4.1.1 Flashcards
How do you calculate vertical differential prism
P= C x F so times sph of one eye with decentration(0.5) and do the same to other eye and the difference between the two is the vertical differential prism
What are the tolerances for differential prism
0.25-0.5 = acceptable/minimal discomfort
0.5-1.0 = mild adaptation, noticeable symptoms like eye strain/HAs
>1 = discomfort/DIP/eye strain and req gradual adaptation
How do you check lens thickness without an order form
Calipers with a display screen, position caliper jaw on surface of lens at centre to measure central thickness
What is VDP and HDP
VDP is vertical differential prism when optical centres are not aligned vertically and HDP is horizontal differential prism when optical centres are not aligned horizontally
What condition causes vertical imbalance when switching from the near portion?
Anisometropia
Anisometropia refers to a condition where the two eyes have unequal refractive power.
What can the difference in lens powers in anisometropia lead to?
Vertical disparity
Vertical disparity can result in symptoms such as double vision or discomfort.
What is used to correct the vertical imbalance in near portion of the lens?
Slab off
Slab off is a technique used in lens design to accommodate differences in prescription between the two eyes.
In which eye is the slab off prism correction applied?
The more myopic eye
Myopia refers to nearsightedness, where distant objects appear blurry.
Where does the prism correction start in bifocals?
At the dividing line between dv and nv
‘dv’ refers to distance vision and ‘nv’ refers to near vision in bifocal lenses.
In bifocals, where is the prism correction present?
Only in the nv
The near vision (nv) portion is designed to assist with close-up tasks.
Fill in the blank: Anisometropia can cause an imbalance in the amount of ______ experienced.
prism
Prism refers to the degree of deviation of light entering the eye.
What is monocular centration
Align each lenses optical centre to the pupil of each eye by measuring the distance from centre of patients nose to pupil of each eye separately. This is crucial if eyes are not symmetrically placed
What is the relationship between good VAs and anisometropia?
Good VAs are more likely to cause anisometropia
Anisometropia refers to a condition where the two eyes have a significant difference in refractive power, which can lead to visual discomfort and difficulties in coordination.
Why are bad VAs less likely to cause anisometropia?
The brain suppresses the image from the poorer eye
This suppression can help to mitigate the effects of unequal visual input from the eyes.
What occurs with large anisometropia and good monocular VAs?
Anisocoria occurs
Anisocoria is a condition where there is a difference in the diameter of the pupils, which can be related to differences in image size or magnification between the eyes.
How does power difference between the eyes affect prismatic effect?
Each 1D in power difference induces 1D of vertical differential prismatic effect
This effect is notable at the near viewing point (NVP), typically located 8-10mm below the optical centers of the lenses.
What are common problems for anisometropia/anisokonia
Vertical differential prism
Horizontal decentration
Large change in rx effecting BV status
Frame size change (smaller to bigger)
Frame shape change(more bowing effect)
Solutions for anisometropia/anisokonia
Reduce rx to help adaptation
Prescribe prism - calc amount of vertical decentration and it’s over 1D and disp is multis then check if the compensating prism makes the vision more comfortable
What do the following frame markings mean = 47.21 145
And how would u calculate frame OC
47 = lens diameter
21 = bridge width
145 = arm length
Frame OC = 47+ 21
= 68 /2
= 34
Why does changing to higher index lens also require adaptation
Higher index lenses have more chromatic aberrations (different wavelengths of light are refracted at diff angles)
What are the symptoms of Aniseikonia?
Visual discomfort, visual distortion, difficulty with stereopsis/BV, dizziness, H/A, nausea
H/A stands for headache.
What type of lens is used for single vision dispensing for an anisometropic patient?
Aspheric lens for the most positive powered eye
The aspheric lens reduces retinal image size by altering lens curvature to control magnification
How does an aspheric lens affect the retinal image size?
The retinal image size is reduced as the aspheric surface is approximately 2.00D flatter
This flattening helps in managing Aniseikonia.
What is the purpose of an Iseikonic lens?
To increase the spectacle magnification of the least positive eye
This is achieved by increasing the base curve.
How is the thickness of an Iseikonic lens adjusted?
By increasing the base curve to approximately 8.00D
This adjustment helps to accommodate the least positive eye.
What is the formula for Spectacle Magnification?
Spectacle Magnification = Shape Factor x Power Factor
This formula is essential for understanding lens design in Aniseikonia.
Define Shape Factor in the context of lenses.
A ratio of surface curvatures that describe the shape of the lens, typically ranging from -2 to +2
Shape Factor influences how the lens shapes the image.
What does the Power Factor represent in lens design?
The dioptric power of the lens considering the power and vertex distance of the lens
Power Factor plays a crucial role in determining lens effectiveness.
What is slab off prisms?
A method of grinding the lens to add a specific amount of prism to one lens to compensate for the difference in prescription between the two eyes.
In what types of lenses is slab off used?
Bifocals and varifocals.
Where is the slab off added in the lens?
To the lower part of the stronger lens.
What effect does looking down have for an anisometropic patient?
It causes a vertical prismatic effect.
What symptoms can slab off prevent?
DIP/eyestrain.
How is slab off calculated?
By halving the difference between the two eyes and adding to the most negative lens.
Why is the most negative lens used in slab off calculation?
It induces the most base down (BD) prism.
What does base down (BD) prism correct?
Image alignment when a person looks down.
What are polarizing lenses designed to reduce?
Glare
Polarizing lenses filter out specific orientations of light waves.
What do polarizing lenses contain that blocks certain types of light?
A polarizing film made from polymer or iodine crystals. The material is stretched to align the crystals in a vertical direction to help block out the horizontal light
How do light waves travel?
In waves, vibrating in many directions
Light from the sun scatters in all directions.
What happens to light when it reflects off flat surfaces?
It becomes polarized
This means the light waves align mostly in a horizontal direction.
What type of light do polarizing lenses allow to pass through and how does this make the glasses polarised
Only Vertically oriented light can pass through
Natural light travels in all directions but when it hits a flat surface it reflects off at a horizontal angle causing horizontal polarised light which causes intense glare
What does the ‘D’ in the D28 segment refer to?
The shape of the near-vision portion
It is D-shaped.
What does the ‘28’ in D28 segment refer to?
The width of the segment in millimeters
In this case, it is 28 mm wide at its widest point.
Where is the D-shaped segment typically placed in the lens?
Towards the bottom of the lens
The flat side of the ‘D’ faces upwards.
What is one characteristic that differentiates the C segment from the D segment?
Smaller diameter of 22mm
The C segment has a more discrete appearance.
What is a preferred use for the C segment?
More specific tasks requiring a smaller reading area
It has a round topped design.
What is aniseikonia?
A condition where there is a significant difference in the perceived size of images between the two eyes
What causes aniseikonia?
It can occur when the refractive power or shape of the lenses in each eye differs
What are anisekonic glasses used for?
They are used for correction of aniseikonia
How do anisekonic glasses work?
The lenses change how big or small the images look when viewed through them
What is the effect of wearing anisekonic glasses?
They help make the sizes look more similar between both eyes
