What is the wavelength range of UVA?
What is the wavelength range of UVB?
What is a diverging pencil of light rays?
A set of light rays that emerges from a point source
What is a converging pencil of light rays?
A set of light rays focused at a point
What is the nature of light rays generated by an object at infinity?
An object at infinity produces a parallel pencil of light rays
Are light rays perpendicular or parallel to wavefronts?
What is vergence?
A measure of the curvature of a wavefront
What is the sign convention for the vergence of diverging light?
What is the sign convention for the vergence of converging light?
Is the magnitude of vergence smaller or larger as one approaches a light source or focal point?
What are the units for vergence?
diopters = 1/meters
What is the vergence of a light source located infinitely far away?
0 (Zero). The curvature of the wavefront goes to 0 as the distance from the source goes to infinity.
What is considered the cutoff for being "infinitely far away" for clinical purposes?
20 ft (6m)
What is the refractive index of air?
What is the refractive index of water?
What is the refractive index of polycarbonate?
How does the speed of light change as it enters a more optically dense medium?
The speed of light slows
What is a refractive index?
The ratio of the speed of light in a vacuum to its speed in a given medium.
What is the phenomenon of refraction?
The deviation of a ray of light as light travels from one optical medium into another medium of different optical density. It is related to the change in the speed of light due to differing optical densities.
When a light ray travels into a material with a higher index of refraction, is it deviated toward or away from the surface normal?
Toward the surface normal
What is a "critical angle?"
The angle of incidence beyond which a light ray undergoes total internal reflection when entering one medium from another.
How can the critical angle be derived?
What is Snell's Law?
n_1 sin (theta_1) = n_2 sin (theta_2)
What is the secondary focal point of light traveling from one medium into another?
The point to which the light converges or from which the light appears to diverge as light enters the "secondary" medium from the primary medium.
What is the primary focal point of light traveling from one medium into another?
The point to which the light converges or from which the light appears to diverge as light enters the "primary" medium from the secondary medium.
What is the secondary focal length?
The distance from the surface apex to the secondary focal point
By what expression is the dioptric power (or refractive power) of a spherical surface given?
F = | n_2/f_2 |
Is the power of a converging optical system positive or negative?
What is a common sign convention for solving optical problems?
Light is assumed to travel from left to right. Distances to the right of a surface are positive, and those to the left are negative.
Does the primary focal length equal the secondary focal length of a spherical surface?
No. Power is "conserved," since F = | n_1/f_1 | = | n_2/f_2 |. That is, the spherical surface has the same power regardless of the direction of the light rays. This indicates that focal length scales with index of refraction.
What is the sign convention for the radius of curvature of a spherical surface?
If the center of curvature is to the right of the apex of the surface, the radius of curvature is positive. If the center of curvature is to the left of the apex of the surface, the radius of curvature is negative.
Is a real image formed by converging or diverging light rays?
Converging light rays
Is a virtual image formed by converging or diverging light rays?
Is an image formed by diverging light rays able to be focused on a screen?
No. This means that a virtual image cannot be focused on a screen.
Are real images upright or inverted?
Always inverted. Consider the light ray from the tip of the object through the center of curvature of a spherical surface (this ray does not deviate since it is normal to the surface) for intuitive confirmation of this.
Are virtual images upright or inverted?
What is another term for the "vergence relationship?"
The "paraxial relationship." This term derives from the use of the assumption that rays are paraxial (i.e., in relatively close proximity to the optical axis of the surface). This assumption is required to derive the paraxial or vergence relationship.
What is the expression for the vergence of the light rays emanating from an object in a medium with index of refraction n_1 at distance l_1?
L_1 = n_1 / l_1
What is the expression for the vergence of an image at distance l_2 from the apex of a spherical surface in medium with index of refraction n_2?
L_2 = n_2 / l_2
What is the vergence relationship?
image vergence = object vergence + surface power; L_2 = L_1 + F
What does the sign of image vergence indicate?
Whether the image is real or virtual (if positive, converging rays, then real; if negative, virtual)
What is the dioptric power of a flat surface?
0 diopters. Its radius of curvature is infinite.
What is the magnification of an optical system in terms of vergence?
M_L = L_1 / L_2. That is, vergence of object / vergence of image. Alternative notation could be M_L = L_o/L_i (mnemonic: lowly).
What is the term used for the back surface of an ophthalmic lens?
The ocular surface
What is the nominal or approximate power of a thin lens?
F_T = F_1 + F_2, where F_1 and F_2 are the powers of the front and back surfaces of the lens, respectively.
How do the primary and secondary focal points of a thin lens compare if the media on both sides of the lens are the same?
The primary and secondary focal points are equidistant from the lens.
How is a ray passing through the optical center (intersection of lens and optical axis) of a thin lens deviated?
It is NOT deviated
Is the image formed by an object greater than 1 focal length but less than 2 focal lengths away from a thin lens real or virtual?
Real, inverted, minified
Is the image formed by an object less than 1 focal length away from a thin lens real or virtual?
Virtual, upright, magnified
Is the image formed by an object greater than 2 focal lengths away from a thin lens real or virtual?
Real, inverted, magnified
What kind of image of an object is formed by a minus lens?
Virtual (on same side of lens as object), upright, minified
If the media on both sides of a thin lens is the same, what is a simplified expression for the lateral magnification?
M_L = l_i / l_o = l_2 / l_1
Where are the object vergence and lens power commonly noted in a lens diagram?
Above the lens. The image vergence is commonly noted below the lens.
For what purpose is Newton's relation used?
Newton's relation can be used to locate an object and its image with respect to the focal points of a plus lens.
What is Newton's relation?
xx' = f^2, where x is the distance of the object from the primary focal point (F), x' is the distance of the image from the secondary focal point (F'), and f is the focal length
What is the total power of a set of thin lenses without significant distance between the lenses?
The sum of the powers of the individual lenses.
What is the total lateral magnification of a system of lenses?
The product of the magnifications for each lens:
What is a virtual object?
An object that would have been formed by a converging lens if a second lens were not in the way. A virtual object always has positive vergence and can only be created by a converging surface.
What is a thick lens?
A lens whose thickness istaken into account in calculating its impact on light rays.
In which medium do refracted rays of light exist after passing through the first surface of a thick lens?
In the medium of the lens material.
For a thick lens, what constitutes the object space for the second surface of the lens?
The image space of the first surface (from left to right) of the lens.
What is the lateral magnification for an optical system with two surfaces?
The product of the mangification for each surface:
What is an equivalent lens?
An alternative representation of a thick lens involving the superposition of 2 thin lenses located at "principal planes." The planes are referred to as H and H'
What are the "equivalent" focal lengths of a thick lens?
The focal lengths (f_e and f'_e) measured from the equivalent planes H and H' to the primary and secondary focal points, respectively.
What is the back vertex focal length of a thick lens?
The distance, f_v, from the back surface of the lens to the secondary focal point.
What is the front vertex focal length of a thick lens?
The distance, f_n, measured from the front surface of the lens to the primary focal point.
When using a lensometer to neutralize the distance prescription in a patient's spectacle lenses, what power are we measuring?
The back vertex power, F_v.
By what expression can back vertex power be given?
F_1 = first surface power
F_2 = second surface power
t = thickness of lens in meters
n = index of refraction of lens
By what expression can front vertex power be given?
How do the front and back vertex powers (F_n and F_v) differ from the powers of the front and back surfaces (F_1 and F_2) of a thick lens?
The front and back vertex powers are two different approximations of the overall power of the thick lens. F_1 and F2 are the individual powers of the first and second surfaces of the thick lens.
By what expression is Equivalent Power of a thick lens given?
What are the nodal points of a thick lens?
A pair of points such that a light ray directed at one nodal point emerges from the other nodal point at an angle equal to the angle of incidence. For a thick lens surrounded by a single medium, the primary and secondary nodal points are located at the intersections of the of the prinicipal planes (H and H') with the optical axis.
What is the nodal point of a thin lens?
The intersection of the lens plane with the optical axis.
What is the nodal point of a spherical refracting surface?
The center of the radius of curvature (rays passing through the center of the radius of curvature are normal to the surface and thus are not deviated).
What are the locations of the principal planes with regard to the apices of the first and second surfaces (A_1 and A_2)?
What is the refracting surface in the "reduced eye?"
There is a single spherical refracting surface in the reduced eye, with a radius of curvature of 5.55mm. The surface separates air from aqueous humor, with index of refraction 1.333.
Where is the nodal point of the reduced eye?
At the center of curvature of the spherical refracting surface. The axial length of the reduced eye is approximately 22.22mm, and the location of the nodal point is often approximated as being 17mm in front of the retina.
What is the power of the reduced eye?
Is the reduced eye emmetropic?
What is the axial length of the reduced eye?
What are the secondary and primary focal lengths of the reduced eye?
f' = 22.22mm; f = -16.67
What is emmetropia?
A state of an eye such that an infinitely distant object is focused on the retina.
What is ametropia?
A state of an eye such that an infinitely distant object is not focused on the retina. Two common forms of ametropia are myopia and hyperopia.
What does it mean that the Far Point (FP) of the eye is conjugate with the retina?
An object placed at the far point is focused on the retina. For the emmetropic eye, the far point is at infinity. For the hyperopic eye, the far point is behind the eye. For the myopic eye, the far point is in front of the eye, but not infinitely distant.
What is the far-point vergence of an eye?
The vergence required of an eye in order for the image of the object to be focused on the retina. The far point vergence is equivalent to the correction required (in the plane of the refracting surface of the reduced eye) to achieve emmetropia (focusing of object at infinity onto the retina)
Is the far point vergence for a myope positive or negative?
Negative. Far point vergence for a hyperope is positive.
How can ametropia be corrected with a lens in the plane of the eye's refracting surface?
With a lens that focuses objects at infinity at the far point of of the eye. That is, the secondary focal point of the lens must be coincident with the far point of the eye.
What is the vertex distance?
The distance between the cornea and the back (ocular) surface of a (spectacle) lens.
What is a common range for vertex distance?
Is greater lens power required to correct myopia in the spectacle plane than in the corneal plane?
Yes (more minus power)
Is a spectacle lens or closer or farther from the far point of the myopic eye than a contact lens?
Closer. Thus, it needs to be stronger (more minus) to achieve emmetropia.
Is a spectacle lens closer or farther from the far point of the hyperopic eye than a contact lens?
Farther. Thus, the power required of a spectacle lens for hyperopic correction is smaller (less plus) than that of a contact lens.
What is accommodation (for the purpose of optics)?
The process by which the refractive power of the eye can be increased.
What is the typical amplitude of accommodation for a 10 year old?
What is the typical amplitude of accommodation for a 30 year old?
What is the typical amplitude of accommodation of a 40 year old?
What is the typical amplitude of accommodation of a 70 year old?
What is presbyopia?
The reduction in amplitude of accommodation associated with aging.
What is a bifocal add?
The plus lens power used to allow a presbyopic patient to focus on objects at near.
Where is accommodation represented in the reduced eye?
At the single refracting surface of the reduced eye, located at the front surface of the eye (technically 1.67mm posterior to the cornea, but typically assumed to be at the anterior surface for the sake of calculations).
In order for a near object to be focused on the retina, the vergence leaving the accommodative lens must be equal to what?
To the far-point vergence
What is the vergence relationship for accommodation?
What is the far-point vergence of an emmetropic eye?
The far-point is located at infinity for an emmetropic eye. Thus, the far-point vergence is 0 for an emmetropic eye.
Why might aging myopies sometimes remove their distance spectacles to view near objects?
A myope with uncorrected myopia can focus on objects at near (-3D myope can focus on objects at 33.33cm)
If an eye is corrected in the corneal plane, what is the required accommodation in comparison to the stimulus to accommodation?
Equal and opposite.
What is the Near Point of Accommodation (NPA)?
The minimum distance an object can be from the eye while still being imaged clearly on the retina.
Is the NPA closer for an uncorrected myope or uncorrected hyperope?
closer for the uncorrected myope
How is the reading "add" of a presbyopic patient determined?
The goal is to prescribe an add that requires the patient to use one-half of his/her accommodative amplitude (dioptrically, not linearly) at his/her habitual reading distance.
What are the benefits of centering a patient's reading add dioptrically at his/her usual reading distance?
It will allow the patient to accommodate for slightly nearer objects or relax accommodation for farther objects. It will also allow for mild variation in reading distance that can be dealt with purely by adjusting accommodation.
What is Negative Relative Accommodation (NRA)?
The maximum amount of plus power (beyond a tentative add based on age and history) that the patient accepts while reading at her normal reading distance before experiencing blurring of letters. This is the amount that the patient is able to relax accommodation from the combined power of her distance correction and tentative add.
What is Positive Relative Accommodation?
The maximum amount of minus power (over a tentative add based on age and history) that the patient accepts while reading at her normal reading distance before experiencing blurring of letters. This is the amount that the patient is able to accommodate beyond the combined power of her distance correction and tentative add.
What is the amplitude of accommodation in terms of NRA and PRA?
Amplitude of accommodation = NRA + PRA
If a patient's PRA > NRA, should the add be increased or decreased?
Decreased. The patient has greater ability to accommodate than relax, and thus does not require as much add.
What is the radius of curvature of a cylindrical lens along its axis?
What is the orientation of the power meridian of a cylindrical lens compared to its axis meridian?
The power meridian is orthogonal to the axis meridian
How many secondary focal points does a spherocylindrical lens have?
2 secondary focal points. The focal points are different for the axis and power meridians of the cylindrical component.
What is the customary label for the horizontal meridian (0 degrees) in discussing lens meridians?
What are the 3 components of a spherocylindrical lens formula?
1) Sphere, 2) Cylinder, 3) Axis meridian of cylinder
What is a quick, 3-step method for converting between plus and minus cylinder formulae?
1) Spherical power = sum of old spherical and cylindrical powers, 2) Change sign of cylindrical component, 3) Rotate axis by 90 degrees
In discussing spherocylindrical lenses, is an axis of greater than 180 degrees ever used?
No. The interval used is (0, 180]
What is the circle of least confusion for a spherocylindrical lens?
A circle dioptrically equidistant from the two image planes of a spherocylindrical lens. The circle of least confusion is in the plane at which the principal meridians are equally defocused.
What is the interval of Sturm?
The interval between the two image planes of a spherocylindrical lens (one image plane for each principal meridian).
What are the principal meridians of a spherocylindrical lens?
The meridians of maximum and minimum power. The principal meridians are orthogonal to each other.
What is the expression for power of a cylindrical lens in an oblique meridian?
If theta is the angle between the oblique meridian and the cylinder's axis meridian:
What is the relationship between the principal meridians of an eye with regular astigmatism?
The principal meridians are orthogonal to each other.
What is the relationship between the principal meridians of an eye with irregular astigmatism?
The principal meridians are not orthogonal to each other.
Can irregular astigmatism be fully corrected with a spherocylindrical lens?
What are the 5 forms of astigmatism?
Compound myopic/hyperopic (both principal meridians require correction in same direction)
Simple myopic/hyperopic (only one principal meridian requires correction)
Mixed (both principal meridians require correction, but in opposite directions)
What is "with-the-rule" astigmatism?
Astigmatism such that the vertical meridian has maximum power.
What is "against-the-rule" astigmatism?
Astigmatism such that the horizontal meridian is of greatest power.
What is oblique astigmatism?
Astigmatism such that the meridian of greatest power is not horizontal or vertical.
At what stage is cylindrical power refinement complete with the Jackson crossed cylinder test?
If positions 1 and 2 of the crossed cylinder produce no difference in image quality.
How can a spherical equivalent be computed?
Sph equiv = sph + cyl/2
Why does the spherical component of a correction require modification when changing the cylindrical component?
If the spherical correction has already been refined, the circle of least confusion should be placed on the retina. When changing the cylindrical component, the circle of least confusion moves by half the dioptric amount of the cylindrical change in the same direction. Accordingly, the spherical component must be increased by half the dioptric amount of the cylindrical change in order to maintain the circle of least confusion on the retina. E.g., for -0.5 D cyl change, make +0.25 sph change to re-center Interval of Sturm and place CoLC on retina.
Which optotype is particularly difficult to resolve for a patient demonstrating simple myopic with-the-rule astigmatism?
F -- the horizontal segments of the optotype will be blurry.
In which direction are light rays deviated by a prism?
Toward the base of the prism.
Are images formed by prisms real or virtual?
What is the minimum angle of deviation for a prism?
d_min = alpha * (n - 1)
How is the power of a prism in prism diopters defined?
P = (100)(y/x) = tan (d)
where d is the angle of deviation and y and x are the vertical and horizontal deviations, respectively, measured in the same distance units
A prism that deviates a light ray 1 cm at a distance of 1m has what prism power?
1 prism diopter
What is Prentice's rule for the prismatic power of a lens?
P = cF
where P is the prismatic power, c is the distance in centimeters from the optical axis, and F is the lens refractive power.
Through what relation can Prentice's rule be derived?
P = (100)(c/f')
where P = power in prism diopters
c = distance in centimeters from optical axis
f' = secondary focal point of lens
What effect does decentering of a lens on the eye's optical center have?
Induction of prism
What are the main determinants of angle of deviation for a prism?
1) apical angle, 2) index of refraction
The greater the prism thickness, the greater variation in angle of deviation occurs with variation in angle of incidence.
What is a Fresnel prism?
A thin, light, easily changed series of prisms that can be cut to shape and applied (with adhesive) directly to the surface of spectacle lenses.
In which direction should prisms be oriented to help a patient with a right homonymous hemianopia?
Base-right. This will allow images from the right visual field to be deviated on to the right hemiretinas.
What is absolute presbyopia?
Complete inability to accommodate
What is the approximate size of a pinhole used in visual acuity testing?
What is a retinal blur circle?
A circle formed on the retina due to a point source of light that is not located at the far point of the eye. Size of the blur circle grows with distance (possibly dioptric, rather than linear; remains to be elucidated) from the far point.
How does a pinhole reduce the size of a retinal blur circle?
A pinhole reduces the incidence of peripheral rays on the retina, reducing the size of the blur circle. It does sacrifice illumination and result in diffractive effects, but can be useful for acuity testing.
What is senile miosis?
A tendency toward reduction in pupil size in elderly individuals.
What is diffraction?
A phenomenon whereby light appears to bend around opaque objects, such as the pupil.
What is an Airy's disk?
The distance in a diffraction pattern from the center of the light peak to the first trough.
What is the minimum distance between peaks of the diffraction patterns of two point sources to allow resolution of the two pint sources?
The distance between the peaks must be >= the radius of an Airy's disk
How does a diffraction pattern (i.e. the distribution of light) formed on the retina from a monochromatic source change with wavelength and pupil diameter?
where theta (radians) is the radius of the Airy's disk, lambda is the wavelength of light, and d is the pupillary diameter.
Airy's disk radius rises with longer wavelength and smaller pupillary diameter.
What is the resolution that can be achieved based on packing density of retinal photoreceptors alone?
What is the limiting factor for resolution with pupillary diameters less than 2mm?
What is "depth of field?"
The range of distance from the eye within which a given optotype can be resolved (whether or not it is focused directly on the retina). It is customarily measured in diopters, but can be measured in meters.
What is "depth of focus?"
The range centered on the retina within which objects located in the depth of field of the retina are focused. It is customarily measured in diopters, but can be measured in meters.
How does depth of field relate to depth of focus?
Depth of field is dioptrically equal to depth of focus. The limits of field are conjugate to the limits of focus. The linear depth of field (in meters) is usually larger than the linear depth of focus (in meters).
What is the difference between "true" and "apparent" amplitudes of accommodation?
True amplitude of accommodation is purely due to accommodative effort. Apparent amplitude of accommodation is due to the combination of accommodative effort and depth of field.
What is hyperfocal distance?
The nearest distance at which the eye can be focused and still resolve infinitely distant objects. It is limited by the depth of field.
How can the hyperfocal distance of an eye be determined?
The hyperfocal distance is dioptrically half of the depth of field. For example, for an eye with depth of field of 1 diopter, the hyperfocal distance is 0.5 diopters, or 2m.
What is low vision?
Vision that is not satisfactory for everyday tasks even when ametropia is fully corrected.
What unit of measurement is commonly used to specify the magnification of print/typeface?
What is the definition of 1M print?
Print that subtends an angle of 1 minute of arc at a viewing distance of 1 meter.
How many minutes of arc does 5 M print subtend at 0.5 meters?
5 minarc * 1/0.5 = 10 minarc
What is a "reference distance" in the context of low vision?
The viewing distance at which a patient's near vision is known (i.e., what minimum M-size the patient is able to view at that distance).
What is a an equivalent viewing distance?
The distance at which one appears to be viewing an object when looking through a plus lens held with the object at its primary focal point
What is the equivalent viewing distance of a +20 D lens?
0.05m (5 cm)
What is "minimum angle of resolution?"
The angle subtended at the eye of the smallest detail the patient can resolve. It is found by taking the reciprocal of Snellen visual acuity.
How many minutes of arc does 2 M print subtend at 1m?
2 minutes of arc
How can the "add power" needed in conjunction with a fixed-focus stand magnifier be determined?
With the "add equation:"
Equivalent viewing power = M * (add power)
How can the effective magnification at 25cm be computed?
What is an afocal optical system?
An optical system without focal points, such as a telescope used to view an infinitely distant object.
What is an objective lens?
A converging lens that is generally the first lens through which light rays pass in traditional Galilean and Keplerian telescopes.
What is an eyepiece?
A plus or minus lens that is generally the second lens through which light rays pass in a traditional Galilean or Keplerian telescope.
What is another term for objective lens?
What two lenses comprise a Galilean telescope?
A plus objective lens and minus eyepiece lens. The eyepiece is located such that its primary focal plane is coincident with the secondary focal plane of the objective lens. That is, F_2 is coincident with F_1'.
What two types of lenses comprise a Keplerian telescope?
A plus objective lens and stronger plus eyepiece lens. As with the Galilean telescope, the eyepiece is located such that its primary focal plane is coincident with the secondary focal plane of the objective lens. That is, F_2 is coincident with F_1'.
What is the tube length of a telescope?
The distance between the objective and eyepiece
What is the angular magnification of a Galilean or Keplerian telescope in terms of lens powers?
What is the angular magnification of a Galilean or Keplerian telescope in terms of tube length?
What is the angular magnification of a Galilean or Keplerian telescope in terms of entrance and exit pupil diameters?
What is a lens cap?
A plus lens placed over the objective lens in order to allow an object at the primary focal point of the lens (rather than at infinity) to be imaged through the telescope.
What is a telemicroscope?
A telescope that has been adapted for near use by placing a lens cap (plus lens) over the objective and positioning the object at the primary focal point of the lens cap. The lens rays leaving the lens cap are parallel, so a nearby object can be viewed by a telescope as though it were at inifnity.
How do telescopes compare to plus lenses for use in low vision or magnification?
Plus lenses have shorter working distance (when placed on spectacles), but provide a greater field of view.
What is aniseikonia?
A difference between the size or shape of the images of an object formed on the retinas of the two eyes.
What are the two factors contributing to spectacle lens magnification?
1) Power factor, 2) Shape factor
What is a size lens?
A lens that has no refractive power, but does produce angular magnification
What is the equation for magnification due to power?
where F_v is the back vertex power of the lens and d is the vertex distance
What is the equation for mangification due to lens shape?
where t is the thickness, n is the refractive index, and F_1 is the front surface power.
What is the equation for spectacle mangification?
If two lenses of equal power are made of the same material and have the same thickness and vertex distance, will the one with the more curved or less curved front surface produce the greater magnification?
The spectacle lens with the more curved front surface will produce greater magnification.
Do contact lenses affect retinal image size?
No. Contact lenses have a vertex distance of 0, making the power magnification factor 1. Due to the thinness of contact lenses, the shape magnification factor is very nearly 1. The product of these factors is very nearly 1, meaning that contact lenses do not meaningfully affect retinal image size.
What is the difference between axial and refractive myopia?
Axial myopia indicates that the axial length is greater than normal, whereas refractive myopia indicates that the refractive power of the eye is greater than normal. Both can be corrected with minus lenses, but involve different retinal image sizes.
Is the retinal image size in uncorrected axial myopia larger or smaller than that in uncorrected refractive myopia?
Larger in uncorrected axial myopia. The longer the axial length, the larger the image size.
Is the retinal image size in uncorrected axial myopia larger or smaller than that in emmetropia?
Larger in uncorrected axial myopia. The longer the axial length, the larger the retinal image size.
How does the image size in uncorrected refractive ametropia compare to that in emmetropia?
The image sizes are the same. The image in uncorrected refractive ametropia is blurred, but of the same size since the axial length is the same as in the emmetropic eye.
Does a plus spectacle lens cause magnification or minification?
Does a minus spectacle lens produce magnification or minification?
Is the retinal image size in axial hyperopia larger or smaller than that in emmetropia?
Smaller than in emmetropia.
In attempting to avoid changes in retinal image size, what is an effective dichotomy in managing axial vs. refractive ametropia?
- Correct axial ametropia with spectacles
- Axial myopia produces larger retinal image that is minified by minus spectacle lenses --> nearly normal image size
- Axial hyperopia produces smaller retinal image that is magnified by plus spectacle lenses --> nearly normal image size
- Correct refractive ametropia with contact lenses
- Image size already normal if refractive
- Contact lenses introduce no change in image size
- Axial myopia produces larger retinal image that is minified by minus spectacle lenses --> nearly normal image size
- Axial hyperopia produces smaller retinal image that is magnified by plus spectacle lenses --> nearly normal image size
- Image size already normal if refractive
- Contact lenses introduce no change in image size
In order for the retinal image size in ametropia corrected with spectacles to be the same as in emmetropia, what should the vertex distance be (in the reduced eye)?
How can correction of anisometropia lead to aniseikonia?
Depending on the etiology of the ametropia (axial vs. refractive) in the two eyes, correction may improve or worsen aniseikonia.
Correcting refractive ametropia with spectacle lenses in anisometropia can create disparity in retinal image size due to greater magnfication or minification in one eye than the other.
What factors can be modified intentionally to attempt to achieve iseikonia with spectacle correction?
Thickness and front surface power can significantly affect the shape factor of magnification.
What is a specular surface?
A smooth, shiny surface that reflects light.
What is a perfectly diffusing surface?
A surface that has the same brightness regardless of the angle from which it is viewed. A blackboard approximates a perfectly diffusing surface.
For a specular surface is the angle of reflected light equal to the angle of incidence?
What is the power of a mirror in air?
F = -2/r
where r is the radius of curvature of the mirror
What is the relationship between a mirror's power and its secondary focal length?
F = -1 / f'
What is the relationship between the radius of curvature of a mirror in air and its secondary focal length?
r = 2f'
What is the relationship between image vergence and image distance for a mirror in air?
L' = -1 / l'
where L' is the image vergence and l' is the image distance.
Note the (-) sign, which is needed due to the reversal of direction of travel light after reflection
What is the lateral magnification of a mirror in air?
Are images formed by minus (convex) mirrors real or virtual?
Always virtual, since convex mirrors diverge light.
Are images formed by plano mirrors real or virtual?
What is the magnification of a plano mirror?
What is the magnification of a minus (convex) mirror?
0 < M_L < 1x
What is the magnification of a plus (concave) mirror for an object located at 2 times its focal length (2f)?
What proportion of light rays orthogonal to a transparent surface are reflected by that surface?
where n is the RI of the medium surrounding the surface (generally air) and n' is the RI of the surface
What relationship between the RI of an anti-reflection coating and the RI of a lens maximizes the destructive interference of reflected light?
This is derived by setting the proportion of light reflected from the coating equal to the proportion of light reflected from the lens.
What is the source of Purkinje image I?
Reflection of light from the anterior surface of the cornea. It is used in measuring angle kapp and performing the Hirschberg test.
What is an aspheric lens?
A lens that does not have a single radius of curvature. The cornea is actually aspheric, with greater curvature centrally than peripherally.
Which Purkinje image is used by corneal topographers?
Purkinje image I, as generated by concentric luminous rings.
What is the difference between the measurements obtained by corneal topography and keratometry?
Topography provides corneal curvature information in all meridians, while keratometry provides corneal curvature information only in the two principal meridians.
What are the five deviations from the predictions of the paraxial assumption that are found when using the 3rd degree Taylor approximation of the sin function?
The Seidel, or classic, aberrations:
- spherical aberration
- oblique astigmatism
- curvature of field (power error)
What is the Coddington shape factor?
A measure of the amount of spherical aberration of a lens, based on the radii of curvature of its front and back surfaces.
What is "positive (undercorrected) longitudinal spherical aberration?"
The characteristic of spherical lenses to focus light rays striking the periphery of the lens (nonparaxial rays) closer to the lens than the central (paraxial) rays. It is present in both plus and minus lenses.
What is the lens aberration "coma?"
The characteristic of a spherical lens to image off-axis point objects as a comet-like shape due to non-uniform refractive power of the lens surface.
What is the lens aberration "oblique astigmatism?"
A lens aberration resulting from differing angles of incidence for horizontally and vertically diverging rays from an off-axis object.
What is the lens aberration "curvature of field?"
The characteristic of a spherical lens imaging a flat object to create a curved image plane due to varying distances from the lens of the object itself.
What is the lens aberration "distortion?"
The characteristic of the central and peripheral portions of a spherical lens to produce differing amounts of lateral magnification.
What are the two basic types of distortion due to a spherical lens?
1) barrel distortion (minus lenses), 2) pincushion distortion (plus lenses).
Minification in a minus lens is greater in the peripher than its center.
What is a Hartmann-Shack aberrometer?
An apparatus that focuses a point of light on the retina that serves as a point source and measures the pattern of monochromatic aberrations after the light passes back through the eye's optical system.
What are Zernike polynomials?
A manner of quantifying the aberration of an eye, particularly with respect to data obtained from an aberrometer. Zernike polynomials are not moer commonly used in describing aberrations than Seidel aberrations.
What is meant by the term adaptive optics?
"Adaptive optics" refers to the usage of a deformable mirror to compensate for wavefront distortions as measured by an aberrometer. Utilizing adaptive optics allows for improved contrast sensitivity and resolution.
What is chromatic dispersion?
The separation of white light into its component elements by a prism or other optical element.
What are the wavelengths typically used to quantify the amount of dispersion produced by a prism or lens?
486, 589, and 656 nm. These wavelengths are absent in the sunlight that reaches the surface of the earth, creating dark lines in the solar spectrum called Fraunhofer lines.