In the Lab Flashcards
What are the 6 different types of lens forms?
- Plano convex
- Plan concave
- Bi-convex
- Bi concave
- Periscopic
- Meniscus
When do you use biconvex lenses?
For VERY high plus prescriptions, binoculars, telescopes and camera lenses
When do you use biconcave lenses?
For VERY high minus prescriptions
What are the rules for periscopic lenses?
One power must be either +1.25 of -1.25 and you varies the other power to get the desired total power. This is not used anymore as it had a lot of aberration
What are the rules for meniscus lenses?
One power must be either +6.00 of -6.00 and you varies the other power to get the desired total power. This is not used anymore as it had a lot of aberration
What is the most common type of lens form we use now?
The corrective curve series. This is a series of lenses that determines the best combination of front and back curves to get the best optics for a given prescription.
What are 5 names for a lens with cylinder?
- Cylindrical
- Astigmatic
- Compound
- Sphero-cylindrical
- Toric (CL only)
How are Cylindrical lenses designed? Why?
Sphere on the front surface and two difference curves on the back surface. you could put them on the front surface, but this would create worse optics
How can you tell if a lens blank has cylinder?
The lens blank will rock on a flat surface
What is a distometer?
It measures the Back Vertex Power: the distance between the back of a lens and the closed eye lid
Fill in the blank: Plus lens moved out is effectively ________, so compensate __________.
1) stronger
2) weaker
What is the Interval of Sturm?
The difference in two focal points created by an astigmatic eye.
What is the circle of least confusion?
The dioptic midpoint in the interval of Sturm where in image is equally blurred in all meridians
Define Base Curve
The curve of a lens that determines the other curves on a lens therefore it is the Front Curve of a lens
What 4 changes can you make to increase magnification?
- Increase centre thickness
- Steepen base curve
- Increase vertex distance of plus lens
- Decrease vertex distance of minus lens
What 4 changes can you make to decrease magnification?
- Flatten base curve
- Decrease centre thickness
- Decrease vertex distance of a plus lens
- Increase vertex distance of a minus lens
What is the A box?
The “eye size” - The horizontal distance between the furthest temporal and furthest nasal edges of the lens
What is the B Box?
The “frame depth” - The vertical distance between the furthest top and furthest bottom edges of the lens
What is DBL?
The “Bridge size” (Least Distance Between Lenses)- the shortest distance between the nasal edges of each lens
What is DBC?
The “Distance between centres” - The Horizontal distance between the geometric centres of the lenses. The “Frame PD”
What is the ED?
The “Effective Diameter” - The distance from geometric centre to the furthest point of the eye wire (x2)
Typically measure the widest distance between edges of the eye wire. It is used to determine the minimum blank size
ALWAYS greater than or equal to A
What is the C measurement?
The eye size along the Datum Line
What is the C Size?
The circumference of the entire lens
What is the datum line?
The horizontal line that runs through the vertical centre of the frame (the horizontal line that is drawn at the B box mid point)
What modification do you have to account for when measuring A, B and ED?
The bevel (1/2 mm on each side) so whatever your ruler reads, add 1 mm
What modification do you have to account for when measuring DBL?
The bevel (1/2 mm on each lens) so whatever your ruler reads, subtract 1 mm
What is GC?
The “Geometric Centre” - meeting of two diagonals after boxing the lens. This is equal to 1/2 A
What is the marking for a Safety standard?
Z-87 on the frame and lenses
What are the 4 types of Segments?
- Kryptok (Round Top or K seg)
- Straight Top (Flat Top or Mono Step D)
- Ultex A
- Executive (Presidents, Franklin, E-seg)
What are the parameters of a Kryptok Segment?
- 22 mm in diameter, so the OC is in the middle (11 mm down)
- Distance OC to segment OC is 15 mm (distance OC is measured 4 mm above the top curve)
What are some issues of a Round Top?
Image jump due to prism present between the OC heights - this has one of the worst optics, even though it is cosmetically the best
What are the parameters of a Straight Top Segment?
- OC is 4 mm below the line (distance OC is 4 mm above the line)
- Distance between OC to OC is 8 mm
- Comes in many sizes (28, 35, 40 and 45)
What are the parameters of an Ultex A?
- The segment is on the back - fused to one piece
- Segment OC is 19 mm below the top of the curve
- OC to OC is 23 mm
- Basically the worst segment option because there is too much prism - the eye naturally only drop 9 mm
What are the parameters for an Executive Bifocal?
- One piece, the segment line is right at the distance OC so there is no image jump
Why are Executives not a good design?
they are UGLY and the ledge chips and easily gets dirty
What is the wanted top of tilt for lenses?
Pantoscopic tilt
What is the ideal pantoscopic tilt for SV and PALs?
SV: 8-10º
PAL: 12-15º
When a frame is on someone’s face, how is pantoscopic tilt influenced?
By the patient’s ears. To increase pantoscopic tilt bend the temples down. To decrease the pantoscopic tilt, bend the temples up
How do you measure Binocular PDs?
Measured from pupil centre to pupil centre (or nasal limbus to temporal limbus if the patient has VERY dark irises)
How do you measure Monocular PDs?
Measured from pupil centre to middle of the patient’s bridge
Define Distance Binocular PD
Measuring pupil centre to pupil centre when looking at an object at infinity
Define Near Binocular PD
Measuring pupil. centre to pupil centre when looking at an object closer than 20 ft (usually 40 cm or 14 inches)
What is the frame PD?
This is the geometric centre to geometric centre = A + DBL
When does decentration affect frame selection?
Affects the edge thickness so when the patient’s Rx is above ± 4.00 D
How do you calculate Size of Blank?
Minimum = ED + 2(Horizontal Decentration) + 2 mm
size of bank.
What is Joe’s Rule?
How to calculate Size of Blank based on double decentration (horizontal and vertical).
SOB = Vertical + ED + 2(Horiz. Decent.) + 2 mm
Decentration
Define Prism
Image displacement
How do you measure prism?
Measured in Diopters, but it does not have a sign… it is measured in Base Direction
How much horizontal prism can the brain handle before they start complaining of fusion?
- 25 ∆ in each eye
0. 50 ∆ total
How long can it take for a patient to start complaining of fusion?
A couple days - over time the brain gives up on muscling through/counteracting prism imbalance. They can come back after pick up complaining of headaches, eye strain and double vision
Why would you split prism?
For cosmetics only
What are the steps to split vertical prism?
- Split prism in half and give 1/2 to each eye.
- Give the original base prescribed from the doctor to the eye that originally had prescribed prism and give the opposite base to the other eye
- You must keep the imbalance the same value to keep fusion
How do you split horizontal prism?
- Split prism in half and give 1/2 to each eye
2. Keep both the base directions the same
What is fresnel prism?
Stick on temporary prism
Describe an aspheric lens
- Not spherical
- Eccentricity on the front surface (only horizontal and vertical)
- Better peripheral vision only when looking up/down and side to side
Describe Atoric lens
- More complex aspheric lens
- Eccentricity on the BACK surface in all meridians
- Better vision in all periphery
- All 1.74 lens are atoric
What are the rules for Aspheric/Atoric lens fitting?
- Use monocular PDs
- Mark OCs and subtract 1 mm for every 2º of pantos (to a max of 5 mm)
- Prism cannot be achieved with decentration
Define computer syndrome
Neck.shoulder pain, headaches, eye fatigue, blue light radiation associated with 3+ hours on a screen/day
What is a Double D lens design?
Designed for mature presbyopia (+2.00 or more) that work/read above their heads - librarians, pharmacists, electricians, plumber, carpenter, mechanic
What is an anti fatigue lens?
A lens that has a boost (0.4 0.6 0.85 D) to keep eyes from fatiguing
How would you order a Nikon Online lens?
- Order the complete Rx, mono distance PDs, pupil centre height, coatings
How would you verify a Nikon Online lens?
Verify at the NRP (it is made with degression values).
Add: ≤ 1.50 - 1.00 degression
1.75 to 2.25 - 1.50 degression
≥ 2.50 2.00 degression
