topic 4 optical characteristics

Question 1

what is lens aberration?

Answer 1

• Aberration causes degradation of image quality.
• Aberration happens mostly when the eyes are viewing through the lens periphery, away from the optical centre.
• Lens Aberration can be reduced but not totally eliminated.

Question 2

what is Lateral chromatic aberration?

Answer 2

Due to the material of the lens, when polychromatic lights is used.

The inability of a lens material to focus different colours of light to the same spot.

Question 3

what effects does LCA have on the wearer?

Answer 3

Lateral CA causes wearers to complain about colour fringes around the images of high contrast objects

Make the image blurry especially when the contrast is low.

Question 4

what is abbe number?

Answer 4

Abbe number is a measure of a transparent lens material to disperse lights.

Traditional lens materials has the highest Abbe number.

higher abbe number= lower dispersion= lower chromatic aberration

Question 5

what is Chromatic Aberration Formula?

Answer 5

chromatic aberration(Lateral) = cF/V

c= distance away from OC, c increase, CA increase
F= power of a lens, F increase, CA increase
V= Abbe number, Abbe number increase, CA decrease

Question 6

what is the solution to LCA?

Answer 6

choose lens material with high Abbe number

Question 7

how does Oblique astigmatism occur?

Answer 7

Rays of light from an object strike the lens obliquely.

These light rays will not form a sharp, point image.

Oblique astigmatism is minimum when the lens is in a certain lens forms.

Best Form Lens are made to have zero oblique astigmatism within 30 degree from the lens optical axis

Question 8

What is Distortion

Answer 8

Lens magnification/
minification effect increases with the increase of distance away from Optical center

minus lens distortion=barrel

plus lens distortion= pin cushion

Question 9

Describe the 3 positions of wear (POW) can affect vision and visual comfort.

Answer 9

1. Vertex Distance changes effective power
2. Pantoscopic tilt is the vertical tilt of lens axis
3. Wrap angle (face form) is the horizontal tilt of the lens axis

Question 10

how does pantoscopic tilt angle differ depending on ear and frame?

Answer 10

• higher ear=more tilt

- the forward tilt angle differs depending on the tilt angle of the frames

Question 11

how does OC need to be changed depending on pantoscopic tilt?

Answer 11

Position of the OC needs adjustment as the tilt changes,

for every 2 degree tilt, lower the OC by 1 mm

Question 12

what are the 3 types of face forms?

Answer 12

1. zero (patient’s PD= frame PD)
2. positive (ideal, wraps around face, frame PD is larger than patient’s PD and is correctly adjusted on face)
3. negative (patient PD > frame PD)

Question 13

what are the 5 properties of lens materials?

Answer 13

1. Refractive index
2. Density
3. abbe number
4. UV cut off
5. Tensile Strength

Question 14

what is refractive index and how does it affect lens thickness?

Answer 14

Ratio of the velocity of light of a given wavelength in air, to the velocity of light of the same wavelength in the refracting medium.

The higher the refractive index, the thinner the lens.

effect of high index material in reducing lens thickness is only evident when the power is greater than ± 4.00D

Question 15

what is the formula for predicting effect of higher RI?

Answer 15

n1-1 / n2-1

result will show the thickness of new lens as percentage of current lens thickness

n1=current lens material RI
n2=new lens material RI

Question 16

what is the RI of CR39, Trivex, ormex, polycarbonate and polyurethane ?

Answer 16

CR39=1.49
trivex= 1.53
omex=1.56
polycarbonate=1.59
polyurethane= 1.60

• Polyurethane is more commonly called plastic HI 1.6
• The rest of the materials all have RI in their names

Low-index: <1.53 (1.49)
Mid-index: 1.53 to 1.58
High-index: 1.59, 1.60, 1.61, 1.66
Super-high index: >1.66 (1.67, 1.71, 1.74)

Question 17

what is the rs between density and RI?

Answer 17

In general, the higher the RI the higher the density

Question 18

what is the rs between density and glass materials? what material has the least density?

Answer 18

All glass materials is much denser and therefore heavier. Glass lens is around 2-4 times the weight of plastic.

Trivex has the least density

Question 19

what is abbe value? what is the formula?

Answer 19

• Degree of chromatic aberration which the wearer will experience
• Amount of dispersion caused by lens material
• purely a lens material property, not affected by power or thickness

formula;
v = 1/dispersion

*Higher the Abbe value, lower the chromatic aberrations

Question 20

what is considered low, medium and high dispersion?

Answer 20

> 45 : low dispersion
39 < 45 : medium dispersion
< 39: high dispersion

Question 21

what is the uv cut off for glass and plastic lens?

Answer 21

plastic lenses have UV cut off ranging from 355 to 400

glass lenses have UV cut off ranging from 320-340nm
All glass lens materials provide insufficient protection against UV

Question 22

what is tensile strength? what instrument is used to measure it? what is considered good tensile strength?

Answer 22

Tensile strength is the resistance of a lens against breakage when it is mounted into rimless frames.
It is measured using an instrument called Instron (in unit of kilogram of force)
Tensile strength of 35 kg and above is considered good.

Question 23

List the plastic materials from lowest to highest tensile strength
*lower RI=lower tensile strength generally

Answer 23

CR39, omex=HI plastic 1.74, polycarbonate, HI plastic 1.71, trivex, HI plastic 1.66, polyurethane

(Since glass materials are not indicated for rimless., no values given)

Question 24

GO LOOK AT THEFUKVING TABLE OK

Answer 24

ok