Capacidad de carga real (CCR)

4. Rotating the axis of the correcting cylinder toward 90 or 180 degrees.

 Most distortions of aphakic spectacle lenses stem from their position anterior to the pupil and include image magnification, ring scotoma, pincushion distortion, and the so-called jack-in-the-box phenomenon. Aphakic spectacle lenses can also create cosmetic problems not only because of their size but also because they can make the patient's eyes look

magnified.

 The magnification associated with any corrective lens diminishes as the lens approaches the eye's nodal point.

o IOL decreases image magnification to <4%

o 20% to 30% with aphakic spectacle correction o 7% to 12% with aphakic contact lens correction.

 At the correct axis, the following are true:

1. The break phenomenon disappears (the intercept and reflex are parallel).

2. The width of the streak is narrowest.

3. The intensity is brightest.

4. Skew motion is no longer observed.

Intensity and skew are primarily useful for small astigmatic errors, while break and width are best judged with an enhanced streak. Astigmatic errors of <1.0D do not enhance well, so break and width are not readily appreciable.

 The spherical equivalent is discovered by fogging the eye (adding a lot of plus sphere) to relax accommodation, then adding minus sphere until vision is sharpest. At this point, the circle of least confusion is on the retina. Then, the cross-cylinder is introduced to find axis and power. The circle of least confusion must remain on the retina throughout cross-cylinder testing. When fogged, the circle of least confusion is anterior to the retina.

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 The sphere end point can be verified by the duochrome test, but this test does not relax accommodation. Therefore, the test should be introduced with the patient slightly fogged, such that the letters on the red side are clearer (the red letters will focus behind the green letters, closer to the retina). Then, minus sphere is added until letters on the green and red sides are equally clear.

An emmetropic person sees red and green colors equally sharp. After giving correction patient is asked to read FRIEND. FIN letters are written in green and RED letters are written in red color. If myopia is over corrected or hypermetropia is under corrected patient will appreciate green letters much better than red letters. On the other hand if myopia is under corrected or hypermetropia is over corrected patient will appreciate red color much better.

Or simply you can remember, If RED clear, add MINUS If GREEN clear, add PLUS.

 Image jump can occur in any correction whose add does not have its optical center at the top of the segment. Round-top segments typically produce more image jump than flat-top segments, as they have much lower optical centers.

Image displacement occurs in all corrections as gaze moves away from the optical center of the correcting lens; this is unlikely to be significantly distressing unless an imbalanced displacement occurs, as in anisometropia. The other situation where image displacement is troublesome is when an increased demand is made on already taxed vertical fusional system ability, as in compensated vertical phoria.

 Holding the radius of curvature constant, larger diameter lenses are effectively steeper.

At a given diameter, radius of curvature and lens steepness is inversely proportional.

 If the center of curvature of the refracting surface is on the same side as the medium of higher index of refraction, the surface is positive, regardless of direction of light

propagation.

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 Candela is the unit of measure of luminous intensity, which is defined as the light emitted per unit of solid angle.

Luminous flux is the quantity of light leaving a source or passing through a region of space, and it is measured in lumens.

Illuminance is the quantity of light per unit area incident on a surface or at an image, and it is measured in lux.

Luminance is the light reflected or emitted by a surface per unit area and per unit solid angle, and it is measured in apostilbs.

 Why vision is 6/6?

Two points discriminable as long as they stimulate two separate cones.

Width of a foveal cone = 2.5u (30sec angular separation) Individual part of 6/6 letter = 1 min angle at fovea

In order to properly represent the peaks and troughs in the intensity profile of a sinewave, there must be at least two cones for each cycle of the grating. This is known as the Nyquist limit.

 Types of Visual Acuity

 Minimum visible: Best threshold is 1 sec arc

 Minimum resolvable/recognizable: Best threshold 30 sec arc

 Minimum discriminable(hyperacuity): Best threshold is 2 sec arc

 optical correction of presbyopia can also be achieved with hyperopic [orthokeratology]

lenses, bringing the advantages of waking hour visual correction without the need for glasses or contact lenses to a rapidly growing sector of the optical market.

 MRI Prism Glasses: Non-Magnetic Prism Glasses allow the patient to see the room outside the bore of magnet during their scan, allowing a companion or scenic picture to be viewed.

A simple and cost effective comfort measure to be used alone, or in conjunction with a sound system for ultimate relaxation in an MR environment. Also known as recumbent spectacles.

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 The development of presbyopia is independent of ocular parasympathetic nerve paresis.

 Visual acuity is usually better in blue cone monochromatism than rod monochromatism.

 Becker's test: A test for astigmatism that uses diagrams of sets of three lines radiating in different meridians.

 Retina is most sensitive to yellow light in photopic conditions. In scotopic conditions it is most sensitive to blue light.

 The Sloan Letters: 10 letters C, D, H, K, N, O, R, S, V, Z

 X notation for magnification:

o Multiply the amount of magnification by 4 to get the ―Dioptric Power‖ of the lens.

Divide the Dioptric power into the number 40 to get the number of inches from the eye.

o For example, a 4X lens is 16 Diopters. Divide into 40, we get a 2.5" working distance; a 5X lens is 20 Diopters and has a reading distance of 2 inches

 Aspheric lenses are lenses which have a relatively flat surface, yet still offer the same vision quality as non aspheric lenses. This is because although the lens is thinner and flatter, it still refracts light to exactly the same degree as a conventional lens.

Double aspheric lenses are essentially an even better version of the already useful

aspheric lenses. Not only is the front of the lens aspheric, but the back of the lens has the same aspheric design, hence the name, ‗double aspheric‘. Typically, when you look out from the edge of your lenses, the image you see can be distorted. Double aspheric lenses limit this distortion of image right up to the edges of the lens. This is done by ensuring that a large area of the lens is the focal point.

 Photoreceptors:

145 o "red" cones (64%), "green" cones (32%), and "blue" cones (2%)

o The green and red cones are concentrated in the fovea centralis.

o The "blue" cones have the highest sensitivity and are mostly found outside the fovea, leading to some distinctions in the eye's blue perception.

o The cones are less sensitive to light than the rods

o visual acuity or visual resolution is much better with the cones, the rods are better motion sensors

o Rods Do Not See Red!  The light response of the rods peaks sharply in the blue;

they respond very little to red light.

 The ship captain has red instrument lights. Since the rods do not respond to red, the captain can gain full dark-adapted vision with the rods with which to watch for icebergs and other obstacles outside.

 Dispersion is the variation of refractive index with wavelength. In general, the shorter the wavelength, the higher the refractive index. For instance, in spectacle crown glass, light of wavelength 656 nm (corresponding to red) has an index of 1.520, whereas light of

wavelength 480 nm (corresponding to blue) has an index of 1.531. In the crown glass example, the dispersion is 0.011 (= 1.531 - 1.520).

 Light scattering:

o If light strikes a structure larger than 1000 nm, the light is absorbed.

o If the structure is less than 1 nm, the light passes by unaffected.

o However, if the object has a size between 1 and 1000 nm, light will be absorbed and re-emitted as a ray of similar wavelength, but in a different direction.

 Magnesium fluoride (MgF2), a durable substance with an index of 1.38 at 550 nm, is the most commonly used coating material for single-layer antireflection coatings. Instead of one reflection at the air-glass interface, there is now a reflection at the air-MgF2 interface and a second reflection at the MgF2-glass interface. These reflections interfere

destructively, resulting in a minimum reflectance within the visible spectrum (at normal incidence) of 1.5% or less, as compared with the original 4% for the uncoated glass surface

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 The rods have peak sensitivity at 500 nm that corresponds to blue-green, and the foveal cones have peak sensitivity at 562 nm or in yellow-green. The significance of this shift in sensitivity is that the foveal cones respond more strongly to longer wavelengths where chromatic aberration gradually increases, whereas rods respond more strongly to the region of rapidly increasing chromatic aberration.

 When a glassblower or other glass worker heats glass directly in a flame, the flame surrounding the glass emits a yellow light termed sodium flare.

Didymium or neodymium glass absorbs this light, which is emitted at approximately 589 nm, making it easier for the glass worker to view his or her work.

 Most common mirror coating materials are chromium, aluminum, and copper.

 Laser light is mainly absorbed in ocular tissue by three pigments

o Melanin strongly absorbs all ultraviolet and invisible wavelengths. However, this absorption by melanin decreases with increasing wavelengths.

o Hemoglobin has strong absorption in the violet (420 nm) and green (514 nm) wavelength. Deoxygenated hemoglobin absorbs red more strongly than does oxyhemoglobin.

o Xanthophyll which is the pigment most densely distributed in the macular area, absorbs the blue wavelength (460 nm).

 Factors that modify color perception

o Brightness (bezold-brücke phenomenon): As brightness increases, most hues appear to change. At low intensities, blue green, green, and yellow green appear greener than they do at high intensities, which make them appear bluer. At low intensities, reds and oranges appear redder, and at high intensities, they appear yellower. The exceptions are a blue of about 478 nm, a green of about 503 nm, and a yellow of about 578 nm. These are the wavelengths of invariant hue.

o Saturation (abney effect): As white is added to a hue, the hue appears to change slightly in color. The effect is similar to adding yellow. Blue greens become greener and yellow greens become yellower. Reds and oranges also become yellower. The exception is a yellow of 570 nm.

147 o State of dark adaptation (purkinje effect): The relative luminosity curve illustrates

the eye's sensitivity to different wavelengths of light. When the eye is light adapted (daytime), yellow, yellow green, and orange appear brighter than do blues, greens, and reds. The cones' peak sensitivity is to light of 555 nm. A relative luminosity curve can also be constructed for the rods in a dark-adapted eye. The lights are so dim that the observer cannot name the various wavelengths used. Rods are most sensitive to light of 505 nm (blue). It has been postulated that rods share the pathways used by blue cones. As the eye dark adapts and rods begin to send messages, more blue messages are sent to the hue center. Therefore, at dusk, although the brightness of all colors decreases, blues and greens appear to gain in relative brightness when compared with yellows and reds. This phenomenon is called the Purkinje effect after the Czechoslovakian scientist Purkinje, who first described it while watching blue and green flowers become relatively brighter (as compared with red and yellow) at dusk.

 Power of air filled eye: (-150) D

 Eigengrau: (―dark light,‖ retinal ―self-light‖): Complete dark adaptation of the eye does not produce a sensation of absolute black. Instead, a uniform gray with superimposed phosphene-like dots is generally noted. The dots observed in this entoptic phenomenan are more mobile than phosphenes and may form colored patterns that drift about the field.

 Troxler fading

o refers to the spontaneous suppression of the visibility of an image that occurs when one stares intently at a point in a scene. After some seconds or even minutes, parts (but not all) of the field will lose contrast and merge into a misty blur. Moving the eyes restores clear vision.

o Ignaz Paul Vital Troxler 1804

o E.g: Lilac chasec (card with disappearing dots)

 Haag-streit slit lamp decreases binocular viewing angle from 13.5 degrees to 4.5 degrees.

 Cornea absorbs rays shorter than 295 nm. Therefore rays between 600 and 295 nm only can reach the crystalline lens.

148 The normal human eye is insensitive to wavelengths between 400 and 350 nm (ultraviolet rays) because they are absorbed by the crystalline lens of the eye.

In aphakic eyes the light rays between 350 and 400 nm can also pass on the retina.

Therefore, the Aphakic eyes are sensitivc to those wavelengths which give rise to the sensation of blue or violet colour so the newly aphakic patients often complain that every thing looks bluer than visualized before the operation.

 Persistence of the eye is 0.1 second: i.e. if the time interval between two successive light pulses is lesser than 0.1 second, eye cannot distinguish them separately

 Raman Effect: The light (or photons) impinging on a molecule interacts in various ways but the final outcome always results in the scattering of light. For example, we do not see light directly. We always see light and objects as a result of scattered light. Scattering is

absorbance of incident light used in exciting the atom and reradiation of this light. The Raman scattering is the result of inelastic collisions in which the scattered photons exchange energy with the vibrational energy modes of an atom.

 In a toric surface, one principal meridian is more curved than the second principal

meridian. The principal meridian with minimum curvature, and therefore with minimum power, is called base curve of a toric lens.

 A reduction of 1 mm in the depth of the anterior chamber (through a forward shift of the crystalline lens) would increase the eye's total power by about 1.4 D.

 Accommodation in uncorrected myopes is not developed normally. Since they need not accommodate to see the near objects clearly. For this reason they may suffer from convergence insufficiency, exophoria and early presbyopia as they grow older.

 Night myopia or twilight myopia: The shift from photopic to scotopic vision at twilight is associated with increased sensitivity to the shorter wavelengths of light. The

emmetropic eye, if accommodated for the middle range of the visual spectrum, will be slightly myopic for the shorter wavelengths.

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 Symmetric astigmatism: refers to the regular astigmatism in which the principal meridia in each eye have similar but opposite axes, e.g. 15° in left eye and 165° in right eye which together add up to 180 +- 15°.

 Depth of field: the range of distance from the eye in which an object appears clear without change of accommodation.

 Depth of Focus: The range at the retina in which an optical image may move without impairment of clarity is termed as depth of focus.

 Reaction time:

Accommodation: Far to near: 0.64 s Accommodation: Near to Far: 0.56 s Direct light reflex: 0.26 s

Convergence response: 0.2 s

 Causes of premature presbyopia

o Uncorrected hypermetropia

o Premature sclerosis of the crystalline lens

o General debility causing presenile weakness of ciliary muscle o Chronic simple glaucoma

 Angle of convergence becomes smaller with increasing fixation distance and becomes larger with increasing IPD.

 In air, the speed of light remains relatively constant. When the light passes through a higher index of refraction, its properties change and aberrations are formed. This can be explained by the following equation:

F = Vn/ λ

150 F = frequency, V = velocity, n = index of refraction, λ = wavelength.

 The waves of light are joined at a single point in time by what is called a wavefront and always travel perpendicular to it. The distorted shape that a wavefront takes after emerging from an irregular optical media is called a wavefront aberration.

 A practical example of spherical aberration is the comparison between the optical properties of the +20D trial frame lens and the aspheric indirect binocular

ophthalmoscope (IBO) +20D lens. Both lenses focus light coming from infinity to a focal point of 5 centimeters behind the lens. However, when analyzing the quality of the entire image, one can observe a deformity in the periphery of the image formed by the trial frame lens, which is not present with the aspheric lens. The higher the power of a lens, the more the distortion produced by spherical aberration.

 Point spread function (PSF) is the intensity with which an optical system distributes an image from a point source onto the retina. The point source is influenced by the pupil size. The larger the pupil, the more irregular the shape of the point source imaged on the retina

 Modulation transfer function (MTF): MTF is the ability of the eye's optics to focus a sharp image on the retina with high contrast. As light passes through optical structures of the eye, it undergoes a process of ―degradation‖ which can be measured by MTF.

 Optical limitations to vision

• Pupil size

• Nyquist sampling limit: The spatial frequency of the images entering the eye can be influenced by the pupil size – the wider the pupil, the higher the spatial frequency of an object that can be perceived by the eye. However, the highest spatial frequency that can be detected by the visual system is also limited by the number of photoreceptors densely packed in the fovea also known as the Nyquist Sampling Limit.

• Diffraction: a phenomenon which occurs when light waves are bent as they enter an aperture – in the case of the human eye, the pupil. In 1896, the German physicist, Arnold Sommerfeld, defined diffraction as ―any deviation of light rays from a rectilinear path which cannot be interpreted as a reflection or refraction‖.

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• Styles-Crawford effect: This is the effect of light entering the cones transversely from the pupil margin, which is perceived half as bright as the light entering the center of the pupil. In simpler terms, light that passes through the edge of the pupil contributes less to image quality than light entering the center of the pupil.

 There are two classes of aberrations: chromatic and monochromatic. Theoretically, correcting both chromatic and monochromatic aberrations increases the contrast of images focused on the retina (contrast sensitivity).

o Chromatic aberrations

 also known as ―achromatism‖

 inability of a lens to focus all colors of light on a single point.

 arises because the index of refraction of the media is not the same for all wavelengths.

o Monochromatic aberrations

 defects of an image caused by the nature of a lens

 Types:

1. Piston 2. Tilt 3. Defocus

4. Spherical aberration 5. Coma

6. Astigmatism

 Wavefront sensing devices

1. Shack–Hartmann 2. Tscherning aberrometer 3. Laser ray tracing

4. Slit skiascopy

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 The main focus of laser refractive surgery nowadays is to not only correct refractive errors such as defocus and astigmatism, but to create customized ablations which compensate for induced higher-order aberrations (HOA) such as spherical aberration (optimized ablation) and also correct pre-existing aberrations (customized ablation).

 Visual effects of aberrations o Glare

o Starbursts o Haloes o Ghost images o Poor image contrast o Poor night vision

 The term ―aberrometry‖ is used to describe the science of the detection and analysis of

 The term ―aberrometry‖ is used to describe the science of the detection and analysis of

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