D: Ophthalmic Lens Forms - Week 3

Question 1

Under what circumstances might we see coloured fringes surrounding the image through our spectacles?

Answer 1

As a result of decent transverse chromatic aberration for a HIGH CONTRAST target

Question 2

What effect might we see when there is noticeable transverse chromatic aberration looking at a low contrast target?

Answer 2

Reduction in visual acuity/blurry edges

Question 3

How does the Abbe’ Number relate to coloured fringes?

Answer 3

A low Abbe’ number means higher chromatic aberration therefore more likely to observe colour fringes

Question 4

What is the formula used for calculating Abbe Number? (V)

Answer 4

V = 1/w

• *w = dispersive power = (nf-nc)/(nd - 1)
• *nf = ref index blue light, nc = ref index red light, nd = ref index yellow light

Question 5

What is a Field Diagram? What is it used for?

Answer 5

A plot of the tangential and sagittal oblique vertex sphere powers against the ocular rotation of the eye viewing through the lens

– are used to help reduce oblique astigmatism

Question 6

How would an ideal +4D lens look on a field diagram?

Answer 6

Will have tangential and sagittal powers that remain at +4D for all zones of the lens

note: In a real scenario however, when the eye rotates, these 2 powers become different from each other

Question 7

Briefly describe how lenses are made (i.e. how the curves in the lens are made)

Answer 7

1. ‘blanks’ (pieces of plastic) are obtained and the front is sufaced into a ‘base curve’, with fixed powers
2. laboratories hold onto these and then ship to order in accordance to the most accurate fixed power blank
3. The back curve is then produced

Question 8

What are the 3 possible lens designs using only spherical curves to get best vision (i.e. “Best Form Lenses”)

Answer 8

1. Point focal lens – correct oblique astig. completely, leaving power error uncorrected
2. Percival form lens – correct power error, but leave residual oblique astig (= zero mean oblique error)
3. T-error lens – lens with minimum tangential error

Question 9

What are Tscherning’s ellipses used for?

Answer 9

To plot the ideal curves for eliminating oblique astig for a spherical lens form

Question 10

What are Tscherning’s ellipses?

Answer 10

A plot of back surface power required to minimise desired aberrations for a given back vertex power or equivalent power

Question 11

What are the 2 lens forms possible for each lens form (Tscherning ellipses)?

Answer 11

1. Shallow form = Ostwalt point-focal curve (most ophthalmic lenses)
2. Steeper form = Wollasten point-focal curve (highly curved and hard to manufacture, cosmetically unappealing)

**both lenses have good oblique astig control (Wollaston has better TCA and less distortion but is highly curved)

Question 12

How does the refractive index of our material relate to tscherning ellipses?

Answer 12

As we increase R.I, our tscherning ellipse grows in the negative direction

– so we get an increased range of powers where we can reduce astig to zero

Question 13

With field diagrams, how would we reduce oblique astig in a toric lens?

Answer 13

Know that there are 2 principle meridia and look at the aberration profile for each meridia separately

Question 14

What are aspheric lenses useful for?

Answer 14

Flatter and lighter lens form with elimination of oblique astig (i.e. a superior type of best form lens)

– i.e. for lens powers beyond the upper limit of best form spherical lenses (i.e. high plus lenses)

Question 15

Describe an Aspheric Surface?

Answer 15

a surface which is rotationally symmetric but at the same time not spherical

Question 16

How does an aspheric lens work to reduce oblique astigmatism? Is there any drawbacks to this?

Answer 16

the aspheric lens introduces negative oblique astig. to counteract the positive oblique astig from oblique incidence

– however (in high plus lenses), the mean power of the lens drops off rapidly as the eye rotates from the optical axis, resulting in some spherical error

Question 17

What is the most commonly used type of Best Form lens?

Answer 17

T-error form

however aspheric lenses are becoming increasingly popular

Question 18

What main optical aberrations are responsible for degrading off-axis performance in spectacles? (4)

Answer 18

• TCA (transverse chrom. aberration)
• Oblique astig.
• Field curvature
• Distortion

Question 19

What is the difference between axial and transverse chromatic aberration?

Answer 19

Axial (longitudinal): different wavelengths focus at different distances from lens

Transverse (lateral): different wavelengths focus at different positions in focal plane

Question 20

How do you calculate TCA at any given point?

Answer 20

TCA = p/V

**p = prismatic effect, where the threshold where TCA starts being a problem is p = 0.1 x V

** V = Abbe number

Question 21

What is the Abbe number for normal index plastic? What does this mean for the threshold prismatic effect?

Answer 21

around 60

(hence the prismatic effect at any one point would need to reach 0.1x60 = 6 prism diopters before the chromatic aberration effect was noticed, though note some people are more or less sensitive)