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