contact lens measurements
Eye exams for contact lenses include special tests that are not typically performed during a comprehensive eye exam.
An instrument called a keratometer will be used to measure the curvature of your eye's clear front surface (cornea). The keratometer analyzes light reflections from your cornea and determines the curvature of your eye's surface. These measurements help your eye doctor choose the proper curve and size for your contact lenses.
pupil and iris measurements
The size of your eye's pupil may also be measured. In a simple approach, a card or ruler showing different pupil sizes is held next to your eye to determine the best match. Automated instruments that measure pupil size also exist. These instruments are capable of extremely precise measurements, and some simultaneously measure the horizontal and vertical diameter of your pupil. Similar technologies also may be used to measure the diameter of the colored portion of your eye (iris). Pupil and iris measurements help your ECP choose contact lenses that are of a proper size to fit well and look best on your eyes.
tear film evaluation
Your body's ability to produce tears may be evaluated through use of small strip of paper inserted underneath your lower eyelid. You close your eyes for about five minutes, and then the paper is removed. The length of the paper moistened by your tears is measured to assess your tear production and determine if you have dry eyes. If you don't produce enough tears and you have severe dry eye, contact lenses may not be right for you. In some cases, certain contact lenses for dry eyes such as those made of silicone hydrogel material might work better for you if you have mild dryness-related discomfort when wearing conventional contact lenses.
Evaluation of your eye's surface and contact lens fit
The health of your cornea will be evaluated using a biomicroscope (also called a slit lamp). This lighted instrument provides a highly magnified view of the cornea and other tissues to enable your eye doctor to evaluate the health of the front of your eyes and detect any changes caused by contact lens wear.
The biomicroscope also is used to evaluate the fit of a trial contact lens, because it enables your doctor to observe the alignment and movement of the lens as it rests on the surface of your eye.
When trial lenses are used, you typically will need to wear them a few minutes so that initial tearing of the eye stops and the lenses stabilize. Your eye doctor can then make a proper evaluation of how the lenses fit without the presence of excess moisture caused by tearing.
In follow-up visits, your optometrist may stain your eye with fluorescein to check for defects and make sure your contact lenses are not damaging your eye's surface. You usually will need to remove your contact lenses before this test is performed.
After finding contact lenses that fit properly, are comfortable for you, and provide good vision, your eye doctor can write your contact lens prescription. This prescription will designate contact lens power, a shape matching the curvature of your eye (base curve), and diameter.
Usually, it takes two or three follow-up visits to complete an uncomplicated contact lens fitting. After that, you should have annual contact lens exams so your eye doctor can monitor the health of your eyes. In some cases, you may need more frequent exams or additional follow-up visits.
Keep in mind that if you wear contact lenses, your annual eye exams typically will cost more than a routine exam for someone who doesn't wear contacts, due to the additional contact lens-related tests that are included.