Physiological Optics II Flashcards
GUllstrands exact eye model
- made up of the cornea, the humor, the lens.
* In total there are 6 refractive surfaces and 4 chambers.
Reduced eye model
single spherical refraction interface • N=1.33 • Cornea to anterior focal point=16.67mm • Cornea to posterior focal point (axial length)=22.22mm • Total eye power=60.00D
Far point of the eye
the point conjugate to the axial retinal point. A point object at the far point will result in the formation of a point image on the retina. Where is the eye looking WITHOUT accommodation to have light perfectly focused on the retina. 1/F=far point.
Far point sphere of the eye
when the eye rotates, it traces out a spherical surface known at the far point sphere. Center of curvature is the center of rotation of the eye (about 27mm behind the tropical spectacle plane)
Near point of eye
point conjugate to the retina when the eye is exerting maximum accommodation. Similar to far point except the eye is fully accommodating
Emmetropia
- incoming plane waves should converge to a point on the retina.
- Far point is infinity
- The retina should be located about 22mm from the lens to be emmetropic.
Myopia
- eye is too strong (>60D)
- Light from incoming plane waves to converge to a point in front of the retina
- Far point: located between the eye and infinity. It will sit somewhere in front of the retina.
- L (object vergence) has to be negative (real object)
Night myopia
◦ Low light levels
◦ Due to combination of increased spherical aberration as well as light levels which are too low to fully relax accommodation.
When is there the highest prevalence of myopia
Newborns
Prevalence of myopia with age
◦ 0y=25-50% ◦ 1y=1% ◦ 6y=2% ◦ 20y=20% ◦ 30y=30%
Hyperopia
- far sighted
- Eye too weak (<60D)
- Plane waves converge somewhere behind the retina
- Axial distance and refractive power play a role.
- Far point: located behind the retina=virtual object
- Optically speaking, a correcting lens should be places so that its secondary focal point coincides with the far point of the ametropic eye
Latent hyperopia
◦ Difference between subjective and objective
◦ Patient may show a lower amount of hyperopia in subjective refraction tha during objective refraction
Manifest hyperopia
◦ What is present in the refraction
Absolute hyperopia
◦ The amount that cannot be overcome with accommodation
Facultative hyperopia
◦ Hyperopia that can be neutralized with accommodation
Trends with hyperopia
◦ 5yr >1.50D=hyperopic at 14 ◦ 5yr -0.50-1.25D=emmetropic at 14 ◦ 5yr <0.50D=myopic at 14 ◦ 6% of 6-15yr olds ◦ Between ages 20-40, hyperopia tends to remain constant
Interpupillary distance
◦ Use a PD ruler or pupillometer
◦ From one edge of one pupil to the same edge of another
◦ Examiner sits 40cm away from pt and closes one eye
◦ Pt fixates on examiners left eye: measure, this is near PD
◦ Pt fixates on other eye while rule remains stationary, this is the distance PD
◦ Pupillometer reduces errors resulting from parallax
Resolution acuity
‣ Teller
‣ Determined by asking a patient to distinguish a pattern from a uniform patch of equal luminance
‣ Normal cut off is 40-60cpd
Recognition acuity
‣ Snellen acuity
‣ Information about our abiltiy to resolve high frequencies
‣ Not good for cataract patients
Minimum detectable acuity
‣ Thinnest possible wire that is detectable (Cardiff?)
Hyperacuity
‣ Vernier
‣ Person’s abiltiy to sense directional relationships
‣ Telling if two lines are perfectly aligned or not
‣ Higher cortical processing
MAR
‣ Minimum angle of resolution
‣ Arcminutes
Snellen fraction
‣ 1/MAR, multiplied by 20 to get the standard optometry form
LogMAR
‣ Log(MAR)
‣ 20/20=1 MAR=0 logMAR
‣ Each letter on the 20/20 line of snellen chart subtends an angle of 5 arcminutes when viewed at 20 feet. Each distinct bar making up the letter subtends 1 arc minute
Charts for VA research
‣ EDTRS is a common one used in clinical research because the geometrical progression of letters is based on logMAR scaling
Acuity in kids
◦ Birthday cake slide: recognition, for kids who don’t know letters and numbers
◦ Tumbling E chart: ages 3-6, recognition
◦ Landolt C: simialr to tumbling E, recognition
◦ Allen Vision Test: 2 and older. Child names a series of images on cards while viewing them at close range. Then occlude one eye and determine the longest distance at which the child can resolve the imaged
‣ Acuity expressed as x/30
◦ STYCAR: matching
◦ HOTV: matching
◦ Lea Symbols: Matching
‣ Uncorrected high hyperopes and those with accommodative infacility may have trouble with matching acuity tests
Who may have trouble with matching acuity tests
‣ Uncorrected high hyperopes and those with accommodative infacility may have trouble with matching acuity tests
Corneal curvature
◦ Keratometer
◦ Corneal topographer
◦ Central 3mm of cornea
Corneal thickness
◦ Pachymeter
◦ Measures the distance between the purkinje images
◦ Important for corneal refractive surgery. Also plays an increasingly important role in glaucoma risk assessment
Static retinoscopy
◦ Determine a patient’s spherocylindrical refractive error
◦ Optical principles
‣ Retinal reflex and the streak
‣ With add plus, against add minus
‣ Myopes will have against motion
‣ Hyperopes will have with motion
◦ Rx=F-working distance
◦ Astigmatism
‣ Use sphere lens to neutralize the most positive meridian of the eye, and then neutralize the perpendicular axis using a minus cyl lens
‣ Remember that horizontal streak scopes the vertical meridian and the vertical streak scopes the horizontal meridian
Dynamic retinoscopy
◦ Accommodative accuracy/response
◦ MEM
‣ The fixation target is located at a typical reading distance
‣ Lenses are quickly placed in front of the patient to achieve neutrality of the reflex.
◦ Nott’s method
‣ Moving the retinoscope backwards instead of using lenses
◦ Mohindra: near ret. if no cyclo available. In the dark at 50cm. Monocular. 1.25 must be subtracted.
Stenopaic slit
‣ 1mm rectangular slit
‣ Decreases the diameter of the entrance pupil the meridian perpendicular to the slit.
‣ Rotate until a “best” position is found
• This is perpendicular to the most plus meridian of the eye
‣ use spherical lenses to get information about the astigmatism.
JCC
‣ Most common method for measuring astigmatic correction
‣ JCC is a lens whose principle meridians have equal power but opposite signs
‣ Red is positive meridian (or negative axis), and white is the negative meridian (or positive axis)
‣ 1. Refine sphere power so that the CoLC falls on the retina. This corresponds to fogging then decreasing plus power until best acuity is achieved
‣ 2. Chase the red
‣ 3. Refine the power
‣ 4. With each +/-0.50D change of cyl power, one must change the sphere power +/-0.25D to maintain a constant spherical equivalent.
‣ A +/-0.25D JCC is typically used for patients with acuity of 20/30 or better, while higher powers (+/- 0.50D or +/-1.00D) are used for patients with poorer vision.
Definition of a JCC lens
It has to have SE of PLANO!
Astigmatic dial
‣ Spoke like pattern
‣ 1. Fog the eye to 20/50
‣ 2. Find the lines of the dial that appear the sharpest and darkest
‣ 3. Add minus cyl until all lines appear equal
‣ 4. Add minus sphere until BCVA obtained
‣ Remember that we do not need to adjust the sphere power to maintain the spherical equivalent as we add minus cyl with the astigmatic dial because the added cyl power is collapsing the interval of sturm.
‣ The rule of 30: lowest number on the clock dial x 30 gives the axis in minus cyl
Refining the spherical component of subjective refraction
◦ 1. Following JCC, add +0.25D increments until acuity is compromised
◦ 2. Subtract 0.25D increments from each eye, checking acuity after each step until max acuity is reached
◦ 3. If an astigmatic dial is used for the astigmatic portion, the patient should be sufficiently fogged that only step 2 is necessary.
◦ 4. Verify spherical endpoint using the duo chrome test
Duochrome test
◦ If too much minus power is RXed, the patient can accommodate to offset the excess minus spheres. The combination of a minus lens and a plus lens is optical similar to a galilean telescope, and hence the patient will see images that appear increasingly smaller as minus is added
◦ Utilizes chromatic aberrations
‣ Independent of color perception and therefor works with color vision impaired individuals. However, it cannot be used until acuity has been corrected to at least 20/30 or better
◦ Split background (half red, half green)
◦ Green should focus in front of the retina, red should focus behind the retina. We want the retina to be positioned perfectly between these two points.
◦ A fogged patient will perceive the red image to be clearer, while a patient that is overminsued will perceive the green image to be clearer.
Fogging technique for equalizing
‣ Fog both eyes to reduce acuity to 20/200-20/100. Place a -0.25D lens in front of one eye at a time switching eyes in rapid succession
Prism techniques for equalizing
‣ Risley prism to dissociate the eyes.
‣ Fog both eyes and look for differences in the images seen by each eye. When they are balanced, they will report equal clarity between the two eyes at any point during the testing
Cycloplegic techniques for refraction
◦ Ensures that accommodation is fully relaxed
◦ use for
‣ Child with convergent strabismus
‣ Child with significant eso at near
‣ Young adult complaint of HA but with no uncorrected hyperopia (there could be latent)
Accommodation
- an object located closer to the eye than the far point results in rays striking the eye that are too divergent.
- Accommodation must offset the change in vergence which results from moving an object closer to the eye
- A person uses only about 50% of his/her accommodation ability comfortably
A person can use ____ of his/her accommodative demand comfortable
50%
Ocular acommodative demand
amount of accommodation needed to see an object clearly
Ocular accommodation
accommodation that the eye actually provides
Amplitude of accommodation
maximum amount of accommodation possible for a given eye
Near point of eye
point conjugate to the retina when the eye is achieving its maximum accommodation. Exactly like the far point, but now the eye is fully accommodating
Range of clear vision
refers to all other points analogous to the near and far points, but at varying degrees of accommodation. It covers all points between the near and the far point.
Presbyopia
- loss of accommodation abiltiy with age
* Decrease in the amplitude of accommodation
Amplitude of accommodation estimated
18.5-0.3(age)
Measuring add for presbyopia
◦ 1. Half amp method: the patient should have half the amplitude of accommodation in reserve in order to maintain clear and comfortable vision. The theoretical add power is the working distance minus one half of the patients AoA.
◦ 2. Balancing NRA/PRA: patients RXed add power is in the middle of the NRA/PRA range
◦ 3. FCC:
‣ Add plus until they report that the vertical lines appear darker
‣ Gradually decrease plus until they say the horizontal and vertical lines are equally dark.
‣ The total plus power over the patient’s subjective refraction that is required to reach the endpoint is the patient’s add power for that given working distance
◦ 4. Age based plus power
Aphakia
◦ Aphakia=without a lens
‣ Similar to having a large minus error, leading to aniseikonia and diplopia, or reduced VF and puncushion distortion. Correction associated with a ring scotoma resulting in visual stimuli appearing an disappearing briefly as they pass in and out of the scotoma (jumping effect or jack in the box effect). They also have additional aconvergence demands at near, large BO prismatic effects
‣ Mag=-F1/F2
Patient can no longer accommodate
Pseudophakia
lens has been replaced with an implant
‣ An IOL can be placed at the anteiror chamber
◦ Patient can no longer accommodate
Simple astigmat
one line falls on the retina, and one line falls in front of or behind the retina.
Mixed astigmat
one line falls in front of the retina and one line falls behind the retina
Compound astigmat
both lines fall in front of or behind the retina
Equally mixed astigmat
CoLC falls on retina
Major meridian of the eye
the meridian with the most plus power and steepest corneal meridian
WTR astigmatism
Corneas major meridian is vertical 90+/- 30
ATR
major meridian is horizontal (0 +/- 30)
Oblique astigmatism
zone not covered by WTR or ATR
In terms of lens RX in minus forms, WTR and WTR..
WTR astigmatism corresponds to an axis that’s us appx horizontal, and ATR corresponds to an axis that is approximately vertical (things are opposite in glasses!)
