more about prescriptions and and lens types

Question 1

bifocals,

Answer 1

Bifocals have two lens powers

Question 2

trifocals

Answer 2

have three lens powers.

Question 3

Progressive lenses

Answer 3

which many people prefer, because there’s no visible line in the lens—gradually change in power from top to bottom, so they combine several powers in a single lens.

Question 4

The bottom part of the bifocal lens that contains the reading add is called what?

Answer 4

a segment, or seg for short.

Question 5

The depth of a bifocal segment (that is, the distance from the top to the bottom of the segment) is called what?

Answer 5

the seg depth.

Question 6

The seg height is what?

Answer 6

The seg height is the distance from the top of the segment to the bottom of the main carrier lens.

Question 7

The seg drop

Answer 7

The seg drop is the distance from the top of the full lens to the top of the segment.

Question 8

flat-top bifocals.

Answer 8

Traditional lined bifocals have a half-circle design These are called flat-top bifocals.

Question 9

We categorize flat-top bifocal lenses by their width, as measured across there flat tops these are?

Answer 9

We categorize flat-top bifocal lenses by their width, as measured across their flat tops. Some segments are 22 millimeters (mm) across the top; we call these Flat Top 22 bifocals. Other segments are 25 mm, 28 mm, 35 mm, and 45 mm across the top. As you can predict, we call these Flat Top 25 bifocals, Flat Top 28 bifocals, and so on.

Question 10

you’ll also see flat-top bifocal lenses referred to what?

Answer 10

half moon,
straight top,
or D segment lenses.

Question 11

a ribbon seg.

Answer 11

One type of bifocal seg is flat on the top and bottom both. It’s typically 13 or 14 mm deep, and it’s called a ribbon seg.

Question 12

kryptok segment.

Answer 12

Another common type of bifocal segment you’ll see is a kryptok segment. It’s round,

Question 13

why airline pilots like double D bifocal lenses

Answer 13

because they need to be able to see at arm’s length when they glance upward.

Question 14

Executive bifocals

Answer 14

Executive bifocals are like flat-top bifocals, but the bifocal line goes all the way across the lens. Also, the lens is one piece as opposed to a fused bifocal that has a reading segment fused to the front of the lens.These lenses provide a very wide reading area. However they’re very heavy, have a thick ledge at the top of the reading segment, and are cosmetically unappealing because of the wide line across the lens.

Question 15

No matter what style of bifocals people prefer, there’s one thing most people dislike what is it:

Answer 15

the visible line of the segment. As a result, some people choose blended or curved-top bifocals. In these lenses, the edges of the bifocal segment are slightly buffed so there’s no hard line even though the segment is still there.

Question 16

what is the drawback to blended or curved top bifocals

Answer 16

There’s a zone about one to two millimeters wide around the buffed-out line that isn’t clear to look through. However, this style often works well for people who can’t adapt to a progressive lens,

Question 17

progressive lenses are also called what?

Answer 17

progressive lenses, also called invisible bifocals or no-line bifocals.

Question 18

progressive lenses

Answer 18

These lenses gradually and continuously change power from top to bottom, so wearers can get the power they need simply by looking straight ahead to see in the distance and lowering their eyes to see closer and closer in. There’s no image jump, because the change is smooth and gentle. And these lenses are pleasing cosmetically because of the lack of lines.

Question 19

drawbacks to progressive lenses

Answer 19

First, progressive lenses have some distortion at the outer parts of the lens. For instance, when people are looking over their shoulders while driving with progressive lenses, their vision will be a bit blurry. Thus, these lenses aren’t the best choice for people whose jobs or hobbies require excellent peripheral vision.

Also some people have a hard time adapting to progressive lenses and may even complain of headaches or dizzy spells. Most problems like these occur because the seg heights or the monocular pupillary distances (or both) weren’t measured accurately. This is one reason you’ll want to master using the pupillometer to accurately measure the monocular PDs (pupil distances) we talked about a while back.

And here’s another tip: With progressive lenses, as the add power increases, the peripheral distortions increase, and the size of the reading area decreases.

Question 20

And here’s another tip: With progressive lenses, as the add power increases, the peripheral distortions increase, and the size of the reading area decreases. You can reduce these problems by fitting a client with a style with a shorter transition zone between the distance and the near powers. Styles like these are called compact progressive designs. Because the glasses will sit closer to the face, the central part of the glasses and the reading area will effectively be larger. (You’ll learn how to measure the resulting power of the lenses in Chapter 4.)

Answer 20

And here’s another tip: With progressive lenses, as the add power increases, the peripheral distortions increase, and the size of the reading area decreases. You can reduce these problems by fitting a client with a style with a shorter transition zone between the distance and the near powers. Styles like these are called compact progressive designs. Because the glasses will sit closer to the face, the central part of the glasses and the reading area will effectively be larger. (You’ll learn how to measure the resulting power of the lenses in Chapter 4.)

Question 21

Dozens of companies offer their own progressive designs. They include:

Answer 21

Varilux, 
Kodak, 
Adaptar, 
Definity, 
Autograph, 
and Reveal.

Question 22

The newer designs are free-form, meaning what

Answer 22

The newer designs are free-form, meaning that each prescription has a unique progressive design.

Question 23

Remember that not all progressive designs do not what?

Answer 23

perform the same optically, and some people will prefer some designs over others.

Question 24

Progressive brands are identified by what?

Answer 24

Progressive brands are identified by a tiny symbol that’s typically etched or engraved into the temporal side of the lens surface. The first two numbers of the add power are also etched by the symbol, so +2.50 add is etched as 25 and +2.25 is etched as 22.

Question 25

the top portion of a typical trifocal lens contains what?

Answer 25

the distance prescription.

Question 26

The top portion of the trifocal segment is for viewing what?

Answer 26

The top portion of the segment is for viewing objects at intermediate distances, such as looking at a computer or at shelves in a grocery store.

Question 27

the bottom portion of a trifocal is for what?

Answer 27

for reading.

Question 28

The inverse

Answer 28

1 divided by the power of the lens.