Physiological Optics 2 Flashcards
Gullstrand’s Eye model
-# of surfaces and # of n
6 surfaces (2 cornea, 4 lens) 4 n
Parameters of a Reduced Eye Model
n = 1.33 Total Power = +60.00D Axial Length (K to post. focal pt) = 22.22mm
The center of curvature of the far point sphere of the eye is the same as…
The eye’s center of rotation
Far Point (Punctum Remotum)
Point in space that forms a point image on the retina
Emmetropes: infinity
Myopes: negative (in front of retina)
Hyperopes: positive (behind retina)
Near Point (punctum proximum)
The point in space that forms a point image on the retina when the eye is using maximum accommodation
Factors contributing to night myopia
increased spherical aberration
low light levels hinder relaxation of accommodation
% of people with myopia of at lease -0.50D at age 6 and 20
6: 2%
20: 20%
Latent Hyperopia
Hyperopia eliminated by accommodation
Shows more hyperopia on objective refraction than subjective refraction
Manifest Hyperopia
Amount of hyperopia on a subjective refraction
Absolute Hyperopia
Amount of hyperopia unable to be eliminated by accommodation
Facultative Hyperopia
Maximum amount of hyperopia that can be eliminated by accommodation
Trends of Hyperopia
%6 of kids 6-15
Constant from 20-40 but may increase after age 45
Expected refractive condition at 14 based on hyperopia at age 5
>1.5: hyperope
0.5-1.25: emmetrope
<0.5: myope
PD readings from pupilometer reduce influence of…
Parallax
Equally Mixed Astigmatism
Mixed astigmatism where the COLC falls on the retina
Major Meridian of the Eye
The STEEPEST corneal meridian (one with most plus power)
Recognition Acuity
Resolution at high frequencies and high contrast
-not suitable for pt with problems at other frequencies (cataracts)
Minimum Detectable Acuity
Minimum visible line
- usually better than recognition and resolution
- tested via veneer acuity
Resolution Acuity
Distinguish pattern (sine wave) from a uniform patch -Cutoff usually 40-60 cpd (MAR = 0.75)
Hyperacuity
Sense of directional relationships (are two lines parallel?)
-Higher cortical processing
MAR
Minimum angle of resolution in arcmin (1/60 degree)
MAR = 1/snellen (so 1 MAR = 20/20 = 0logMAR)
Which acuity chart is used in research because of its logMAR scaling?
ETDRS
Allen Vision Test
Good for kids
-Determine longest distance they can resolve the image monocularly
Acuity = distance/30
Which Acuity methods require a child to match the test letter to a “key”
STYCAR, HOTV, LEA
Sheridan- Gardiner is similar but includes near vision
Static Retinoscopy
-motion
The good ol’ phoropter kind (no accommodation involved)
-horizontal streak scopes the vertical meridian and a vertical streak scopes the horizontal meridian
Hyperope: with motion
Myope: against motion
Dynamic Retinoscopy
-methods
Measures accuracy of accommodation (lead/lag)
MEM: uses quick presentation of lenses
Nott: moves the retinoscope (away = lag)
JND calculation
Snellen denominator/100
-present lenses +/- one half of JND
Stenopaic Slit
Reduces entrance pupil perpendicular to slit orientation
- clearer slit orientation = SRx axis
- good for determining astigmatism
Astigmatic Dial
Fog, ask which lines are darkest, add minus cyl until all lines are equal, then add minus sphere til best acuity
-Lowest # on clear meridian x 30 gives Cyl axis
Duochrome Test Acuity Threshold
RAM-GAP pts only if they are 20/30 or better
-deficiencies in color vision do not affect ability to do test
Methods for balancing Accommodation
Fogging: fog +2.00 then present -0.25 in succession until clarity b/t eyes switches back and forth
Risley Prism: the one you do
Prism Dissociated Bi-Ocular: best if acuities are different
Reasons to use Cycloplegic refraction
convergent strabismus
significant eso at near
young adult complaining of HA with no hyperopia
How much accommodation can a person use comfortably
50%
Average Accommodation equation
A = 18.5 - 0.3 age
Half Amp method equation
Determine 1/2 amp then subtract from working distance to find add
Magnification effects in aphakic pts
M = -F1 / F2
Symptoms of Aphakia
Pincushion distortion Ring scotoma Convergence demands due to large BO effects Diplopia Anisekonia
Why does over minusing make images appear smaller?
Combo of a minus lens and a plus lens (eye) acts like a Galilean telescope
Mohindra/Near retinoscopy
Dynamic Ret technique used to determine kids refractive status
-monocular, in the dark at 50 cm, and must subtract 1.25 adjustment factor
Anisometropia
Difference in refractive errors b/t eye usually > 1.00D
