Final Flashcards
What are the causes of aberrations?
- the way light is refracted by a particular surface
- index of the medium
- location of the object point
What are the two types of aberrations (and what is each dependent on)?
- chromatic aberrations (lens material)
2. monochromatic aberrations (lens form design)
What chromatic aberration occurs in lenses that have a vergence effect? 1. What is the test that uses these aberrations? 2
- longitudinal chromatic aberration
2. Duochrome (red/green) test
What is used to control chromatic aberrations?
a doublet
What is the aberration that results from differences in the prismatic effects (angular dispersion) of the lens for the various wavelengths?
transverse chromatic aberration
What, in theory, would eliminate chromatic aberrations?
one wavelength (monochromatic)
What two factors in lenses are related to more dispersion and therefore more chromatic aberrations?
- high index lenses
2. low Abbe value
What are the five monochromatic Seidel aberrations?
- spherical
- coma
- oblique astigmatism
- curvature of field
- distortion
What causes spherical aberrations? 1. What does it affect? 2
- different zones of the aperture have different focal lengths
- sharpness of image point
What type of optical system is spherical aberrations usually involved with? 1. What is the path of the light rays that are impacted? 2. Is it usually ignored or addressed? 3. When is it a problem (and what is done to fix it)? 4
- large aperture
- parallel to optic axis
- ignored
- high plus lenses (aspheric lenses)
What causes coma? 1 What does it affect? 2
- oblique rays refracted by a large aperture optical system
2. sharpness of image points
What type of optical system is coma aberrations usually involved with? 1. What is the path of the light rays that are impacted? 2. Is it usually ignored or addressed? 3.
- large aperture
- oblique to optic axis
- ignored
Which is more significant: coma or spherical aberrations?
spherical
What causes oblique astigmatism?
narrow pencil of light passes obliquely through a spherical surface and form interval of sturm with 2 line foci and circle of least confusion
What are the two line images formed in oblique astigmatism (and which is steeper)?
- tangential focus (steeper)
2. sagittal focus
What equation provides the mathematical solution to eliminate oblique astigmatism?
Jalie’s equation
What are the two possible lens designs using Jalie’s equation (and what is the distinguishing factor in each)?
- Wollaston (steep base curves)
2. Ostwalt (flatter base curves)
Is oblique astigmatism usually ignored or addressed? 1. How is it addressed? 2
- most significant aberration so not ignored
2. lens form (base curve)
What is the benefit of Wollaston form lenses? 1. What is the benefit of Ostwalt? 2
- also minimizes distortion
2. more cosmetically appealing
What must be sacrificed to address oblique astigmatism?
curvature of image distorted
What is the aberration where the lens does not form a plane image for a plane object? 1. What is this image known as? 2
- curvature of image
2. Petzval’s image
Is curvature of image usually ignored or addressed? 1. How is it addressed? 2
- second most significant aberration so not ignored
2. lens form (base curve)
When is there no curvature of image aberration?
when far-point sphere coincides with Petzval surface
Why does distortion occur? 1. What does it affect? 2
- variable magnification across lens
2. shape of object
What is it called when there is no distortion?
orthoscopy
What type of distortion occurs when the image size to object size ratio decreases with an increase in object size? 1. What type of lenses cause this? 2
- barrel distortion
2. minus lenses
What type of distortion occurs when the image size to object size ratio increases with an increase in object size? 1. What type of lenses cause this? 2
- pincushion distortion
2. plus lenses
Is distortion usually ignored or addressed? 1. How is it addressed? 2
- ignored
2. steep back surfaces
What are the advantages of aspheric lenses?
- Correct off axis astigmatism
- Better cosmetic appearance
- Reduced magnification
- Thinner lenses
What are the disadvantages of aspheric lenses?
- Require more precision fitting
- No prism by decentration
- More expensive
What are lenses that concentrate of oblique astigmatism and ignores curvature of image?
point-focal lenses
What are the advantages of minus cylinder lens design?
- less meridional magnification
2. better cosmetic appearance
Which spectacle wearer will most likely be first to require an add power: hyperope, emmetrope, myope? 1. Which will be last? 2
- hyperope
2. myope
What are the advantages of the solid upcurve bifocal? 1. Disadvantages? 2
- “invisible” bifocal
2. decentered optical system so image jump
What are the theories for the occurrence of presbyopia?
- mechanical changes in lens and capsule
- inc size and curvature of lens
- changes in ciliary muscle
- changes in elastic components of zonules and CB
How much amplitude is there typically left in order to be considered presbyopic?
less than 5D
Add power is usually added to the patient in +0.25 steps after how many years?
2-3
What are the procedures for determining tentative or working add?
- Amplitude of Accommodation (allow patient to use half)
- Age Tables (ex: +0.75 at 40 + 0.1D per year afterward)
- Binocular Cross Cylinder (BCC)
- NRA/PRA (the plus power that balances NRA/PRA is the Add)
- Plus Build Up (best VA with least plus at 40cm)
Does BCC typically over or underestimate the add power for a patient?
overestimate
What id the difference between near power and add power?
near power is the add through the current prescription while add power is just what is added to the prescription
When is the near power the same as the add power?
an emmetrope
If there is an increase in distance power (dec in minus) what happens to the near power? 1. To the add power? 2
- increases
2. may stay same
If there is an decrease in distance power (dec in minus) what happens to the near power? 1. To the add power? 2
- decreases
2. may need to increase add power
What are the two types of multifocal lenses?
- fused
2. one piece
What are the types of one piece bifocals (and what is the difference between them)?
- ultex (curvature change on back)
2. executive (curvature change on front)
How are one piece bifocal lenses measured?
- lens clock
2. lensometer
How are fused bifocal lenses measured?
lensometer
What are the relevant curved surfaces of a fused bifocal?
- front suface
- back surface
- countersink
Where is the segment position normally located? 1. What is the exception? 2
- set at lower limbus (1 to 2mm below lower lid margin)
2. Fit slightly higher (1-2mm) for round segments
What is the distance from distance optical center to top of segment?
segment drop
What is the lateral distance between distance OC and segment OC?
Segment inset
Where is the location of the segment optical center for an executive lens?
at segment line (top)
Where is the location of the segment optical center for a flat top lens?
5mm below segment top
Where is the location of the segment optical center for a round bifocal?
at the center of the segment
What are the three prismatic effects of segmented bifocals (and where are they from)?
- Image Jump (prism at segment line (top))
- Differential Displacement (prism from segment alone)
- Total Displacement (total prism (distance and near))
Where is the location of the segment optical center for a ultex bifocal?
19mm below segment top
What is the equation used to find the segment inset?
Inset = (dist PD - near PD)/2
What are the advantages of the flat top bifocal?
- Less image jump than round segment
- Better cosmetic appearance than executive
- Least differential displacement of all segmented multifocals
- wider reading field near top of lens
What are the disadvantages of the flat top bifocal?
- more image jump than executive
2. segment more visible than round
What is the widest width of the flat top bifocal?
35mm
What are the advantages of the executive bifocal?
- No image jump
- Widest segment available (reading)
- Works well when using slab off prism
What are the disadvantages of the executive bifocal?
- Poor cosmetic appearance
2. Thicker, heavier lenses
What type of lenses are better for an executive bifocal?
better minus than plus
What are the advantages of the round bifocal?
Good cosmetic appearance
What are the disadvantages of the round bifocal?
- High image jump for larger segments
- Have to fit higher
- narrower reading field at top of segment
What type of lenses are better for a round bifocal?
Better for plus than minus Rx with large segment sizes
What is the advantage of blended bifocals? 1. What is the disadvantage? 2
- cannot see line
2. Creates a zone of unwanted astigmatism that is an unusable transition zone
How are PAL designs evaluated and categorized?
using contour plots of lens power
For the hard design of PAL’s what is the relative amount of unwanted to astigmatism relative to soft PALs? 1. Amount of distance zone? 2. Amount of near zone? 3. Length of corridor? 4
- smaller portions of lens
- larger distance zone
- wider near zone
- short corridor
For the soft design of PAL’s what is the relative amount of unwanted to astigmatism relative to hard PALs? 1. Amount of distance zone? 2. Amount of near zone? 3. Length of corridor? 4
- larger portions of lens
- smaller distance zone
- narrower near zone
- long corridor
What are the characteristics that a person posses that is better for hard PAL design?
- More distance tasks
- Not bothered by swimming motion
- Needs wider near zone
- eye mover
What are the characteristics that a person posses that is better for soft PAL design?
- More intermediate tasks
- Bothered by swimming motion
- Wants easier adaptation
- Head Mover
What are the advantages of PALs?
- no image jump
- invisible segment
- continuous vision from distance to near
What are the disadvantages of PALs?
- unwanted astigmatism
2. narrower reading field
What are the most important facts to know about the patient before prescribing multifocals?
- near vision requirements
- occupational needs
- avocational needs
- habitual reading distance
Is the add power higher or lower for shorter working distances?
higher
What type of rays are ionizing radiation?
- x-rays
2. gamma rays
What are the effects of optical radiation (and what are examples of each)?
- thermal effect (solar retinopathy)
- photochemical effect (vision, photokeratitis, UV absorption by cornea)
- photoluminescence (fluorescence of lens by UV light)
What type of UV light is able to get through the lens? 1. Which is able to get through the cornea but not the lens? 2
- UV A
2. IV B
What wavelength of UV rays have partial transmission through the cornea? 1. IR rays? 2
- 290-315nm
2. 1000-3000nm
Does the transmittance of shorter wavelengths increase or decrease with age?
decreases
What are the effects of UV radiation on the eye?
- photokeratitis
- pterygia
- pingueculae
- climate droplet keratopathy
- cataract
What type of IR radiation is harmful?
high intensity IR radiation (but not ambient)
What are the occupations that lead to high intensity IR radiation?
- glass blower
2. blast furnace operators
What are the ocular effects of high intensity IR radiation?
- opacification of cornea
- congestion, depigmentation, and atrophy or iris
- exfoliation and cataract of lens
- necrotic burn of retina
What is the reciprocal of the transmission?
opacity
What is the best tint for nonselective absorption of visible light?
grey
What lens color absorbs all UV radiation, used for shooting, driving?
yellow
What lens color helps with indoor glare of fluorescent lighting and helps hide multifocal segments?
pink
What are lenses that darken when exposed t long wavelength UV radiation?
photochromic lenses
What lenses have analyzers that are oriented vertically to eliminate horizontal plane polarized light?
polarized lenses
What lenses protect against excessive UV and IR radiation?
occupational tinted lenses
What types of lens materials do not provide adequate UV protection?
- CR39
2. glass
What may reflected light produce for a patient?
- ghost images
- falsification of image position
- haze
- loss of contrast
Does the intensity of reflected light increase or decrease as the index of refraction increases?
increases
Which side of the lens is AR coating placed?
both
What two conditions must on effective coating meet?
- amplitude condition
2. path condition
What does the amplitude condition determine for the lens?
index of coating
What does the path condition determine for the lens?
thickness of coating
What color do AR coatings usually appear? 1. When the coating is too thin what color? 2. Too thick? 3
- purple
- amber
- pale blue
What % transmission is too low for driving?
What occurs when the spherical equivalent refraction of the two eyes differs by 1.00D or more?
anisometropia
What are the problems associated with anisometropia?
- suppression
- amblyopia
- asthenopia
- abnormal binocularity
- reduced stereopsis
What are the problems that can arise when correcting anisometropia?
- accommodative system
- vergence system
- relationship between retinal image sizes
What problem when correcting anisometropia arises as a result of different prismatic effect present when patient looks through non-optical center?
vergence problems
What are the ways to compensate for difference in vertical prism at the reading level for corrected anisometropes (and which are the best)?
- Lowering Optical Centers
- Dissimilar Multifocal Segments
- Compensated Multifocal Segments
- Prism Segments
- Fresnel Prism
- Single Vision Reading Rx
- Slab Off Prism (best)
- Contact Lenses (best)
What is the amount of prism compensation equal to in dissimilar segments for the compensation of vertical prism?
the difference in segment OC position times the add power
What does slab-off prism do to the lenses?
removes base down prism of more minus lens by thinning bottom half of lens
What is the relative differences in the size and/or shapes of the ocular images of the two eyes called?
aniseikonia
What percentage of image size difference is related to the most frequent complaints of aniseikonia?
1.0-2.5%
At what percentage of image size difference is poor fusion usually prevalent?
> 3%
What are the symptoms of patients with >5% image size difference?
- diplopia
- suppression
- confusion
What are the most common symptoms of aniseikonia?
- asthenopia
2. headache
What type of aniseikonia is a progressive increase or decrease in image size in one eye in all directions?
overall aniseikonia
What type of aniseikonia is a progressive increase or decrease in image size in one eye in one direction?
meridional aniseikonia
What is the monocular phenomenon that is a change in RIS caused by lens to correct ametropia?
spectacle magnification
Which component of the spectacle magnification has no vergence power and is the magnifying power?
afocal component (shape power)
Which component of the spectacle magnification contributes the vergence power?
power component (power factor)
For a hyperope with spectacle correction, as vertex distance increase, what happens to spectacle magnification?
increases
For a myope with spectacle correction, as vertex distance increase, what happens to spectacle magnification?
decreases
For an axial anisometrope, is aniseikonia present or absent when uncorrected?
present
For an axial anisometrope, is aniseikonia present or absent when wearing spectacles?
absent
For an axial anisometrope, is aniseikonia present or absent when wearing CL?
present
For an refractive anisometrope, is aniseikonia present or absent when uncorrected?
absent
For an refractive anisometrope, is aniseikonia present or absent when wearing spectacles?
present
For an refractive anisometrope, is aniseikonia present or absent when wearing CL?
absent
What are the tools for detecting and measuring aniseikonia?
- Leaf Room
- Space Eikonometer
- Orthoscope
- Other screening tests
- Clinical Estimate (1% per diopter)
What does Knapp’s Law say to do to minimize magnification effects?
- fit axial ametropes with spectacles
2. fit refractive ametropes with contact lenses
What tests are generally used to determine if anisometropia is axial or refractive?
- compare K readings
- A scans (ultrasound)
- astigmatism is generally refractive
What are lenses that are designed to eliminate or minimize aniseikonia called?
iseikonic lenses or eikonic lenses
What parts of the lens do eikonic lenses manipulate?
- front surface power
- lens thickness
- vertex distance
For plus lenses, what do eikonic lenses manipulate to increase magnification?
- increase front curve
- increase thickness
- increase vertex distance
What are the eikonic lens design basic guidelines?
- try equal base curves and thickness first
- don’t inc BC more than 6.00D above normal
- no more than 6.00D diff in BC btw lenses
- front surface no flatter than +1.00D
- back surface no flatter than -2.00D
If both lenses are plus lenses what should be done to the more plus lenses to correct aniseikonia? 1. Less plus lenses? 2
- dec vertex dist, flatter base curve, dec thickness
2. steepen BC, inc thickness
If both lenses are minus lenses what should be done to the more minus lenses to correct aniseikonia? 1. Less minus lenses? 2
- dec vertex dist, steepen BC, inc thickness
2. inc vertex dist, do NOT thin lens
According to the cookbook, what should be done to the Least plus or most minus lens?
inc shape magnification until greater than other eye by desired amount
What is the role of an optometrist in occupational vision?
- Identify & Establish Visual Standards
- Certifying Visual Standards
- Helping patients relieve symptoms due to workplace
What type of eye injuries are usually from large, relatively slow moving objects?
non-industrial eye injuries
What type of eye injuries are typically from objects less then 6mm in size and are at high velocities?
industrial eye injuries
What is the only lens material that can adequately resist high energy impact of both large and small objects?
polycarbonate
What are the two sports that have the most injuries?
- basketball
2. baseball
For tasks in the workplace, what does increasing the illuminance lead to?
- decreased blink rate
- decreased nervous muscular tension
- decreased fatigue of convergence ability with reading
What are the symptoms of computer vision syndrome?
- eyestrain
- blurred vision
- dry and irritated eyes
- tired eyes
- headaches
- double vision
- glare sensitivity
What is the treatments for computer vision syndrome?
- Correct even marginal refractive errors
- Give presbyopes a special Rx
- Watch for dry eye
- Watch for glare
- Look for oculomotor dysfunctions
- Improve the workplace environment
For a 24 inch viewing distance where should the center of the computer screen be?
4-9 inches lower than eyes
