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.