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40 Terms
what was the fundamental difference in design strategy between Wollaston and Ostwalt
Wollaston attempted to eliminate all monochromatic aberrations at the same time but Ostwalt ignored all other primary aberrations and focused on eliminating oblique astigmatism
what are the physical traits and manufacturing flaws of the Wollaston lens design
periscopic lens with extremely steep curved surfaces which made the lenses very difficult to manufacture with historic tools and were cosmetically unacceptable becuase they bulked out too far from the patients face
why was ostwalt mathematically justified in ignoring spherical aberration and coma in spectacle design
because the human pupil acts as a small aperture that only selects a tiny area of the lens while viewing and coma can be ignored becuase oblique astigmatism worsens at a square rate as the eye looks away from the centre meaning its negative impact on peripheral vision is far greater than the linear change caused by coma
what were the physical advantages of the ostwalt lens design
flatter meniscus lens forms and because the curves were shallow the lenses were easier to manufacture in late quantities and were cosmetically acceptable to patients
what is the definition of a base curve in lens manufacturing
initial surface curve that serves as the foundation or starting point from which the final lens prescription is made
how do manufacturing labs use semi-finished blanks to create a patient’s prescription
they hold an inventory of semi-finished blanks that already have a known base curve edged onto the front surface. to create the final customised prescription the lab surfaces and grinds the required power onto the back surface of the blank
how is a specific base curve chosen for a patient and what is the goal of this choice
it is chosen based on the patient’s prescription. the manufacturer selects the best possible base curve to produce a lens with minimal peripheral lens aberration which is known as a corrected curve or best form lens design
why are there two recommended best form spherical base curves for each individual lens power
for each lens power there are two recommended best form spherical base curves which results in a steeper lens design called Wollaston brand and flatter lens design called ostwalt branch
what is the tscherning ellipse and how is it formed on a graph
it is the locus points created when you plot the two recommended best form base curves across a range of different lens powers. when these steeper Wollaston and flatter ostwalt base curves values are plotted together, they naturally connect to form the physical shape of an ellipse
which section of the tscherning ellipse is used as the foundation for most modern spectacle lenses
most modern spectacle lenses are manufactured using base curves that sit close to the flatter ostwalt section of the tscherning ellipse becuase they are much thinner, lighter and cosmetically appealing
what is the far point sphere of the eye and how is it formed
imaginary spherical surface formed in space by the far point of the eye as it rotates around using the eye’s centre of rotation as its pivot point
what is the ultimate function of every optical correction prescribed for the human eye
bend incoming light rays so they form a perfectly focused image directly on the eye’s far point sphere
where is the far point sphere physically located for myopic eyes compared to hyperopic eyes
located in the front of the eye for myopes and behind for hyperopes
why do oblique astigmatism and field curvature always occur together in a spectacle lens
they are both mathematically related to the square of the field angle as the eye looks away from the centre of the lens
what is the petzval surface and why is it an issue for image formation
paraboloid image surface where light naturally focuses when you correct a lens for oblique astigmatism. this creates an issue because this curved image surface does not perfectly align with the eye’s true far point sphere leaving the image slightly out of focus
how did the Percival solution of 1926 solve peripheral lens blur
adjusted the lens design so that the two separated focal lines of oblique astigmatism would straddle the far point sphere ensuring the circle of least confusion lands directly on the far point sphere
what real world factors are completely ignored when a lens design only focuses on correcting oblique astigmatism
all other Seidel aberrations, the changing distance between the lens and eye’s centre of rotation for children versus adults, full peripheral field of view, specific distance and shape of object being viewed
why are historical Seidel aberration calculations unable to create a perfectly flawless lens in terms of raw physics
seidel aberrations are mathematically limited because they fail to account for the natural diffraction of light passing through a lens and are typically only calculated using one single wavelength of light at one specific field angle
what is the flawed theoretical assumption regarding how the five primary Seidel aberrations occur in a lens
they are distinct and isolated whereas they all occur simultaneously across the lens surface
what practical, real-world limitations are left out when designing a lens purely based on Seidel aberration equations
ignore non-optical factors such as thickness and weight of the lens, cost and ease of manufacturing the products for patients
why do modern lens manufacturers still choose to make single vision lenses using a meniscus form based on the historical ostwalt and Percival concepts
good cosmetic properties (minimal surface properties and good eyelash clearance), claimable basis for an underlying optical design principle, little understanding of the flaws in the marketing claims and long history of use
how can the structure of progressive addition lens be conceptually compared to an executive style bifocal
the seperate spherical distance and near zones are seamlessly joined together by a corridor of intermediate powers rather than a sharp visible line
how is the power change physically achieved between the distance and near zones of progressive lens
it is achieved along the umbilical where the surface curvature transitions continuously from a flatter radius at the top to a steeper radius at the bottom
what is the umbilic line in a progressive lens design
narrow, vertical blending path down the centre of a progressive lens where the surface remains perfectly spherical and free of unwanted astigmatism as the power increases from top to bottom
how do progressive addition lenses incorporate a smooth increase in plus power without any visible lines or segment breaks
by replacing the physical ledge between the flatter distance curve and steeper near curve with a smoothly blended surface that transitions continuously from the top to the bottom of the lens
what optical modification is physically used to blend the transition between the distance and near zones in the lateral regions of a progressive lens and why
incorporating varying amount of surface cylindrical power oriented at oblique axes into the outer regions of the lens. this is because it is impossible to blend a flat distance curve into steep near curve down the centre without warping the sides of the plastic
why do progressive lenses have a complex surface geometry on either side of the viewing corridor and what is the consequence for the patient
because the cylinder power varies in both magnitude and axis direction across the lateral regions creating areas of unwanted peripheral distortion
how do the astigmatism contour plots visually differ between a hard and soft progressive lens design
hard pals have closely spaced cylinder power contour lines that create rapid, high gradients of distortion whereas soft pals have widely spaced cylinder power contour lines which spreads the distortion out into much gentler, lower gradients
how does the concentration of unwanted peripheral astigmatism differ between a hard and soft pal and how is it caused
caused by blending cylinder section and is tightly concentrated into small lateral areas of a hard pals whereas the distortion is less concentrated because it is spread widely across the peripheral surfaces
how do the speed of power transition and presence of image jump compare between hard and soft progressive designs
hard pals have fast power transition that can cause more noticeable sense of image jump for the patient whereas soft pals have a slow, gradual power transition that minimises image jump
how does patient adaptation time vary between hard and soft pals designs
hard pals require longer adaptation time because patients must get used to the harsh boundaries of peripheral distortion whereas soft pals usually have a shorter, easier adaptation time because the transitions are gentler
how does the physical surface curvature of an aspheric lens differ from a standard spherical lens
in an aspheric lens the surface curvature change gradually from the optical centre toward the edge of the lens whereas a spherical lens maintains one uniform constant curve
why are aspheric lenses manufactured for patients and how do they benefit plus versus minus prescriptions
manufactured to reduce centre thickness of plus lenses and the edge thickness of minus lens. this provides a flatter and better cosmetic appearance which reducing the physical weight of the eyewear
which surface of a spectacle lens can be engineered with an aspheric design
front surface, back surface or both
why are aspheric spectacle lenses ineffective at correcting spherical aberrations for the wearer
because the wearer only looks through a small isolated area of the lens at ay given moment and this viewing zone continuously shifts as the eye rotates behind the frame
do aspheric lens design reduce peripheral distortion for a patient and why
they do not because the wearer only uses a small area of the surface at a time as the eye rotates meaning a highly steep historical Wollaston design is still required to control distortion