Prescription Writing & Simple Transposition Flashcards
Which prescription is written & measured 1st?
right eye
2 options for prescription writing:
text format or table format.
no x used in table format
eg RE +2.00/+2.00x180 Base Down
2 numbers after decimal point
+or- sign for sphere AND cylinder
axis in degrees but no degrees sign
for ‘x’ say “axis” or “at”
never say “positive two”
unusual to say “plus two point two five” or “plus two point five”
Sphere and Cylinder
sph&cyl increments: usually 0.25D, 0.12D also available
cyl axis: degrees (no sign), all cyl axes lie between 0&180 but 0 is not used clinically- same as 180 for cyls
Prisms
0 and 180 NOT same
need full 360 degrees
prism (base) orientation- up/down/in/out
Lens Centration
- all linear measurements in mm (no other unit used in optical dispensing)
- related to position of lenses in frame
Near addition (“Add)
extra power for close work
always plus
does NOT normally affect cyl or axis
(R Distance +2.00/+1.75x120 R Near +4.00/+1.75x120) is same prescription as (R Distance +2.00/+1.75x120 Add +2.00)
Back Vertex Distance (BVD)
how far back surface of lens is from front to eye (cornea)
What does sphero-cylindrical mean?
sphere, cylinder and axis
What happens if axis is at 90? (transposition)
axis becomes 180
+2.00/+1.50x135 (plus cylinder form)
or +3.50/-1.50x45 (minus cylinder form)
Step 1: add sphere and cylinder (always old power + new power (always plus as -ve sign will be involved)) +3.50/…
Step 2: change sign of cylinder +3.50/-1.50x…
Step 3: add or subtract 90 from axis (to keep within 0&180)
Result: +3.50/-1.50x45
+1.25/-2.50x95 (minus cylinder)
- add sphere and cylinder -1.25/…
- change sign of cylinder -1.25/+2.50x…
- add or subtract 90 (to keep within 0&180) -1.25/+2.50x5 (plus cylinder)
