Optics Flashcards
For patient wearing RGP CL if you steepen the base curve by 0.75D how should you adjust the contact lens power?
Prescription should increase by -0.75D
** remember SAM FAP Steepen add minus
Flattening the base curve of a lens induces?
Additional minus power of the tear film
**therefore to offset this power change need to ADD plus power to the Rx
**SAM FAP (Flat add plus)
What adjustments can be made to a GP lens if it’s loose and moves too much? (4)
Options:
1. Steepen base curve
2. Increase optic zone diameter
3. Increase overall diameter (OAD)
4. Narrow peripheral curve system or steepen it
Modifications to lens that fits too tight
- Flatten base curve
- Decrease OAD
- Decrease optic zone diameter
- Flatten peripheral curve
- Widen width of peripheral curves
What formula is used to calculate induced prism?
P=dF
d in cm
F in diopters
Tolerance for sphere powers +/- 6.50
+/- 0.13 D
(According to ANSI standards)
What does ANSI stand for?
American National Standards Institute
True or false
More strict tolerance for powers in lower power
True
Error tolerance for cyl powers of 2.00 DC or less
+/- 0.13 D
Over correction of near reading power will result in?
Reduced focal length
Causing patient to bring reading materials closer to
Antimetropia
One eye is myopic, while other is hyperopic
Refractive index of CR-39
1.498
* plastic
Refractive index of crown glass
1.523
Refractive index of Trivex
1.523
Abbe value of polycarbonate
30
* low abbe= high chromatic aberration
Abbe value of trivex
43-45
Abbe value of CR-39
58
Abbe value of crown glass
58
Frame adjustment
If right lens is in
Move right temple in (or left temple out)
Frame adjustment
If left lens is in
Move left temple in (or right temple out)
Frame adjustment
If right lens is out
Move right temple out (or left temple in)
Frame adjustment
If left lens is out
Move left temple out (or right temple in)
If frame not straight on face
Right lens is up how would you fix it?
Bend right temple up (or left temple down)
If frame not straight on face
If left lens is up
Bend the left temple up (or right temple down)
If frame not straight on face
If right lens is down
Bend right temple down or left temple up
If frame not straight on face
If left lens is down
Bend left temple down or right temple up
Drop ball test for dress lenses to be considered impact resistant
5/8 inch steel ball weighing 0.56 oz dropped from distance of 50 inches
Terrien’s marginal degeneration causes what type of astigmatism?
Against the rule (ATR)
True or false
Increasing index of refraction of lens material will allow for decrease in lens thickness
TRUE
* thinner lenses and may also lower specific gravity which minimizes overall weight of glasses
What is the far point when uncorrected?
Assume constant pupil size and relaxed accommodation.
OD: -6.75 DS
** take inverse of subjective refraction
1/ -6.75 =-0.148 or 14.8 cm
- if object is placed at this distance from the eye, pt will perceive a clear image if left uncorrected
Which ophthalmic instrument will provide the most accurate method of assessing spectacle lens warpage?
Lens clock
* used to calculate base curve of spectacle lenses, determine add power of Multifocal lens, determine slab-off prism, calculate thickness of GPCL
Which lens material has the greatest amount of reflection at its incident surface?
Polycarbonate n= 1.586 (refractive index)
Jackson cross cylinder
RX will have spherical equivalent of 0
Example: +1.00 -2.00 x 180
What factors contribute to development/progression of myopia in young children? (5)
- Family history of myopia (especially if both parents are myopic)
- Prolonged near activities on a consistent basis
- Presence of ATR astigmatism
- Children emmetropic or have less than 0.50 D hyperopia at school age
- Long axial length
What is the annual progression rate of myopia in children?
0.25 to 0.50 diopters annually
Marfan syndrome, ROP and Ehlers Danlos syndrome are most associated with what refractive error?
High myopia
Leber congenital amaurosis, cornea plans and nanophthalmos present with what refractive error?
High hyperopia
Normal A-scan length
22-25 mm
Patients with high myopia have axial lengths of ____mm or longer
26.5 mm
Which population have the highest predilection to pathological high myopia?
Japanese
Chinese
Jewish
Arab
* rare in AAs
What is a common characteristic of pathologic myopia related to the eye’s length?
Increased axial length
Increased axial length is a key indicator of pathologic myopia.
What is observed in patients with pathologic myopia concerning the lens?
Premature lenticular pacification
This refers to the early clouding of the lens in myopic patients.
What type of eye alignment issues may be present in pathologic myopia?
Large exophoria or exotropia
Exophoria and exotropia are conditions related to outward eye deviation.
Posterior staphyloma
outpouching of the sclera that can occur in high myopia.
What abnormality may occur regarding the optic discs in pathologic myopia?
Oblique insertion of the discs
This refers to the atypical positioning of the optic nerve head.
What are lacquer cracks in the context of pathologic myopia?
breaks in Bruch’s membrane
*can cause CNVM
* seen in pathological myopia due to stretching long axial length
What change in the vitreous is noted in pathologic myopia?
Syneresis of the vitreous
Syneresis refers to the shrinkage of the vitreous body, which can lead to complications.
What type of membranes can develop subretinally in pathologic myopia?
Subretinal neovascular membranes
These membranes can lead to vision loss due to their growth and subsequent complications.
What serious retinal condition can occur in patients with pathologic myopia?
Retinal detachments
Retinal detachments are a critical complication that can severely impact vision.
What is foveal retinoschisis?
Foveal retinoschisis
This condition involves splitting of the retinal layers at the fovea and is associated with high myopia.
What visual symptom may patients with pathologic myopia report?
Decreased visual acuity
Decreased visual acuity is a common complaint among individuals with pathologic myopia.
Fill in the blank: Patients with pathologic myopia may experience image _______.
minification
Image minification is the perception of images being smaller than they are.
What impairment in color perception might occur in pathologic myopia?
Impaired color vision
This refers to difficulties in distinguishing colors, potentially due to retinal damage.
What abnormal adaptation to darkness may patients with pathologic myopia experience?
Abnormal dark adaptation
Abnormal dark adaptation refers to the difficulty in adjusting to low light conditions.
FDA class of SCL Group 1
Low water, non-ionic
Which group of the FDA classes of SCL is least likely to develop deposits?
Group 1
*low water, non-ionic
What is the recommended method for measuring distance PDs?
Use a pupillometer
The pupillometer is noted as the most accurate method for measuring distance PDs.
What should be done to the frame before taking fitting measurements?
Fit and fully adjust the desired frame on the patient’s face
Adjustments include pantoscopic tilt, frame height, vertex distance, face-form wrap, and nose-pad alignment.
Why is it important to have the clear demo lenses in place?
To ensure accurate fitting measurements
If lenses are not present or are darkly tinted, use clear tape across the eyewire.
Where should the fitter’s eyes be positioned during the fitting?
At the level of the patient’s eyes
This ensures accurate alignment and measurements.
What should the patient look at during the fitting process?
The bridge of the fitter’s nose
This helps in drawing accurate fitting lines.
What is drawn on the lens or tape during the fitting process?
A horizontal line through the center of the pupil for each eye
This differs from bifocal fittings, where the lower eyelid margin is used.
What should be done with the frame on the manufacturer’s centration chart?
Align the bridge and marked horizontal fitting lines
There is no standard progressive lens cut-out chart for alignment.
How is the fitting height of the lenses measured?
As the vertical distance between the fitting cross and the inside bevel of the lower eyewire
This measurement is crucial for proper lens placement.
What should be verified regarding the add power circle?
It should fit within the boundaries of the frame
This ensures proper lens functionality.
What must be checked regarding the overall circles on the centration chart?
One of the large overall circles must completely enclose the frame’s lens shape
This is vital for ensuring the correct lens size and shape.
Fill in the blank: The standard method for taking progressive lens fitting measurements includes measuring distance PDs _______.
monocularly
True or False: The fitting height measurements can be accurate if the frame is not pre-adjusted.
False
Incorrect adjustments can lead to inaccurate fitting height measurements.
What adjustments can be made for a patient reporting decreased field of view through near portion of progressive glasses?
Increase pantoscopic tilt
- 10-12 degree angle is recommended
- field of view in near zones of PAL are widest when near zone sits as close to the eyes as cosmetically and physically as possible
Which area of the progressive lens should be used to verify the distance prescription when glasses arrive from the lab?
Center of the distance arc
What does a progressive lens prescription typically contain?
Removable markings for prescription verification and fitting purposes
These markings are essential for ensuring the lenses are made according to the specified prescription.
Where is the Distance Reference Point (DRP) located?
At the center of the distance arc
The DRP indicates the recommended position of the lens for measuring the distance prescription.
What is the purpose of the Distance Reference Point (DRP)?
Indicates the recommended position for measuring the distance prescription with a lensometer
This ensures accurate fitting and prescription verification.
What is the Fitting Cross used for?
To verify the fitting height
The Fitting Cross should be centered on the patient’s pupil for proper alignment.
What do the two horizontal dashes next to the Fitting Cross indicate?
Help to determine whether the lens is level or tilted
This is important to ensure proper vision correction.
What does PRP stand for?
Prism Reference Point
The PRP is used to verify the prism power and is the same as the Major Reference Point (MRP).
What is the purpose of the Near Reference Point (NRP)?
Used to verify the near add power
The NRP is located at the center of the circle in the lower part of the lens.
Fill in the blank: The PRP is the same as the _______.
Major Reference Point (MRP)
Both points are used for verifying prism power.
How to verify near add power on a PAL that has arrived from the lab?
Near reference point (NRP)
* located at the center of the circle of the lower part of the lens
What parameters can be modified to improve GP contact lens performance?
Comfort, fit, centration, clarity
These parameters can be altered in the office with a modification unit.
How is minus power added to a contact lens?
By holding the suction cup attached to the concave side against the modification unit’s sponge tool
The central convex surface (F1 surface) of the lens must be in contact with the sponge tool.
What should be done while pressing the lens against the sponge tool?
Rotate the lens counterclockwise while the sponge tool revolves clockwise
This ensures even modification of the lens surface.
What is a crucial step after modifying the contact lens?
Remove and evaluate frequently to ensure no warpage is induced
This also helps verify that too much minus power is not added unintentionally.
What procedures can be performed in the office on GP lenses?
Polishing of the F1 surface, re-edging the lens, decreasing overall diameter, truncating the lens
These modifications aim to enhance lens comfort and positioning.
What should be exercised when modifying high Dk lens materials?
Extra caution
High Dk materials may require special handling to avoid damage.
What are the conditions to ensure during the modification process?
Both the modification tool and lens should be sufficiently wet, adequate polish used
This prevents overheating of the lens.
What can happen if too much pressure is applied during the modification process?
The optics may become distorted due to lens warpage
Careful pressure application is essential for maintaining lens integrity.
What is a common characteristic of patients with pathological myopia?
Increased axial length
Pathological myopia is often associated with elongation of the eyeball.
What condition related to the lens is frequently observed in pathological myopia?
Premature lenticular pacification
This refers to the clouding of the lens occurring earlier than usual.
What type of eye alignment issues may be seen in patients with pathological myopia?
A large exophoria or exotropia
Exophoria indicates a tendency for the eye to drift outward.
Which type of glaucoma is associated with pathological myopia?
Open-angle glaucoma
This is the most common form of glaucoma, characterized by a gradual increase in eye pressure.
What structural abnormality is often found in the eyes of those with pathological myopia?
Posterior staphyloma
A posterior staphyloma is an outpouching of the wall of the eyeball.
What abnormality may occur regarding the optic disc in pathological myopia?
Oblique insertion of the discs
This refers to an unusual alignment of the optic nerve head.
What are lacquer cracks in the context of pathological myopia?
Lacquer cracks
These are breaks in the retinal pigment epithelium due to stretching.
What vitreous condition is associated with pathological myopia?
Syneresis of the vitreous
This describes the liquefaction and shrinkage of the vitreous gel.
What type of membranes may develop subretinally in patients with pathological myopia?
Subretinal neovascular membranes
These membranes can lead to serious vision complications.
What serious retinal condition can occur in patients with pathological myopia?
Retinal detachments
This condition can lead to permanent vision loss if not treated promptly.
What visual anomaly might be reported by patients with pathological myopia?
Foveal retinoschisis
This refers to splitting within the retinal layers at the fovea.
What functional issue may patients with pathological myopia experience?
Decreased visual acuity
This means a reduction in the clarity or sharpness of vision.
What visual distortion can patients with pathological myopia report?
Image minification
This describes the perception of images appearing smaller than they are.
What type of visual field changes may occur in pathological myopia?
Visual field defects
These can manifest as blind spots or areas of reduced vision.
What color vision issue might patients with pathological myopia face?
Impaired color vision
This can lead to difficulties in distinguishing between colors.
What abnormality in vision adaptation might be reported by patients with pathological myopia?
Abnormal dark adaptation
This refers to difficulty adjusting to low light conditions.
What happens when prescribing minus cylinder power with an axis of 180 degrees?
Minification of images in the vertical meridian
Objects will appear stretched horizontally.
What is the effect of prescribing minus cylinder power with an axis of 90 degrees?
Minification of images in the horizontal meridian
Objects will appear stretched vertically.
What do patients often note when undergoing a large myopic shift or wearing glasses for the first time?
Overall minification of objects
This observation is generally more pronounced with glasses than with contact lenses.
What visual effect may patients experience during a large hyperopic shift or when wearing correction for hyperopia for the first time?
Objects appear magnified
This perception is typically more noticeable with glasses than with contact lenses.
Slab off prism is base ____
Base up
- slab off prism used to eliminate diplopia, it is applied to the most minus or least plus lens (because minus lenses are thickest at the edges and slab off prism is base-up, helps to minimize thickness difference)
What is acetone commonly used for?
Nail polish remover
Can acetone be used to remove markings from polycarbonate lenses?
No
Why should acetone not be used on polycarbonate lenses?
It can lead to pacification of the lenses and cause small stress fractures
What happens when acetone comes in contact with polycarbonate?
It quickly diffuses into the plastic and causes small stress fractures
Which materials are safe to use for cleaning polycarbonate lenses?
Dish soap, water, and acetic acid (vinegar)
What are the components of many lens cleaner sprays?
Water, isopropyl alcohol, and soap
Fill in the blank: Acetone can lead to _______ of the lenses when used on polycarbonate.
pacification
True or False: Isopropyl alcohol is harmful to polycarbonate lenses.
False
