Flashcards in self study 2 Deck (35)
Identify the refractive index values of regular plastic
Identify the refractive index values of glass
Identify the refractive index values of high-index plastic
Identify the refractive index values of Trivex
Identify the refractive index values of polycarbonate,
describe the purposes of photochromic lenses
Photochromics are lens materials that darken when the lenses are exposed to ultraviolet light. They’re made of substances that react to UV light, so the thicker the material or the lens prescription, the darker the lenses will get.
describe the purposes of polarized lenses
Polarizing lenses eliminate the reflected horizontal wavelengths of light that cause glare—so only the vertical wavelengths of light pass through the lenses. It’s a little like having invisible Venetian blinds on your glasses! These lenses can make driving much safer. You should also recommend them for people who are into fishing or sailing, because they reduce the glare from the water.
describe the purposes of anti-reflective lenses
Antireflective coatings are also very popular these days. These coatings prevent light from reflecting off the surface of a person’s lenses. This makes a big difference in how the glasses look.
Antireflective coatings also increase the amount of visible light going through a lens from 92% to 99%. They’re a good choice for people who work on computers, because computer monitors create a lot of glare. Antireflective coatings also work well with high-index lenses, which reflect more light than regular plastic and glass. Sunglasses with antireflective lenses can help reduce daytime glare, while clear lenses with this coating can reduce the glare from headlights at night.
Describe Prentice’s Rule
Prentice’s Rule (or the Prentice Rule, as one of your textbooks refers to it). This formula, which you read about today, tells us just how powerful our induced prism is. (One reminder, before we dive into this math: If you find any of the calculations tricky, feel free to jump ahead to Lesson 20 and brush up on your algebra skills.)
Explain how you will use Prentice’s Rule to calculate an induced prism or the correct amount of decentration in a lens.
Ready? Then let’s do our math. We’ll start with the original equation we used to solve our earlier problems:
Prism = F x d
.5Δ = 1.50 x d (remember that d is the amount of decentration in centimeters)
Now, as you’ll recall from your algebra classes, we need to get our unknown number—that d, or the decentration amount—all by itself on one side of the equation. When we do that, we’ll be able to find our answer. So let’s divide both sides of our equation by 1.50. Here’s what we get:
.5 ÷ 1.50 = d
.5 ÷ 1.50 = .33 cm
.33 cm = 3.3 mm
So you’ll want to decenter Samantha’s lens 3.3 mm inward from the optical center.
Of course, prisms can be vertical or horizontal. Just remember that the light bends toward the base, and the image will move in the opposite direction. So let’s try our equation with a vertical prism.
Here’s your question. Imagine that you’re looking 5 mm (or .5 cm) above the optical center of a lens with this prescription:
How much prism will you experience?
We’ll start with our optical cross. It looks like this:
So the power in the vertical meridian is +1.50.
Now, let’s use Prentice’s Rule:
Prism = Power x decentration in centimeters
Prism = +1.50 x .5 cm
Prism = +0.75 Δ BD
Describe the uses of a Fresnel prism.
Some people with multiple sclerosis, Graves’ disease (a thyroid problem), or other medical disorders have vision problems that are only intermittent. With a Fresnel lens, we can correct the problem when it occurs.
People with uncontrolled diabetes or some other medical problems may experience rapid vision loss. With Fresnel lenses, we can continually strengthen a prescription without having to order new lenses.
Divers and skiers sometimes want some temporary magnification so they can see details better. Fresnel lenses work perfectly in this situation.
People who are bedridden don’t get the same view of the world as people who spend most of their time sitting or standing. When bedridden patients wear Fresnel lenses, they can see at the same angle as if they were sitting up.
Sometimes, we use Fresnel lenses to test a bifocal prescription on a client and see if it works well. That saves the time and expense of ordering glasses, only to find out that our patient doesn’t like them.
Define chromatic aberration,
you’ll recall that light is made of different wavelengths. Some lenses (particularly high-index lenses) cause a high degree of dispersion, which means that the colors of the rays passing through the lenses separate, much like a light ray passing through a prism separates into the colors of the rainbow.
Remember when we discussed Abbe values? They describe the amount of chromatic aberration for a certain type of lens material. The higher the Abbe value, the less chromatic aberration the material has. Take a look at the table below each lens material stacks up.
When the amount of chromatic aberration in a prescription lens is high because of the material used, it can cause dizziness, discomfort, and altered peripheral vision.
Define pincushion effect
Distortion occurs when the different points at the periphery of a lens have different amounts of magnification. When this happens in a plus lens, the magnification increases from the center outward. The result is what we call a pincushion effect. (Aha—there’s one of those strange terms!)
Define barrel effect
When distortion occurs in a minus lens, the opposite effect occurs. In this case, the magnification decreases as you move outward—what we call a barrel effect. Here’s an illustration:
Define spherical aberration
Spherical aberration occurs when parallel light rays moving along the optical axis—that line that passes straight through the lens from front to back—don’t converge at the same place. As a result, an image appears blurred and out of focus. Luckily, the pupil of the eye helps to limit the amount of light rays entering the eye. Because of this, spherical aberration isn’t often a problem with lenses.
Coma isn’t a word you expect to hear in an eye doctor’s office, is it? But when we talk about coma in the optical field, the word has a meaning that's very different from the one you’re probably thinking of!
The type of lens aberration called coma occurs when parallel light rays going through a lens focus in a ring-shaped pattern with a V-shaped or comet-like flare on one end of the image. If you associate the word coma with the word comet, it’ll help you remember the meaning of this term for the ABO exam.
curvature of field
Curvature of field occurs when the things a person sees through the edge of a lens don’t focus on the retina at the same time as the objects the person sees through the center of the lens. This makes a flat surface look curved.
oblique or marginal astigmatism
Oblique (or marginal) astigmatism occurs when rays of light pass through a lens at a slant, as shown in the picture below. This creates two focal lines that are perpendicular to each other. The result, as you can imagine, is blurry vision and an unhappy client.The difference between the two focal lines is the amount of astigmatism caused by the lens. The bigger the distance, as you can guess, the worse the problem is.
Explain how progressive lenses work,
describe the design of short-corridor progressive lenses.
Describe the parts of a manual lensometer
define the steps for finding the distance power power of a lens using a manual lensometer.
1Start by focusing the eyepiece. Do this before you place the glasses on the spectacle table. When you look into the eyepiece, you’ll see the reticle target. You may want to put a plain white piece of paper on the lens table and turn the eyepiece until the reticle lines are clearest for you.Remember that you can have a prescription in minus cylinder form or plus cylinder form. Most opticians use a lensometer to find the power of a lens in minus cylinder form, so we’ll focus on that method in the remaining steps.i="">
Pull the lens holder lever toward you until it locks in place away from the lens stop. Place the glasses into the lensometer, with the back surface of the lens against the lens stop and the temples pointing away from you.
Turn the power drum toward you in the high plus power direction (about +10.00) before you begin. This ensures that your initial point is well outside the actual power of the glasses.
Slowly turn the wheel in the opposite direction until the sphere lines are in focus. You may need to turn the axis wheel as well to get the best focus, especially if there’s a cylinder in the lens. If the sphere lines get more blurred instead of sharper, go back to step 1 and start at a higher plus power before moving the wheel back. If both the sphere and cylinder lines focus at the same time, you have a lens that is a sphere.
define the steps for finding the add power of a lens using a manual lensometer.
First, read the distance power of the lens, just as you did in the preceding chapter.
Next, in order to find the add power, just gently move the lens and the spectacle table upward so you position the reading portion of the lens in front of the lens stop. Be careful not to scratch the lens.
Read the spherical power of the lens in this position. The difference between the spherical power of the lens in the distance portion of the lens and the spherical power of the lens in the add portion is the add power.
Understand how to convert millimeters and centimeters to inches and vice versa.
Converting from one metric measurement to another is simple. For instance, to convert from meters to centimeters to millimeters, you only need to move the decimal point to the right or left.
1 centimeter = 10 millimeters
1 millimeter = .1 centimeters
Each move of the decimal point to the left means that your number is one-tenth as big, and each move of the decimal point to the right means that it’s ten times as big.
Now let’s look at centimeters and meters.
1 meter = 100 centimeters
1 centimeter = .01 meters
Here, you’ll just move the decimal point to the left two places. Since there are 100 centimeters in a meter, a centimeter is 1/100 of a meter.
Define a spherical equivalent
explain how spherical equivalents are calculated.
For each 0.50 diopter of minus cylinder, add 0.25 diopter of negative sphere power.
Describe the parameters you will consider when ordering soft contact lens
explain the uses of corrective lenses.
explain the uses of therapeutic contact lenses.