D: Ophthalmic Lens Forms - Week 3 Flashcards
Under what circumstances might we see coloured fringes surrounding the image through our spectacles?
As a result of decent transverse chromatic aberration for a HIGH CONTRAST target
What effect might we see when there is noticeable transverse chromatic aberration looking at a low contrast target?
Reduction in visual acuity/blurry edges
How does the Abbe’ Number relate to coloured fringes?
A low Abbe’ number means higher chromatic aberration therefore more likely to observe colour fringes
What is the formula used for calculating Abbe Number? (V)
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
What is a Field Diagram? What is it used for?
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
How would an ideal +4D lens look on a field diagram?
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
Briefly describe how lenses are made (i.e. how the curves in the lens are made)
- ‘blanks’ (pieces of plastic) are obtained and the front is sufaced into a ‘base curve’, with fixed powers
- laboratories hold onto these and then ship to order in accordance to the most accurate fixed power blank
- The back curve is then produced
What are the 3 possible lens designs using only spherical curves to get best vision (i.e. “Best Form Lenses”)
- Point focal lens – correct oblique astig. completely, leaving power error uncorrected
- Percival form lens – correct power error, but leave residual oblique astig (= zero mean oblique error)
- T-error lens – lens with minimum tangential error
What are Tscherning’s ellipses used for?
To plot the ideal curves for eliminating oblique astig for a spherical lens form
What are Tscherning’s ellipses?
A plot of back surface power required to minimise desired aberrations for a given back vertex power or equivalent power
What are the 2 lens forms possible for each lens form (Tscherning ellipses)?
- Shallow form = Ostwalt point-focal curve (most ophthalmic lenses)
- 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)
How does the refractive index of our material relate to tscherning ellipses?
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
With field diagrams, how would we reduce oblique astig in a toric lens?
Know that there are 2 principle meridia and look at the aberration profile for each meridia separately
What are aspheric lenses useful for?
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)
Describe an Aspheric Surface?
a surface which is rotationally symmetric but at the same time not spherical
How does an aspheric lens work to reduce oblique astigmatism? Is there any drawbacks to this?
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
What is the most commonly used type of Best Form lens?
T-error form
however aspheric lenses are becoming increasingly popular
What main optical aberrations are responsible for degrading off-axis performance in spectacles? (4)
- TCA (transverse chrom. aberration)
- Oblique astig.
- Field curvature
- Distortion
What is the difference between axial and transverse chromatic aberration?
Axial (longitudinal): different wavelengths focus at different distances from lens
Transverse (lateral): different wavelengths focus at different positions in focal plane
How do you calculate TCA at any given point?
TCA = p/V
**p = prismatic effect, where the threshold where TCA starts being a problem is p = 0.1 x V
** V = Abbe number
What is the Abbe number for normal index plastic? What does this mean for the threshold prismatic effect?
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)
