DISP - Ophthalmic Lens Forms - Week 3 Flashcards
When does and doesn’t lens form become important when considering where images are formed with modern lenses (2)?
The lens form isnt important when looking through the optical axis as its performance here is excellent.
When images are formed from off-axis regions of a lens, the lens form is important to performance.
Name 4 aberrations that adversely affect performance in off-axis regions.
Transverse chromatic aberrations
Oblique astigmatism
Field curvature
Distortion
Describe two ways in which off-axis aberrations can be reduced via lens form.
Using the best form approach where curves are kept spherical.
Can also use an aspherical approach.
Define chromatic aberration, and why it occurs.
When the lens fails to focus light of different wavelengths to a single point.
This is because lenses have different refractive indices for different wavelengths of light.
How does the refractive index of a lens differ with the wavelength of light?
Refractive index decreases with increasing wavelength.
Name and describe the two kinds of chromatic aberrations.
Axial/longitudinal - different wavelengths are focussed on the optical axis, but not at the focal point.
Transverse/lateral - different wavelengths are focussed at different positions in the focal plane.
Consider transverse chromatic aberration. Distinguish between the effects of high and low contrast.
High contrast - the effect is to cause coloured fringes surrounding the image. The image itself is in focus.
Low contrast - colour fringing may not be noticed, but instead, the effect causes a reduction in acuity.
What is an Abbé number, and what does a high/low number represent?
A number used to represent the amount of chromatic aberration for a given lens material.
A high Abbé number means less chromatic aberration.
A low Abbé number means its more likely that colour fringes will be observed.
Describe the formula for Abbé number.
V = 1 / ω
V = Abbé number
ω = dispersive power
ω = (nF - nC) / (nD - 1)
nF = refractive index of blue light
nC = refractive index of red light
nD = refractive index of yellow light
What lens material has the highest and lowest Abbé number?
Highest - crown glass
Lowest - polycarbonate
How can the amount of transverse chromatic aberration at any given point be calculated?
Calculating the prismatic effect, and dividing by the Abbé number
What threshold value for transverse chromatic aberration will typically cause problems?
~ 0.1Δ
Any less, and patients are unlikely to complain.
If the Abbé number for normal index glass/plastic is ~60, what prism dioptre would be needed to reach the transverse chromatic aberration threshold?
What consequence does this have on low Abbé number lens materials?
p ~0.1 x V
and so prismatic effect would need to be 6Δ before it becomes noticed.The lower the Abbé number, the more likely the aberrations will be noticed.
In oblique astigmatism, which focal line is more anterior (closer to the lens)?
Tangential is more anterior to the lens than the sagittal.
Where are off-axis points brought into focus in field curvature?
The petzval surface.
Consider distortion. What image shape does positive and negative distortion result in, and does it affect image clarity?
Doesnt affect image clarity.
Positive distortion (in positive lenses) results in a pincushion effect, negative distortion (in negative lenses) results in a barrel effect.
In what kind of lens is distortion only a problem in practice?
High powered lenses.
How can distortions be minimised?
By using aspheric lenses.
Why does distortion occur?
Because of the differences in magnification between different parts of the lens.
Describe the pincushion and barrel effect of distortion.
Pincushion - the periphery is magnified.
Barrel - the periphery is minified.
In practice, what kinds of aberrations can a lens designer reduce (3)?
Can only reduce oblique astigmatism and field curvature, but not transverse chromatic aberrations as much.
Describe the process of making a lens beginning with a blank (4).
A spectacle lens is made from a blank.
The front of the lens is surfaced to an appropriate base curve.
These blanks are held in storage until an order arrives.
Most appropriate base curve blank is assigned to an order, and the back curve is produced.
Describe what point focal lenses are.
When oblique astigmatism is completely corrected, but the expense of having power error uncorrected.
Describe what percival form lenses are. What kind of mean oblique error do they have?
When power error is corrected completely, at the expense of having residual oblique astigmatism uncorrected.
These lenses have zero mean oblique error.
Describe what T-error lenses are.
Lens designed to have the minimum tangential error.
Give three examples of best form lenses.
Point focal lenses
Percival form lenses
T-error lenses
What is the most common form of best form lenses today and why?
Minimum T-error lenses, perform well over a wide range of vertex distances.
Define aspheric surface.
A surface that is rotationally symmetrical, but at the same time, not spherical.
Which is lighter and thinner, spheric or aspheric lenses?
Aspheric
Does aspheric surfacing reduce aberrations in high or low powers? what does this come at the expense of?
Reduces aberrations in high plus power lenses, at the expense of a rapid decrease in power as the eye rotates away from the optical axis. This means there is some spherical error.
