Reduced eye model
- interior eye
- cornea to anterior focal point
- cornea to posterior focal point
- total eye power
n = 1.33
- 67mm (axial length)
- 22mm
60D
Far point
aka punctum remotum (PR)
Where the eye is looking without accommodation to have light perfectly focused on the retina
i. e. the point conjugate to the axial retinal point
i. e. a point object object at the far point will result in the formation of a point image and the retina, and vice versa
Far point sphere
When the eye rotates, the far point is no longer just a point, but traces a spherical surface - known as fps
Center of curvature = center of rotation of the eye (~27mm behind typical spectacle plane)
How to find far point of ametropic eye
With correcting prescription (F):
Far point = abs(1 ÷ F)
Near point
aka punctum proximum (PP)
Same as far point, but with MAX ACCOMM
Reduced eye model
-emmetropia
Incoming plane waves converge to a point on the retina
-retina located at Ƒ’ (secondary fp of lens), which must account for the IR used for the interior eye
For reduced eye model: power +60D, internal n=1.3
-retina located at 1.3/60m, or about 22mm from the lens
The far point for an emmetrope is optical infinity (same for corrected my/hyperopes)
Reduced eye model
-myopia
Eye is too strong (>60D) -> light from incoming plane waves converge in front of the retina
The far point for a myope is between the eye and infinity (i.e. will sit in front of the retina)
Describe night myopia
Pt becomes more myopic under low light levels
Due to combo of:
- incr spherical aberrations
- light levels that are too low to fully relax accomm when viewing distance
Myopia trends
At birth ~25-50%
By 1yo, few children are myopic
By 6yo, -0.50 or more in only 2%
-incr in prevalence b/w ages 6 and 20, reaching 20% by 20yo
Incr somewhat in later years due to nuclear sclerotic lens changes
Reduced eye model
-hyperopia
Eye is too weak (<60) -> light from plane waves converge to a point behind the retina
Far point located behind the retina, considered virtual
Optically speaking, a correcting lens should be place so that…
Its secondary focal point coincides with the far point of the ametropic eye
Hyperopia
- latent
- manifest
- absolute
- facultative
Accommodated/covered up with accomm
Found in subjective refraction
Amount that cannot be corrected in a hyperope whose rx is too large to be neutralized by accomm
Amount that can be neutralized by accomm
Hyperopia trends
Prevalence ~6% in children 6-15yo
-unlike myopia, prevalence doesn’t change with age for this range
A hyperopic child approx 5yo, will likely be:
- hyperopic at age 14 if orig >1.50 D
- emmetropic at 14 if orig 0.50 to 1.25 D
- myopic at age 14 if orig <0.50 D
Ages 20-40:
- expected to be relatively constant
- decr in accomm ability may functionally highlight otherwise non-burdensome hyperopia
Over age 45, hyperopes + emmetropes tend to show an incr with age
Measuring interpupillary distance
-PD ruler
1) dr sit at 40cm and closes OD
2) pt fixates on dr OS, PD measured (OS edge -> OD edge) = NEAR
3) with ruler in place dr closes OS/opens OD, pt fixates on dr OD
4) measure again = DISTANCE
Measuring interpupillary distance
-pupilometer
1) dr sets ometer at 40cm/infinity
2) place ometer against bridge of nose/at spec plane
3) ometer light produces corneal reflex visible to dr
4) dr aligns vertical hairline with location of corneal reflexes
5) both binoc and monoc PD can be read directly from instrument
Measuring interpupillary distance
-using a pupilometer reduces…
Errors resulting from parallax
Measuring visual acuity
-resolution vs recognition acuity
Resolution: pt distinguishes a pattern from a uniform patch of equal luminance (e.g. Teller cards)
-typical young adult has cutoff of 40-60 cycles/degree (0.75 MAR)
Recognition: gives info about ability to resolve high frequencies
-not useful for pts who show probs at other frequencies (e.g. cataracts)
Measuring visual acuity
-minimum detectable acuity
Essentially asks what is the thinnest possible wire that’s visible
-e.g. vernier lines
Avg person’s MDA (~1 arcsec) is much better than resolution/recognition
Measuring visual acuity
-hyperacuity
Person’s ability to sense directional relationships
- e.g. whether 2 lines are parallel (ability to so is very good, threshold on order of several arcsecs)
- believed to be the result of higher cortical processing
Measuring visual acuity
-MAR and snellen
MAR = minimum angle of resolution
-typically measured in arcminutes (1/60 of a degree)
Snellen fraction = 1/MAR
-fraction is typically multiplied by 20/20 to obtain standard form
20/20 = 1 MAR = 0 logMAR
Measuring visual acuity
-snellen letter size
Each leter on 20/20 subtends 5 arcmin at 20 ft
Each distinct bar = 1 arcmin (think of letter E)
20/20 = distance/letter size
Measuring children’s visual acuity
- examples of recognition acuity
- examples of recognition acuity matching
Birthday cake, tumbling E, landolt C
STYCAR, HOTV, lea symbols
Measuring children’s visual acuity
-allen vision test
Ages 2+
Child names series of images on cards at close range -> pt closes/occludes one eye, dr determines longest dist at which child can resolve images
Acuity expressed as x/30
-where x is distance (ft) at which child can read
Measuring
- corneal curvature
- corneal thickness
Curv: keratometer (only central 3mm) or topographer (whole k)
Thick: pachymeter
- old measured dist bw purkinje images from ant + post k
- modern methods use ultrasound
