If the lens edges reverse and curve outward, then the lens is inside-out. This phenomenon can usually be observed without pinching the lens.
   
  Some manufacturers print initials near the edge of their lenses.  A keen observer (or someone using a slit-lamp) can then tell if the lens is inside-out.
   
  The soft lens is inserted by positioning the lens on the index finger of the dominant hand.  The lids are opened and held by the remaining fingers of both hands.  The lens is guided to the cornea until contact is made.

   
  It is helpful if all parts of the lens edge make contact with the eye at the same time.  The lids are not released until the lens has adhered to the cornea.  The lens can usually be centered with a few blinks, if not, the lens can be pushed toward center with a finger.  It is helpful for most patients to look at themselves in the mirror during the procedure.  Alternatively, the contact lens can be placed on the conjunctiva below the cornea as the eye looks upward.  The lens is then moved onto the cornea with a finger.
   
 

The soft lens is removed by pinching the lens between the thumb and index finger of one hand.  As with insertion, the lids are held open with the remaining fingers.  It is helpful to look slightly upward, and pinch the lower edge of the lens.  Some find it helpful to first slide the lens downward from the cornea before pinching the lens.
   
  Most contact lens patients eventually become very adept at insertion and removal.  Many are able to use a one handed technique that simply pulls the lower lid down for insertion and removal.  It is best to teach the two handed technique and let them improvise on their own.
   
 

The patient should be taught what to do if the contact lens becomes de-centered. 

 

Knowing ocular anatomy, we know that the lens will not become "lost behind the eye", but the patient may not know this.  Finding a de-centered lens is just a matter of searching the conjunctival area after the lids have been pulled away from the globe.  The lens is then pinched, removed, and then re-inserted.

 

Even though your I&R video may cover these next points, be sure they are part of a printed handout for the patient. You may want to add to or modify this list as your experience may suggest:

 

  • Always wash your hands before handling a contact lens.
  • Short fingernails are better than long nails.  Care must be taken so that long nails do not tear the lens.
  • Hand lotions and creams should not be used prior to contact lens handling.
  • It is best to use hair spray before contact lenses are inserted.
  • It is best to apply makeup after contact lenses are inserted and remove the contact lenses before makeup is removed.
  • Only use appropriate care products for cleaning, wetting, and rinsing contact lenses. Saliva and tap water should not be used as wetting or rinsing agents.  
  • Eye redness, discomfort, or blurry vision should be reported to your doctor immediately.
  • If the contact lens becomes de-centered from the cornea, it cannot travel behind the eye.  Using a mirror, try to locate the lens on the white part of the eye, or under the upper or lower lid.   Move the lens with your finger if necessary.  Remove the lens by pinching it with your fingers.  Re-insert the lens in the usual manner. If you cannot find the lens, it may have fallen out. 
  • It is not a good idea to wear contact lenses while swimming.  The lenses can easily become dislodged and/or soft lenses may soak up some chlorine and irritate your eyes.
  • Do not use eye drops with contact lenses, except for lubricating drops approved for your lenses.
  • Do not sleep with your contact lenses unless they are approved for overnight wear.
  • Wear safety glasses over the contact lenses in appropriate situations.
  Initial wearing time:  This varies with the practitioner.  Many patients tolerate soft contact lenses right away.  Some practitioners like to start the patient with 4 hours the first day and add 2 hours each day until the lenses are worn for all waking hours if desired.
   
 

Soft contact lens care

It is the goal of every contact lens care system to remove foreign matter and microorganisms from the surface of the lens, and to neutralize or kill any remaining microorganisms on the lens.  This is usually called "cleaning" and "disinfection".  Care systems use various combinations of daily cleaners, rinsing agents, and disinfectants.

 

   
 

Daily cleaners work well only if combined with mechanically cleaning the lens.  This is accomplished by putting a drop of cleaner on the lens and rubbing the lens gently with a finger in the cupped palm of the other hand.  The lens can be flipped inside out so that the other surface can be cleaned.

A rinsing agent is used to wash the daily cleaner and other matter from the surface of the lens.  A rinsing agent should be used whether or not a daily cleaner is used.  Rinsing the lens will obviously be much more effective if a daily cleaner is used. 

Disinfection can be accomplished by storing the contact lenses overnight in a disinfecting solution. 

Enzymatic cleaners are used to remove tear protein deposits from the surface of the contact lens.  These are deposits that may not be removed with a daily cleaner.  These cleaners are for the patients who are more susceptible to these deposits than other patients.  Excessive protein deposits can block oxygen transmission through the lens and they may trigger a hypersensitivity reaction under the eyelids.  More frequent replacement of the lenses also reduces this problem.

All-purpose soft contact lens care systems are currently popular for contact lens care.  One solution is used for cleaning, rinsing, and for disinfection (storage in the case).  The idea is to encourage better compliance with a more simple system.  Unfortunately, many patients think all they have to do is remove the lenses and place them in the solution overnight.  Be sure to encourage the patient to rub and rinse the lenses as described above.  

The all-purpose solutions are expensive.  The cost for the patient can be reduced if the all-purpose solution is only used as the cleaning agent and the disinfecting agent (storage).  A much less expensive contact lens saline solution can be purchased to rinse the lens during cleaning and to rinse the lens before insertion.

Some patients may become hypersensitive to the preservatives in chemical care systems.  Symptoms may include soreness, stinging, foreign body sensation, redness of the conjunctiva, redness of lids. swelling of the conjunctiva and/or lids, and punctate staining of the cornea.  A major offender has been the preservative thimerosal.  It is best to avoid care systems with this preservative.  Switching the patient to a chemical care system with a different preservative may solve the problem. Alternatives to chemical care systems are daily disposables or a hydrogen peroxide care system as discussed below.

A hydrogen peroxide based care system requires more steps for the patient, but it may be the only choice for the patient who is hypersensitive to the preservatives in other care systems.  The peroxide is an efficient antimicrobial and it has some protein cleaning activity as well.  The system requires an exposure step of about 10 minutes and a neutralization step that lasts from 20 minutes to a few hours, depending on the system.

General instructions to the patient should include:

  • Only use products that are compatible with your lenses.  Check with your doctor's office.
  • Do not mix care product brands unless recommended by the doctor's office.
  • Wash hands before handling lenses.
  • Do not skip steps in your lens care routine, as instructed by the doctor's office.
  • Keep the lens care environment clean (case, counter, storage bag, etc.).
  • Keep care product bottle tops from touching any surface.
  • Work over a clean surface.  Use paper towels if in a public restroom.
  • If you drop your lens prior to insertion, rinse the lens well before insertion. 
  • Notify your doctor's office if you experience eye or lid redness and/or irritation.
   
 

RGP Lens Insertion and Removal

If this discussion seems familiar, it is because it is very similar to the discussion you read in the soft contact lens modules.  RGP lens care is somewhat less complicated because the RGP lens does not soak up water,  chemicals, or pathogens like the the soft lens can.  RGP insertion is similar to soft lens insertion, but RGP lens removal is somewhat more complicated.

The RGP lens is inserted by positioning the lens on the index finger of the dominant hand.  The lids are opened and held by the remaining fingers of both hands.  The lens is guided to the cornea until contact is made.  It is helpful if all parts of the lens edge make contact with the eye at the same time.  The lids are not released until the lens has adhered to the cornea.  The lens can usually be centered with a few blinks.  It is helpful for most patients to look at themselves in the mirror during the procedure.

   
 

Yes, this is a soft lens, but the insertion technique is very similar.
   
  The RGP lens is removed by placing the finger on the skin at the outer canthus and pulling outward.  This tightens the lids against the the upper and lower edges of the lens.  The patient then blinks, and if all goes well, the lens pops out of the eye.
   
 

It is best to bend over a towel on a flat surface to catch the lens.  After some practice, the patient can pop the lens into the other hand.  If the lens does not pop the first time, make sure the lens is centered in the palpebral fissure by looking straight ahead and be sure to apply equal pulling force to the upper and lower lids.  An alternative to this method is to use a lens removal tool.  This has a small suction cup on the end of a holder.  The suction cup is guided to the lens until suction is achieved and the lens is then removed.  Use of a wetting drop in the eye prior to using the tool usually helps.

The patient should be taught what to do if the contact lens becomes de-centered.  See the instructions in the next section.

Even though your I&R video may cover these points, be sure they are part of a printed handout for the patient. You may want to add to or modify this list as your experience may suggest:

 
  • Always wash your hands before handling a contact lens.
  • Short fingernails are better than long nails. 
  • Hand lotions and creams should not be used prior to contact lens handling.
  • It is best to use hair spray before contact lenses are inserted.
  • It is best to apply makeup after contact lenses are inserted and remove the contact lenses before makeup is removed.
  • Only use appropriate care products for cleaning, wetting, and rinsing contact lenses. Saliva and tap water should not be used as wetting or rinsing agents.  
  • Eye redness, discomfort, or blurry vision should be reported to your doctor immediately.
  • If the contact lens becomes de-centered from the cornea, it cannot travel behind the eye.  Using a mirror, try to locate the lens on the white part of the eye, or under the upper or lower lid.   Move the lens to the cornea by pushing against it with the eyelid, which is massaged with your finger.  The lens may fall out of the eye during this relocation procedure.  Re-insert the lens in the usual manner. If you cannot find the lens, it may have fallen out. 
  • Do not wear contact lenses while swimming.
  • Do not sleep with your contact lenses unless they are approved for overnight wear.
  • Wear safety glasses over the contact lenses in appropriate situations. 
  Initial wearing time:  This varies with the practitioner.  The new RGP wearer must gradually build up wearing time.  Some practitioners like to start the patient with 2 hours the first day and add 2 hours each day until the lenses are worn for all waking hours if desired.
   
 

RGP contact lens care

It is the goal of every RGP contact lens care system to remove foreign matter and microorganisms from the surface of the lens, to neutralize or kill any remaining microorganisms on the lens, and to properly "wet" the RGP lens prior to insertion.   Care systems use various combinations of daily cleaners, disinfectants, and wetting agents.

Daily cleaners work well only if combined with mechanically cleaning the lens.  This is accomplished by putting a drop of cleaner on the lens and rubbing the lens gently with a finger in the cupped palm of the other hand, with the convex side of the lens against the palm.  Unlike with the soft lens, with the RGP lens, the cleaner can be rinsed with warm tap water.  This is best done after the contact lens is removed for the day.

Disinfection can be accomplished by storing the contact lenses overnight in a disinfecting (storage) solution.

Enzymatic cleaners are used to remove tear protein deposits from the surface of the contact lens.  These are deposits that may not be removed with a daily cleaner.  These cleaners are for the patients who are more susceptible to these deposits than other patients.  Excessive protein deposits can block oxygen transmission through the lens and they may trigger a hypersensitivity reaction under the eyelids.  This is less of a problem with RGP lenses than it is with soft lenses.

All-purpose RGP lens care systems are currently popular for contact lens care.  One or two solutions are used for cleaning, disinfection (storage in the case), and wetting prior to insertion.  The idea is to encourage better compliance with a more simple system.  Unfortunately, many patients think all they have to do is remove the lenses and place them in the storage solution overnight.  Be sure to encourage the patient to clean (rub and rinse) the lenses as described above.  

Some patients may become hypersensitive to the preservatives in care systems.  Symptoms may include soreness, stinging, foreign body sensation, redness of the conjunctiva, redness of lids. swelling of the conjunctiva and/or lids, and punctate staining of the cornea.  A major offender has been the preservative thimerosal.  It is best to avoid care systems with this preservative.  Switching the patient to a care system with a different preservative may solve the problem.

 

General instructions to the patient should include:

  • Only use products that are compatible with your lenses.  Check with your doctor's office.
  • Do not mix care product brands unless recommended by the doctor's office.
  • Wash hands before handling lenses.
  • Do not skip steps in your lens care routine, as instructed by the doctor's office.
  • Keep the lens care environment clean (case, counter, storage bag, etc.).
  • Keep care product bottle tops from touching any surface.
  • Work over a clean surface.  Use paper towels if in a public restroom.
  • If you drop your lens prior to insertion, rinse the lens well before insertion. 
  • Notify your doctor's office if you experience eye or lid redness and/or irritation.
   
   
   
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