L4(a) Optical Characteristics Of Opthalmic Lens

Question 1

Define optical / principal axis

Answer 1

The line that passes through the Centre of a lens where the radii of curvature of the front and back surfaces fall

Question 2

Define optical Centre

Answer 2

Point on an ophthalmic prescription lens where no prismatic effect is manifested

Question 3

Explain importance of lens centration

Answer 3

• most effective power
• optimum visual field (wide)
• minimize unwanted prismatic effects (light rays undeviated)
• reduce ghost image

Question 4

What are the three optical characteristics

Answer 4

Aberration, lens effective power, prismatic effects

Question 5

how do lens aberrations happen

Answer 5

• when eyes view lens periphery (not centred), image quality is degraded
• can be reduced but not totally eliminated

Question 6

6 types of aberrations

Answer 6

CHROMATIC, OBLIQUE ASTIG, DISTORTION, spherical aberration, coma, field curvature

Question 7

Explain chromatic aberration

Answer 7

• caused by a material of lens when polychromatic lens is used
• lens material unable to focus different colours of light to the same spot

Question 8

Explain lateral chromatic aberration

Answer 8

Different colours get focused at different distances from the lens,

Patients see coloured fringes

Question 9

Effects of LCA on wearer

Answer 9

• esp high power lens: coloured fringes formed around images of high contrast objects: seen at periphery
• image is blurry especially when contrast is Low

Question 10

What is the measurement for aberration

Answer 10

Abbe number - measure of a transparent lens marterial to DISPERSE lights.

The higher the AN the lower the dispersion

Usually higher refractive index, lower abbe number

Question 11

What is the formula for chromatic (lateral) aberration

Answer 11

Distance away from oc * power of lens / abbe number

Question 12

Lens materials to reduce chromatic aberrations

Answer 12

CR39, Trivex, polycarbonate, polyurethane

Question 13

Explain oblique astigmatism

Answer 13

When rays of light from an object strike PERIPHERY of lens OBLIQUELY (at an oblique angle): light rays will not form sharp point image

Can be reduced in certain lens forms (best form lens: zero oblique astigmatism within 30 degree from the lens optical axis)

Question 14

Explain distortion

Answer 14

• lens mag/minification effect that is seen as we move away from the OC
• increases with increase in distance away from oc
• plus: pincushion distortion
• minus: barrel distortion

Question 15

explain vertex distance

Answer 15

• distance between back of lens surface to the front surface of the cornea.
• changes with effective power
• move away=-d
• move towards=+d

Question 16

What are the 3 positions of wear

Answer 16

1. Vertex distance
2. Pantoscopic tilt: vertical tilt of lens axis
3. Wrap angle (face form): horizontal tilt of the lens

Question 17

Explain pantoscopic tilt

Answer 17

• VERTICAL tilt of lens
• dependent on nose&ear position. higher ears, more obvious tilt
• depends on tilt angle of FRAME even if worn by same person
• normal range (8-10, 20 too much)
• lower part of frame is closer to patient’s cheek: optical centre no longer aligned to patients eye position.
• Hence need to lower the optical centre (every 2 degree, lower by 1mm)

Question 18

Explain face forms

Answer 18

-horizontal tilt of lens
-+ve (ideal) face form is when frame PD > px PD. Frame is bent towards face (matches curve of face)
—ve face form (not ok) when frame PD < px pd. bent away from face