general PMO topics Flashcards
1
Q
Lens having 1 focal length/power
A
Spherical and Toroidal Lens
2
Q
- Use conical curves, notably ellipsoids, to produce flatter and thinner lenses
- most often made in the new high index plastic materials, which further improve the reduction in lens thickness
A
ASPHERIC LENS
3
Q
Advantage of Aspheric lens:
A
- thinner, lighter and flatter as compared to Spherical lens
- Eliminates spherical aberration, oblique astigmatism and distortions
4
Q
two separate lenses held together by the frame
upper halved distance lens and lower halved near correction
-invented by Benjamin franklin
A
Split Bifocal
5
Q
Advantage of Split Bifocals:
A
- Larger field of view
- no vertical image jump
- less chromatic aberration
6
Q
Disadvantage of Split bifocals:
A
- Lenses are held together by eye wire and come apart easily
- dividing line produces annoying reflections
- dust accumulation at dividing line
- as the power increases the segment edge gets bigger and heavier
7
Q
a modified franklins bifocal
reading portion was rounded and reduced in size
A
Perfection Bifocal
8
Q
The first one piece bifocal as it is made from a single piece of glass material
top portion of biconvex lens was ground flat on one surface so that the finished lens was a combination of a Plano convex and biconvex lens
A
Solid Bifocal/ Solid Upcurve Bifocal
9
Q
advantage of Solid Bifocal:
A
- wide field of view for reading
- little chromatic aberration
- structurally stronger
- better appearance than split bifocal
10
Q
disadvantage of Solid Bifocals:
A
- limited surface power
- restricted field of view for Distance
- presence of prismatic effect and image jump
11
Q
- uses two pieces of the same kind of glass, one fixed to the surface of the other
- this bifocal featured a crown glass major lens to which was cemented a small section of crown glass called “wafer”
- Canada Balsam was the adhesive used to cement the two glass
A
Cemented Bifocal
12
Q
advantage of Cemented Bifocal:
A
- Less optical aberrations
- can be made of any power ranges
13
Q
disadvantage of Cemented bifocal:
A
adherence of wafer is affected by the changes in temperature
wafer has a tendency to fall off
14
Q
- the first fused bifocal
- uses two different glass materials
- segment material has higher refractive index than the main lens
- segment is fused into main lens
A
Fused Kryptok
15
Q
advantage of fused kryptok:
A
segment edges do not collect dirt and dust
segment do no fall out, become discolored or chip
16
Q
disadvantage of Fused kyptok:
A
flint segment produces high chromatic aberration
increase possibility of lens tension and strain during manufacturing process
17
Q
Types of Bifocals based on manufacturing method:
A
- Split Bifocals
- Solid Bifocals/Solid Upcurve Bifocal
- Cemented Bifocal
- Fused Bifocal
18
Q
Types of Bifocal Lenses based on segment shape/Design:
A
- Round Segment/KRYPTOK Bifocal
- Flat-Top Bifocal
- Curved-Top Bifocal
- Executive Bifocal
- Ultex Bifocal
*
19
Q
- segment width available in 22mm, 24mm and 28mm
- has a circular arc segment which is least visible compared to other bifocals
A
Round segment bifocal/Kryptok
20
Q
disadvantage of Round segment/Kryptok bifocals:
A
- Segment is not wide enough for reading
- There is presence of noticeable Image Jump
21
Q
Segment widths ranges from 22mm to 45mm
Also known as D Bifocal/D-shaped Bifocal
Available in plastics (CR-39) and fused glass forms
A
Flat-Top Bifocals
22
Q
Advantages of Flat-top bifocals:
A
- more useful reading width
- less image jump than round/Kryptok bifocals
23
Q
disadvantages of Flat-top bifocals:
A
- Less attractive compared to the round segment
24
Q
- Segment widths available in 28mm, 35mm and 45mm
- Also called the C-style segment bifocal
- Segment top has a slight curve rather than a perfectly flat segment top
A
Curved-Top bifocals
25
* it is ***monocentric*** - optical centers for distance and near portion located both on the line segment
* often recommended as an occupational lens for patients involved with a lot of reading or near works
* The ***near add is ground on the front surface*** of the lens
Executive Bifocals
26
advantage of Executive Bifocals:
* Very large reading zone
* No image jump
27
disadvantage of Executive Bifocals:
* **thick** and **Heavy**
* not cosmetically appealing
* dust accumulates in the crevice
28
* a ***one-piece bifocal*** bifocal made out of ***ophthalmic crown glass***
* is ground with a central disk diameter of **38mm** (segment width).
* After grinding the black is cut into two lenses, each having a segment height of **19mm**
Ultex Bifocal
29
advantage of Ultex bifocals:
Recommended as occupational lens as it has a ***LARGE FIELD OF VIEW*** for ***READING PORTION***
30
disadvantage of Ultex Bifocals:
***greater image jump*** than the other bifocal lenses
31
refers to a sudden change or shift in image location that occurs when gaze shifts from the distance to the add segment
Image Jump
32
a sudden introduction of a base down prism at the dividing line of the segment
Image jump
33
Magnitude of image jump is dependent on:
***distance*** of dividing line from ***optical center*** of the segment and ***power addition***
34
lens that help correct the intermediate zone by featuring a second small lens segment directly above the area used to correct near vision
TRIFOCAL LENSES
35
Most popular trifocals:
* Flat-top Trifocal
* Executive trifocal
* E-D line trifocal
36
lens with two flat-top segments
designed for people who need to see objects at an intermediate distance above their heads (librarians, pilots)
Double D Trifocal or ***Librarian Trifocal***
37
lens which the distance power is located in the ***middle band***
Executive Occupational Multifocal
38
Same as Double-D trifocal except that a trifocal ***segment is incorporated at the bottom***
Quadrifocal Lens
39
* also known as ***dental upcurve bifocal***
* the ***segment on top is the distant portion of the lens*** and the main part of the lens being the near zone
Rede Rite Bifocal (Minus Add Up Curve)
40
* has small, ***round segment for near vision*** placed low in the ***outside corner of one lens***
* the lens for the right eye, in the case of a right-handed golfer
Golf/Golfer's Bifocal
41
* the design of the lens is such that the power gradually increases as it goes from the distance zone - through a corridor - near zone.
* were designed to meet visual demands of presbyopes who require spectacles for all distances.
Progressive Lenses
42
advantage of PALs:
* Improved appearance
* Complete range of working distances
* No image jump
* Thinner and Lighter
43
disadvantage of PALs:
* Unwanted astigmatic power at the periphery ***(Peripheral Aberration)***
* adaption problems
* More critical fitting required
* More expensive
44
disadvantage of PALs:
* Unwanted astigmatic power at the periphery ***(Peripheral Aberration)***
* adaption problems
* More critical fitting required
* More expensive
45
PAL that has:
* **Narrower distance** and Near zones
* ***Wider and Longer*** intermediate Corridor
Soft Design Progressive Lens
46
PAL that has:
* ***Wider*** distance and Near zones
* ***narrow and shorter*** intermediate corridor
Hard Design Progressive Lens
47
PAL that has:
* ***rapid increase*** in unwanted astigmatism
* ***more*** peripheral aberrations
Hard Design Progressive Lenses
48
PAL that has:
* ***Gradual increase*** in unwanted astigmatism
* ***Reduced*** peripheral aberrations
Soft Design Progressive Lenses
49
PAL lens ***permanent*** markings:
* Micro etching/ Horizontal Locators
* Lens Logo
* ADD power
50
PAL lens ***Temporary markings*** (ink stamped markings):
* Distance reference circle
* Fitting Cross
* Prism reference point
* Near reference Point
51
PAL markings that are small micro-engraved circles which are positioned about 34mm apart
Micro-Etching / Horizontal Locators
52
PAL temporary marking:
portion of the lens through which the distance power should be read
Distance Reference Circle
53
PAL temporary marking:
the point which should be fitted directly in front of the pupil
Fitting Cross
54
PAL temporary marking:
portion used to check for the presence of prism within the lens or the amount of differential prism
Prism reference Point
55
PAL temporary marking:
portion used for checking the power of the addition through the lensmeter
Near Reference Point
56
a homogeneous glass, free from defects such as striae and bubbles, used for lenses or prisms.
Optical Glass
57
made of 70% Silica (sand),
14-16% Sodium Oxide,
11-13% Calcium Oxide
and 5% of Potassium, borax antimony and arsenic
Ophthalmic Crown glass
58
it is used for the majority of single-vision glass lenses available today, and for the distance portion of most glass bi- and tri- focals.
Ophthalmic CROWN glass
59
made of 45-65% lead oxide,
25-45% silica
and about
10% mixture of soda and potassium oxide
Flint Glass
60
it is used mainly for bifocal segments in some fused bifocals
Flint Glass
61
is made of 25-40% barium oxide and the result is silica and potassium oxide
Barium crown Glass
62
it is mainly used for the segment portion of a specific type of **fused bifocal**
Barium crown Glass
63
CR-39 specific gravity:
1.31
64
crown glass specific gravity:
2.51
65
the best materials for lens impact resistance:
Polycarbonate and Trivex
66
a lens that blocks over 90% of UVA
CR-39
67
a lens that blocks almost 100% UVA:
Polycarbonate lens
68
Characteristics of Lens Materials:
* Refractive index
* Abbe Value
* Impact resistance
* Specific gravity
* Scratch resistance
* Transparency
* UV absorption
* Chemical resistance
69
ratio of the speed of light in vacuum to the speed of light in the medium:
Refractive index
70
Lenses made from a ______ material will be thinner and more cosmetically appealing than _____ materials.
Higher refractive index
Lower index materials
71
High index materials have low ______ value
Abbe Value
72
Lower Abbe Value means that is has more \_\_\_\_\_\_
Chromatic Aberrations
73
is the reciprocal of the dispersive power of the material
Abbe Value
74
Gives a measure of material's ability to hold the spectrum
Abbe Value
75
The higher the Abbe value, the _____ the chromatic aberration
LOWER
76
If lens has a great chromatic aberration, it is because it has \_\_\_\_\_\_\_.
Low abbe value
77
is a material's ability to withstand intense force or shock applied to it over a short period of time.
Impact resitance
78
standard DROP BALL TEST:
Dropping a 5/8 inch steel ball to the lens
From a height of 50 inches
79
a measure of the physical density of the material
Specific Gravity
80
Unit of measurement of Specific Gravity:
grams per cubic centimeter (**g/cm3**)
81
the ability of a lens material to resist and withstand scratches
Scratch Resistance
82
is the oldest and most often quoted test to measure the scratch resistance of a lens material
Moh's hardness scale
83
a function of the refractive index. the higher the refractive index, the lesser the transmittance of light.
Transparency
84
Significance of Transparency:
* reduction of troublesome ghost images
* reduction in the glassy appearance of the lenses
* increased transmittance
85
gives an indication of the lens material's resistance to various solvents.
Chemical resistance
86
Test to measure chemical resistance
simulates prolonged exposure to atmospheric conditions using heat and humidity
Thermodyne test
87
he developed Thermodyne test:
Chance-Pilkington
88
the inability of a lens to bring all incident rays to a desired point:
Aberration
89
a defect in an image due to imperfect focusing of light rays by the lens or an optical system
Monochromatic Aberration or ***Seidel*** Aberration
90
defect in an optic image occurring when an incident light is not confined to paraxial rays (central)
Monochromatic Aberration or ***Seidel*** Aberration
91
Types of ***Monochromatic*** Aberrations:
* Spherical Aberration
* Coma
* Oblique Astigmatism
* Curvature of Field
* Distortion
92
the failure of a lens to focus rays of light at one and same point therefore the image seen is somewhat blurred
Spherical Aberration
93
happens when peripheral rays are focused sooner than the central rays
Spherical Aberration
94
Correction of spherical abberation:
Limiting the aperture of the lens
Use of Plano convex lens
Use of aplanatic surfaces
Use of Aspheric lens
95
* occurs when rays of light from an off-axis object (oblique incident light) are refracted by the periphery of a refracting surface
* also called off-axis spherical aberration
* Spherical aberration for oblique light
Coma
96
forms a comet shape image of a point source
Coma
97
Correction of Coma:
* Eliminating the peripheral rays
* Adjusting the shape of the lens
98
Correction of Coma:
* Eliminating the peripheral rays
* Adjusting the shape of the lens
99
occurs when narrow beam of light enters obliquely to lens axis of a spherical lens, the refracted rays become astigmatic
the emerging rays instead of uniting in a single image point, form the ***interval of sturm***
Oblique astigmatism
100
also known as Marginal astigmatism or Radial Astigmatism
Oblique astigmatism
101
correction of Oblique astigmatism:
* use of meniscus lenses
* orientation of lens such that incident light is parallel to the principal axis
* Using an aspheric lens
102
correction of Oblique astigmatism:
* use of meniscus lenses
* orientation of lens such that incident light is parallel to the principal axis
* Using an aspheric lens
103
it is also known as **disk of confusion**, circle of indistinctness, **blur circle** or blur spot
Circle of Confusion
104
it is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source
Circle of Confusion
105
* the smallest of a **blur circle**
* Found exactly midpoint between the two focal lines
Circle of **Least** Confusion
106
inability of a lens to form a plane image of a plane object
also called “Curvature of the Image”
Curvature of Field
107
presents problem for instrument makers, particularly, camera manufacturers who need to produce an image on a PLANE film
Curvature of Field
108
* the inability of a lens to form an image of the ***same shape*** as the object
* Mostly seen in high power plus and minus lenses
Distortion
109
* it affects **image shape and lateral position**, but not image clarity
* results when the magnification of an extended object varies with its distance from the optical axis
Distortion
110
Term that means “free from distortion”
Orthoscopic/Orthoscopy
111
distortion observed in high power PLUS lenses
Pincushion Distortio
112
distortion observed in high power MINUS lenses
Barrel Distortion
113
occurs as a result of the unequal refraction of the various wavelengths of light
Chromatic Aberration
114
is the ***non-uniform focusing of the different color waves*** in a pencil of light, due to the different refrangibility of these waves:
* short violet waves being refracted most
* long red waves least
* medium waves in proportional degrees
Chromatic Aberration
115
a type of lens made up of two simple lenses paired together designed so that the chromatic aberration of each lens partially offsets the other
Achromatic Doublet
116
Forms of Chromatic aberration:
1. **Longitudinal** chromatic aberration or **Axial** chromatic aberration
2. **Lateral** Chromatic aberration
117
type of chromatic aberration that focuses different spectral hues at different distances along the optical axis
Longitudinal or Axial Chromatic aberration
118
type of chromatic aberration wherein all colors are in focus in the same plane, but the foci are not placed along the optical axis
**Lateral** Chromatic aberration
119
the transmission of electromagnetic radiation can be reduced by _______ or \_\_\_\_\_\_\_.
reflection or absorption
120
* reduce reflection at each surface improving the visual function
* sharpens the patient's vision, because more light is able to pass through the lens instead of being bounced off the lens
Anti-Reflection coatings
121
\_\_\_\_\_\_ coatings are particularly important for high index lenses and flat form lenses, such as the aspheric lenses
Anti-Reflective coatings
122
used to prevent the condensation of water in the form of small droplets on a surface of lens
Anti-Fog coatings
123
coating used by skiers, mountaineers, astronomers and pseudophakics
UV COATING
124
UV that creates a **risk** for the conjunctiva and cornea
UV-B and UV-C (low levels of UV)
125
this UV causes severe damage to the:
* cornea
* contribute to cataract formation
* retinal damage
* macular degeneration
UV-A (High level UV)
126
acute effects of UV-A:
* Photochemical retinopathy
* Tanning of eyelids
127
Chronic effects of UV-A:
* Cataract
* Macular degeneration
* Malignant Melanoma
128
Acute effects of UV B:
Photo-kerato-conjunctivitis
Cataract
Erythema of eyelid skin (redness)
129
Chronic effects of UV B:
* Pterygium
* Pinguecula
* Band Keratopathy
* Corneal degeneration
130
* Highly reflective coatings applied to front surface of sunglasses
* decrease eyestrain by blocking 10-60% of light transmission to the eye
Mirror Coating
131
* are applied by a vacuum application of interference layers made of powdered oxides, metals, lacquered plastics or other available compounds
* this coating is good for highly reflective outdoor activities, especially from water and snow (Functional)
* it shields the appearance of the eyes (Aesthetic reasons)
Mirror Coating
132
* Contains ***Silver chloride*** or ***silver halide*** which when exposed to UV causes a chemical reaction that gives the ability of the lens to darken
* Also known as “**light-adaptive lenses**”
PhotoChromic Filters
133
it means restricting the light waves to one direction
Polarization
134
block horizontally aligned light which causes glare, while allowing vertical lights to transmit
Polarized Lenses
135
* blocks radiant light and all types of glare
* filters out harmful ultraviolet rays
* permits only natural light that is gentle to the eyes to pass
Polarized Lenses
136
Advantage of Polarized lenses:
* Filters glare
* Reduces reflection
* enhance contrast and depth perception
* improves clarity of vision
137
lens coating that combats digital eye strain
Blue Light Coating
138
Beneficial blue light can:
* regulates circadian rhythm
* boost alertness
* elevates mood
139
Harmful blue light can:
* cause digital eyestrain
* too much exposure cause eye damage
140
* a bright yellow brown coating that filters out ultraviolet rays
* also called as **blue blocker tint**
UV 500
141
a clear coat applied the the lens to protect them against scratches
Hard Coating
142
a color applied to the bevel area of a lens that matches the frame camouflaging the edge
Edge Coating
143
used for individuals who are constantly going into and out of changing temperature
Anti Fog coating
144
consists of several coatings of:
* anti reflection
* UV
* hard coating
Multicoated lens
145
advantages of multicoated lens:
* Eliminates reflections
* filters out ultraviolet light
* makes your glasses lens conspicuous and hence more attractive
* reduces eye fatigue in office and artificial light
* helps to avoid glare during night driving from an incoming car headlight
146
Standard uncoated lenses can decrease light transmission by up to ____ of available light
15%
147
Standard uncoated lenses can decrease light transmission caused by visible reflection on the ____ and _____ surface of the lens,
as well as \_\_\_\_\_\_\_
Front and Back surface
Internal reflections
