2. Lens curvature and materials

lens curvature & lens materials

What are the three categories ophthalmic lenses can be divided into

1. single vision lens

2. Bifocal lens

3. Power progressive lens

What does a + lens do

magnify

What does a - lens do

minifies

The greater the diameter of the lens the___________

thicker the lens

Define a base curve

the back surface of the lens

Where is the base curve on a spherical lens

the front sphere

If both lens surfaces are spherical the thin lens power can be determined by

adding the two powers together

What is the power calculation formula

( base curve/ cross curve ) / sphere curve

What is the most commonly worn ophthalmic lens type

meniscus lenses - convex front surface and concave back surface

The larger the radius of the lens the ________ the lens

flatter

What is the height/ depth referred to as

sagitta/ sag

Define pupillary distance

the distance in millimetres (mm) between the centre of one pupil to the centre of the other

in a minus lens the edge thickness

increases

in a plus lens the centre thickness

increases

What is the Minimum size uncut (MSU) formula

(frame PD- patient PD) + widest point of the box lens size + 2mm

What are the two most common materials lenses are made of

1. glass

2. plastic

Glass lenses are often referred to as

mineral lenses

which lens material has the lowest refractive index

plastic

What does the refractive index of a material indicate

How much the material will refract or bend the light as it enters the material from air by comparing the speed of light in a given material to the speed of light in air

The higher the refractive index the more _______

light refracts as it enters the material

How many times more heavy is glass than plastic

4x

what is the formula for reflectance

R=(n-1)/(n+1) x2

What is the reflective index for crown glass

1.5

What is crown glass composed of

1. silica

2. soda

3. lime

What are the properties of crown glass

1. high transparency

2. resists most chemicals

3. resistant to scratching

4. needs to be toughened for use as safety lenses

What is the types of plastics used in lenses

1. CR39

2. polycarbonate

3. Trivex

Which type of plastic is most commonly used in children

trivex plastic

Which factors should be considered when choosing the ideal lens

1. optically clear and durable

2. uv absorption

3. light

4. thin

5. low or no aberrations

6. low or no reflections

7. resistant to abrasions

8. does not shatter with impact

9. able to be tinted

10. low cost

Which type of lens is the best option for high myopia

glass lenses

what is the refractive index of polycarbonate

1.59

what is the refractive index of trivex plastic

1.53

The higher the refractive index the

lighter the weight of a higher power lens