Flashcards in Lecture 4 - anatomy and physiology of a CL Deck (58)
what is the base curve radius (BCR)?
radius of curvature of the central posterior optical section in mm
what is the back vertex power (BVP)?
used to express power of CL -Determined from a fixed position with the concave surface of the lens against the lensometer lens stop
what is the contact lens power?
dioptric power of CL - usually expressed as back vertex power
what is edge clearance?
actual distance from lens edge to cornea (not inherent property of lens) - less than calculated edge lift due to corneal asphericity
what is effective power?
power of CL at the corneal plane
what is the Flat-K?
flatter (longer) of the 2 principal meridians (GP lenses are generally selected with BCR slightly steeper or flatter than Flat-K)
what is fluoro-silicone/acrylate (FSA)?
used in modern GP CL - consists of fluorine, silicone and methyl methacrylate
what is front vertex power (FVP)?
power determined from fixed position with convex surface against lens stop
what is the lacrimal lens (LL)?
tear lens between GP and corneal surface
what is the optical zone diameter (OZD, FOZD, BOZD)?
provides the visual optics for both GP and soft lenses
what is the overall diameter (OAD, TD)?
linear edge-to-edge measurement of the lens in mm
what is oxygen permeability (Dk)?
potential of a CL material to transmit oxygen, considers both solubility and diffusion (unit = barrer)
what is oxygen transmissibility (Dk/t)?
describes oxygen transmission for a specific CL
what is prism (in CL)?
produced by varying thickness from superior to inferior region of CL while maintaining the same front and back surface curvatures (used for cylinder lenses to center themselves)
what is radial edge lift?
measured from the lends edge perpendicular to an extension of the BCR (inherent property)
what is relative spectacle magnification (RSM)?
compares corrected ametropic retinal image to a standard emmetropic eye
what is residual astigmatism?
spherical GP = refractive cylinder - corneal cylinder
spherical soft = refractive cylinder
what is a rigid GP CL?
has rigid material (methyl methacrylate, silicone, fluorine), small in diameter (smaller than cornea) and higher optical quality than soft lenses
what is a silicone hydrogel (SiHy) lens?
current generation of soft lenses - uses silicone in matrix for higher oxygen permeability
what is a soft toric lens?
designed for refractive astigmatism - conforms to cornea (must be stabilized)
what is a soft CL?
flexible (hydrogel or SiHy material), large in diameter (larger than cornea) and some water content
what is spectacle magnification?
ratio of retinal image size of a corrected ametropic eye to retinal image size of the same eye uncorrected
what is the OAD for a GP lens? soft lens?
GP OAD = 9mm - 10mm
soft OAD = 14mm
what is the Bennett scale for Dk values?
low = 25-50
high = 51-99
hyper = equal to or more than 100
how is water content and oxygen related in hydrogel lenses? what about in SiHy lenses?
hydrogel = directly related (higher water = higher Dk)
SiHy = inversely related
what is the HEMA material used in?
soft lenses - only used with other materials to increase water content
what is EGDMA used in?
soft lenses - makes material stiffer/less stretchable
what are NVP and MAA used in?
soft lenses - both increase water content
what is MMA used in?
soft lenses - lowers water content and increases hardness/strength