Clinical Optics Flashcards
Given lens power in diopters, object distance from lens and location of image. How would you calculate the distance from the image to lens?
U + P= V
U = object vergence (always negative)
P = power of lens/mirror
V = vergence of light exiting the lens/mirror to form image
How do you find the object and image vergences?
Reciprocal of distances from the lens/mirror respectively
Spherical equivalent given principle meridians
Average cylindrical powers of a lens.
How do spherical lenses focus light rays?
Focus light rays around the same point producing a BLUR CIRCLE that can be mathematically described by a point spread function.
Image height divided by object height
Transverse magnification
Simple magnifiers
Angular magnification
How do you calculate the magnification produced by simple magnifiers?
Magnifier is held 25 cm from objects and magnification produced is the dioptric power of magnifier divided by 4
Axial magnification
AKA longitudinal magnification
Calculated by square of transverse magnification
How do you calculate transverse magnification?
Transverse magnification (“M”) is given by M = U/V
U is incoming vergence (often from object)
V is output vergence of light leaving lens
“MUV’in out” – M = U/V = i/o “i” is image distance and “o” is object distance from lens of interest.
What is the vergence of light rays emanating from an object left of the lens?
Negative, and reciprocal of its distance from the lens
Kestenbaum’s rule
initial add will be the inverse of the Snellen acuity for the BETTER SEEING EYE (if Snellen acuity were 20/40 OD and 20/100 OS, then add would be +2.00)
Refractive change when oil is placed in eye
Phakic or pseudophakic - hyperopic shift of 3-5 diopters because molds to back of lens producing concave shape (negative powered lens)
Aphakic - variable but convex shape acting as positively powered lens and produces a myopic shift.
Refractive index of crown glass
1.523
Refractive index of aqueous
1.336
Refractive index of the cornea
1.376
Refractive index of acrylic
1.46
Refractive index of silicone oil
1.438
tendency for light to turn or bend when passing by an edge
Diffraction
How does a pinhole work?
limiting light rays entering the eye to those which are mostly perpendicular to the lens and cornea and therefore not refracted before projecting on to the retina.
Uses chromatic aberration to help refine the spherical component of a person’s refractive error
Duochrome test
How do you interpret results of a duochrome test?
if red side is better, current refraction is too hyperopic and more minus power must be added. If Green better, too myopic and more plus power added.
“RAM-GAP”
Shorter wavelengths (green) focused more anteriorly. Longer wavelengths (red) focused more posteriorly.
Duochrome can’t be used on this patient
VA worse than 20/30, since small distance (0.50 diopters) between red and green sides is too small to distinguish.
Where does a positive cylinder lens extert focusing power?
exerts all focusing power 90 degrees away from axis of placement and no focusing power along its axis of placement.
Principle of AcuFocus corneal inlay
Pinholes allow increased depth of field. Ring inserted under LASIK flap in nondominant eye after LASIK. Opaque and matches pupil size. At near, pupil constricts only light rays inside inlay brought into focus, increasing depth of field.
