term 2 useless stuff Flashcards
(22 cards)
optical aberrations
In an ideal optical system all the light rays from all points in the object plane would form a perfect image in the image plane but in reality this doesn’t happen due to aberrations. All aberrations except for chromatic aberration occurs w monochromatic light: spherical, chromatic, coma, oblique, astigmatism, curvature of field+ distortion.
spherical aberrations
Due to parallel light rays, paraxial rays form a focus at a point but light rays further from optical axis do not form a focus in the same position bc lens has a spherical curvature. Spherical aberrations get worse as d from optical axis increases= reduces image quality in lenses and mirrors made w curved surfaces so it makes images blurry. The hubble space telescope mirror suffered w these but corrected w system.
chromatic aberration longitudinal and transverse
longitudinal- Spread of spectrum along optic axis. Duochrome exaggerated. When diff colours of light have diff focal lengths which can cause the diff colours to be focused at different points along optic axis which can produce a coloured halo or fringing effect around objects. Most noticeable when looking at objects that are out of focus or when using lenses w a large aperture.
transverse- Spread of spectrum in plane perpendicular to optical axis. When diff colours of light are bent by different amounts as they pass through a lens causing the colours to focus at slightly different points which can produce a blurred image. Most noticeable near edges or periphery of lens where light passes through at an angle. Coloured fringes are caused by transverse chromatic aberrations mainly.
coma
the image takes an elongated shape like a comet
Similar to spherical aberration but produced by oblique light rays, compared to oblique paraxial rays, peripheral oblique light rays focus closer to optic axis but prod a larger blurry spot. So coma gets worse as distance from optical axis increases.
oblique astigmatism
parallel light rays enter lens obliquely to optic axis to form an astigmatic image. Rays are parallel to each other but not to optic axis. Comes in obliquely and forms 2 different lines of focus w a clc in middle. One image focus is called tangential focus and other image focus= sagittal focus. Both foci are 90 degrees apart. So for ret we need to be on axis otherwise result influenced by this= errors.
curvature of field aberration
focuses in a curved pattern bc object distance varies depending on where light comes from= causes variation in image distance as well. Retina curved so helps us not to notice this aberration as much. Image is on a curved surface (Petzval surface) don’t notice it bc retina curved.
distortion
inability of lens to create a rectilinear image of object. Doesn’t modify the colours or sharpness of the image but it modifies its shape.
Distortion occurs because focal length of lens varies over the Petzval surface= causes variations in transverse mag of an image-> some parts being more magnified than others. Further we go out from centre of lens more mag we have so it stretches the image out.
distortion- positive and negative lenses
pincushion- positive lenses: centre is the same and edges are being magnified and pulled out
barrel distortion- negative lenses- minification at edges and centre looks bigger but it isnt so for neg it seems as though edges are being pulled in
aberrations and eye on axis
only monochromatic aberration on axis is spherical aberration
But the elements of eye don’t all line up perfectly, crystalline lens often tilted which leads to coma and oblique astig at fovea. For off axis light the dominant aberration affecting the monochromatic retinal image is oblique astigmatism. The retinal image is also affected by distortion but compensated by the brain. If distortion pattern changes brain adapts
aberrations
Axial aberrations- Spherical aberrations (but in large aperture systems only a small area of lens is looked through at a time so don’t consider it in lens design)
Longitudinal chromatic aberration- eye has its own chrom abb so not noticed normally
Oblique astigmatism- coma- similar to spherical abb, due to large apertures not considered in lens design as well.
what matters for lens design
oblique astigmatism, distortion, curvature of field, transverse chromatic aberration as well= can be reduced w high abbe number material
spectacle lens forms
lenses come in lots of diff designs eg biconvex or meniscus.
Pairs of surfaces can add up to the same power eg you can make a +6.00 in lots of different ways. Meniscus lenses have a positive curve on front and negative curve on back are called curved lenses. Other lenses= flat form designs even if they aren’t ‘flat’ most spectacles are curved form designs.
So do decide what curves we want to have- do this by using a Best form lens
Lens design that is modified to minimise these aberrations so the surfaces of curvatures have been chosen and varied to minimise the aberrations particularly oblique astigmatism. -> to give best possible vision in oblique gaze (if you look out of edge of lens) may help w secondary considerations (Curv of field, distortion, transverse chromatic abb)
far point sphere and mop and moe oae etc
- spherical surface traced out by eyes far point as eye rotates. Rotated eye may be looking obliquely through the lens, if eye straight ahead perfect but if looking down= far point sphere is further->astig=2 different foci (tangential and sagittal and neither of these land on far point sphere so the image isn’t going to be sharp+ clear)
From T-S= oblique astigmatism, further out= more oblique astig. MOP= halfway through OAE (mean oblique error= how far mean is from far point sphere) ideally no diff between t and s so if person looked down they would lie on top of each other but if eye is turned obliquely and light comes in obliquely patient sees an image as though there was a cyl present.
Vertex sphere lies in front of eye (spherical surface traced out by axial vertex d as the eye rotates.
Lens is flat and the eye is curved around its centre of rotation so vertex distance increasing would also affect aberrations.
differences in t and s
Differences in T and S shows you how much difference there is as we go more oblique. If light comes in obliquely if eye turned down or lens is angled px has more aberrations at edge of lens. Bottom left s and t lines lie on top of each other so no oblique astigmatic error. Choice of lens form alters relative position of these foci and there are two lens forms where t and s foci are coincident. MOE= if we had T and S on top of each other how much do they differ from where we would like them to be.
point focal lens, min tangen error, percival form
point focal-When T and S foci coincide. Blur but comes to point focus, might not be where we want it to be but theres no cylinder induced=spherical blur. Complaints as spherical error at edge of lens. Sphere blurs more than cyl.
min tang error-Some astigmatism but not overall spherical blur. T=+4 sag becomes in front. Half a dioptre of cyl power as we get to 40 degrees. Do have OAE and MOE as eye rotates away from optic axis. Cyl>sphere
percival form- Astigmatism bc diff between t and s but not much. MOE is 0 as its on either side+ cancels out. OAE increases slightly as eye rotates away from optic axis.
Cyl>sphere
tscherning ellipse
2 best form lenses for a given lens power, to choose what powers we need to go for= use a tscherning ellipse. Need to understand it. Used to design best form lenses that are either point focal form (OAE zero), minimum tangential error form, percival lens form (MOE zero)
Front powers plotted as a function of total lens power= tscherning ellipse.
Factors that determine ellipse= n of lens, centre of rotation distance, object distance shallower forms needed for near distance so often compromise of design
If we wanted overall power -5 could have front surface +5 back -17 but ostwalt= lowest front surface power.
wollaston- lens form w highest front surface power
steeper curves and in general what happens as n increases
Generally steeper curves: good off: axis vision, control distortion. More bulbous, thicker, more spectacle magnification, theoretically restricts lens diameter. Ostwalt (shallower) lens form is most commonly used for spectacle lenses
In general as n increases the curvature of lens for best form increases+ range of negative lenses that can be made free from oblique astig increases. Outside max and min total lens power it isn’t possible to design a lens with zero OAE or MOE using spherical surfaces, could use aspheric lenses.
curvature of field and lens form and distortion
Curvature of field and lens form- Reduction of oblique astigmatism by altering lens form and this also reduces curvature of field.
Distortion- brain adapts to distortion pattern but if change in prescript readapts
Younger people adapt faster than older people, myopes find it harder to adapt to distortion bc they may have been accustomed to a plano concave lens form so they may be unable to tolerate changing to a curved lens form eventhough the new lens may have better off axis performance, plano concave lens has about twice the distortion of a best form design but if that’s what px is used to= harder to adapt.
base curve
lowest absolute power of toroidal surface but manufacturers talk about the base curve as being the spherical surface of the semi-finished blank (the front one)
So when we’re choosing a lens form it is this base curve we may wish to avoid altering. We ideally want less curve on the front surface so higher powers on the back.
how else to reduce distortion
wearing spectacles closer to eye/contact lenses or smaller frames or aspheric lenses.
mass production
A best form lens design exists for each power but lens manufacturers choose one to mass produce to minimise costs so mass produces spectacles have a near best form design. Freeform (digital) surfacing enables the form of the lens to be optimised for an individual prescription.
points to consider for optimum lens
lens form, material, field of view, centre thickness, weight, reflections, other tints and coatings.
