Lensometry Multifocal Flashcards
What is the green and orange light on the lensometer for
Green for light lenses
Orange for sunglasses
Lenses that contain two or more loess power to let you see at different distances
Multifocal
Who wears multifocal
Over 40, children
Types of multifocal
Progressive
Bifocal
Trifocals
Must you set the axis on 180 before doing lensometry>
No
Line multifocal design
Front surface design
What is the distance between the optical center and the top of a bifocal
3-5mm
What is the distance between the optical power and the trifocal add power
1-3mm
How to read the add power on lensometer
Go from single to single or from cyl to cyl. Don’t fluctuate between the two
What side of the lens is thicker on multifocal
Front
How do you read the add power on a multifocal in lensometry
Flip the glasses around and find it that way. Also check the distance power again flipped and subtract the difference between the two, to make sure you are using a uniform side to do this.
When should you do the proper front/back vertex reading for add power
When the Rx is: +/-5 with an add of +2.50 or greater
Corrective lenses used in eyewear to correct presbyopia or other accommodation issues
Progressive lenses
Types of progressives
Multifocal, no lines, PAL
Downside of progressive lenses
Distortion on the side
Designates the point on a lens that should be placed at the center of the pupil
Fitting point
Areas used to verify prescribed distance power
Distance reference circle
Optical center of a PAL, used to check prism
Prism reference point
Located on nasal side, under micro-circles/references lenses type/material
Alignment markings and manufacturing logo
Used to verify total near point
Near reference point
Located on temporal side/ used to verify add power
Add power
What side is is the add power always on
Temporal side
When reading progressives, it is important to mark these points on each lense before proceeding
Dot it up
Where is the reference point usually
4mm up from the 180 line
Where is the prism in a progressive lens
Right int he middle of the 180 line
Reading distance on progressive lenses
Distance reference point
Where do you read the add power on a progressive lens
Read add power from the near reference point (down to the nasal)
Some manufacturers tell you not to do this. Short frames will read inaccurate. Digitally surfaced lenses will not be in quarter increments. Can be a +1.56 or something to that effect
How do you read the prism on a progressive
Right in the middle of the 180 line
Triangulated pieces of glass that pass through it and change the direction of light
Prism
Thinner in the middle, thicker on the side
Minus
Thicker inthe middle
Plus
A unit that describes the power of a prism to produce a cone centimeter deviation in a ray of light at a distance of one meter from the lens
Prism diopter
Involves the grinding of the surface of a lens during a manufacturing
Ground in prism
Involves decentering the optical center to utilize the pristmatic effects of sphere-cylindrical lens. Prentice’s rule
Induced prism
How do you know you have ground in prism
It will never lien up with the mires
How do you know you have induced prism
The mires can only be centered when the lens is decentered
What kind of lenses can you induce prisms into
Single vision lenses
What kind of prism has to be in progressives
Ground in
Minor powers and prism
Cannot induce prism
Unwanted prism
Calculate prentice rules
Can be vertical and horizontal imbalance
Too much BD prism
Vertical objects appear taller than they are. Looks like you are walking up a hill
Too much BU prism
Vertical objects will appear smaller
Too much BI or BO prism
Horizontal lines will have a curve to them
PAL and prism thinner
In PAL, surface power power increases as you go down, but it also increases thickness on the opposite side. Manufacturers want their lenses to look as good as possible, use prism thinning which induces prism, whether Rxed or not.
Most thinning is BD
Prism that is up in both eyes, or down in both eyes
Cancel each other out
Prism that is in in one eye, and out in the other eye will
Cancel each other out
How do you read prism on a progressive lens
Read from the prism reference point
3 ways you can get prism from a single vision
Exam history
Lab ticket
Getting optical center while patient wears them
