Optic Flashcards
Optical center of bifocal?
The distance and reading segments have seperate optical centers
Chromatic aberration?
Achromatic lenses can overcome it
Perspex produces greater dispersion than crown glass
the Hruby lens produces an erect diminished image
The panfundoscope consists of 2 convex lenses
Some multi-focal intra-ocular lenses use the principle of?
Some multi-focal intra-ocular lenses use the principle of diffraction, which is the interference of waves of light as they encounter an obstruction or opening, in order to create multi-focality.
.A focimeter uses which colour of light
A focimeter uses green light in order to eliminate chromatic aberration
A convex mirror always produces an erect, virtual, diminished image.
The image produced by a concave mirror is only inverted, real and enlarged if the object is between the centre of curvature and the principal focus.
Sclerotic scatter requires off-axis illumination
retro-illumination is optimised by co-axial illumination
Dispersive power is not related to refractive index.
Correct
Visible light?
400-780nm
UVC200-280 UVB280-315 UVA315-400 Visible400-780 IRA780-1400 IRB1400-3000 IRC3000-10000
UVB,UVC,IRB,IRC جذب توسط قرنیه و اسکلرا
UVAجذب توسط لنز
Visible , IRA(Near infrared)به رتین می رسد
Visible:sensation of light
IRA:thermal effect(eclipse burn)
فوتورسپتورها به طول موج محدوده ی UVAنیز حساس هستند و در افراد آفاک که این نور به رتین می رسد بصورت نور آبی دیده می شود به همین دلیل بیماران آفاک از آبی دیده شدن اشیا شکایت دارند
نور محدوده ی ۳۵۰ تا ۴۴۰ برای رتین آسیب رسان است بنابراین بهتره IOLها برای این طیف فیلتر داشته باشند
Acquired ON disease:red green defects
Exception??
Glaucoma
AD optic neuropathy
باعث نقص آبی زرد در ابتدا می شوند به همین دلیل گلوکوم را میتوان زودتر تشخیص داد اگر برای VFاز تارگت آبی روی زمینه زرد استفاده شود
Acquired retinal disease: blue yellow defects
Exception??
Cone dystrophy
Stargardt
این دو بیشتر نقص سبز قرمز می دهند
Farnsworth munsell hue 100
تنها در hue تفاوت دارند نه brightness و saturation
D15 color test
Does not distinguish mild color defect
چون خیلی sensitive نیست
Lanthony new color test
Can be used by children
ICG?
توسط RPE جذب نمی شود
می تواند ضایعات وسکولار را به لیزر دیود phorosensitise کند
اختلاف فاز دو موج اگر کمتر از نیم سیکل باشد باعث intermediate amplitude and phase می شود.
یک سیکل برابر است با یک دوره oscilation موج که برابر با طول موج می شود
Low reflection coating on lens surface?thin layer of transparent material of appropriate thickness.MOA?
نوری که از لایه ی سطحی بر می گردد و نوری که از لایه ی عمقی reflect می شود یکدیگر را خنثی کرده و باعث destructive interference می شود.
Diffraction?
یک ناحیه ی روشن در وسط است که airy disc نام دارد و توسط ring های روشن و تیره بصورت یکی در میان احاطه شده
Limit of resolution:resolving power?
کوچکترین زاویه ی بین دو نقطه که باعث تشکیل دو تصویر مجزا می شود و زمانی حاصل می شود که دو airy discبه گونه ای مجزا باشند که مرکز یکی روی اولین رینگ تاریک دیگری بیفتد
How can we measure the resolvibg power of the eye?by testing VA
In preverbal children?1)CSM 2)playin in age 15 m and older 3)catford drumفاصله ۶۰ cm وmoving target(overestimation) 4)STYCAR and worth ivory ball(pursuit eye movement indicates that they can see)
5)preferential looking and teller
Age 18 to 24 m? 1)kay picture test 2)STYCAR letter test 3)sheridan gardiner test
Log mar VA charts?
Bailley lovie test
More precise than snellen
Regular progression in the size and spacing of the letters
Vernier acuity?smallest offset of a line which can be detected
This is less than limit of snellen acuity so it is called hyperacuity
Blue feild entoptic phenomenon
Ability to see moving white dots when blue light diffesely illuminates the retina
این پدیده نشانگر نوری است که توسط WBC هادر کاپیلرهای پری فووهآ منتقل می شود،این امر نشانگر intact بودن عملکرد ماکولا است
کاتاراکت و گلوکوم و نوریت اپتیک
ممکنه VA نرمال باشد علی رغم اینکه contrast sensitivity کاهش یافته
Low or very high soatial frequencies must have higher levels of contrast in order to be seen
Contrast sensitivity measurement?
Pellirobson
VISITECH
What is the name of scattered light which reduces visual function?
Glare
Normal stereoacuity?60 sec of arc or better
Which test offers smallest disparity? Frisby
What is stereoacuity that suggests amblyopia?
Worse than 250 sec of arc
Vectigraph?2superimposed view like titmus
نیاز به عینک polarising دارد
Anaglyph?TNO
نیاز به عینک red green دارد
Radiometry:
اندازه گیری انرژی در تمام طیف الکترومغناطیس
How much is emitted from a source?radiant fluxوات
Its intensity?radiant intensity
Amount falling on a surface?irradiance
Amount reflected from surface?radiance
Photometry:
اندازه گیری تنها در قسمتی از طیف بر اساس پاسخ بینایی که تولید می کند
Luminous fluxلومن
Luminous intensity( candle)
Illuminance(lux)
Luminance(candle per square meter)تنها نور رفلکت شده در یک جهت را نشان میدهدو نه نقدار توتال رفلکت شده را
بیشترین photopic sensitivity چشم در چه طول موجی است؟
۵۵۵ زرد سبز
که در آن ۱ وات از نور مونوکروماتیک معادل فوتومتریک۶۸۵ لومن دارد
Max luminous efficacy
E=Icosi/d۲
E: illumination of the surface
I: luminous intensity
i: angle of incidence
d: distance between the source and surface
Luminous flux of of 1 lumen per square meter
1 apostib
Troland?
اندازه گیری retinal illuminatiin زمانی که surface luminance معادل 1 candle/m۲ و سایز پوپیل یک میلی متر مربع باشد
چنانچه یک آینه ی تخت به مقدار xدرجه بچرخد و پرتوی تابش تقییر نکند پرتوی بازتابش چقدر تغییر می کند؟
2x
Dispersive power of the medium is not related to the RI of the material
Correct
رنگین کمان بر اساس چه مکانیسمی است و کدام رنگ بالاتر دیده می شود؟
TIR و dispersion
قرمز
refracting angle or apical angle of the prism (a)
Angle of deviation (D)=(n_1)a
glass prism of refractive index 1.5
the angle of deviation equals half the refracting angle for a glass prism.
Prentice position
پرتوی تابش عمود بر یک سطح است و لذا تمام deviation در سطح دیگر رخ می دهد
Glass prisms
موازی iris plane
مقدار deviationبیشتر از مقدار انراف در پوزیشن min deviation است
Min deviation position
Plastic prism
Frontal plane
اگر این پریزم را در پوزیشن prentice قرار دهیم پاور بیشتری اعمال میشود(مثلا ۷۰دبه جای ۴۰)
a glass prism of refracting angle 10° (a ten-degree prism) deviates light through 5° and has a
power of 10 prism dioptres (10∆), assuming its refractive index is 1.5.
The centrad produces a very slightly greater angle of deviation than the prism dioptre
Risley prism may be used
in conjunction with a Maddox rod to measure phorias, and is included in the refractor heads (instead of a trial lens box).
Correct
Therapeutic Prisms
(1) Convergence insufficiency. The commonest therapeutic use of prisms :BO during the patients’ exercise periods
(2) To relieve diplopia in certain cases of squint
Fresnel prisms
consists of a plastic sheet of parallel tiny prisms of identical refracting angle
The overall prismatic effect is the same as that of a single large prism
Instruments in which prism are used?
slit lamp microscope, the applanation tonometer and
the keratometer
The point at which the principal plane and principal axis intersect is called the principal point or nodal point, N, of the lens.
Rays of light passing through the nodal point are
undeviated.
If the medium on either side of the lens is the same, e.g. air, then f1 = f2.
if the second medium differs from the first, e.g. as in the case of a contact lens, then f1 will not equal f2
M=f/4
commonly used × 8 loupe has a lens power of +32 dioptres.
two types of astigmatic lenses
cylindrical and toric lenses.
two types of astigmatic lenses
cylindrical and toric lenses.
Cylindrical:in one meridian the lens has no vergence power and this is called the axis of the cylinder. In the meridian at right angles to the axis, the cylinder acts as a spherical lens
Toric(sperocylindrical):Between the two line foci the rays of light form a figure known as Sturm’s conoid
Maddox rod
consists of a series of powerful convex cylindrical lenses mounted side by side in a trial lens
Cross cylinder
Type of sphero cylindrical lens
How we use cross cylinder?
the patient is asked to look at the line of test type two lines above the smallest he can see.
This is because the cross-cylinder blurs the vision and larger letters are used to make discrimination
between the positions of the cross-cylinder easier for the patient
A focimeter
two main parts, a focusing system and an observation
system . The focusing system comprises an illuminated target and a collimating lens.The position of the collimating lens is fixed but the target may be moved relative to it.theobservation system which comprises a telescope with an adjustable eyepiece which should be focused at infinity. The eyepiece contains a graticule and a protractor scale for measuring the axes of cylindrical lenses and prismatic power
Tinted lenses?
Tints are either absorptive or reflective; absorptive tints absorb light passing through them, whereas a reflective tint reflects unwanted wavelengths.
The purpose of tinted lenses may be to screen out unwanted or harmful radiation (e.g. laser
protective goggles) or cosmetic.
coloured tint
Unequal absorption of different wavelengths produces a coloured tint. For example, a yellow-tinted filter absorbs all wavelengths of light except those in the yellow part of the spectrum, which it transmits. A neutral density filter absorbs all wavelengths to the same degree and
does not alter the spectral composition of the light.
Tint
Tints may be of fixed colour (cf. fluorescein
angiography) or photochromic (in which transmission characteristics vary with the intensity of incident
light).
tint may be added to a lens by permeation, by means of a coating or as a solid tint. Most plastic lenses
are tinted by immersion in a dye which permeates the lens to a uniform depth to produce an even tint;
darker tints are produced by prolonged immersion. Coating applied to the lens surface may be
absorptive (Cr, NiCr, MgF2, SiO) or reflective (Cr, NiCr). A solid tint is incorporated evenly throughout
the lens and absorption of radiation is therefore greater where the lens is thicker.
Ultraviolet Filters
Ultraviolet light comprises approximately 5% of total solar radiation. Ozone in the earth’s atmosphere
absorbs almost all solar UV-C radiation. Of the remainder which strikes the earth’s surface,
approximately 90% is UV-A and 10% is UV-B .Ultraviolet
light exposure may also come from arc welding and UV-emitting light bulbs. CR39 lenses absorb UV
light shorter than 370 nm.
Infrared (IR) wavelengths near 1400 nm are very hazardous; filters for these wavelengths are usually
incorporated into protective goggles and face masks. Heat absorbing filters also act as IR filters but maximise the transmission of visible light.
Blue light filters vary in tint between yellow and red. They increase contrast and facilitate distinction of
light and dark areas and are used by mountaineers and skiers.
Photochromic Lenses
A photochromic lens changes its transmission characteristics depending the intensity of incident
radiation. The lens becomes darker in brighter light. The process of darkening is more rapid than that of
lightening. The reactions in glass and plastic are different.
Glass photochromic lenses comprise colourless silver halide crystals suspended in borosilicate.
Electromagnetic energy dissociates the silver and halogen to cause darkening. Each type of glass has an
optimum activating (usually UV or blue) and bleaching wavelength; the tint will depend upon which
wavelength predominates. Heat opposes the effect of light. Thus, glass darkens more easily when cold
and lightens more easily when warm. A glass lens becomes gradually darker over time if it is used
repeatedly.
سیلور هیالید در glassو مواد ارگانیک در plastic کاربرد دارد
Anti-reflective Coatings
The reflection of light from the surface of a lens may be reduced by coating it with a material the
thickness of which is a quarter of the wavelength of the incident light. Light rays which are reflected from the surface of the lens travel a distance of one half of a wavelength further than those which are reflected from the surface of the antireflective coating. This causes destructive interference and reduces the reflection of light .
In contrast, a coating which has a thickness half the wavelength of the incident light produces a mirror
coating because of constructive interference. Any wavelength may be selectively reflected by a coating
which is half a wavelength thick. Mirror coatings are usually combined with an absorptive tint.
Entopic phenomenon
visual perceptions that are produced by the structures of one’s own eyes e.g. cells in the vitreous or through eyeball palpation. The edge of one’s pupil can be seen when a uniformly illuminated background is viewed through a pinhole.
کریستالین لنز طول موج های کوتاهتر (UV)را بهتر از بلند(lR) جذب می کند
طول موج با انرژی و فرکانس ارتباط عکس دارد
شایعترین congenital color defect?
Deutranomaly
Light scattering?
در خلا رخ نمی دهد و در اثر پارتیکل های موجود در یک مدیوم ایجاد می شود و با طول موج رابطه ی عکس دارد،نور با طول موج کمتر بیشتر scatter می شود به همین دلیل ویتره بهتر از همه با نور آبی و سبز دیده می شود چون بیشتر scatter می شود. قرنیه طبیعی ۱۰ درصد نور را scatter می کند که این مقدار در ادم قرنیه افزایش میابد
زمانی که نور وارد محیط دنس تر می شود طول موج کوتاه می شود ولی فرکانس تغییری نمی کند
زمانی که نور وارد محیط دنس تر می شود طول موج های کوتاه بیشتر شکسته می شوند
طول موج کوتاه تر ،diffraction کمتر
FA?
یک فوندوس cameraدارد که اپتیک آن شبیه افتالموسکوپ غیر مستقیم است،یک فیلتر دارد که اجازه ی عبور طول موج زیر ۵۱۰ یعنی آبی را میدهد و یک فیلتر جلوی فیلم دارد که طول موج زیر ۵۱۰ را بلوک می کند(بنابراین نورهای آبی را میتاباند ولی نورهای آبی رفلکت شده دریافت نمیشود و تنها نور سبز ثبت می شود)
ICG ?
نسبت به FA مولکولهای بزرگتری دارد و باند با پروتئین های پلاسما بیشتر است و عروق کوروئید را بهتر نشان میدهد.ICGحاوی ید است و در افراد با حساسیت به ماده ی حاجب ممنوع است(غذای دریایی ممنوعیتی ندارد)
کدام پدیده باعث می شود تا از یک point source یک point image ساخته نشود؟
Diffraction
Diffraction?
Airy disc receives 90% of luminance flux
سایز دیسک با طول موج رابطه ی مستقیم و با سایز پوپیل رابطه ی عکس دارد
Visual acuity?
it can be represented a reciprocal of the minimum
angle of resolution
it varies with the region of the retina
it is affected by general illumination
it is affected by the colour of the test objects
it is affected by the time of exposure
PH
نیاز به فوکوس کردن را کاهش می دهد،آمتروپی زیر ۴ دیوپتر را اصلاح می کند،ولی اگر سایز آن از ۱mmکوچکتر باشد به دلیل diffraction کیفیت تصویر خراب می شود.
هم depth of focus و هم depth of feild را زیاد می کند
refers to the amount of light arriving at a given point?
Illuminance
Irradiance
Titmus 3000-40
Frisby 600-15
TNO 450-15
Lang 1200-550
Stereoscopic vision in normal person, the value is about 60 seconds of arcor better
What is used in pleoptics to produce Haidinger’s brushes
Polarized light
polarimetry uses the birefringence of the nerve fibre
layer to quantitate its thickness indirectly
ماده ای با خصوصیات birefrengence می تواند نور غیر پلاریزه را به دو بیم پلاریزه در دو جهت مختلف تبدیل کند بنابراین گفته می شود که ۲ RIمتفاوت دارد
کاربرد های نور پلاریزه؟
لنزهای اپتیکی،pleoptics،slit lamp
ATR Anaglyph TNO Red green glass تا ۱۵ secرا هم می تواند تشخیص بدهد
Disparity rang تا ۱۵
1) TNO
2) frisby
The actual luminous flux emitted by a laser is relatively small (lasers are very inefficient in that a great
deal of energy has to be ‘pumped’ into them in order to maintain the lasing process). However, because
the luminous flux is not scattered in all directions but is concentrated in a fine parallel beam, the beam
of light is exceedingly bright.
A laser producing approximately 5 lumens of light may have a beam of
luminous intensity 500 million candela. Another useful comparison is that a 1 watt laser produces a
retinal irradiance 100 million times greater than that of a 100 watt incandescent bulb.
The point along the path of the laser beam where it is least divergent is the point at which energy can be
focused to the smallest spot; this is called the fundamental mode. At the point of focus, energy is most concentrated at the centre of the laser beam and diminishes peripherally in a distribution described by a
Gaussian curve
Non-fundamental modes representing divergent waves may be cancelled by an aperture inside the laser
cavity to allow only the passage of parallel rays.
The beam from a continuous wave
ophthalmic laser (e.g. argon) has a constant power output and its energy output depends on the shutter
speed used. Output is therefore more conveniently measured in watt
the beam from a pulsed laser (e.g. Nd:YAG) has a peak of power and the output is
therefore more conveniently measured in joules
Laser light is in fact less than pure because, for several reasons, it comprises more than a single
wavelength
First, the length of the laser tube, often of the order of 1 m, is enormous compared with the
wavelength of laser light. It is therefore possible for multiples of several different wavelengths to ‘fit’
into the tube length. Secondly, in the case of solid state lasers, the heat generated during operation may
cause expansion of the laser crystal, altering the distance between the mirrors. Thirdly, gas lasers have
wavelength impurities caused by the Doppler effect: gas molecules have random motion and the
wavelength of light which they emit depends on whether or not the direction of their random motion is
the same as the emission. For these reasons, the various ‘sub-wavelengths’ are not in phase, a situation
called free running mode. Mode locking is a refinement of Q-switching which synchronises the various
wavelengths so that periodically they are in phase and summate as a train of high energy pulses
Effects of Laser Energy on Tissue
1) ionising(photodisruption)(plasma formation)(Nd YAG , argon fluride eximer laser)
2) thermal(photocoagulation)(pro denaturation)(آرگون)
3) photochemical(sublimation)(شکستن باند کوالان)(PDT)
Aiming beam?used in laser with invisible output(Nd YAG ,diod)
لیزر هلیوم نئون
لیزر هلیوم نئون طول موج ۶۳۰ دارد که در محدوده ی مرئی است
Used for aiming beam
لیزر Nd YAGطول موج۱۰۶۵ دارد و لیزر دیود طول موج در ۸۱۰ دارد که هر دو در محدوده ی IRاست و دیده نمیشوند بنابراین نیاز به aiming beam دارند
جمع بندی فوتومتری؟
مقدار نور خروجی از منبع luminous flux است که واحد آن لومن است ۱ لومن در واحد استریدین معادل ۱ کندل است (واحد intensity یا I کندل میباشد) و در واحد ۱ مترمربع معارل ۱ آپوستیب است و در واحد یک ft۲ معادل یک ft lambert است
مقداری که سطح روشن می شود با illuminance بیان می شود با E نشان داده می شود و واحد آن لوکس است
مقداری که از سطح رفلکت میشود luminanceاست که معادل یک کندل در واحد متر مربع است
catoptric images are images reflected from the surfaces of the eye
تصویر ۱ مجازی نزدیک کپسول قدامی لنز، تصویر ۲ مجازی و کمی عقبتر ، تصویر۴ حقیقی و عقبتر از ۲ در ماتریال قدامی لنز تشکیل می شود
تصویر ۳ مجازی بوده و در ویتره می افتد
For an object situated within the principal focus of a concave mirror, the image?
مستقیم مجازی بزرگتر و laterally invert است
For an object situated anywhere in front of a convex mirror, the image?
همیشه مجازی مستقیم کوچکتر و laterally invert است
the absolute refractive index of a material is always greater than itsrefractive index
absolute refractive index=سرعت نور در خلا تقسیم بر سرعت نور در مدیوم
refractive index=سرعت نور در هوا تقسیم بر سرعت نور در مدیوم
نور در خلا سریعتر از هوا حرکت می کند
The argon blue-green gas laser emits a mixture of 70% 488 nm (blue) and 30% 514 nm (green) light.
Argon lasers are most commonly employed for retinal photocoagulation
Photocoagulation aims to treat the outer retina
and spare the inner retina to avoid damaging the nerve fibre layer. Argon green (blue screened out)
photocoagulation of the macula does not cause direct retinal damage.
Laser photocoagulation?
فوندوس پیگمانته نیاز به پاور کمتری دارد
لنز کاتاراکته به دلیل ایجاد scattering در نور لیزر نیاز به پاور بیشتری دارد
In the eye, diode laser light is absorbed only by melanin and consequently is most commonly used for retinal
photocoagulation. Low scattering of this wavelength ensures good penetration of the ocular media and
of oedematous retina. The 810 nm wavelength also penetrates the sclera. Thus, even if the retina is
obscured from view through the pupil, photocoagulation may still be performed by placing the
delivering probe on the surface of the eye
The transparency of the sclera to diode laser also allows photocycloablation of the ciliary body in ‘end stage’ glaucoma. Diode photocoagulation of vascular
structures (e.g. neovascular membranes and tumours) is enhanced by intravenous indocyanine green
dye with an absorption peak of 800–810 nm. Diode laser light has been used endoscopically to create a
dacryocystorhinostomy (DCR).
The neodymium–yttrium–aluminium–garnet (Nd–YAG) laser emits 1064 nm infrared radiation.
It is a powerful continuous wave (CW) laser which is usually Q-switched when used to treat the eye
The Frequency-Doubled Nd-YAG Laser
The photocoagulation effect is similar to that of continuous wave argon green laser
Excimer lasers in clinical use employ an argon–fluorine
(Ar–F) dimer laser medium to emit 193 nm ultraviolet (UV) radiation. High absorption of UV by the
cornea limits its penetration
erbium:YAG laser delivers 2940 nm infrared radiation which is absorbed by water and penetrates
tissue by less than 1 µm. results in the
explosive evaporation of tissue, and thermal effects are limited to the surrounding 5–15 µm. This laser
has been used experimentally to emulsify the lens in cataract surgery
The carbon dioxide (CO2) laser emits 10600 nm mid-infrared wavelength which is strongly absorbed by
water, and therefore by most tissues. The only effects are thermal; the diffusion of heat from the target
coagulates adjacent tissues,and water vaporisation releases steam
Such lasers are used in other branches of surgery to produce a nearly bloodless incision but have yet to find a use in ophthalmology
مزیت بارز گونیولنز کوپه؟
مقایسه همزمان دو زاویه
Rodenstock panfundoscope consists of?
A high power convex lens
علت کاهش دید در frenzel prism?
پریزم هایپلاستیکی که اغلب از پلی وینیل کلرید ساخته شده اند.
Mainly chromatic aberration
Another thing is scattering
Dove prism
Porro prism
تصویر را ۱۸۰ درجه بصورت عمودی می چرخانند
مسئول چه مقدار از Ab چشمی chromatic Ab است؟
۱/۵ تا ۲ دیوپتر
Cross-cylinders are named by the power of the cylinder, and this is marked on the handle
The axes marked on the lens are the axes of no power of the individual cylinders. The power of each
cylinder lies at 90° to the marked axis and coincides with the marked axis (of no power) of the other
cylinder (of opposite sign)
When lenses are used in instruments, it is desirable to eliminate chromatic aberration
Refraction by the human eye is also subject to chromatic aberration, the total dispersion from the red to
the blue image being approximately 2.00D. The emmetropic eye focuses for the yellow–green (555 nm)
as this is the peak wavelength of the photopic relative luminosity curve. This wavelength focus lies
between the blue and red foci, being slightly nearer to the red
Achromatic Lens Systems
The earliest achromatic lenses were made by combining elements of flint
and crown glass.
کاهشSA؟
occluding the periphery of the lens by the use of ‘stops’
plano-convex is better than biconvex
aplanatic surfaces, that is, the peripheral curvature is less than the central curvature
doublet
effect of spherical aberration in the human eye is reduced by several factors
1) The anterior corneal surface is flatter peripherally than at its centre, and therefore acts as an aplanatic surface
2) The nucleus of the lens of the eye has a higher refractive index than the lens cortex
3)the iris acts as a stop to reduce spherical aberration. The impairment of VA that occurs when the pupil is dilated is almost entirely due to SA.
Optimum pupil size is 2–2.5 mm.
(4)the retinal cones are much more sensitive to light which enters the eye paraxially than to light which enters obliquely through the peripheral cornea (Stiles–Crawford effect). This directional sensitivity of the cone photoreceptors limits the visual effects of the residual SA in the eye
Which Ab dacreased with Doublet ?
Chromatic and spherical Ab
Oblique Astigmatism?
Oblique astigmatism is an aberration which occurs when rays of light traverse a spherical lens
obliquely.
Oblique astigmatism occurs with spectacle lenses when the line of sight is not parallel with the principal
axis of the lens. This is unavoidable in the case of the near portion of a multifocal lens. It may
also be a cause of reduced acuity in patients with restricted eye movement who adopt a compensatory
head posture and look obliquely through peripheral portions of their spectacle lenses
In daily life adults spend most time looking slightly downward from the primary position, and
spectacles are therefore made with the lower borders of the lenses tilted towards the cheek (pantoscopic
tilt). This also slightly reduces the obliquity of the reading portion of multifocal lenses
much
OA worse in biconvex and biconcave lenses than in meniscus lenses.
The factors which reduce
ocular oblique astigmatism are as follows:
(1) The aplanatic curvature of the cornea reduces oblique astigmatism as well as spherical aberration.
(2) The retina is not a plane surface, but a spherical surface. In practice the radius of curvature of the
retina in the emmetropic eye means that the circle of least confusion of the Sturm’s conoid formed by
oblique astigmatism falls on the retina.
(3) Finally, the astigmatic image falls on peripheral retina which has relatively poor resolving power
compared with the retina at the macula. Visual appreciation of the astigmatic image is therefore limited
Correction of Coma Aberration
As in the case of oblique astigmatism this aberration can be avoided by limiting rays to the axial area of
the lens, and by using the principal axis of the lens rather than a subsidiary axis.
Ocular coma aberration is not of practical importance for the reasons given under oblique astigmatism.
A concave lens causes ‘barrel’ distortion
convex lens causes ‘pincushion’ distortion
Also causes jack in box effect
The term ‘curvature of field’ indicates that a plane object gives rise to a curved image
حتی زمانی که SAو comaو OAحذف شده باشند،این باقی می ماند که بستگی به RIو curvature لنز دارد
Ocular Curvature of Field
In the eye the curvature of the retina compensates for curvature of field.
- The angle of deviation of a prism is determined by:
a. the refracting angle
b. the angle of incidence of the ray
c. the refractive index of the prism material
Torsional diplopia can not be controlled by prism
برای محاسبه ی پاور پریزم به ۲ چیز نیاز است:
RI,refracting angle
The vergence power of a lens is dependent on
vergence power of each surface
thickness of the lens
the medium on either side of the lens
The increasing prismatic effect of the more peripheral parts of a spherical lens isresponsible for:
a. ring scotoma
spherical aberration
jack-in-the-box effect
image distortion
cylindrical lens:
a. it has one plane surface and one curve surface
b. it has no power along its axis
c. it can be used to measure phoria in maddox rod
d. it causes meridian magnification or minification at 900to its axis
e. it forms a focal line parallel to its axis
Maddox rod:distant phoria
Maddox wing:near phoria
Double maddox:torsion
اگر axis مادوکس ۹۰ درجه باشد انتظار می رود که focal line عمودی جلوتر و افقی عقبتر تشکیل شود
از آنجا که جلویی بسیار به چشم نزدیک است توسط چشم دیده نمی شود و فرد فقط خط افقی را میبیند
Irregular astigmatism:
occurs when there are many radii in one meridian
spectacles can not correct it
usually corneal in nature
can be caused by different indices of refraction in different portionsof the crystalline lens
can be treated with laser refractive surgery,RGP,corneal graft
طول موج كوتاه تر مثل ابي
بيشتر شكسته ميشود،بيشتر scatter مي شود،فركانس و انرژي آن هم بيشتره،در مديا آهسته تر حركت مي كندوdispersion بيشتره ولي diffraction كمتره
وقتي نور از خلا وارد محيط ديگر مي شود؟
هم طول موج هم سرعت كم مي شود ولي فركانس تغييري نميكند.
وقتي نور از خلا وارد محيط ديگر مي شود؟
هم طول موج هم سرعت كم مي شود ولي فركانس تغييري نميكند.
وقتي نور از خلا وارد محيط ديگر مي شود؟
هم طول موج هم سرعت كم مي شود ولي فركانس تغييري نميكند.
همواره محد حقيقت كمه
همواره شي مجازي در لنز محدب تصوير حقيقي و كوچك و مستقيم مي دهد
همواره همت كاوه كمه
همواره شي حقيقي تصوير مجازي در لنز كاو مي دهد كه مستقيم و كوچكتره
مقنعه در كانون حق مبين است ولي با مقنعه در خارج مي توان عكس مجاز گرفت
شي مجازي در لنز مقعر اگر در فاصله ي كانوني باشد تصويرش حقيقي مستقيم و بزرگتر است ولي اگر در فاصله ي خارج كانوني باشد تصوير مجازي و معكوس است
هه هه عكس تو محدكودك ولي تو كانون مه مه
شي هقيقي تصوير هقيقي معكوس در لنز محدب مي دهد مگر در فاصله ي كانوني كه تصويرش مستقيم و مجازي است
كداميك از انواع monochromatic ab مي تواند در اجسام on axis هم رخ دهد؟
Sperical aberration
فيلد بينايي در آينه ي تخت متاثر از چه فاكتووي است؟
سايز آينه
دور كردن آينه از خودمان فيلد رابيشتر نمي كند
Reflecting power of the cornea
-250
Refracting power of the cornea
+50
Geneva lens measure:
it measures the base curve of a lens
it makes use of the principle that the total power ofa thin lens is equal to the sum of its surface power
it can be used to measure the thickness of a lens
it can be used to measure the diameter of a lens
Chromatic aberration?
it can be reduced by combining crown and flint glass
spherical aberration:
a. it is responsible for reduced vision in dilated pupil
b. it results from the prismatic effect of the peripheral parts of a spherical lens
c. light passing through the periphery of a spherical lens is deviated more than those through the paraxial zone of the lens
D.it can be reduced with a doublet.
Best form lenses is used to reduce:
oblique astigmatism
spherical aberration
Oblique astigmatism in the eye is reduced by::
aplanatic curvature of the cornea
the spherical surface of the retina
the peripheral retina has less resolving power than the central cornea
انواع چشم شماتیک؟
در همگی2nd focal pointروی رتین می افتد
در همگی refractive power تقزیبا یکی است
1)gullstrand
دوتا nodal point در ACو ۲ تا principle planeدر سطح خلفی لنز دارند.
2)reduced models
یک nodal point در ACو یک principle plane در سطح خلفی لنز دارند
Back vertex power یک تقسیم بر فاصله ی سطح خلفی لنز تا F2 True power یک تقسیم بر F length2 Ant power یک تقسیم بر F length1
Back》true》front
زیر یک سالteller و killer
۲ سال cardiff,kay,sheridan gardiner
۳و۴ سال و عقب افتاده stycar
۵ به بالا اسنلن
Contrast sen test: vistech,cambridge
پاسخ به OKN drum تا ۱/۵ ماهگی در کودک به وجود می آید
صحیح
Rp ,CSR
more decrease in contrast sensitivity
Retinal degeneration
More decrease in visual acuity
Inadequate acc?
داون،دیابت،کم کاری تیروئید،میاستنی،مردمک تونیک،کلروکین،سیکلوپلژیک،فنوتیازین
Acute intermittent ET
در فرد جوان درس خوان
Spasm of ACC
RD?
باعث AST نمی شود بلکه باعث Distortion و non astigmatic blur می شود
مزایای کاهش BVD?
کاهش oblique ast و distortion
کاهش آنیزوکونی
)به عبارتی کاهش BVDسایز تصویر را به مقدار واقعی اش نزدیک می کند)
دور کردن لنز مثبت از چشم(عینک مامان بزرگ روی نوک بینی)
پاور مثبت را زیاد می کند و لذا بزرگنمایی را هم زیاد میکند
(به همین دلیل از راهکارهای کاهش آنیزوکونی کاهش BVDاست)
نزدیک کردن لنز منفی به چشم(میوپ های هیز کنار دریا)
پاور منفی را زیاد می کند و لذا کوچک نمایی را کم می کند
(به همین دلیل از راهکارهای کاهش آنیزوکونی کاهش BVDاست)
dynamic refraction refers to the dioptric power of the accommodated eye
static refraction refers to the dioptric power of a resting eye
AC/A ratio?
Heterophoria method
باید IPD را بدانیم.مقدار به دست آمده بیشتره
Gradiant method برعکس
manifest hypermetropia is the strongest plus lens which the patientcan accept for clear distant vision
latent hypermetropia is the residual hypermetropia masked by ciliarytone and involuntary accommodation and can be unmasked by cycloplegic refraction
falcultative hypermetropia refers to hypermetropia that can not beovercome by accommodation
absolute hypermetropia cannot be overcome by accommodation
Facultative+absolute=manifest
Regular Ast
زمانی که زاویه بین دو مریرین ۹۰ درجه باشد
Oblique Ast
زمانی که زاویه بین آنها باز هم ۹۰ دره باشد ولی به ۱۸۰ و ۹۰ نزدیک نباشند
Irregular Ast
زاویه بین دو مریدین ۹۰ نباشد
چه بیمارانی با PHدیدشان بدتر می شود؟
بیماری ماکولا،PSC، اسکار قرنیه
too small a pin-hole can affect vision through interference
Correct
when a correcting lens is moved forward in a the image is moved forward
چه در لنز مثبت چه در لنز منفی
در تست ها منظور از move backward به سمت چشم و forward برعکس است
contact lens magnifies the image in a patient with axial myopia
Correct
Relative spectacle magnification(RMS)
the ratio of corrected image size to ammetropic image size
Spectacle magnification
the ratio of corrected image size to uncorrected image size1
The stenopaeic slit
is an elongated pinhole that reduces blur resulting from ametropiain the meridian perpendicular to the slit
RSM?
axial ametropia if the correcting lens is placed at the anterior focal point:RSM=1
index or refractive ametropia if the correcting lens is placed atthe anterior focal point: RSM》1 for hyperopia
RSM《1 for myopia
Aphakia with glass:RSM=1.3
Aphakia with CL:RSM=1.1
Aphakia with IOL:RSM=1
correction of aphakia with glasses:
a. there is an increasing image magnification as the correcting glasses
is moved forward from the position of the natural crystalline lens.
b. reduction in visual field is a common problem
c. “pincushion” distortion of the visual field
d. ring scotoma of aphakia is created by the prismatic effect of the lens
e. using head turning rather than eye movement to change gaze direction
can prevent ‘Jack-in-the-box’ phenomenon
Intraocular lens calculation:
is more affected by an error in axial length calculation than keratometricmeasurement
uses a lower A constant for anterior chamber than posterior chamber lens
is affected by the velocity of the ultrasound through the cataract
is affected by the absence of crystalline lens
چه شماره ی میوپی را میتوان lens extraction کرد و هیچ iol ای را نذاشت؟
بین منفی ۱۸ تا منفی ۲۰
The anterior lens surface is less curved
Correct
onset of presbyopia depends on:
the size of the pupil
task required
the refractive state of the patients
amplitude of accommodation
for comfortable near vision one-third of the accommodation should be kept in reserve
True
Split (Franklin) bifocals were the earliest design and comprise a distance lens whose flat bottom abuts
the flat top of a separate near lens
In newer types of bifocal lenses, the near portion is
constructed by modifying the main lens to incorporate a near addition. This modification is called a
segment and is achieved either by attaching a supplementary lens, or by changing the surface curvature
Cemented bifocals have a near portion constructed by attaching a supplementary lens to the surface of a
distance lens of the same refractive index
In fused bifocals, the near portion is made by heat-fusing a button of flint glass to a corresponding
depression in a crown glass main lens which has a lower refractive index
Solid bifocals are of single piece construction. The near addition is produced by a different curvature of
either the front or back surface of this portion. The executive-style solid bifocal has a full-width
horizontal junction between the near and distance portions . Plastic bifocals are always of the
solid type.
Split (Franklin) bifocals were the earliest design and comprise a distance lens whose flat bottom abuts
the flat top of a separate near lens
In newer types of bifocal lenses, the near portion is
constructed by modifying the main lens to incorporate a near addition. This modification is called a
segment and is achieved either by attaching a supplementary lens, or by changing the surface curvature
Cemented bifocals have a near portion constructed by attaching a supplementary lens to the surface of a
distance lens of the same refractive index
In fused bifocals, the near portion is made by heat-fusing a button of flint glass to a corresponding
depression in a crown glass main lens which has a lower refractive index
Solid bifocals are of single piece construction. The near addition is produced by a different curvature of
either the front or back surface of this portion. The executive-style solid bifocal has a full-width
horizontal junction between the near and distance portions . Plastic bifocals are always of the
solid type.
When prescribing for children, for whom executive bifocals may be prescribed to overcome convergence excess esotropia, the segment edge should be at the lower margin of the pupil to ensure that the near portion is used for all near tasks
the problem of prismatic jump is related to the power of the lenses
Reduce prismatic jump in bifocals?
Prismatic jump can be reduced by moving the optical centres tward the junction of the two
portions(executive)
Alternatively, image jump and prismatic effect are reduced by
incorporating a base-up prism in the near segment.
Varifocal lenses
Progressive lenses
of the soft designs are better than the hard designs for prolonged VDUworks(due to wider corridor)
power of a contact lens is determined by its:
thickness
posterior curvature
Anterior curvature
refractive index
The base curve of a contact lens:
refers to its posterior central curvature
is measured with a radiuscope
determines its movement with blinking
for a particular patient is choosen according to the central keratometry measurement
Contact lenses?
they are usually tinted to make them more visible for handling
tear lens can neutralize astigmatism
truncation of a contact lens is used to prevent lens
rotation
piggyback contact lens involves the use of softcontact lense and RGP
corneal warpage:
it refers to a change in the corneal curvature associated with contact lens wear
it is more common with rigid gas permeable contact lens
it interferes with the result of biometry
it is a reversible condition
پتوز و staining ساعت ۳ و ۹ با RGPشایع تره
اولسر باکتریایی و کنژونکتیویت Giant papillary با soft cl شایع تره
To reduce the lens movement
Increase thickness
Decrease BC
Increase diameter
Ultrasound?
the ultrasound used in diagnosis has a frequency of greater than 20 kHz
piezoelectric effect is used to emit ultrasound as wellas analysing the echo
ultrasound with longer wavelength penetrates the tissue deeperbut has poorer resolution
Fresnel prism
The power is determined by the apex angle
Horizontal fusional reserve
مثبت:مقدار پاور BOکه لازمه تا دیپلوپی را القا کند(این رزرو تحت تاثیر ACC reserve بیمار است و لذا با افزایش سن کاهش می یابد)برای نزدیک این مقدار بالاتر است
منفی:مقدار پاور BI که لازمه تا دیپلوپی را القا کند
Horizontal fusional reserve is more than vertical
Resolution:is the least separation of two images so that they are seenas separate and is affected by the cone separation
in normal subject is 6/6 or betteris dependent on the size of the pupil. A small pupil causesincreased diffraction and therefore reduced resolution, pupil ofabout 2.5 mm is best for resolution above which optical aberrationcan affect the resolving poweris 1 minute of arc in normal subjectcan be tested with Sheridan-Gardner test which is a modified formof Snellen chart for testing children and the illiterate
Vernier acuity:
is the determination of the eye to detect a break in a line andthis may be as little as 3 to 5 seconds of arcis not determined by cone separation because the normal acuityis only a fraction of the width of the cone, the mechanisminvolves in Vernier acuity is still unclear does notrequires binocular single vision.
is required in using optical instruments such as keratometeror applanation tonometer
Diffraction:
is increased with smaller pupil
is increased with longer wavelength(red light diffracts more )
Fraunhofer diffraction refers to the production of an interferencepattern by diffraction through a single slit (pupil) and this givesan Airy’s disc
the Airy’s disc affects the resolving power of the eye. The minimumresolvable distance is about the radius of the Airy disc
the Airy’s disc is increased with a smaller pupil (and red light).The resolving power is therefore decreased witha smaller pupil
Accommodation spasm:
can masquerade hypermetropia by the moving the far point forward.does not cause presbyopia which is caused by accommodationinsufficiencymore common in hypermetropia due to the attempted accommodativeeffort to bring the far point to he front.accommodation is closely linked to convergence. In accommodationspasm, there is esotropia which is variable.can be alleviated by mydriatic
Schemic eye
Post lens surface is more curved
Stenopaeic slit:
can be used to determine subjective astigmatic refractive error,especially when the retinoscopic reflex is dim.
gives the best visual acuity when it is in the axis of the astigmatismand may be used to decide where iridotomy need to beperformed in corneal scarring to allow light passage
can be considered as an elongated pinhole and the narrower the slitwidth the more diffraction can occur which can affect the visualacuity
Myopic shift
occurs when the far point is moved forwardoccurs in 1)keratoconus because the increased curvature of thecornea increase the refractive power2)spasm of the ciliary body increases accommodation which bringsthe far point forward3)staphyloma occurs in conditions such as myopia or rheumatoidarthritis and moves the retina away from the far point4)forward lens subluxation causes the far point to move forward5)poorly controlled diabetes mellitus can cause lens swellingresulting in increased refractive power(brittle diabetes)
Diseases associated with myopia include:
Down's syndrome Marfan's syndrome homocystinuria Weill-Marchesani's syndrome Ehlers-Danlos syndrome Stickler's syndrome Pierre-Robin's syndrome Pader-Will's syndrome congenital glaucoma
Visual fields:
cecocentral scotoma is typically seen in toxic optic neuropathysuch as tobacco-alcohol amblyopia, ethambutoland lead poisoning
optic disc pit can produce arcuate scotoma
bilateral retinoschisis in the temporal quadrants can causebinasal hemianopia
homonymous hemianopia suggestsretrochiasmal lesion.
The following optical devices may be used for lowvisual aids:
telescope
High-add bifocal Prismatic half-frame spectacle Head-mounted magnifier Hand-held magnifier Stand magnifier Spectacle-mounted telemicroscope Closed-circuit television (CCTV) Text scanner, enlarged font on computer monitor
steady
Disadvantages of optical magnifying glasses includes:
reduced depth of focus, reduced visual fields, objectsneed to be placed near the eye unsteadiness ofimages if the objects are not held steady.
There are two optical ways which can help patients withconstricted visual fields:
minification and image relocation.Minification of image allows the remaining functionalretina to process more information and this can beachieved with 1)concave lens or 2)inverted Galileantelescope. Image relocation relocates visual informationfrom the scotoma closer to the area of functioning visualfield. This has the effect of minimizing the size of the eyeor head movement required to detect the visualobject. This technique typically uses 3)prisms placed nasallyand temporally with the base tward thescotoma but can also use 4)mirrors.Lastly, the non-optical way involves making the patientsaware of the spatial area to be processed ie. behaviouralapproach and this involves increasing the efficiency ofeye movements.
اصلاح پريزماتيك در عينك bifocal
روي لنز less plus سگمان flat top و يا پريزم slab off🤪
در تمام انواع bifocal سگمان روی سطح محدب قرار می گیرد به جز؟
Round segment
در تمام انواع bifocal آستیگمات در سطح مقعر اصلاح می شود به جز؟
Ultra type
در چه مواردي سگمان bifocal بايد داخلتر قرار گيرد؟
Lower Working distance
Higher PD
More plus power
Exophoria
Soft PAL
Wider corridor
Quick adaptation
Less peripheral distortion
در trial frameلنزهای با پاور بالا را در پوستریور قرار می دهیم
صحیح
در رتینوسکوپی بیمار با ASTبالا
با نزدیک شدن به نقطه نوترالیته محور باید به میزان اندک Adjust شودچرا که مقدار misalignment رفله نور بسیار بیشتر از misalignmentلنز سیلندر است.
IPD?
۱ میلی متر بیشتر از فاصله ی بین visual axis دو چشم برای دور است
بزرگ بودن پوپیل و viewing aperture رتینوسکوپ accuracy را کم میکند( نقطه ی neutralization با اطمینان کمتر به دست می آید)
بزرگ بودن پوپیل و viewing aperture رتینوسکوپ sensitivity را بیشتر می کند به عبارت دیگر RE راحتتر به دست می آید
Congenital nystagmus
WTR Ast
BO prism improves VA
Blind spot VF if we use plus lens?
کوچکتر و نزدیکتر به fixation
Best form lenses
BC in meniscus6
BC in perioscopic 1.25
plus lenses have negative BC
concave surface is always placed next to the eye
در چه مواردی استفاده از bifocal مناسب نیست؟
Oblique Ast،آنیزومتروپی شدید،muscle imbalance
اختلاف محور AST بین دو چشم ؟
اگر بالای ۲۰ درجه باشد ممکن است باعث عدم تحمل عینک شود
Retoid lens?
باعث بهبود دید محیطی در بیماران آفاکی می شود
Steep curve
Soft CL
اگر دید بلافاصله بعد از پلک زدن خوب شود یعنی too steep
۲ میلیمتر lag در gaze ها و زمان طولانی تر برای recenter شدن یعنی loose fit
در موارد thick lens بهترین فیت loose fit است
لنزهای high water برای بیماران dry eye مناسب نیست
می توان به آنها BD prism اضافه کرد
در CL پایین بودن RI باعث ضخیم شدن لنز می شود
صحیح
در irregular corneal ast ؟؟؟
CL is better
در Oblique ast ؟؟؟
عینک بهتره
الگوی فلوئورسئین در early KCN
Central corneal touch
Midperipheral pooling
Head band loup
High power spectacle magnifier
BI prism
Compound microscope?
كاربرد در كراتومتر پاكومتر و تونومتر
اسپكولار ميكروسكوپي هم يك فرم مديفيه از آن است
اسليت هم يك فرم از آن با پاور كم است
Power of lens in loup
Magnification*4
بزرگنمايي در لوپ ٨ است لذا پاور ميشود ٣٢
Acc with telescope for 33 cm
3* mag ٢
Wollaston prism in javal
Porro prism in compound microscope
كراتوسكوپ يا placido disc يك flat disc است
توپوگرافي ارزيابي تصوير يك placido disc درون يك سطح مقعر است
Macular function test
PAM
Lens interferometer
Noise chart
مربوط به contrast sensitivity است و بيشتر نشانگر آسيب نورال است تا opacity مديا
كاربرد optical doubling?
پاكي متري
تونوكتري
كراتومتري
Indirect ophthalmoscopy in small pupil
جابجا كردن آينه مثلثي به سمت پزشك
دور كردن دو چشمي از هم
دور كردن سر پزشك از بيمار
Axial mag=TM٢
True axial mag=AM/4
Depth distortion=true AM/TM
بيشترين mag و كمترين depth distortion(١)مربوط به لنز ١٥ است
بيشترين فيلد و بيشترين depth distortion(٠/٥)مربوط به لنز ٣٠ است يعني تصوير را ٥٠ درصد flat تر مي كند
Compound microscope?
كاربرد در كراتومتر پاكومتر و تونومتر
اسپكولار ميكروسكوپي هم يك فرم مديفيه از آن است
اسليت هم يك فرم از آن با پاور كم است
Power of lens in loup
Magnification*4
بزرگنمايي در لوپ ٨ است لذا پاور ميشود ٣٢
Acc with telescope for 33 cm
3* mag ٢
Wollaston prism in javal
Porro prism in compound microscope
كراتوسكوپ يا placido disc يك flat disc است
توپوگرافي ارزيابي تصوير يك placido disc درون يك سطح مقعر است
Macular function test
PAM
Lens interferometer
Noise chart
مربوط به contrast sensitivity است و بيشتر نشانگر آسيب نورال است تا opacity مديا
كاربرد optical doubling?
پاكي متري
تونوكتري
كراتومتري
Indirect ophthalmoscopy in small pupil
جابجا كردن آينه مثلثي به سمت پزشك
دور كردن دو چشمي از هم
دور كردن سر پزشك از بيمار
Axial mag=TM٢
True axial mag=AM/4
Depth distortion=true AM/TM
بيشترين mag و كمترين depth distortion(١)مربوط به لنز ١٥ است
بيشترين فيلد و بيشترين depth distortion(٠/٥)مربوط به لنز ٣٠ است يعني تصوير را ٥٠ درصد flat تر مي كند
