Lensometry Flashcards
What can a lensometer be used to determine?
Power
Optical center
Ground prism
What is the range for a normal add? How can we calculate near add on a bifocal during lensometry?
+0.50 to +4.00
Near fvp - distance fvp
How do prisms affect light? Where is light deviated? How does this compare to lenses?
Prisms bend light, they don’t change the vergence of the beam (Parallel comes out parallel)
Light deviated to base, image towards apex
Lenses change vergence
How do we calculate prism diopter?
Displacement of 1cm at a viewing distance of 1 meter
Displacement (cm)= prism power x viewing distance (m)
Compare ground prism to prism by decentration.
Ground is the same all over the lens
Decentration varies across the lens
If the image is displaced down within the lensometer what kind of base does this indicate?
Base down prism b/c the lenso inverts the image
What are the values of the reticles within the lensometer?
0.5, 1, 2, 3, 4 prism diopters
What is prentice’s rule?
P=cF
For prism by decentration
C in centimeters
What is the optical center of the lens?
Point on the lens where there is zero prism by decentration
What happens when the prisms are in front of the same eye?
They are added together
Define near contraction distance.
The distance between visual axes measured in the spectacle plane when viewing a near object
By how much less is the near CD compared to the distance PD?
CD is usually 3-4mm less
What is the order of planes from farthest from retina to closest when considering calculations for calculating near PB.
Spectacle plane (near CD)
Corneal plane
Pupil plane (near PD)
Distance PD
